3 perlfunc - Perl builtin functions
7 The functions in this section can serve as terms in an expression.
8 They fall into two major categories: list operators and named unary
9 operators. These differ in their precedence relationship with a
10 following comma. (See the precedence table in L<perlop>.) List
11 operators take more than one argument, while unary operators can never
12 take more than one argument. Thus, a comma terminates the argument of
13 a unary operator, but merely separates the arguments of a list
14 operator. A unary operator generally provides a scalar context to its
15 argument, while a list operator may provide either scalar or list
16 contexts for its arguments. If it does both, the scalar arguments will
17 be first, and the list argument will follow. (Note that there can ever
18 be only one such list argument.) For instance, splice() has three scalar
19 arguments followed by a list, whereas gethostbyname() has four scalar
22 In the syntax descriptions that follow, list operators that expect a
23 list (and provide list context for the elements of the list) are shown
24 with LIST as an argument. Such a list may consist of any combination
25 of scalar arguments or list values; the list values will be included
26 in the list as if each individual element were interpolated at that
27 point in the list, forming a longer single-dimensional list value.
28 Elements of the LIST should be separated by commas.
30 Any function in the list below may be used either with or without
31 parentheses around its arguments. (The syntax descriptions omit the
32 parentheses.) If you use the parentheses, the simple (but occasionally
33 surprising) rule is this: It I<looks> like a function, therefore it I<is> a
34 function, and precedence doesn't matter. Otherwise it's a list
35 operator or unary operator, and precedence does matter. And whitespace
36 between the function and left parenthesis doesn't count--so you need to
39 print 1+2+4; # Prints 7.
40 print(1+2) + 4; # Prints 3.
41 print (1+2)+4; # Also prints 3!
42 print +(1+2)+4; # Prints 7.
43 print ((1+2)+4); # Prints 7.
45 If you run Perl with the B<-w> switch it can warn you about this. For
46 example, the third line above produces:
48 print (...) interpreted as function at - line 1.
49 Useless use of integer addition in void context at - line 1.
51 A few functions take no arguments at all, and therefore work as neither
52 unary nor list operators. These include such functions as C<time>
53 and C<endpwent>. For example, C<time+86_400> always means
56 For functions that can be used in either a scalar or list context,
57 nonabortive failure is generally indicated in a scalar context by
58 returning the undefined value, and in a list context by returning the
61 Remember the following important rule: There is B<no rule> that relates
62 the behavior of an expression in list context to its behavior in scalar
63 context, or vice versa. It might do two totally different things.
64 Each operator and function decides which sort of value it would be most
65 appropriate to return in scalar context. Some operators return the
66 length of the list that would have been returned in list context. Some
67 operators return the first value in the list. Some operators return the
68 last value in the list. Some operators return a count of successful
69 operations. In general, they do what you want, unless you want
72 An named array in scalar context is quite different from what would at
73 first glance appear to be a list in scalar context. You can't get a list
74 like C<(1,2,3)> into being in scalar context, because the compiler knows
75 the context at compile time. It would generate the scalar comma operator
76 there, not the list construction version of the comma. That means it
77 was never a list to start with.
79 In general, functions in Perl that serve as wrappers for system calls
80 of the same name (like chown(2), fork(2), closedir(2), etc.) all return
81 true when they succeed and C<undef> otherwise, as is usually mentioned
82 in the descriptions below. This is different from the C interfaces,
83 which return C<-1> on failure. Exceptions to this rule are C<wait>,
84 C<waitpid>, and C<syscall>. System calls also set the special C<$!>
85 variable on failure. Other functions do not, except accidentally.
87 =head2 Perl Functions by Category
89 Here are Perl's functions (including things that look like
90 functions, like some keywords and named operators)
91 arranged by category. Some functions appear in more
96 =item Functions for SCALARs or strings
98 C<chomp>, C<chop>, C<chr>, C<crypt>, C<hex>, C<index>, C<lc>, C<lcfirst>,
99 C<length>, C<oct>, C<ord>, C<pack>, C<q/STRING/>, C<qq/STRING/>, C<reverse>,
100 C<rindex>, C<sprintf>, C<substr>, C<tr///>, C<uc>, C<ucfirst>, C<y///>
102 =item Regular expressions and pattern matching
104 C<m//>, C<pos>, C<quotemeta>, C<s///>, C<split>, C<study>, C<qr//>
106 =item Numeric functions
108 C<abs>, C<atan2>, C<cos>, C<exp>, C<hex>, C<int>, C<log>, C<oct>, C<rand>,
109 C<sin>, C<sqrt>, C<srand>
111 =item Functions for real @ARRAYs
113 C<pop>, C<push>, C<shift>, C<splice>, C<unshift>
115 =item Functions for list data
117 C<grep>, C<join>, C<map>, C<qw/STRING/>, C<reverse>, C<sort>, C<unpack>
119 =item Functions for real %HASHes
121 C<delete>, C<each>, C<exists>, C<keys>, C<values>
123 =item Input and output functions
125 C<binmode>, C<close>, C<closedir>, C<dbmclose>, C<dbmopen>, C<die>, C<eof>,
126 C<fileno>, C<flock>, C<format>, C<getc>, C<print>, C<printf>, C<read>,
127 C<readdir>, C<rewinddir>, C<seek>, C<seekdir>, C<select>, C<syscall>,
128 C<sysread>, C<sysseek>, C<syswrite>, C<tell>, C<telldir>, C<truncate>,
131 =item Functions for fixed length data or records
133 C<pack>, C<read>, C<syscall>, C<sysread>, C<syswrite>, C<unpack>, C<vec>
135 =item Functions for filehandles, files, or directories
137 C<-I<X>>, C<chdir>, C<chmod>, C<chown>, C<chroot>, C<fcntl>, C<glob>,
138 C<ioctl>, C<link>, C<lstat>, C<mkdir>, C<open>, C<opendir>,
139 C<readlink>, C<rename>, C<rmdir>, C<stat>, C<symlink>, C<umask>,
142 =item Keywords related to the control flow of your perl program
144 C<caller>, C<continue>, C<die>, C<do>, C<dump>, C<eval>, C<exit>,
145 C<goto>, C<last>, C<next>, C<redo>, C<return>, C<sub>, C<wantarray>
147 =item Keywords related to scoping
149 C<caller>, C<import>, C<local>, C<my>, C<our>, C<package>, C<use>
151 =item Miscellaneous functions
153 C<defined>, C<dump>, C<eval>, C<formline>, C<local>, C<my>, C<our>, C<reset>,
154 C<scalar>, C<undef>, C<wantarray>
156 =item Functions for processes and process groups
158 C<alarm>, C<exec>, C<fork>, C<getpgrp>, C<getppid>, C<getpriority>, C<kill>,
159 C<pipe>, C<qx/STRING/>, C<setpgrp>, C<setpriority>, C<sleep>, C<system>,
160 C<times>, C<wait>, C<waitpid>
162 =item Keywords related to perl modules
164 C<do>, C<import>, C<no>, C<package>, C<require>, C<use>
166 =item Keywords related to classes and object-orientedness
168 C<bless>, C<dbmclose>, C<dbmopen>, C<package>, C<ref>, C<tie>, C<tied>,
171 =item Low-level socket functions
173 C<accept>, C<bind>, C<connect>, C<getpeername>, C<getsockname>,
174 C<getsockopt>, C<listen>, C<recv>, C<send>, C<setsockopt>, C<shutdown>,
175 C<sockatmark>, C<socket>, C<socketpair>
177 =item System V interprocess communication functions
179 C<msgctl>, C<msgget>, C<msgrcv>, C<msgsnd>, C<semctl>, C<semget>, C<semop>,
180 C<shmctl>, C<shmget>, C<shmread>, C<shmwrite>
182 =item Fetching user and group info
184 C<endgrent>, C<endhostent>, C<endnetent>, C<endpwent>, C<getgrent>,
185 C<getgrgid>, C<getgrnam>, C<getlogin>, C<getpwent>, C<getpwnam>,
186 C<getpwuid>, C<setgrent>, C<setpwent>
188 =item Fetching network info
190 C<endprotoent>, C<endservent>, C<gethostbyaddr>, C<gethostbyname>,
191 C<gethostent>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
192 C<getprotobyname>, C<getprotobynumber>, C<getprotoent>,
193 C<getservbyname>, C<getservbyport>, C<getservent>, C<sethostent>,
194 C<setnetent>, C<setprotoent>, C<setservent>
196 =item Time-related functions
198 C<gmtime>, C<localtime>, C<time>, C<times>
200 =item Functions new in perl5
202 C<abs>, C<bless>, C<chomp>, C<chr>, C<exists>, C<formline>, C<glob>,
203 C<import>, C<lc>, C<lcfirst>, C<map>, C<my>, C<no>, C<our>, C<prototype>,
204 C<qx>, C<qw>, C<readline>, C<readpipe>, C<ref>, C<sub*>, C<sysopen>, C<tie>,
205 C<tied>, C<uc>, C<ucfirst>, C<untie>, C<use>
207 * - C<sub> was a keyword in perl4, but in perl5 it is an
208 operator, which can be used in expressions.
210 =item Functions obsoleted in perl5
212 C<dbmclose>, C<dbmopen>
218 Perl was born in Unix and can therefore access all common Unix
219 system calls. In non-Unix environments, the functionality of some
220 Unix system calls may not be available, or details of the available
221 functionality may differ slightly. The Perl functions affected
224 C<-X>, C<binmode>, C<chmod>, C<chown>, C<chroot>, C<crypt>,
225 C<dbmclose>, C<dbmopen>, C<dump>, C<endgrent>, C<endhostent>,
226 C<endnetent>, C<endprotoent>, C<endpwent>, C<endservent>, C<exec>,
227 C<fcntl>, C<flock>, C<fork>, C<getgrent>, C<getgrgid>, C<gethostent>,
228 C<getlogin>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
229 C<getppid>, C<getprgp>, C<getpriority>, C<getprotobynumber>,
230 C<getprotoent>, C<getpwent>, C<getpwnam>, C<getpwuid>,
231 C<getservbyport>, C<getservent>, C<getsockopt>, C<glob>, C<ioctl>,
232 C<kill>, C<link>, C<lstat>, C<msgctl>, C<msgget>, C<msgrcv>,
233 C<msgsnd>, C<open>, C<pipe>, C<readlink>, C<rename>, C<select>, C<semctl>,
234 C<semget>, C<semop>, C<setgrent>, C<sethostent>, C<setnetent>,
235 C<setpgrp>, C<setpriority>, C<setprotoent>, C<setpwent>,
236 C<setservent>, C<setsockopt>, C<shmctl>, C<shmget>, C<shmread>,
237 C<shmwrite>, C<sockatmark>, C<socket>, C<socketpair>,
238 C<stat>, C<symlink>, C<syscall>, C<sysopen>, C<system>,
239 C<times>, C<truncate>, C<umask>, C<unlink>,
240 C<utime>, C<wait>, C<waitpid>
242 For more information about the portability of these functions, see
243 L<perlport> and other available platform-specific documentation.
245 =head2 Alphabetical Listing of Perl Functions
249 =item I<-X> FILEHANDLE
255 A file test, where X is one of the letters listed below. This unary
256 operator takes one argument, either a filename or a filehandle, and
257 tests the associated file to see if something is true about it. If the
258 argument is omitted, tests C<$_>, except for C<-t>, which tests STDIN.
259 Unless otherwise documented, it returns C<1> for true and C<''> for false, or
260 the undefined value if the file doesn't exist. Despite the funny
261 names, precedence is the same as any other named unary operator, and
262 the argument may be parenthesized like any other unary operator. The
263 operator may be any of:
264 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>
265 X<-S>X<-b>X<-c>X<-t>X<-u>X<-g>X<-k>X<-T>X<-B>X<-M>X<-A>X<-C>
267 -r File is readable by effective uid/gid.
268 -w File is writable by effective uid/gid.
269 -x File is executable by effective uid/gid.
270 -o File is owned by effective uid.
272 -R File is readable by real uid/gid.
273 -W File is writable by real uid/gid.
274 -X File is executable by real uid/gid.
275 -O File is owned by real uid.
278 -z File has zero size (is empty).
279 -s File has nonzero size (returns size in bytes).
281 -f File is a plain file.
282 -d File is a directory.
283 -l File is a symbolic link.
284 -p File is a named pipe (FIFO), or Filehandle is a pipe.
286 -b File is a block special file.
287 -c File is a character special file.
288 -t Filehandle is opened to a tty.
290 -u File has setuid bit set.
291 -g File has setgid bit set.
292 -k File has sticky bit set.
294 -T File is an ASCII text file.
295 -B File is a "binary" file (opposite of -T).
297 -M Age of file in days when script started.
298 -A Same for access time.
299 -C Same for inode change time.
305 next unless -f $_; # ignore specials
309 The interpretation of the file permission operators C<-r>, C<-R>,
310 C<-w>, C<-W>, C<-x>, and C<-X> is by default based solely on the mode
311 of the file and the uids and gids of the user. There may be other
312 reasons you can't actually read, write, or execute the file. Such
313 reasons may be for example network filesystem access controls, ACLs
314 (access control lists), read-only filesystems, and unrecognized
317 Also note that, for the superuser on the local filesystems, the C<-r>,
318 C<-R>, C<-w>, and C<-W> tests always return 1, and C<-x> and C<-X> return 1
319 if any execute bit is set in the mode. Scripts run by the superuser
320 may thus need to do a stat() to determine the actual mode of the file,
321 or temporarily set their effective uid to something else.
323 If you are using ACLs, there is a pragma called C<filetest> that may
324 produce more accurate results than the bare stat() mode bits.
325 When under the C<use filetest 'access'> the above-mentioned filetests
326 will test whether the permission can (not) be granted using the
327 access() family of system calls. Also note that the C<-x> and C<-X> may
328 under this pragma return true even if there are no execute permission
329 bits set (nor any extra execute permission ACLs). This strangeness is
330 due to the underlying system calls' definitions. Read the
331 documentation for the C<filetest> pragma for more information.
333 Note that C<-s/a/b/> does not do a negated substitution. Saying
334 C<-exp($foo)> still works as expected, however--only single letters
335 following a minus are interpreted as file tests.
337 The C<-T> and C<-B> switches work as follows. The first block or so of the
338 file is examined for odd characters such as strange control codes or
339 characters with the high bit set. If too many strange characters (>30%)
340 are found, it's a C<-B> file, otherwise it's a C<-T> file. Also, any file
341 containing null in the first block is considered a binary file. If C<-T>
342 or C<-B> is used on a filehandle, the current stdio buffer is examined
343 rather than the first block. Both C<-T> and C<-B> return true on a null
344 file, or a file at EOF when testing a filehandle. Because you have to
345 read a file to do the C<-T> test, on most occasions you want to use a C<-f>
346 against the file first, as in C<next unless -f $file && -T $file>.
348 If any of the file tests (or either the C<stat> or C<lstat> operators) are given
349 the special filehandle consisting of a solitary underline, then the stat
350 structure of the previous file test (or stat operator) is used, saving
351 a system call. (This doesn't work with C<-t>, and you need to remember
352 that lstat() and C<-l> will leave values in the stat structure for the
353 symbolic link, not the real file.) Example:
355 print "Can do.\n" if -r $a || -w _ || -x _;
358 print "Readable\n" if -r _;
359 print "Writable\n" if -w _;
360 print "Executable\n" if -x _;
361 print "Setuid\n" if -u _;
362 print "Setgid\n" if -g _;
363 print "Sticky\n" if -k _;
364 print "Text\n" if -T _;
365 print "Binary\n" if -B _;
371 Returns the absolute value of its argument.
372 If VALUE is omitted, uses C<$_>.
374 =item accept NEWSOCKET,GENERICSOCKET
376 Accepts an incoming socket connect, just as the accept(2) system call
377 does. Returns the packed address if it succeeded, false otherwise.
378 See the example in L<perlipc/"Sockets: Client/Server Communication">.
380 On systems that support a close-on-exec flag on files, the flag will
381 be set for the newly opened file descriptor, as determined by the
382 value of $^F. See L<perlvar/$^F>.
388 Arranges to have a SIGALRM delivered to this process after the
389 specified number of seconds have elapsed. If SECONDS is not specified,
390 the value stored in C<$_> is used. (On some machines,
391 unfortunately, the elapsed time may be up to one second less than you
392 specified because of how seconds are counted.) Only one timer may be
393 counting at once. Each call disables the previous timer, and an
394 argument of C<0> may be supplied to cancel the previous timer without
395 starting a new one. The returned value is the amount of time remaining
396 on the previous timer.
398 For delays of finer granularity than one second, you may use Perl's
399 four-argument version of select() leaving the first three arguments
400 undefined, or you might be able to use the C<syscall> interface to
401 access setitimer(2) if your system supports it. The Time::HiRes
402 module (from CPAN, and starting from Perl 5.8 part of the standard
403 distribution) may also prove useful.
405 It is usually a mistake to intermix C<alarm> and C<sleep> calls.
406 (C<sleep> may be internally implemented in your system with C<alarm>)
408 If you want to use C<alarm> to time out a system call you need to use an
409 C<eval>/C<die> pair. You can't rely on the alarm causing the system call to
410 fail with C<$!> set to C<EINTR> because Perl sets up signal handlers to
411 restart system calls on some systems. Using C<eval>/C<die> always works,
412 modulo the caveats given in L<perlipc/"Signals">.
415 local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
417 $nread = sysread SOCKET, $buffer, $size;
421 die unless $@ eq "alarm\n"; # propagate unexpected errors
430 Returns the arctangent of Y/X in the range -PI to PI.
432 For the tangent operation, you may use the C<Math::Trig::tan>
433 function, or use the familiar relation:
435 sub tan { sin($_[0]) / cos($_[0]) }
437 =item bind SOCKET,NAME
439 Binds a network address to a socket, just as the bind system call
440 does. Returns true if it succeeded, false otherwise. NAME should be a
441 packed address of the appropriate type for the socket. See the examples in
442 L<perlipc/"Sockets: Client/Server Communication">.
444 =item binmode FILEHANDLE, DISCIPLINE
446 =item binmode FILEHANDLE
448 Arranges for FILEHANDLE to be read or written in "binary" or "text" mode
449 on systems where the run-time libraries distinguish between binary and
450 text files. If FILEHANDLE is an expression, the value is taken as the
451 name of the filehandle. DISCIPLINE can be either of C<":raw"> for
452 binary mode or C<":crlf"> for "text" mode. If the DISCIPLINE is
453 omitted, it defaults to C<":raw">.
455 binmode() should be called after open() but before any I/O is done on
458 On many systems binmode() currently has no effect, but in future, it
459 will be extended to support user-defined input and output disciplines.
460 On some systems binmode() is necessary when you're not working with a
461 text file. For the sake of portability it is a good idea to always use
462 it when appropriate, and to never use it when it isn't appropriate.
464 In other words: Regardless of platform, use binmode() on binary
465 files, and do not use binmode() on text files.
467 The C<open> pragma can be used to establish default disciplines.
470 The operating system, device drivers, C libraries, and Perl run-time
471 system all work together to let the programmer treat a single
472 character (C<\n>) as the line terminator, irrespective of the external
473 representation. On many operating systems, the native text file
474 representation matches the internal representation, but on some
475 platforms the external representation of C<\n> is made up of more than
478 Mac OS, all variants of Unix, and Stream_LF files on VMS use a single
479 character to end each line in the external representation of text (even
480 though that single character is CARRIAGE RETURN on Mac OS and LINE FEED
481 on Unix and most VMS files). Consequently binmode() has no effect on
482 these operating systems. In other systems like OS/2, DOS and the various
483 flavors of MS-Windows your program sees a C<\n> as a simple C<\cJ>, but
484 what's stored in text files are the two characters C<\cM\cJ>. That means
485 that, if you don't use binmode() on these systems, C<\cM\cJ> sequences on
486 disk will be converted to C<\n> on input, and any C<\n> in your program
487 will be converted back to C<\cM\cJ> on output. This is what you want for
488 text files, but it can be disastrous for binary files.
490 Another consequence of using binmode() (on some systems) is that
491 special end-of-file markers will be seen as part of the data stream.
492 For systems from the Microsoft family this means that if your binary
493 data contains C<\cZ>, the I/O subsystem will regard it as the end of
494 the file, unless you use binmode().
496 binmode() is not only important for readline() and print() operations,
497 but also when using read(), seek(), sysread(), syswrite() and tell()
498 (see L<perlport> for more details). See the C<$/> and C<$\> variables
499 in L<perlvar> for how to manually set your input and output
500 line-termination sequences.
502 =item bless REF,CLASSNAME
506 This function tells the thingy referenced by REF that it is now an object
507 in the CLASSNAME package. If CLASSNAME is omitted, the current package
508 is used. Because a C<bless> is often the last thing in a constructor,
509 it returns the reference for convenience. Always use the two-argument
510 version if the function doing the blessing might be inherited by a
511 derived class. See L<perltoot> and L<perlobj> for more about the blessing
512 (and blessings) of objects.
514 Consider always blessing objects in CLASSNAMEs that are mixed case.
515 Namespaces with all lowercase names are considered reserved for
516 Perl pragmata. Builtin types have all uppercase names, so to prevent
517 confusion, you may wish to avoid such package names as well. Make sure
518 that CLASSNAME is a true value.
520 See L<perlmod/"Perl Modules">.
526 Returns the context of the current subroutine call. In scalar context,
527 returns the caller's package name if there is a caller, that is, if
528 we're in a subroutine or C<eval> or C<require>, and the undefined value
529 otherwise. In list context, returns
531 ($package, $filename, $line) = caller;
533 With EXPR, it returns some extra information that the debugger uses to
534 print a stack trace. The value of EXPR indicates how many call frames
535 to go back before the current one.
537 ($package, $filename, $line, $subroutine, $hasargs,
538 $wantarray, $evaltext, $is_require, $hints, $bitmask) = caller($i);
540 Here $subroutine may be C<(eval)> if the frame is not a subroutine
541 call, but an C<eval>. In such a case additional elements $evaltext and
542 C<$is_require> are set: C<$is_require> is true if the frame is created by a
543 C<require> or C<use> statement, $evaltext contains the text of the
544 C<eval EXPR> statement. In particular, for an C<eval BLOCK> statement,
545 $filename is C<(eval)>, but $evaltext is undefined. (Note also that
546 each C<use> statement creates a C<require> frame inside an C<eval EXPR>)
547 frame. C<$hasargs> is true if a new instance of C<@_> was set up for the
548 frame. C<$hints> and C<$bitmask> contain pragmatic hints that the caller
549 was compiled with. The C<$hints> and C<$bitmask> values are subject to
550 change between versions of Perl, and are not meant for external use.
552 Furthermore, when called from within the DB package, caller returns more
553 detailed information: it sets the list variable C<@DB::args> to be the
554 arguments with which the subroutine was invoked.
556 Be aware that the optimizer might have optimized call frames away before
557 C<caller> had a chance to get the information. That means that C<caller(N)>
558 might not return information about the call frame you expect it do, for
559 C<< N > 1 >>. In particular, C<@DB::args> might have information from the
560 previous time C<caller> was called.
564 Changes the working directory to EXPR, if possible. If EXPR is omitted,
565 changes to the directory specified by C<$ENV{HOME}>, if set; if not,
566 changes to the directory specified by C<$ENV{LOGDIR}>. If neither is
567 set, C<chdir> does nothing. It returns true upon success, false
568 otherwise. See the example under C<die>.
572 Changes the permissions of a list of files. The first element of the
573 list must be the numerical mode, which should probably be an octal
574 number, and which definitely should I<not> a string of octal digits:
575 C<0644> is okay, C<'0644'> is not. Returns the number of files
576 successfully changed. See also L</oct>, if all you have is a string.
578 $cnt = chmod 0755, 'foo', 'bar';
579 chmod 0755, @executables;
580 $mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to
582 $mode = '0644'; chmod oct($mode), 'foo'; # this is better
583 $mode = 0644; chmod $mode, 'foo'; # this is best
585 You can also import the symbolic C<S_I*> constants from the Fcntl
590 chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
591 # This is identical to the chmod 0755 of the above example.
599 This safer version of L</chop> removes any trailing string
600 that corresponds to the current value of C<$/> (also known as
601 $INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
602 number of characters removed from all its arguments. It's often used to
603 remove the newline from the end of an input record when you're worried
604 that the final record may be missing its newline. When in paragraph
605 mode (C<$/ = "">), it removes all trailing newlines from the string.
606 When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is
607 a reference to an integer or the like, see L<perlvar>) chomp() won't
609 If VARIABLE is omitted, it chomps C<$_>. Example:
612 chomp; # avoid \n on last field
617 If VARIABLE is a hash, it chomps the hash's values, but not its keys.
619 You can actually chomp anything that's an lvalue, including an assignment:
622 chomp($answer = <STDIN>);
624 If you chomp a list, each element is chomped, and the total number of
625 characters removed is returned.
633 Chops off the last character of a string and returns the character
634 chopped. It is much more efficient than C<s/.$//s> because it neither
635 scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
636 If VARIABLE is a hash, it chops the hash's values, but not its keys.
638 You can actually chop anything that's an lvalue, including an assignment.
640 If you chop a list, each element is chopped. Only the value of the
641 last C<chop> is returned.
643 Note that C<chop> returns the last character. To return all but the last
644 character, use C<substr($string, 0, -1)>.
648 Changes the owner (and group) of a list of files. The first two
649 elements of the list must be the I<numeric> uid and gid, in that
650 order. A value of -1 in either position is interpreted by most
651 systems to leave that value unchanged. Returns the number of files
652 successfully changed.
654 $cnt = chown $uid, $gid, 'foo', 'bar';
655 chown $uid, $gid, @filenames;
657 Here's an example that looks up nonnumeric uids in the passwd file:
660 chomp($user = <STDIN>);
662 chomp($pattern = <STDIN>);
664 ($login,$pass,$uid,$gid) = getpwnam($user)
665 or die "$user not in passwd file";
667 @ary = glob($pattern); # expand filenames
668 chown $uid, $gid, @ary;
670 On most systems, you are not allowed to change the ownership of the
671 file unless you're the superuser, although you should be able to change
672 the group to any of your secondary groups. On insecure systems, these
673 restrictions may be relaxed, but this is not a portable assumption.
674 On POSIX systems, you can detect this condition this way:
676 use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
677 $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
683 Returns the character represented by that NUMBER in the character set.
684 For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
685 chr(0x263a) is a Unicode smiley face. Within the scope of C<use utf8>,
686 characters higher than 127 are encoded in Unicode; if you don't want
687 this, temporarily C<use bytes> or use C<pack("C*",...)>
689 For the reverse, use L</ord>.
690 See L<utf8> for more about Unicode.
692 If NUMBER is omitted, uses C<$_>.
694 =item chroot FILENAME
698 This function works like the system call by the same name: it makes the
699 named directory the new root directory for all further pathnames that
700 begin with a C</> by your process and all its children. (It doesn't
701 change your current working directory, which is unaffected.) For security
702 reasons, this call is restricted to the superuser. If FILENAME is
703 omitted, does a C<chroot> to C<$_>.
705 =item close FILEHANDLE
709 Closes the file or pipe associated with the file handle, returning true
710 only if stdio successfully flushes buffers and closes the system file
711 descriptor. Closes the currently selected filehandle if the argument
714 You don't have to close FILEHANDLE if you are immediately going to do
715 another C<open> on it, because C<open> will close it for you. (See
716 C<open>.) However, an explicit C<close> on an input file resets the line
717 counter (C<$.>), while the implicit close done by C<open> does not.
719 If the file handle came from a piped open C<close> will additionally
720 return false if one of the other system calls involved fails or if the
721 program exits with non-zero status. (If the only problem was that the
722 program exited non-zero C<$!> will be set to C<0>.) Closing a pipe
723 also waits for the process executing on the pipe to complete, in case you
724 want to look at the output of the pipe afterwards, and
725 implicitly puts the exit status value of that command into C<$?>.
727 Prematurely closing the read end of a pipe (i.e. before the process
728 writing to it at the other end has closed it) will result in a
729 SIGPIPE being delivered to the writer. If the other end can't
730 handle that, be sure to read all the data before closing the pipe.
734 open(OUTPUT, '|sort >foo') # pipe to sort
735 or die "Can't start sort: $!";
736 #... # print stuff to output
737 close OUTPUT # wait for sort to finish
738 or warn $! ? "Error closing sort pipe: $!"
739 : "Exit status $? from sort";
740 open(INPUT, 'foo') # get sort's results
741 or die "Can't open 'foo' for input: $!";
743 FILEHANDLE may be an expression whose value can be used as an indirect
744 filehandle, usually the real filehandle name.
746 =item closedir DIRHANDLE
748 Closes a directory opened by C<opendir> and returns the success of that
751 DIRHANDLE may be an expression whose value can be used as an indirect
752 dirhandle, usually the real dirhandle name.
754 =item connect SOCKET,NAME
756 Attempts to connect to a remote socket, just as the connect system call
757 does. Returns true if it succeeded, false otherwise. NAME should be a
758 packed address of the appropriate type for the socket. See the examples in
759 L<perlipc/"Sockets: Client/Server Communication">.
763 Actually a flow control statement rather than a function. If there is a
764 C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
765 C<foreach>), it is always executed just before the conditional is about to
766 be evaluated again, just like the third part of a C<for> loop in C. Thus
767 it can be used to increment a loop variable, even when the loop has been
768 continued via the C<next> statement (which is similar to the C C<continue>
771 C<last>, C<next>, or C<redo> may appear within a C<continue>
772 block. C<last> and C<redo> will behave as if they had been executed within
773 the main block. So will C<next>, but since it will execute a C<continue>
774 block, it may be more entertaining.
777 ### redo always comes here
780 ### next always comes here
782 # then back the top to re-check EXPR
784 ### last always comes here
786 Omitting the C<continue> section is semantically equivalent to using an
787 empty one, logically enough. In that case, C<next> goes directly back
788 to check the condition at the top of the loop.
794 Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
795 takes cosine of C<$_>.
797 For the inverse cosine operation, you may use the C<Math::Trig::acos()>
798 function, or use this relation:
800 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
802 =item crypt PLAINTEXT,SALT
804 Encrypts a string exactly like the crypt(3) function in the C library
805 (assuming that you actually have a version there that has not been
806 extirpated as a potential munition). This can prove useful for checking
807 the password file for lousy passwords, amongst other things. Only the
808 guys wearing white hats should do this.
810 Note that C<crypt> is intended to be a one-way function, much like breaking
811 eggs to make an omelette. There is no (known) corresponding decrypt
812 function. As a result, this function isn't all that useful for
813 cryptography. (For that, see your nearby CPAN mirror.)
815 When verifying an existing encrypted string you should use the encrypted
816 text as the salt (like C<crypt($plain, $crypted) eq $crypted>). This
817 allows your code to work with the standard C<crypt> and with more
818 exotic implementations. When choosing a new salt create a random two
819 character string whose characters come from the set C<[./0-9A-Za-z]>
820 (like C<join '', ('.', '/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>).
822 Here's an example that makes sure that whoever runs this program knows
825 $pwd = (getpwuid($<))[1];
829 chomp($word = <STDIN>);
833 if (crypt($word, $pwd) ne $pwd) {
839 Of course, typing in your own password to whoever asks you
842 The L<crypt> function is unsuitable for encrypting large quantities
843 of data, not least of all because you can't get the information
844 back. Look at the F<by-module/Crypt> and F<by-module/PGP> directories
845 on your favorite CPAN mirror for a slew of potentially useful
850 [This function has been largely superseded by the C<untie> function.]
852 Breaks the binding between a DBM file and a hash.
854 =item dbmopen HASH,DBNAME,MASK
856 [This function has been largely superseded by the C<tie> function.]
858 This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
859 hash. HASH is the name of the hash. (Unlike normal C<open>, the first
860 argument is I<not> a filehandle, even though it looks like one). DBNAME
861 is the name of the database (without the F<.dir> or F<.pag> extension if
862 any). If the database does not exist, it is created with protection
863 specified by MASK (as modified by the C<umask>). If your system supports
864 only the older DBM functions, you may perform only one C<dbmopen> in your
865 program. In older versions of Perl, if your system had neither DBM nor
866 ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
869 If you don't have write access to the DBM file, you can only read hash
870 variables, not set them. If you want to test whether you can write,
871 either use file tests or try setting a dummy hash entry inside an C<eval>,
872 which will trap the error.
874 Note that functions such as C<keys> and C<values> may return huge lists
875 when used on large DBM files. You may prefer to use the C<each>
876 function to iterate over large DBM files. Example:
878 # print out history file offsets
879 dbmopen(%HIST,'/usr/lib/news/history',0666);
880 while (($key,$val) = each %HIST) {
881 print $key, ' = ', unpack('L',$val), "\n";
885 See also L<AnyDBM_File> for a more general description of the pros and
886 cons of the various dbm approaches, as well as L<DB_File> for a particularly
889 You can control which DBM library you use by loading that library
890 before you call dbmopen():
893 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
894 or die "Can't open netscape history file: $!";
900 Returns a Boolean value telling whether EXPR has a value other than
901 the undefined value C<undef>. If EXPR is not present, C<$_> will be
904 Many operations return C<undef> to indicate failure, end of file,
905 system error, uninitialized variable, and other exceptional
906 conditions. This function allows you to distinguish C<undef> from
907 other values. (A simple Boolean test will not distinguish among
908 C<undef>, zero, the empty string, and C<"0">, which are all equally
909 false.) Note that since C<undef> is a valid scalar, its presence
910 doesn't I<necessarily> indicate an exceptional condition: C<pop>
911 returns C<undef> when its argument is an empty array, I<or> when the
912 element to return happens to be C<undef>.
914 You may also use C<defined(&func)> to check whether subroutine C<&func>
915 has ever been defined. The return value is unaffected by any forward
916 declarations of C<&foo>. Note that a subroutine which is not defined
917 may still be callable: its package may have an C<AUTOLOAD> method that
918 makes it spring into existence the first time that it is called -- see
921 Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
922 used to report whether memory for that aggregate has ever been
923 allocated. This behavior may disappear in future versions of Perl.
924 You should instead use a simple test for size:
926 if (@an_array) { print "has array elements\n" }
927 if (%a_hash) { print "has hash members\n" }
929 When used on a hash element, it tells you whether the value is defined,
930 not whether the key exists in the hash. Use L</exists> for the latter
935 print if defined $switch{'D'};
936 print "$val\n" while defined($val = pop(@ary));
937 die "Can't readlink $sym: $!"
938 unless defined($value = readlink $sym);
939 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
940 $debugging = 0 unless defined $debugging;
942 Note: Many folks tend to overuse C<defined>, and then are surprised to
943 discover that the number C<0> and C<""> (the zero-length string) are, in fact,
944 defined values. For example, if you say
948 The pattern match succeeds, and C<$1> is defined, despite the fact that it
949 matched "nothing". But it didn't really match nothing--rather, it
950 matched something that happened to be zero characters long. This is all
951 very above-board and honest. When a function returns an undefined value,
952 it's an admission that it couldn't give you an honest answer. So you
953 should use C<defined> only when you're questioning the integrity of what
954 you're trying to do. At other times, a simple comparison to C<0> or C<""> is
957 See also L</undef>, L</exists>, L</ref>.
961 Given an expression that specifies a hash element, array element, hash slice,
962 or array slice, deletes the specified element(s) from the hash or array.
963 In the case of an array, if the array elements happen to be at the end,
964 the size of the array will shrink to the highest element that tests
965 true for exists() (or 0 if no such element exists).
967 Returns each element so deleted or the undefined value if there was no such
968 element. Deleting from C<$ENV{}> modifies the environment. Deleting from
969 a hash tied to a DBM file deletes the entry from the DBM file. Deleting
970 from a C<tie>d hash or array may not necessarily return anything.
972 Deleting an array element effectively returns that position of the array
973 to its initial, uninitialized state. Subsequently testing for the same
974 element with exists() will return false. Note that deleting array
975 elements in the middle of an array will not shift the index of the ones
976 after them down--use splice() for that. See L</exists>.
978 The following (inefficiently) deletes all the values of %HASH and @ARRAY:
980 foreach $key (keys %HASH) {
984 foreach $index (0 .. $#ARRAY) {
985 delete $ARRAY[$index];
990 delete @HASH{keys %HASH};
992 delete @ARRAY[0 .. $#ARRAY];
994 But both of these are slower than just assigning the empty list
995 or undefining %HASH or @ARRAY:
997 %HASH = (); # completely empty %HASH
998 undef %HASH; # forget %HASH ever existed
1000 @ARRAY = (); # completely empty @ARRAY
1001 undef @ARRAY; # forget @ARRAY ever existed
1003 Note that the EXPR can be arbitrarily complicated as long as the final
1004 operation is a hash element, array element, hash slice, or array slice
1007 delete $ref->[$x][$y]{$key};
1008 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
1010 delete $ref->[$x][$y][$index];
1011 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
1015 Outside an C<eval>, prints the value of LIST to C<STDERR> and
1016 exits with the current value of C<$!> (errno). If C<$!> is C<0>,
1017 exits with the value of C<<< ($? >> 8) >>> (backtick `command`
1018 status). If C<<< ($? >> 8) >>> is C<0>, exits with C<255>. Inside
1019 an C<eval(),> the error message is stuffed into C<$@> and the
1020 C<eval> is terminated with the undefined value. This makes
1021 C<die> the way to raise an exception.
1023 Equivalent examples:
1025 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
1026 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
1028 If the value of EXPR does not end in a newline, the current script line
1029 number and input line number (if any) are also printed, and a newline
1030 is supplied. Note that the "input line number" (also known as "chunk")
1031 is subject to whatever notion of "line" happens to be currently in
1032 effect, and is also available as the special variable C<$.>.
1033 See L<perlvar/"$/"> and L<perlvar/"$.">.
1035 Hint: sometimes appending C<", stopped"> to your message
1036 will cause it to make better sense when the string C<"at foo line 123"> is
1037 appended. Suppose you are running script "canasta".
1039 die "/etc/games is no good";
1040 die "/etc/games is no good, stopped";
1042 produce, respectively
1044 /etc/games is no good at canasta line 123.
1045 /etc/games is no good, stopped at canasta line 123.
1047 See also exit(), warn(), and the Carp module.
1049 If LIST is empty and C<$@> already contains a value (typically from a
1050 previous eval) that value is reused after appending C<"\t...propagated">.
1051 This is useful for propagating exceptions:
1054 die unless $@ =~ /Expected exception/;
1056 If C<$@> is empty then the string C<"Died"> is used.
1058 die() can also be called with a reference argument. If this happens to be
1059 trapped within an eval(), $@ contains the reference. This behavior permits
1060 a more elaborate exception handling implementation using objects that
1061 maintain arbitrary state about the nature of the exception. Such a scheme
1062 is sometimes preferable to matching particular string values of $@ using
1063 regular expressions. Here's an example:
1065 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
1067 if (ref($@) && UNIVERSAL::isa($@,"Some::Module::Exception")) {
1068 # handle Some::Module::Exception
1071 # handle all other possible exceptions
1075 Because perl will stringify uncaught exception messages before displaying
1076 them, you may want to overload stringification operations on such custom
1077 exception objects. See L<overload> for details about that.
1079 You can arrange for a callback to be run just before the C<die>
1080 does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
1081 handler will be called with the error text and can change the error
1082 message, if it sees fit, by calling C<die> again. See
1083 L<perlvar/$SIG{expr}> for details on setting C<%SIG> entries, and
1084 L<"eval BLOCK"> for some examples. Although this feature was meant
1085 to be run only right before your program was to exit, this is not
1086 currently the case--the C<$SIG{__DIE__}> hook is currently called
1087 even inside eval()ed blocks/strings! If one wants the hook to do
1088 nothing in such situations, put
1092 as the first line of the handler (see L<perlvar/$^S>). Because
1093 this promotes strange action at a distance, this counterintuitive
1094 behavior may be fixed in a future release.
1098 Not really a function. Returns the value of the last command in the
1099 sequence of commands indicated by BLOCK. When modified by a loop
1100 modifier, executes the BLOCK once before testing the loop condition.
1101 (On other statements the loop modifiers test the conditional first.)
1103 C<do BLOCK> does I<not> count as a loop, so the loop control statements
1104 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1105 See L<perlsyn> for alternative strategies.
1107 =item do SUBROUTINE(LIST)
1109 A deprecated form of subroutine call. See L<perlsub>.
1113 Uses the value of EXPR as a filename and executes the contents of the
1114 file as a Perl script. Its primary use is to include subroutines
1115 from a Perl subroutine library.
1123 except that it's more efficient and concise, keeps track of the current
1124 filename for error messages, searches the @INC libraries, and updates
1125 C<%INC> if the file is found. See L<perlvar/Predefined Names> for these
1126 variables. It also differs in that code evaluated with C<do FILENAME>
1127 cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1128 same, however, in that it does reparse the file every time you call it,
1129 so you probably don't want to do this inside a loop.
1131 If C<do> cannot read the file, it returns undef and sets C<$!> to the
1132 error. If C<do> can read the file but cannot compile it, it
1133 returns undef and sets an error message in C<$@>. If the file is
1134 successfully compiled, C<do> returns the value of the last expression
1137 Note that inclusion of library modules is better done with the
1138 C<use> and C<require> operators, which also do automatic error checking
1139 and raise an exception if there's a problem.
1141 You might like to use C<do> to read in a program configuration
1142 file. Manual error checking can be done this way:
1144 # read in config files: system first, then user
1145 for $file ("/share/prog/defaults.rc",
1146 "$ENV{HOME}/.someprogrc")
1148 unless ($return = do $file) {
1149 warn "couldn't parse $file: $@" if $@;
1150 warn "couldn't do $file: $!" unless defined $return;
1151 warn "couldn't run $file" unless $return;
1159 This function causes an immediate core dump. See also the B<-u>
1160 command-line switch in L<perlrun>, which does the same thing.
1161 Primarily this is so that you can use the B<undump> program (not
1162 supplied) to turn your core dump into an executable binary after
1163 having initialized all your variables at the beginning of the
1164 program. When the new binary is executed it will begin by executing
1165 a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1166 Think of it as a goto with an intervening core dump and reincarnation.
1167 If C<LABEL> is omitted, restarts the program from the top.
1169 B<WARNING>: Any files opened at the time of the dump will I<not>
1170 be open any more when the program is reincarnated, with possible
1171 resulting confusion on the part of Perl.
1173 This function is now largely obsolete, partly because it's very
1174 hard to convert a core file into an executable, and because the
1175 real compiler backends for generating portable bytecode and compilable
1176 C code have superseded it.
1178 If you're looking to use L<dump> to speed up your program, consider
1179 generating bytecode or native C code as described in L<perlcc>. If
1180 you're just trying to accelerate a CGI script, consider using the
1181 C<mod_perl> extension to B<Apache>, or the CPAN module, Fast::CGI.
1182 You might also consider autoloading or selfloading, which at least
1183 make your program I<appear> to run faster.
1187 When called in list context, returns a 2-element list consisting of the
1188 key and value for the next element of a hash, so that you can iterate over
1189 it. When called in scalar context, returns only the key for the next
1190 element in the hash.
1192 Entries are returned in an apparently random order. The actual random
1193 order is subject to change in future versions of perl, but it is guaranteed
1194 to be in the same order as either the C<keys> or C<values> function
1195 would produce on the same (unmodified) hash.
1197 When the hash is entirely read, a null array is returned in list context
1198 (which when assigned produces a false (C<0>) value), and C<undef> in
1199 scalar context. The next call to C<each> after that will start iterating
1200 again. There is a single iterator for each hash, shared by all C<each>,
1201 C<keys>, and C<values> function calls in the program; it can be reset by
1202 reading all the elements from the hash, or by evaluating C<keys HASH> or
1203 C<values HASH>. If you add or delete elements of a hash while you're
1204 iterating over it, you may get entries skipped or duplicated, so
1205 don't. Exception: It is always safe to delete the item most recently
1206 returned by C<each()>, which means that the following code will work:
1208 while (($key, $value) = each %hash) {
1210 delete $hash{$key}; # This is safe
1213 The following prints out your environment like the printenv(1) program,
1214 only in a different order:
1216 while (($key,$value) = each %ENV) {
1217 print "$key=$value\n";
1220 See also C<keys>, C<values> and C<sort>.
1222 =item eof FILEHANDLE
1228 Returns 1 if the next read on FILEHANDLE will return end of file, or if
1229 FILEHANDLE is not open. FILEHANDLE may be an expression whose value
1230 gives the real filehandle. (Note that this function actually
1231 reads a character and then C<ungetc>s it, so isn't very useful in an
1232 interactive context.) Do not read from a terminal file (or call
1233 C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
1234 as terminals may lose the end-of-file condition if you do.
1236 An C<eof> without an argument uses the last file read. Using C<eof()>
1237 with empty parentheses is very different. It refers to the pseudo file
1238 formed from the files listed on the command line and accessed via the
1239 C<< <> >> operator. Since C<< <> >> isn't explicitly opened,
1240 as a normal filehandle is, an C<eof()> before C<< <> >> has been
1241 used will cause C<@ARGV> to be examined to determine if input is
1244 In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to
1245 detect the end of each file, C<eof()> will only detect the end of the
1246 last file. Examples:
1248 # reset line numbering on each input file
1250 next if /^\s*#/; # skip comments
1253 close ARGV if eof; # Not eof()!
1256 # insert dashes just before last line of last file
1258 if (eof()) { # check for end of current file
1259 print "--------------\n";
1260 close(ARGV); # close or last; is needed if we
1261 # are reading from the terminal
1266 Practical hint: you almost never need to use C<eof> in Perl, because the
1267 input operators typically return C<undef> when they run out of data, or if
1274 In the first form, the return value of EXPR is parsed and executed as if it
1275 were a little Perl program. The value of the expression (which is itself
1276 determined within scalar context) is first parsed, and if there weren't any
1277 errors, executed in the lexical context of the current Perl program, so
1278 that any variable settings or subroutine and format definitions remain
1279 afterwards. Note that the value is parsed every time the eval executes.
1280 If EXPR is omitted, evaluates C<$_>. This form is typically used to
1281 delay parsing and subsequent execution of the text of EXPR until run time.
1283 In the second form, the code within the BLOCK is parsed only once--at the
1284 same time the code surrounding the eval itself was parsed--and executed
1285 within the context of the current Perl program. This form is typically
1286 used to trap exceptions more efficiently than the first (see below), while
1287 also providing the benefit of checking the code within BLOCK at compile
1290 The final semicolon, if any, may be omitted from the value of EXPR or within
1293 In both forms, the value returned is the value of the last expression
1294 evaluated inside the mini-program; a return statement may be also used, just
1295 as with subroutines. The expression providing the return value is evaluated
1296 in void, scalar, or list context, depending on the context of the eval itself.
1297 See L</wantarray> for more on how the evaluation context can be determined.
1299 If there is a syntax error or runtime error, or a C<die> statement is
1300 executed, an undefined value is returned by C<eval>, and C<$@> is set to the
1301 error message. If there was no error, C<$@> is guaranteed to be a null
1302 string. Beware that using C<eval> neither silences perl from printing
1303 warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
1304 To do either of those, you have to use the C<$SIG{__WARN__}> facility. See
1305 L</warn> and L<perlvar>.
1307 Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1308 determining whether a particular feature (such as C<socket> or C<symlink>)
1309 is implemented. It is also Perl's exception trapping mechanism, where
1310 the die operator is used to raise exceptions.
1312 If the code to be executed doesn't vary, you may use the eval-BLOCK
1313 form to trap run-time errors without incurring the penalty of
1314 recompiling each time. The error, if any, is still returned in C<$@>.
1317 # make divide-by-zero nonfatal
1318 eval { $answer = $a / $b; }; warn $@ if $@;
1320 # same thing, but less efficient
1321 eval '$answer = $a / $b'; warn $@ if $@;
1323 # a compile-time error
1324 eval { $answer = }; # WRONG
1327 eval '$answer ='; # sets $@
1329 Due to the current arguably broken state of C<__DIE__> hooks, when using
1330 the C<eval{}> form as an exception trap in libraries, you may wish not
1331 to trigger any C<__DIE__> hooks that user code may have installed.
1332 You can use the C<local $SIG{__DIE__}> construct for this purpose,
1333 as shown in this example:
1335 # a very private exception trap for divide-by-zero
1336 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1339 This is especially significant, given that C<__DIE__> hooks can call
1340 C<die> again, which has the effect of changing their error messages:
1342 # __DIE__ hooks may modify error messages
1344 local $SIG{'__DIE__'} =
1345 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
1346 eval { die "foo lives here" };
1347 print $@ if $@; # prints "bar lives here"
1350 Because this promotes action at a distance, this counterintuitive behavior
1351 may be fixed in a future release.
1353 With an C<eval>, you should be especially careful to remember what's
1354 being looked at when:
1360 eval { $x }; # CASE 4
1362 eval "\$$x++"; # CASE 5
1365 Cases 1 and 2 above behave identically: they run the code contained in
1366 the variable $x. (Although case 2 has misleading double quotes making
1367 the reader wonder what else might be happening (nothing is).) Cases 3
1368 and 4 likewise behave in the same way: they run the code C<'$x'>, which
1369 does nothing but return the value of $x. (Case 4 is preferred for
1370 purely visual reasons, but it also has the advantage of compiling at
1371 compile-time instead of at run-time.) Case 5 is a place where
1372 normally you I<would> like to use double quotes, except that in this
1373 particular situation, you can just use symbolic references instead, as
1376 C<eval BLOCK> does I<not> count as a loop, so the loop control statements
1377 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1381 =item exec PROGRAM LIST
1383 The C<exec> function executes a system command I<and never returns>--
1384 use C<system> instead of C<exec> if you want it to return. It fails and
1385 returns false only if the command does not exist I<and> it is executed
1386 directly instead of via your system's command shell (see below).
1388 Since it's a common mistake to use C<exec> instead of C<system>, Perl
1389 warns you if there is a following statement which isn't C<die>, C<warn>,
1390 or C<exit> (if C<-w> is set - but you always do that). If you
1391 I<really> want to follow an C<exec> with some other statement, you
1392 can use one of these styles to avoid the warning:
1394 exec ('foo') or print STDERR "couldn't exec foo: $!";
1395 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
1397 If there is more than one argument in LIST, or if LIST is an array
1398 with more than one value, calls execvp(3) with the arguments in LIST.
1399 If there is only one scalar argument or an array with one element in it,
1400 the argument is checked for shell metacharacters, and if there are any,
1401 the entire argument is passed to the system's command shell for parsing
1402 (this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1403 If there are no shell metacharacters in the argument, it is split into
1404 words and passed directly to C<execvp>, which is more efficient.
1407 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1408 exec "sort $outfile | uniq";
1410 If you don't really want to execute the first argument, but want to lie
1411 to the program you are executing about its own name, you can specify
1412 the program you actually want to run as an "indirect object" (without a
1413 comma) in front of the LIST. (This always forces interpretation of the
1414 LIST as a multivalued list, even if there is only a single scalar in
1417 $shell = '/bin/csh';
1418 exec $shell '-sh'; # pretend it's a login shell
1422 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1424 When the arguments get executed via the system shell, results will
1425 be subject to its quirks and capabilities. See L<perlop/"`STRING`">
1428 Using an indirect object with C<exec> or C<system> is also more
1429 secure. This usage (which also works fine with system()) forces
1430 interpretation of the arguments as a multivalued list, even if the
1431 list had just one argument. That way you're safe from the shell
1432 expanding wildcards or splitting up words with whitespace in them.
1434 @args = ( "echo surprise" );
1436 exec @args; # subject to shell escapes
1438 exec { $args[0] } @args; # safe even with one-arg list
1440 The first version, the one without the indirect object, ran the I<echo>
1441 program, passing it C<"surprise"> an argument. The second version
1442 didn't--it tried to run a program literally called I<"echo surprise">,
1443 didn't find it, and set C<$?> to a non-zero value indicating failure.
1445 Beginning with v5.6.0, Perl will attempt to flush all files opened for
1446 output before the exec, but this may not be supported on some platforms
1447 (see L<perlport>). To be safe, you may need to set C<$|> ($AUTOFLUSH
1448 in English) or call the C<autoflush()> method of C<IO::Handle> on any
1449 open handles in order to avoid lost output.
1451 Note that C<exec> will not call your C<END> blocks, nor will it call
1452 any C<DESTROY> methods in your objects.
1456 Given an expression that specifies a hash element or array element,
1457 returns true if the specified element in the hash or array has ever
1458 been initialized, even if the corresponding value is undefined. The
1459 element is not autovivified if it doesn't exist.
1461 print "Exists\n" if exists $hash{$key};
1462 print "Defined\n" if defined $hash{$key};
1463 print "True\n" if $hash{$key};
1465 print "Exists\n" if exists $array[$index];
1466 print "Defined\n" if defined $array[$index];
1467 print "True\n" if $array[$index];
1469 A hash or array element can be true only if it's defined, and defined if
1470 it exists, but the reverse doesn't necessarily hold true.
1472 Given an expression that specifies the name of a subroutine,
1473 returns true if the specified subroutine has ever been declared, even
1474 if it is undefined. Mentioning a subroutine name for exists or defined
1475 does not count as declaring it. Note that a subroutine which does not
1476 exist may still be callable: its package may have an C<AUTOLOAD>
1477 method that makes it spring into existence the first time that it is
1478 called -- see L<perlsub>.
1480 print "Exists\n" if exists &subroutine;
1481 print "Defined\n" if defined &subroutine;
1483 Note that the EXPR can be arbitrarily complicated as long as the final
1484 operation is a hash or array key lookup or subroutine name:
1486 if (exists $ref->{A}->{B}->{$key}) { }
1487 if (exists $hash{A}{B}{$key}) { }
1489 if (exists $ref->{A}->{B}->[$ix]) { }
1490 if (exists $hash{A}{B}[$ix]) { }
1492 if (exists &{$ref->{A}{B}{$key}}) { }
1494 Although the deepest nested array or hash will not spring into existence
1495 just because its existence was tested, any intervening ones will.
1496 Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
1497 into existence due to the existence test for the $key element above.
1498 This happens anywhere the arrow operator is used, including even:
1501 if (exists $ref->{"Some key"}) { }
1502 print $ref; # prints HASH(0x80d3d5c)
1504 This surprising autovivification in what does not at first--or even
1505 second--glance appear to be an lvalue context may be fixed in a future
1508 See L<perlref/"Pseudo-hashes: Using an array as a hash"> for specifics
1509 on how exists() acts when used on a pseudo-hash.
1511 Use of a subroutine call, rather than a subroutine name, as an argument
1512 to exists() is an error.
1515 exists &sub(); # Error
1519 Evaluates EXPR and exits immediately with that value. Example:
1522 exit 0 if $ans =~ /^[Xx]/;
1524 See also C<die>. If EXPR is omitted, exits with C<0> status. The only
1525 universally recognized values for EXPR are C<0> for success and C<1>
1526 for error; other values are subject to interpretation depending on the
1527 environment in which the Perl program is running. For example, exiting
1528 69 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
1529 the mailer to return the item undelivered, but that's not true everywhere.
1531 Don't use C<exit> to abort a subroutine if there's any chance that
1532 someone might want to trap whatever error happened. Use C<die> instead,
1533 which can be trapped by an C<eval>.
1535 The exit() function does not always exit immediately. It calls any
1536 defined C<END> routines first, but these C<END> routines may not
1537 themselves abort the exit. Likewise any object destructors that need to
1538 be called are called before the real exit. If this is a problem, you
1539 can call C<POSIX:_exit($status)> to avoid END and destructor processing.
1540 See L<perlmod> for details.
1546 Returns I<e> (the natural logarithm base) to the power of EXPR.
1547 If EXPR is omitted, gives C<exp($_)>.
1549 =item fcntl FILEHANDLE,FUNCTION,SCALAR
1551 Implements the fcntl(2) function. You'll probably have to say
1555 first to get the correct constant definitions. Argument processing and
1556 value return works just like C<ioctl> below.
1560 fcntl($filehandle, F_GETFL, $packed_return_buffer)
1561 or die "can't fcntl F_GETFL: $!";
1563 You don't have to check for C<defined> on the return from C<fnctl>.
1564 Like C<ioctl>, it maps a C<0> return from the system call into
1565 C<"0 but true"> in Perl. This string is true in boolean context and C<0>
1566 in numeric context. It is also exempt from the normal B<-w> warnings
1567 on improper numeric conversions.
1569 Note that C<fcntl> will produce a fatal error if used on a machine that
1570 doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
1571 manpage to learn what functions are available on your system.
1573 =item fileno FILEHANDLE
1575 Returns the file descriptor for a filehandle, or undefined if the
1576 filehandle is not open. This is mainly useful for constructing
1577 bitmaps for C<select> and low-level POSIX tty-handling operations.
1578 If FILEHANDLE is an expression, the value is taken as an indirect
1579 filehandle, generally its name.
1581 You can use this to find out whether two handles refer to the
1582 same underlying descriptor:
1584 if (fileno(THIS) == fileno(THAT)) {
1585 print "THIS and THAT are dups\n";
1588 (Filehandles connected to memory objects via new features of C<open> may
1589 return undefined even though they are open.)
1592 =item flock FILEHANDLE,OPERATION
1594 Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
1595 for success, false on failure. Produces a fatal error if used on a
1596 machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
1597 C<flock> is Perl's portable file locking interface, although it locks
1598 only entire files, not records.
1600 Two potentially non-obvious but traditional C<flock> semantics are
1601 that it waits indefinitely until the lock is granted, and that its locks
1602 B<merely advisory>. Such discretionary locks are more flexible, but offer
1603 fewer guarantees. This means that files locked with C<flock> may be
1604 modified by programs that do not also use C<flock>. See L<perlport>,
1605 your port's specific documentation, or your system-specific local manpages
1606 for details. It's best to assume traditional behavior if you're writing
1607 portable programs. (But if you're not, you should as always feel perfectly
1608 free to write for your own system's idiosyncrasies (sometimes called
1609 "features"). Slavish adherence to portability concerns shouldn't get
1610 in the way of your getting your job done.)
1612 OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1613 LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
1614 you can use the symbolic names if you import them from the Fcntl module,
1615 either individually, or as a group using the ':flock' tag. LOCK_SH
1616 requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
1617 releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
1618 LOCK_SH or LOCK_EX then C<flock> will return immediately rather than blocking
1619 waiting for the lock (check the return status to see if you got it).
1621 To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
1622 before locking or unlocking it.
1624 Note that the emulation built with lockf(3) doesn't provide shared
1625 locks, and it requires that FILEHANDLE be open with write intent. These
1626 are the semantics that lockf(3) implements. Most if not all systems
1627 implement lockf(3) in terms of fcntl(2) locking, though, so the
1628 differing semantics shouldn't bite too many people.
1630 Note also that some versions of C<flock> cannot lock things over the
1631 network; you would need to use the more system-specific C<fcntl> for
1632 that. If you like you can force Perl to ignore your system's flock(2)
1633 function, and so provide its own fcntl(2)-based emulation, by passing
1634 the switch C<-Ud_flock> to the F<Configure> program when you configure
1637 Here's a mailbox appender for BSD systems.
1639 use Fcntl ':flock'; # import LOCK_* constants
1642 flock(MBOX,LOCK_EX);
1643 # and, in case someone appended
1644 # while we were waiting...
1649 flock(MBOX,LOCK_UN);
1652 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1653 or die "Can't open mailbox: $!";
1656 print MBOX $msg,"\n\n";
1659 On systems that support a real flock(), locks are inherited across fork()
1660 calls, whereas those that must resort to the more capricious fcntl()
1661 function lose the locks, making it harder to write servers.
1663 See also L<DB_File> for other flock() examples.
1667 Does a fork(2) system call to create a new process running the
1668 same program at the same point. It returns the child pid to the
1669 parent process, C<0> to the child process, or C<undef> if the fork is
1670 unsuccessful. File descriptors (and sometimes locks on those descriptors)
1671 are shared, while everything else is copied. On most systems supporting
1672 fork(), great care has gone into making it extremely efficient (for
1673 example, using copy-on-write technology on data pages), making it the
1674 dominant paradigm for multitasking over the last few decades.
1676 Beginning with v5.6.0, Perl will attempt to flush all files opened for
1677 output before forking the child process, but this may not be supported
1678 on some platforms (see L<perlport>). To be safe, you may need to set
1679 C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
1680 C<IO::Handle> on any open handles in order to avoid duplicate output.
1682 If you C<fork> without ever waiting on your children, you will
1683 accumulate zombies. On some systems, you can avoid this by setting
1684 C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
1685 forking and reaping moribund children.
1687 Note that if your forked child inherits system file descriptors like
1688 STDIN and STDOUT that are actually connected by a pipe or socket, even
1689 if you exit, then the remote server (such as, say, a CGI script or a
1690 backgrounded job launched from a remote shell) won't think you're done.
1691 You should reopen those to F</dev/null> if it's any issue.
1695 Declare a picture format for use by the C<write> function. For
1699 Test: @<<<<<<<< @||||| @>>>>>
1700 $str, $%, '$' . int($num)
1704 $num = $cost/$quantity;
1708 See L<perlform> for many details and examples.
1710 =item formline PICTURE,LIST
1712 This is an internal function used by C<format>s, though you may call it,
1713 too. It formats (see L<perlform>) a list of values according to the
1714 contents of PICTURE, placing the output into the format output
1715 accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
1716 Eventually, when a C<write> is done, the contents of
1717 C<$^A> are written to some filehandle, but you could also read C<$^A>
1718 yourself and then set C<$^A> back to C<"">. Note that a format typically
1719 does one C<formline> per line of form, but the C<formline> function itself
1720 doesn't care how many newlines are embedded in the PICTURE. This means
1721 that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
1722 You may therefore need to use multiple formlines to implement a single
1723 record format, just like the format compiler.
1725 Be careful if you put double quotes around the picture, because an C<@>
1726 character may be taken to mean the beginning of an array name.
1727 C<formline> always returns true. See L<perlform> for other examples.
1729 =item getc FILEHANDLE
1733 Returns the next character from the input file attached to FILEHANDLE,
1734 or the undefined value at end of file, or if there was an error.
1735 If FILEHANDLE is omitted, reads from STDIN. This is not particularly
1736 efficient. However, it cannot be used by itself to fetch single
1737 characters without waiting for the user to hit enter. For that, try
1738 something more like:
1741 system "stty cbreak </dev/tty >/dev/tty 2>&1";
1744 system "stty", '-icanon', 'eol', "\001";
1750 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
1753 system "stty", 'icanon', 'eol', '^@'; # ASCII null
1757 Determination of whether $BSD_STYLE should be set
1758 is left as an exercise to the reader.
1760 The C<POSIX::getattr> function can do this more portably on
1761 systems purporting POSIX compliance. See also the C<Term::ReadKey>
1762 module from your nearest CPAN site; details on CPAN can be found on
1767 Implements the C library function of the same name, which on most
1768 systems returns the current login from F</etc/utmp>, if any. If null,
1771 $login = getlogin || getpwuid($<) || "Kilroy";
1773 Do not consider C<getlogin> for authentication: it is not as
1774 secure as C<getpwuid>.
1776 =item getpeername SOCKET
1778 Returns the packed sockaddr address of other end of the SOCKET connection.
1781 $hersockaddr = getpeername(SOCK);
1782 ($port, $iaddr) = sockaddr_in($hersockaddr);
1783 $herhostname = gethostbyaddr($iaddr, AF_INET);
1784 $herstraddr = inet_ntoa($iaddr);
1788 Returns the current process group for the specified PID. Use
1789 a PID of C<0> to get the current process group for the
1790 current process. Will raise an exception if used on a machine that
1791 doesn't implement getpgrp(2). If PID is omitted, returns process
1792 group of current process. Note that the POSIX version of C<getpgrp>
1793 does not accept a PID argument, so only C<PID==0> is truly portable.
1797 Returns the process id of the parent process.
1799 =item getpriority WHICH,WHO
1801 Returns the current priority for a process, a process group, or a user.
1802 (See L<getpriority(2)>.) Will raise a fatal exception if used on a
1803 machine that doesn't implement getpriority(2).
1809 =item gethostbyname NAME
1811 =item getnetbyname NAME
1813 =item getprotobyname NAME
1819 =item getservbyname NAME,PROTO
1821 =item gethostbyaddr ADDR,ADDRTYPE
1823 =item getnetbyaddr ADDR,ADDRTYPE
1825 =item getprotobynumber NUMBER
1827 =item getservbyport PORT,PROTO
1845 =item sethostent STAYOPEN
1847 =item setnetent STAYOPEN
1849 =item setprotoent STAYOPEN
1851 =item setservent STAYOPEN
1865 These routines perform the same functions as their counterparts in the
1866 system library. In list context, the return values from the
1867 various get routines are as follows:
1869 ($name,$passwd,$uid,$gid,
1870 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
1871 ($name,$passwd,$gid,$members) = getgr*
1872 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
1873 ($name,$aliases,$addrtype,$net) = getnet*
1874 ($name,$aliases,$proto) = getproto*
1875 ($name,$aliases,$port,$proto) = getserv*
1877 (If the entry doesn't exist you get a null list.)
1879 The exact meaning of the $gcos field varies but it usually contains
1880 the real name of the user (as opposed to the login name) and other
1881 information pertaining to the user. Beware, however, that in many
1882 system users are able to change this information and therefore it
1883 cannot be trusted and therefore the $gcos is tainted (see
1884 L<perlsec>). The $passwd and $shell, user's encrypted password and
1885 login shell, are also tainted, because of the same reason.
1887 In scalar context, you get the name, unless the function was a
1888 lookup by name, in which case you get the other thing, whatever it is.
1889 (If the entry doesn't exist you get the undefined value.) For example:
1891 $uid = getpwnam($name);
1892 $name = getpwuid($num);
1894 $gid = getgrnam($name);
1895 $name = getgrgid($num;
1899 In I<getpw*()> the fields $quota, $comment, and $expire are special
1900 cases in the sense that in many systems they are unsupported. If the
1901 $quota is unsupported, it is an empty scalar. If it is supported, it
1902 usually encodes the disk quota. If the $comment field is unsupported,
1903 it is an empty scalar. If it is supported it usually encodes some
1904 administrative comment about the user. In some systems the $quota
1905 field may be $change or $age, fields that have to do with password
1906 aging. In some systems the $comment field may be $class. The $expire
1907 field, if present, encodes the expiration period of the account or the
1908 password. For the availability and the exact meaning of these fields
1909 in your system, please consult your getpwnam(3) documentation and your
1910 F<pwd.h> file. You can also find out from within Perl what your
1911 $quota and $comment fields mean and whether you have the $expire field
1912 by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
1913 C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password
1914 files are only supported if your vendor has implemented them in the
1915 intuitive fashion that calling the regular C library routines gets the
1916 shadow versions if you're running under privilege or if there exists
1917 the shadow(3) functions as found in System V ( this includes Solaris
1918 and Linux.) Those systems which implement a proprietary shadow password
1919 facility are unlikely to be supported.
1921 The $members value returned by I<getgr*()> is a space separated list of
1922 the login names of the members of the group.
1924 For the I<gethost*()> functions, if the C<h_errno> variable is supported in
1925 C, it will be returned to you via C<$?> if the function call fails. The
1926 C<@addrs> value returned by a successful call is a list of the raw
1927 addresses returned by the corresponding system library call. In the
1928 Internet domain, each address is four bytes long and you can unpack it
1929 by saying something like:
1931 ($a,$b,$c,$d) = unpack('C4',$addr[0]);
1933 The Socket library makes this slightly easier:
1936 $iaddr = inet_aton("127.1"); # or whatever address
1937 $name = gethostbyaddr($iaddr, AF_INET);
1939 # or going the other way
1940 $straddr = inet_ntoa($iaddr);
1942 If you get tired of remembering which element of the return list
1943 contains which return value, by-name interfaces are provided
1944 in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
1945 C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
1946 and C<User::grent>. These override the normal built-ins, supplying
1947 versions that return objects with the appropriate names
1948 for each field. For example:
1952 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
1954 Even though it looks like they're the same method calls (uid),
1955 they aren't, because a C<File::stat> object is different from
1956 a C<User::pwent> object.
1958 =item getsockname SOCKET
1960 Returns the packed sockaddr address of this end of the SOCKET connection,
1961 in case you don't know the address because you have several different
1962 IPs that the connection might have come in on.
1965 $mysockaddr = getsockname(SOCK);
1966 ($port, $myaddr) = sockaddr_in($mysockaddr);
1967 printf "Connect to %s [%s]\n",
1968 scalar gethostbyaddr($myaddr, AF_INET),
1971 =item getsockopt SOCKET,LEVEL,OPTNAME
1973 Returns the socket option requested, or undef if there is an error.
1979 Returns the value of EXPR with filename expansions such as the
1980 standard Unix shell F</bin/csh> would do. This is the internal function
1981 implementing the C<< <*.c> >> operator, but you can use it directly.
1982 If EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is
1983 discussed in more detail in L<perlop/"I/O Operators">.
1985 Beginning with v5.6.0, this operator is implemented using the standard
1986 C<File::Glob> extension. See L<File::Glob> for details.
1990 Converts a time as returned by the time function to a 8-element list
1991 with the time localized for the standard Greenwich time zone.
1992 Typically used as follows:
1995 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday) =
1998 All list elements are numeric, and come straight out of the C `struct
1999 tm'. $sec, $min, and $hour are the seconds, minutes, and hours of the
2000 specified time. $mday is the day of the month, and $mon is the month
2001 itself, in the range C<0..11> with 0 indicating January and 11
2002 indicating December. $year is the number of years since 1900. That
2003 is, $year is C<123> in year 2023. $wday is the day of the week, with
2004 0 indicating Sunday and 3 indicating Wednesday. $yday is the day of
2005 the year, in the range C<0..364> (or C<0..365> in leap years.)
2007 Note that the $year element is I<not> simply the last two digits of
2008 the year. If you assume it is, then you create non-Y2K-compliant
2009 programs--and you wouldn't want to do that, would you?
2011 The proper way to get a complete 4-digit year is simply:
2015 And to get the last two digits of the year (e.g., '01' in 2001) do:
2017 $year = sprintf("%02d", $year % 100);
2019 If EXPR is omitted, C<gmtime()> uses the current time (C<gmtime(time)>).
2021 In scalar context, C<gmtime()> returns the ctime(3) value:
2023 $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
2025 Also see the C<timegm> function provided by the C<Time::Local> module,
2026 and the strftime(3) function available via the POSIX module.
2028 This scalar value is B<not> locale dependent (see L<perllocale>), but
2029 is instead a Perl builtin. Also see the C<Time::Local> module, and the
2030 strftime(3) and mktime(3) functions available via the POSIX module. To
2031 get somewhat similar but locale dependent date strings, set up your
2032 locale environment variables appropriately (please see L<perllocale>)
2033 and try for example:
2035 use POSIX qw(strftime);
2036 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
2038 Note that the C<%a> and C<%b> escapes, which represent the short forms
2039 of the day of the week and the month of the year, may not necessarily
2040 be three characters wide in all locales.
2048 The C<goto-LABEL> form finds the statement labeled with LABEL and resumes
2049 execution there. It may not be used to go into any construct that
2050 requires initialization, such as a subroutine or a C<foreach> loop. It
2051 also can't be used to go into a construct that is optimized away,
2052 or to get out of a block or subroutine given to C<sort>.
2053 It can be used to go almost anywhere else within the dynamic scope,
2054 including out of subroutines, but it's usually better to use some other
2055 construct such as C<last> or C<die>. The author of Perl has never felt the
2056 need to use this form of C<goto> (in Perl, that is--C is another matter).
2058 The C<goto-EXPR> form expects a label name, whose scope will be resolved
2059 dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
2060 necessarily recommended if you're optimizing for maintainability:
2062 goto ("FOO", "BAR", "GLARCH")[$i];
2064 The C<goto-&NAME> form is quite different from the other forms of C<goto>.
2065 In fact, it isn't a goto in the normal sense at all, and doesn't have
2066 the stigma associated with other gotos. Instead, it
2067 substitutes a call to the named subroutine for the currently running
2068 subroutine. This is used by C<AUTOLOAD> subroutines that wish to load
2069 another subroutine and then pretend that the other subroutine had been
2070 called in the first place (except that any modifications to C<@_>
2071 in the current subroutine are propagated to the other subroutine.)
2072 After the C<goto>, not even C<caller> will be able to tell that this
2073 routine was called first.
2075 NAME needn't be the name of a subroutine; it can be a scalar variable
2076 containing a code reference, or a block which evaluates to a code
2079 =item grep BLOCK LIST
2081 =item grep EXPR,LIST
2083 This is similar in spirit to, but not the same as, grep(1) and its
2084 relatives. In particular, it is not limited to using regular expressions.
2086 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2087 C<$_> to each element) and returns the list value consisting of those
2088 elements for which the expression evaluated to true. In scalar
2089 context, returns the number of times the expression was true.
2091 @foo = grep(!/^#/, @bar); # weed out comments
2095 @foo = grep {!/^#/} @bar; # weed out comments
2097 Note that C<$_> is an alias to the list value, so it can be used to
2098 modify the elements of the LIST. While this is useful and supported,
2099 it can cause bizarre results if the elements of LIST are not variables.
2100 Similarly, grep returns aliases into the original list, much as a for
2101 loop's index variable aliases the list elements. That is, modifying an
2102 element of a list returned by grep (for example, in a C<foreach>, C<map>
2103 or another C<grep>) actually modifies the element in the original list.
2104 This is usually something to be avoided when writing clear code.
2106 See also L</map> for a list composed of the results of the BLOCK or EXPR.
2112 Interprets EXPR as a hex string and returns the corresponding value.
2113 (To convert strings that might start with either 0, 0x, or 0b, see
2114 L</oct>.) If EXPR is omitted, uses C<$_>.
2116 print hex '0xAf'; # prints '175'
2117 print hex 'aF'; # same
2119 Hex strings may only represent integers. Strings that would cause
2120 integer overflow trigger a warning.
2124 There is no builtin C<import> function. It is just an ordinary
2125 method (subroutine) defined (or inherited) by modules that wish to export
2126 names to another module. The C<use> function calls the C<import> method
2127 for the package used. See also L</use>, L<perlmod>, and L<Exporter>.
2129 =item index STR,SUBSTR,POSITION
2131 =item index STR,SUBSTR
2133 The index function searches for one string within another, but without
2134 the wildcard-like behavior of a full regular-expression pattern match.
2135 It returns the position of the first occurrence of SUBSTR in STR at
2136 or after POSITION. If POSITION is omitted, starts searching from the
2137 beginning of the string. The return value is based at C<0> (or whatever
2138 you've set the C<$[> variable to--but don't do that). If the substring
2139 is not found, returns one less than the base, ordinarily C<-1>.
2145 Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
2146 You should not use this function for rounding: one because it truncates
2147 towards C<0>, and two because machine representations of floating point
2148 numbers can sometimes produce counterintuitive results. For example,
2149 C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
2150 because it's really more like -268.99999999999994315658 instead. Usually,
2151 the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
2152 functions will serve you better than will int().
2154 =item ioctl FILEHANDLE,FUNCTION,SCALAR
2156 Implements the ioctl(2) function. You'll probably first have to say
2158 require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph
2160 to get the correct function definitions. If F<ioctl.ph> doesn't
2161 exist or doesn't have the correct definitions you'll have to roll your
2162 own, based on your C header files such as F<< <sys/ioctl.h> >>.
2163 (There is a Perl script called B<h2ph> that comes with the Perl kit that
2164 may help you in this, but it's nontrivial.) SCALAR will be read and/or
2165 written depending on the FUNCTION--a pointer to the string value of SCALAR
2166 will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
2167 has no string value but does have a numeric value, that value will be
2168 passed rather than a pointer to the string value. To guarantee this to be
2169 true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
2170 functions may be needed to manipulate the values of structures used by
2173 The return value of C<ioctl> (and C<fcntl>) is as follows:
2175 if OS returns: then Perl returns:
2177 0 string "0 but true"
2178 anything else that number
2180 Thus Perl returns true on success and false on failure, yet you can
2181 still easily determine the actual value returned by the operating
2184 $retval = ioctl(...) || -1;
2185 printf "System returned %d\n", $retval;
2187 The special string "C<0> but true" is exempt from B<-w> complaints
2188 about improper numeric conversions.
2190 Here's an example of setting a filehandle named C<REMOTE> to be
2191 non-blocking at the system level. You'll have to negotiate C<$|>
2192 on your own, though.
2194 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
2196 $flags = fcntl(REMOTE, F_GETFL, 0)
2197 or die "Can't get flags for the socket: $!\n";
2199 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
2200 or die "Can't set flags for the socket: $!\n";
2202 =item join EXPR,LIST
2204 Joins the separate strings of LIST into a single string with fields
2205 separated by the value of EXPR, and returns that new string. Example:
2207 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
2209 Beware that unlike C<split>, C<join> doesn't take a pattern as its
2210 first argument. Compare L</split>.
2214 Returns a list consisting of all the keys of the named hash. (In
2215 scalar context, returns the number of keys.) The keys are returned in
2216 an apparently random order. The actual random order is subject to
2217 change in future versions of perl, but it is guaranteed to be the same
2218 order as either the C<values> or C<each> function produces (given
2219 that the hash has not been modified). As a side effect, it resets
2222 Here is yet another way to print your environment:
2225 @values = values %ENV;
2227 print pop(@keys), '=', pop(@values), "\n";
2230 or how about sorted by key:
2232 foreach $key (sort(keys %ENV)) {
2233 print $key, '=', $ENV{$key}, "\n";
2236 The returned values are copies of the original keys in the hash, so
2237 modifying them will not affect the original hash. Compare L</values>.
2239 To sort a hash by value, you'll need to use a C<sort> function.
2240 Here's a descending numeric sort of a hash by its values:
2242 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
2243 printf "%4d %s\n", $hash{$key}, $key;
2246 As an lvalue C<keys> allows you to increase the number of hash buckets
2247 allocated for the given hash. This can gain you a measure of efficiency if
2248 you know the hash is going to get big. (This is similar to pre-extending
2249 an array by assigning a larger number to $#array.) If you say
2253 then C<%hash> will have at least 200 buckets allocated for it--256 of them,
2254 in fact, since it rounds up to the next power of two. These
2255 buckets will be retained even if you do C<%hash = ()>, use C<undef
2256 %hash> if you want to free the storage while C<%hash> is still in scope.
2257 You can't shrink the number of buckets allocated for the hash using
2258 C<keys> in this way (but you needn't worry about doing this by accident,
2259 as trying has no effect).
2261 See also C<each>, C<values> and C<sort>.
2263 =item kill SIGNAL, LIST
2265 Sends a signal to a list of processes. Returns the number of
2266 processes successfully signaled (which is not necessarily the
2267 same as the number actually killed).
2269 $cnt = kill 1, $child1, $child2;
2272 If SIGNAL is zero, no signal is sent to the process. This is a
2273 useful way to check that the process is alive and hasn't changed
2274 its UID. See L<perlport> for notes on the portability of this
2277 Unlike in the shell, if SIGNAL is negative, it kills
2278 process groups instead of processes. (On System V, a negative I<PROCESS>
2279 number will also kill process groups, but that's not portable.) That
2280 means you usually want to use positive not negative signals. You may also
2281 use a signal name in quotes. See L<perlipc/"Signals"> for details.
2287 The C<last> command is like the C<break> statement in C (as used in
2288 loops); it immediately exits the loop in question. If the LABEL is
2289 omitted, the command refers to the innermost enclosing loop. The
2290 C<continue> block, if any, is not executed:
2292 LINE: while (<STDIN>) {
2293 last LINE if /^$/; # exit when done with header
2297 C<last> cannot be used to exit a block which returns a value such as
2298 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2299 a grep() or map() operation.
2301 Note that a block by itself is semantically identical to a loop
2302 that executes once. Thus C<last> can be used to effect an early
2303 exit out of such a block.
2305 See also L</continue> for an illustration of how C<last>, C<next>, and
2312 Returns an lowercased version of EXPR. This is the internal function
2313 implementing the C<\L> escape in double-quoted strings.
2314 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
2317 If EXPR is omitted, uses C<$_>.
2323 Returns the value of EXPR with the first character lowercased. This is
2324 the internal function implementing the C<\l> escape in double-quoted strings.
2325 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
2327 If EXPR is omitted, uses C<$_>.
2333 Returns the length in characters of the value of EXPR. If EXPR is
2334 omitted, returns length of C<$_>. Note that this cannot be used on
2335 an entire array or hash to find out how many elements these have.
2336 For that, use C<scalar @array> and C<scalar keys %hash> respectively.
2338 =item link OLDFILE,NEWFILE
2340 Creates a new filename linked to the old filename. Returns true for
2341 success, false otherwise.
2343 =item listen SOCKET,QUEUESIZE
2345 Does the same thing that the listen system call does. Returns true if
2346 it succeeded, false otherwise. See the example in
2347 L<perlipc/"Sockets: Client/Server Communication">.
2351 You really probably want to be using C<my> instead, because C<local> isn't
2352 what most people think of as "local". See
2353 L<perlsub/"Private Variables via my()"> for details.
2355 A local modifies the listed variables to be local to the enclosing
2356 block, file, or eval. If more than one value is listed, the list must
2357 be placed in parentheses. See L<perlsub/"Temporary Values via local()">
2358 for details, including issues with tied arrays and hashes.
2360 =item localtime EXPR
2362 Converts a time as returned by the time function to a 9-element list
2363 with the time analyzed for the local time zone. Typically used as
2367 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
2370 All list elements are numeric, and come straight out of the C `struct
2371 tm'. $sec, $min, and $hour are the seconds, minutes, and hours of the
2372 specified time. $mday is the day of the month, and $mon is the month
2373 itself, in the range C<0..11> with 0 indicating January and 11
2374 indicating December. $year is the number of years since 1900. That
2375 is, $year is C<123> in year 2023. $wday is the day of the week, with
2376 0 indicating Sunday and 3 indicating Wednesday. $yday is the day of
2377 the year, in the range C<0..364> (or C<0..365> in leap years.) $isdst
2378 is true if the specified time occurs during daylight savings time,
2381 Note that the $year element is I<not> simply the last two digits of
2382 the year. If you assume it is, then you create non-Y2K-compliant
2383 programs--and you wouldn't want to do that, would you?
2385 The proper way to get a complete 4-digit year is simply:
2389 And to get the last two digits of the year (e.g., '01' in 2001) do:
2391 $year = sprintf("%02d", $year % 100);
2393 If EXPR is omitted, C<localtime()> uses the current time (C<localtime(time)>).
2395 In scalar context, C<localtime()> returns the ctime(3) value:
2397 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
2399 This scalar value is B<not> locale dependent, see L<perllocale>, but
2400 instead a Perl builtin. Also see the C<Time::Local> module
2401 (to convert the second, minutes, hours, ... back to seconds since the
2402 stroke of midnight the 1st of January 1970, the value returned by
2403 time()), and the strftime(3) and mktime(3) functions available via the
2404 POSIX module. To get somewhat similar but locale dependent date
2405 strings, set up your locale environment variables appropriately
2406 (please see L<perllocale>) and try for example:
2408 use POSIX qw(strftime);
2409 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
2411 Note that the C<%a> and C<%b>, the short forms of the day of the week
2412 and the month of the year, may not necessarily be three characters wide.
2418 This function places an advisory lock on a variable, subroutine,
2419 or referenced object contained in I<THING> until the lock goes out
2420 of scope. This is a built-in function only if your version of Perl
2421 was built with threading enabled, and if you've said C<use Threads>.
2422 Otherwise a user-defined function by this name will be called. See
2429 Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
2430 returns log of C<$_>. To get the log of another base, use basic algebra:
2431 The base-N log of a number is equal to the natural log of that number
2432 divided by the natural log of N. For example:
2436 return log($n)/log(10);
2439 See also L</exp> for the inverse operation.
2445 Does the same thing as the C<stat> function (including setting the
2446 special C<_> filehandle) but stats a symbolic link instead of the file
2447 the symbolic link points to. If symbolic links are unimplemented on
2448 your system, a normal C<stat> is done.
2450 If EXPR is omitted, stats C<$_>.
2454 The match operator. See L<perlop>.
2456 =item map BLOCK LIST
2460 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2461 C<$_> to each element) and returns the list value composed of the
2462 results of each such evaluation. In scalar context, returns the
2463 total number of elements so generated. Evaluates BLOCK or EXPR in
2464 list context, so each element of LIST may produce zero, one, or
2465 more elements in the returned value.
2467 @chars = map(chr, @nums);
2469 translates a list of numbers to the corresponding characters. And
2471 %hash = map { getkey($_) => $_ } @array;
2473 is just a funny way to write
2476 foreach $_ (@array) {
2477 $hash{getkey($_)} = $_;
2480 Note that C<$_> is an alias to the list value, so it can be used to
2481 modify the elements of the LIST. While this is useful and supported,
2482 it can cause bizarre results if the elements of LIST are not variables.
2483 Using a regular C<foreach> loop for this purpose would be clearer in
2484 most cases. See also L</grep> for an array composed of those items of
2485 the original list for which the BLOCK or EXPR evaluates to true.
2487 C<{> starts both hash references and blocks, so C<map { ...> could be either
2488 the start of map BLOCK LIST or map EXPR, LIST. Because perl doesn't look
2489 ahead for the closing C<}> it has to take a guess at which its dealing with
2490 based what it finds just after the C<{>. Usually it gets it right, but if it
2491 doesn't it won't realize something is wrong until it gets to the C<}> and
2492 encounters the missing (or unexpected) comma. The syntax error will be
2493 reported close to the C<}> but you'll need to change something near the C<{>
2494 such as using a unary C<+> to give perl some help:
2496 %hash = map { "\L$_", 1 } @array # perl guesses EXPR. wrong
2497 %hash = map { +"\L$_", 1 } @array # perl guesses BLOCK. right
2498 %hash = map { ("\L$_", 1) } @array # this also works
2499 %hash = map { lc($_), 1 } @array # as does this.
2500 %hash = map +( lc($_), 1 ), @array # this is EXPR and works!
2502 %hash = map ( lc($_), 1 ), @array # evaluates to (1, @array)
2504 or to force an anon hash constructor use C<+{>
2506 @hashes = map +{ lc($_), 1 }, @array # EXPR, so needs , at end
2508 and you get list of anonymous hashes each with only 1 entry.
2510 =item mkdir FILENAME,MASK
2512 =item mkdir FILENAME
2514 Creates the directory specified by FILENAME, with permissions
2515 specified by MASK (as modified by C<umask>). If it succeeds it
2516 returns true, otherwise it returns false and sets C<$!> (errno).
2517 If omitted, MASK defaults to 0777.
2519 In general, it is better to create directories with permissive MASK,
2520 and let the user modify that with their C<umask>, than it is to supply
2521 a restrictive MASK and give the user no way to be more permissive.
2522 The exceptions to this rule are when the file or directory should be
2523 kept private (mail files, for instance). The perlfunc(1) entry on
2524 C<umask> discusses the choice of MASK in more detail.
2526 Note that according to the POSIX 1003.1-1996 the FILENAME may have any
2527 number of trailing slashes. Some operating and filesystems do not get
2528 this right, so Perl automatically removes all trailing slashes to keep
2531 =item msgctl ID,CMD,ARG
2533 Calls the System V IPC function msgctl(2). You'll probably have to say
2537 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
2538 then ARG must be a variable which will hold the returned C<msqid_ds>
2539 structure. Returns like C<ioctl>: the undefined value for error,
2540 C<"0 but true"> for zero, or the actual return value otherwise. See also
2541 L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::Semaphore> documentation.
2543 =item msgget KEY,FLAGS
2545 Calls the System V IPC function msgget(2). Returns the message queue
2546 id, or the undefined value if there is an error. See also
2547 L<perlipc/"SysV IPC"> and C<IPC::SysV> and C<IPC::Msg> documentation.
2549 =item msgrcv ID,VAR,SIZE,TYPE,FLAGS
2551 Calls the System V IPC function msgrcv to receive a message from
2552 message queue ID into variable VAR with a maximum message size of
2553 SIZE. Note that when a message is received, the message type as a
2554 native long integer will be the first thing in VAR, followed by the
2555 actual message. This packing may be opened with C<unpack("l! a*")>.
2556 Taints the variable. Returns true if successful, or false if there is
2557 an error. See also L<perlipc/"SysV IPC">, C<IPC::SysV>, and
2558 C<IPC::SysV::Msg> documentation.
2560 =item msgsnd ID,MSG,FLAGS
2562 Calls the System V IPC function msgsnd to send the message MSG to the
2563 message queue ID. MSG must begin with the native long integer message
2564 type, and be followed by the length of the actual message, and finally
2565 the message itself. This kind of packing can be achieved with
2566 C<pack("l! a*", $type, $message)>. Returns true if successful,
2567 or false if there is an error. See also C<IPC::SysV>
2568 and C<IPC::SysV::Msg> documentation.
2572 =item my EXPR : ATTRIBUTES
2574 A C<my> declares the listed variables to be local (lexically) to the
2575 enclosing block, file, or C<eval>. If
2576 more than one value is listed, the list must be placed in parentheses. See
2577 L<perlsub/"Private Variables via my()"> for details.
2583 The C<next> command is like the C<continue> statement in C; it starts
2584 the next iteration of the loop:
2586 LINE: while (<STDIN>) {
2587 next LINE if /^#/; # discard comments
2591 Note that if there were a C<continue> block on the above, it would get
2592 executed even on discarded lines. If the LABEL is omitted, the command
2593 refers to the innermost enclosing loop.
2595 C<next> cannot be used to exit a block which returns a value such as
2596 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2597 a grep() or map() operation.
2599 Note that a block by itself is semantically identical to a loop
2600 that executes once. Thus C<next> will exit such a block early.
2602 See also L</continue> for an illustration of how C<last>, C<next>, and
2605 =item no Module LIST
2607 See the L</use> function, which C<no> is the opposite of.
2613 Interprets EXPR as an octal string and returns the corresponding
2614 value. (If EXPR happens to start off with C<0x>, interprets it as a
2615 hex string. If EXPR starts off with C<0b>, it is interpreted as a
2616 binary string.) The following will handle decimal, binary, octal, and
2617 hex in the standard Perl or C notation:
2619 $val = oct($val) if $val =~ /^0/;
2621 If EXPR is omitted, uses C<$_>. To go the other way (produce a number
2622 in octal), use sprintf() or printf():
2624 $perms = (stat("filename"))[2] & 07777;
2625 $oct_perms = sprintf "%lo", $perms;
2627 The oct() function is commonly used when a string such as C<644> needs
2628 to be converted into a file mode, for example. (Although perl will
2629 automatically convert strings into numbers as needed, this automatic
2630 conversion assumes base 10.)
2632 =item open FILEHANDLE,EXPR
2634 =item open FILEHANDLE,MODE,EXPR
2636 =item open FILEHANDLE,MODE,EXPR,LIST
2638 =item open FILEHANDLE
2640 Opens the file whose filename is given by EXPR, and associates it with
2641 FILEHANDLE. If FILEHANDLE is an undefined lexical (C<my>) variable the variable is
2642 assigned a reference to a new anonymous filehandle, otherwise if FILEHANDLE is an expression,
2643 its value is used as the name of the real filehandle wanted. (This is considered a symbolic
2644 reference, so C<use strict 'refs'> should I<not> be in effect.)
2646 If EXPR is omitted, the scalar
2647 variable of the same name as the FILEHANDLE contains the filename.
2648 (Note that lexical variables--those declared with C<my>--will not work
2649 for this purpose; so if you're using C<my>, specify EXPR in your call
2650 to open.) See L<perlopentut> for a kinder, gentler explanation of opening
2653 If three or more arguments are specified then the mode of opening and the file name
2654 are separate. If MODE is C<< '<' >> or nothing, the file is opened for input.
2655 If MODE is C<< '>' >>, the file is truncated and opened for
2656 output, being created if necessary. If MODE is C<<< '>>' >>>,
2657 the file is opened for appending, again being created if necessary.
2658 You can put a C<'+'> in front of the C<< '>' >> or C<< '<' >> to indicate that
2659 you want both read and write access to the file; thus C<< '+<' >> is almost
2660 always preferred for read/write updates--the C<< '+>' >> mode would clobber the
2661 file first. You can't usually use either read-write mode for updating
2662 textfiles, since they have variable length records. See the B<-i>
2663 switch in L<perlrun> for a better approach. The file is created with
2664 permissions of C<0666> modified by the process' C<umask> value.
2666 These various prefixes correspond to the fopen(3) modes of C<'r'>, C<'r+'>,
2667 C<'w'>, C<'w+'>, C<'a'>, and C<'a+'>.
2669 In the 2-arguments (and 1-argument) form of the call the mode and
2670 filename should be concatenated (in this order), possibly separated by
2671 spaces. It is possible to omit the mode in these forms if the mode is
2674 If the filename begins with C<'|'>, the filename is interpreted as a
2675 command to which output is to be piped, and if the filename ends with a
2676 C<'|'>, the filename is interpreted as a command which pipes output to
2677 us. See L<perlipc/"Using open() for IPC">
2678 for more examples of this. (You are not allowed to C<open> to a command
2679 that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
2680 and L<perlipc/"Bidirectional Communication with Another Process">
2683 For three or more arguments if MODE is C<'|-'>, the filename is interpreted as a
2684 command to which output is to be piped, and if MODE is
2685 C<'-|'>, the filename is interpreted as a command which pipes output to
2686 us. In the 2-arguments (and 1-argument) form one should replace dash
2687 (C<'-'>) with the command. See L<perlipc/"Using open() for IPC">
2688 for more examples of this. (You are not allowed to C<open> to a command
2689 that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
2690 and L<perlipc/"Bidirectional Communication"> for alternatives.) In 3+ arg form of
2691 pipe opens then if LIST is specified (extra arguments after the command name) then
2692 LIST becomes arguments to the command invoked if the platform supports it.
2693 The meaning of C<open> with more than three arguments for non-pipe modes
2694 is not yet specified. Experimental "layers" may give extra LIST arguments meaning.
2696 In the 2-arguments (and 1-argument) form opening C<'-'> opens STDIN
2697 and opening C<< '>-' >> opens STDOUT.
2700 nonzero upon success, the undefined value otherwise. If the C<open>
2701 involved a pipe, the return value happens to be the pid of the
2704 If you're unfortunate enough to be running Perl on a system that
2705 distinguishes between text files and binary files (modern operating
2706 systems don't care), then you should check out L</binmode> for tips for
2707 dealing with this. The key distinction between systems that need C<binmode>
2708 and those that don't is their text file formats. Systems like Unix, MacOS, and
2709 Plan9, which delimit lines with a single character, and which encode that
2710 character in C as C<"\n">, do not need C<binmode>. The rest need it.
2712 In the three argument form MODE may also contain a list of IO "layers" (see L<open> and
2713 L<PerlIO> for more details) to be applied to the handle. This can be used to achieve the
2714 effect of C<binmode> as well as more complex behaviours.
2716 When opening a file, it's usually a bad idea to continue normal execution
2717 if the request failed, so C<open> is frequently used in connection with
2718 C<die>. Even if C<die> won't do what you want (say, in a CGI script,
2719 where you want to make a nicely formatted error message (but there are
2720 modules that can help with that problem)) you should always check
2721 the return value from opening a file. The infrequent exception is when
2722 working with an unopened filehandle is actually what you want to do.
2724 As a special case the 3 arg form with a read/write mode and the third argument
2727 open(TMP, "+>", undef) or die ...
2729 opens a filehandle to an anonymous temporary file.
2735 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
2736 while (<ARTICLE>) {...
2738 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
2739 # if the open fails, output is discarded
2741 open(DBASE, '+<', 'dbase.mine') # open for update
2742 or die "Can't open 'dbase.mine' for update: $!";
2744 open(DBASE, '+<dbase.mine') # ditto
2745 or die "Can't open 'dbase.mine' for update: $!";
2747 open(ARTICLE, '-|', "caesar <$article") # decrypt article
2748 or die "Can't start caesar: $!";
2750 open(ARTICLE, "caesar <$article |") # ditto
2751 or die "Can't start caesar: $!";
2753 open(EXTRACT, "|sort >/tmp/Tmp$$") # $$ is our process id
2754 or die "Can't start sort: $!";
2756 # process argument list of files along with any includes
2758 foreach $file (@ARGV) {
2759 process($file, 'fh00');
2763 my($filename, $input) = @_;
2764 $input++; # this is a string increment
2765 unless (open($input, $filename)) {
2766 print STDERR "Can't open $filename: $!\n";
2771 while (<$input>) { # note use of indirection
2772 if (/^#include "(.*)"/) {
2773 process($1, $input);
2780 You may also, in the Bourne shell tradition, specify an EXPR beginning
2781 with C<< '>&' >>, in which case the rest of the string is interpreted as the
2782 name of a filehandle (or file descriptor, if numeric) to be
2783 duped and opened. You may use C<&> after C<< > >>, C<<< >> >>>,
2784 C<< < >>, C<< +> >>, C<<< +>> >>>, and C<< +< >>. The
2785 mode you specify should match the mode of the original filehandle.
2786 (Duping a filehandle does not take into account any existing contents of
2787 stdio buffers.) If you use the 3 arg form then you can pass either a number,
2788 the name of a filehandle or the normal "reference to a glob".
2790 Here is a script that saves, redirects, and restores STDOUT and
2794 open(my $oldout, ">&", \*STDOUT);
2795 open(OLDERR, ">&STDERR");
2797 open(STDOUT, '>', "foo.out") || die "Can't redirect stdout";
2798 open(STDERR, ">&STDOUT") || die "Can't dup stdout";
2800 select(STDERR); $| = 1; # make unbuffered
2801 select(STDOUT); $| = 1; # make unbuffered
2803 print STDOUT "stdout 1\n"; # this works for
2804 print STDERR "stderr 1\n"; # subprocesses too
2809 open(STDOUT, ">&OLDOUT");
2810 open(STDERR, ">&OLDERR");
2812 print STDOUT "stdout 2\n";
2813 print STDERR "stderr 2\n";
2815 If you specify C<< '<&=N' >>, where C<N> is a number, then Perl will do an
2816 equivalent of C's C<fdopen> of that file descriptor; this is more
2817 parsimonious of file descriptors. For example:
2819 open(FILEHANDLE, "<&=$fd")
2821 open(FILEHANDLE, "<&=", $fd)
2823 Note that if perl is using the standard C libaries fdopen() then on many UNIX systems,
2824 fdopen() is known to fail when file descriptors
2825 exceed a certain value, typically 255. If you need more file
2826 descriptors than that, consider rebuilding Perl to use the C<PerlIO>.
2828 If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'>
2829 with 2-arguments (or 1-argument) form of open(), then
2830 there is an implicit fork done, and the return value of open is the pid
2831 of the child within the parent process, and C<0> within the child
2832 process. (Use C<defined($pid)> to determine whether the open was successful.)
2833 The filehandle behaves normally for the parent, but i/o to that
2834 filehandle is piped from/to the STDOUT/STDIN of the child process.
2835 In the child process the filehandle isn't opened--i/o happens from/to
2836 the new STDOUT or STDIN. Typically this is used like the normal
2837 piped open when you want to exercise more control over just how the
2838 pipe command gets executed, such as when you are running setuid, and
2839 don't want to have to scan shell commands for metacharacters.
2840 The following triples are more or less equivalent:
2842 open(FOO, "|tr '[a-z]' '[A-Z]'");
2843 open(FOO, '|-', "tr '[a-z]' '[A-Z]'");
2844 open(FOO, '|-') || exec 'tr', '[a-z]', '[A-Z]';
2845 open(FOO, '|-', "tr", '[a-z]', '[A-Z]');
2847 open(FOO, "cat -n '$file'|");
2848 open(FOO, '-|', "cat -n '$file'");
2849 open(FOO, '-|') || exec 'cat', '-n', $file;
2850 open(FOO, '-|', "cat", '-n', $file);
2852 The last example in each block shows the pipe as "list form", which is
2853 not yet supported on all platforms.
2855 See L<perlipc/"Safe Pipe Opens"> for more examples of this.
2857 Beginning with v5.6.0, Perl will attempt to flush all files opened for
2858 output before any operation that may do a fork, but this may not be
2859 supported on some platforms (see L<perlport>). To be safe, you may need
2860 to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
2861 of C<IO::Handle> on any open handles.
2863 On systems that support a
2864 close-on-exec flag on files, the flag will be set for the newly opened
2865 file descriptor as determined by the value of $^F. See L<perlvar/$^F>.
2867 Closing any piped filehandle causes the parent process to wait for the
2868 child to finish, and returns the status value in C<$?>.
2870 The filename passed to 2-argument (or 1-argument) form of open()
2871 will have leading and trailing
2872 whitespace deleted, and the normal redirection characters
2873 honored. This property, known as "magic open",
2874 can often be used to good effect. A user could specify a filename of
2875 F<"rsh cat file |">, or you could change certain filenames as needed:
2877 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
2878 open(FH, $filename) or die "Can't open $filename: $!";
2880 Use 3-argument form to open a file with arbitrary weird characters in it,
2882 open(FOO, '<', $file);
2884 otherwise it's necessary to protect any leading and trailing whitespace:
2886 $file =~ s#^(\s)#./$1#;
2887 open(FOO, "< $file\0");
2889 (this may not work on some bizarre filesystems). One should
2890 conscientiously choose between the I<magic> and 3-arguments form
2895 will allow the user to specify an argument of the form C<"rsh cat file |">,
2896 but will not work on a filename which happens to have a trailing space, while
2898 open IN, '<', $ARGV[0];
2900 will have exactly the opposite restrictions.
2902 If you want a "real" C C<open> (see L<open(2)> on your system), then you
2903 should use the C<sysopen> function, which involves no such magic (but
2904 may use subtly different filemodes than Perl open(), which is mapped
2905 to C fopen()). This is
2906 another way to protect your filenames from interpretation. For example:
2909 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
2910 or die "sysopen $path: $!";
2911 $oldfh = select(HANDLE); $| = 1; select($oldfh);
2912 print HANDLE "stuff $$\n";
2914 print "File contains: ", <HANDLE>;
2916 Using the constructor from the C<IO::Handle> package (or one of its
2917 subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous
2918 filehandles that have the scope of whatever variables hold references to
2919 them, and automatically close whenever and however you leave that scope:
2923 sub read_myfile_munged {
2925 my $handle = new IO::File;
2926 open($handle, "myfile") or die "myfile: $!";
2928 or return (); # Automatically closed here.
2929 mung $first or die "mung failed"; # Or here.
2930 return $first, <$handle> if $ALL; # Or here.
2934 See L</seek> for some details about mixing reading and writing.
2936 =item opendir DIRHANDLE,EXPR
2938 Opens a directory named EXPR for processing by C<readdir>, C<telldir>,
2939 C<seekdir>, C<rewinddir>, and C<closedir>. Returns true if successful.
2940 DIRHANDLEs have their own namespace separate from FILEHANDLEs.
2946 Returns the numeric (ASCII or Unicode) value of the first character of EXPR. If
2947 EXPR is omitted, uses C<$_>. For the reverse, see L</chr>.
2948 See L<utf8> for more about Unicode.
2952 =item our EXPR : ATTRIBUTES
2954 An C<our> declares the listed variables to be valid globals within
2955 the enclosing block, file, or C<eval>. That is, it has the same
2956 scoping rules as a "my" declaration, but does not create a local
2957 variable. If more than one value is listed, the list must be placed
2958 in parentheses. The C<our> declaration has no semantic effect unless
2959 "use strict vars" is in effect, in which case it lets you use the
2960 declared global variable without qualifying it with a package name.
2961 (But only within the lexical scope of the C<our> declaration. In this
2962 it differs from "use vars", which is package scoped.)
2964 An C<our> declaration declares a global variable that will be visible
2965 across its entire lexical scope, even across package boundaries. The
2966 package in which the variable is entered is determined at the point
2967 of the declaration, not at the point of use. This means the following
2971 our $bar; # declares $Foo::bar for rest of lexical scope
2975 print $bar; # prints 20
2977 Multiple C<our> declarations in the same lexical scope are allowed
2978 if they are in different packages. If they happened to be in the same
2979 package, Perl will emit warnings if you have asked for them.
2983 our $bar; # declares $Foo::bar for rest of lexical scope
2987 our $bar = 30; # declares $Bar::bar for rest of lexical scope
2988 print $bar; # prints 30
2990 our $bar; # emits warning
2992 An C<our> declaration may also have a list of attributes associated
2993 with it. B<WARNING>: This is an experimental feature that may be
2994 changed or removed in future releases of Perl. It should not be
2997 The only currently recognized attribute is C<shared> which indicates
2998 that a single copy of the global is to be used by all interpreters
2999 should the program happen to be running in a multi-interpreter
3000 environment. (The default behaviour would be for each interpreter to
3001 have its own copy of the global.) In such an environment, this
3002 attribute also has the effect of making the global readonly.
3005 our @EXPORT : shared = qw(foo);
3006 our %EXPORT_TAGS : shared = (bar => [qw(aa bb cc)]);
3007 our $VERSION : shared = "1.00";
3009 Multi-interpreter environments can come to being either through the
3010 fork() emulation on Windows platforms, or by embedding perl in a
3011 multi-threaded application. The C<shared> attribute does nothing in
3012 all other environments.
3014 =item pack TEMPLATE,LIST
3016 Takes a LIST of values and converts it into a string using the rules
3017 given by the TEMPLATE. The resulting string is the concatenation of
3018 the converted values. Typically, each converted value looks
3019 like its machine-level representation. For example, on 32-bit machines
3020 a converted integer may be represented by a sequence of 4 bytes.
3023 sequence of characters that give the order and type of values, as
3026 a A string with arbitrary binary data, will be null padded.
3027 A An ASCII string, will be space padded.
3028 Z A null terminated (asciz) string, will be null padded.
3030 b A bit string (ascending bit order inside each byte, like vec()).
3031 B A bit string (descending bit order inside each byte).
3032 h A hex string (low nybble first).
3033 H A hex string (high nybble first).
3035 c A signed char value.
3036 C An unsigned char value. Only does bytes. See U for Unicode.
3038 s A signed short value.
3039 S An unsigned short value.
3040 (This 'short' is _exactly_ 16 bits, which may differ from
3041 what a local C compiler calls 'short'. If you want
3042 native-length shorts, use the '!' suffix.)
3044 i A signed integer value.
3045 I An unsigned integer value.
3046 (This 'integer' is _at_least_ 32 bits wide. Its exact
3047 size depends on what a local C compiler calls 'int',
3048 and may even be larger than the 'long' described in
3051 l A signed long value.
3052 L An unsigned long value.
3053 (This 'long' is _exactly_ 32 bits, which may differ from
3054 what a local C compiler calls 'long'. If you want
3055 native-length longs, use the '!' suffix.)
3057 n An unsigned short in "network" (big-endian) order.
3058 N An unsigned long in "network" (big-endian) order.
3059 v An unsigned short in "VAX" (little-endian) order.
3060 V An unsigned long in "VAX" (little-endian) order.
3061 (These 'shorts' and 'longs' are _exactly_ 16 bits and
3062 _exactly_ 32 bits, respectively.)
3064 q A signed quad (64-bit) value.
3065 Q An unsigned quad value.
3066 (Quads are available only if your system supports 64-bit
3067 integer values _and_ if Perl has been compiled to support those.
3068 Causes a fatal error otherwise.)
3070 f A single-precision float in the native format.
3071 d A double-precision float in the native format.
3073 p A pointer to a null-terminated string.
3074 P A pointer to a structure (fixed-length string).
3076 u A uuencoded string.
3077 U A Unicode character number. Encodes to UTF-8 internally.
3078 Works even if C<use utf8> is not in effect.
3080 w A BER compressed integer. Its bytes represent an unsigned
3081 integer in base 128, most significant digit first, with as
3082 few digits as possible. Bit eight (the high bit) is set
3083 on each byte except the last.
3087 @ Null fill to absolute position.
3089 The following rules apply:
3095 Each letter may optionally be followed by a number giving a repeat
3096 count. With all types except C<a>, C<A>, C<Z>, C<b>, C<B>, C<h>,
3097 C<H>, and C<P> the pack function will gobble up that many values from
3098 the LIST. A C<*> for the repeat count means to use however many items are
3099 left, except for C<@>, C<x>, C<X>, where it is equivalent
3100 to C<0>, and C<u>, where it is equivalent to 1 (or 45, what is the
3103 When used with C<Z>, C<*> results in the addition of a trailing null
3104 byte (so the packed result will be one longer than the byte C<length>
3107 The repeat count for C<u> is interpreted as the maximal number of bytes
3108 to encode per line of output, with 0 and 1 replaced by 45.
3112 The C<a>, C<A>, and C<Z> types gobble just one value, but pack it as a
3113 string of length count, padding with nulls or spaces as necessary. When
3114 unpacking, C<A> strips trailing spaces and nulls, C<Z> strips everything
3115 after the first null, and C<a> returns data verbatim. When packing,
3116 C<a>, and C<Z> are equivalent.
3118 If the value-to-pack is too long, it is truncated. If too long and an
3119 explicit count is provided, C<Z> packs only C<$count-1> bytes, followed
3120 by a null byte. Thus C<Z> always packs a trailing null byte under
3125 Likewise, the C<b> and C<B> fields pack a string that many bits long.
3126 Each byte of the input field of pack() generates 1 bit of the result.
3127 Each result bit is based on the least-significant bit of the corresponding
3128 input byte, i.e., on C<ord($byte)%2>. In particular, bytes C<"0"> and
3129 C<"1"> generate bits 0 and 1, as do bytes C<"\0"> and C<"\1">.
3131 Starting from the beginning of the input string of pack(), each 8-tuple
3132 of bytes is converted to 1 byte of output. With format C<b>
3133 the first byte of the 8-tuple determines the least-significant bit of a
3134 byte, and with format C<B> it determines the most-significant bit of
3137 If the length of the input string is not exactly divisible by 8, the
3138 remainder is packed as if the input string were padded by null bytes
3139 at the end. Similarly, during unpack()ing the "extra" bits are ignored.
3141 If the input string of pack() is longer than needed, extra bytes are ignored.
3142 A C<*> for the repeat count of pack() means to use all the bytes of
3143 the input field. On unpack()ing the bits are converted to a string
3144 of C<"0">s and C<"1">s.
3148 The C<h> and C<H> fields pack a string that many nybbles (4-bit groups,
3149 representable as hexadecimal digits, 0-9a-f) long.
3151 Each byte of the input field of pack() generates 4 bits of the result.
3152 For non-alphabetical bytes the result is based on the 4 least-significant
3153 bits of the input byte, i.e., on C<ord($byte)%16>. In particular,
3154 bytes C<"0"> and C<"1"> generate nybbles 0 and 1, as do bytes
3155 C<"\0"> and C<"\1">. For bytes C<"a".."f"> and C<"A".."F"> the result
3156 is compatible with the usual hexadecimal digits, so that C<"a"> and
3157 C<"A"> both generate the nybble C<0xa==10>. The result for bytes
3158 C<"g".."z"> and C<"G".."Z"> is not well-defined.
3160 Starting from the beginning of the input string of pack(), each pair
3161 of bytes is converted to 1 byte of output. With format C<h> the
3162 first byte of the pair determines the least-significant nybble of the
3163 output byte, and with format C<H> it determines the most-significant
3166 If the length of the input string is not even, it behaves as if padded
3167 by a null byte at the end. Similarly, during unpack()ing the "extra"
3168 nybbles are ignored.
3170 If the input string of pack() is longer than needed, extra bytes are ignored.
3171 A C<*> for the repeat count of pack() means to use all the bytes of
3172 the input field. On unpack()ing the bits are converted to a string
3173 of hexadecimal digits.
3177 The C<p> type packs a pointer to a null-terminated string. You are
3178 responsible for ensuring the string is not a temporary value (which can
3179 potentially get deallocated before you get around to using the packed result).
3180 The C<P> type packs a pointer to a structure of the size indicated by the
3181 length. A NULL pointer is created if the corresponding value for C<p> or
3182 C<P> is C<undef>, similarly for unpack().
3186 The C</> template character allows packing and unpacking of strings where
3187 the packed structure contains a byte count followed by the string itself.
3188 You write I<length-item>C</>I<string-item>.
3190 The I<length-item> can be any C<pack> template letter,
3191 and describes how the length value is packed.
3192 The ones likely to be of most use are integer-packing ones like
3193 C<n> (for Java strings), C<w> (for ASN.1 or SNMP)
3194 and C<N> (for Sun XDR).
3196 The I<string-item> must, at present, be C<"A*">, C<"a*"> or C<"Z*">.
3197 For C<unpack> the length of the string is obtained from the I<length-item>,
3198 but if you put in the '*' it will be ignored.
3200 unpack 'C/a', "\04Gurusamy"; gives 'Guru'
3201 unpack 'a3/A* A*', '007 Bond J '; gives (' Bond','J')
3202 pack 'n/a* w/a*','hello,','world'; gives "\000\006hello,\005world"
3204 The I<length-item> is not returned explicitly from C<unpack>.
3206 Adding a count to the I<length-item> letter is unlikely to do anything
3207 useful, unless that letter is C<A>, C<a> or C<Z>. Packing with a
3208 I<length-item> of C<a> or C<Z> may introduce C<"\000"> characters,
3209 which Perl does not regard as legal in numeric strings.
3213 The integer types C<s>, C<S>, C<l>, and C<L> may be
3214 immediately followed by a C<!> suffix to signify native shorts or
3215 longs--as you can see from above for example a bare C<l> does mean
3216 exactly 32 bits, the native C<long> (as seen by the local C compiler)
3217 may be larger. This is an issue mainly in 64-bit platforms. You can
3218 see whether using C<!> makes any difference by
3220 print length(pack("s")), " ", length(pack("s!")), "\n";
3221 print length(pack("l")), " ", length(pack("l!")), "\n";
3223 C<i!> and C<I!> also work but only because of completeness;
3224 they are identical to C<i> and C<I>.
3226 The actual sizes (in bytes) of native shorts, ints, longs, and long
3227 longs on the platform where Perl was built are also available via
3231 print $Config{shortsize}, "\n";
3232 print $Config{intsize}, "\n";
3233 print $Config{longsize}, "\n";
3234 print $Config{longlongsize}, "\n";
3236 (The C<$Config{longlongsize}> will be undefine if your system does
3237 not support long longs.)
3241 The integer formats C<s>, C<S>, C<i>, C<I>, C<l>, and C<L>
3242 are inherently non-portable between processors and operating systems
3243 because they obey the native byteorder and endianness. For example a
3244 4-byte integer 0x12345678 (305419896 decimal) would be ordered natively
3245 (arranged in and handled by the CPU registers) into bytes as
3247 0x12 0x34 0x56 0x78 # big-endian
3248 0x78 0x56 0x34 0x12 # little-endian
3250 Basically, the Intel and VAX CPUs are little-endian, while everybody
3251 else, for example Motorola m68k/88k, PPC, Sparc, HP PA, Power, and
3252 Cray are big-endian. Alpha and MIPS can be either: Digital/Compaq
3253 used/uses them in little-endian mode; SGI/Cray uses them in big-endian
3256 The names `big-endian' and `little-endian' are comic references to
3257 the classic "Gulliver's Travels" (via the paper "On Holy Wars and a
3258 Plea for Peace" by Danny Cohen, USC/ISI IEN 137, April 1, 1980) and
3259 the egg-eating habits of the Lilliputians.
3261 Some systems may have even weirder byte orders such as
3266 You can see your system's preference with
3268 print join(" ", map { sprintf "%#02x", $_ }
3269 unpack("C*",pack("L",0x12345678))), "\n";
3271 The byteorder on the platform where Perl was built is also available
3275 print $Config{byteorder}, "\n";
3277 Byteorders C<'1234'> and C<'12345678'> are little-endian, C<'4321'>
3278 and C<'87654321'> are big-endian.
3280 If you want portable packed integers use the formats C<n>, C<N>,
3281 C<v>, and C<V>, their byte endianness and size are known.
3282 See also L<perlport>.
3286 Real numbers (floats and doubles) are in the native machine format only;
3287 due to the multiplicity of floating formats around, and the lack of a
3288 standard "network" representation, no facility for interchange has been
3289 made. This means that packed floating point data written on one machine
3290 may not be readable on another - even if both use IEEE floating point
3291 arithmetic (as the endian-ness of the memory representation is not part
3292 of the IEEE spec). See also L<perlport>.
3294 Note that Perl uses doubles internally for all numeric calculation, and
3295 converting from double into float and thence back to double again will
3296 lose precision (i.e., C<unpack("f", pack("f", $foo)>) will not in general
3301 If the pattern begins with a C<U>, the resulting string will be treated
3302 as Unicode-encoded. You can force UTF8 encoding on in a string with an
3303 initial C<U0>, and the bytes that follow will be interpreted as Unicode
3304 characters. If you don't want this to happen, you can begin your pattern
3305 with C<C0> (or anything else) to force Perl not to UTF8 encode your
3306 string, and then follow this with a C<U*> somewhere in your pattern.
3310 You must yourself do any alignment or padding by inserting for example
3311 enough C<'x'>es while packing. There is no way to pack() and unpack()
3312 could know where the bytes are going to or coming from. Therefore
3313 C<pack> (and C<unpack>) handle their output and input as flat
3318 A comment in a TEMPLATE starts with C<#> and goes to the end of line.
3322 If TEMPLATE requires more arguments to pack() than actually given, pack()
3323 assumes additional C<""> arguments. If TEMPLATE requires less arguments
3324 to pack() than actually given, extra arguments are ignored.
3330 $foo = pack("CCCC",65,66,67,68);
3332 $foo = pack("C4",65,66,67,68);
3334 $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);
3335 # same thing with Unicode circled letters
3337 $foo = pack("ccxxcc",65,66,67,68);
3340 # note: the above examples featuring "C" and "c" are true
3341 # only on ASCII and ASCII-derived systems such as ISO Latin 1
3342 # and UTF-8. In EBCDIC the first example would be
3343 # $foo = pack("CCCC",193,194,195,196);
3345 $foo = pack("s2",1,2);
3346 # "\1\0\2\0" on little-endian
3347 # "\0\1\0\2" on big-endian
3349 $foo = pack("a4","abcd","x","y","z");
3352 $foo = pack("aaaa","abcd","x","y","z");
3355 $foo = pack("a14","abcdefg");
3356 # "abcdefg\0\0\0\0\0\0\0"
3358 $foo = pack("i9pl", gmtime);
3359 # a real struct tm (on my system anyway)
3361 $utmp_template = "Z8 Z8 Z16 L";
3362 $utmp = pack($utmp_template, @utmp1);
3363 # a struct utmp (BSDish)
3365 @utmp2 = unpack($utmp_template, $utmp);
3366 # "@utmp1" eq "@utmp2"
3369 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
3372 $foo = pack('sx2l', 12, 34);
3373 # short 12, two zero bytes padding, long 34
3374 $bar = pack('s@4l', 12, 34);
3375 # short 12, zero fill to position 4, long 34
3378 The same template may generally also be used in unpack().
3380 =item package NAMESPACE
3384 Declares the compilation unit as being in the given namespace. The scope
3385 of the package declaration is from the declaration itself through the end
3386 of the enclosing block, file, or eval (the same as the C<my> operator).
3387 All further unqualified dynamic identifiers will be in this namespace.
3388 A package statement affects only dynamic variables--including those
3389 you've used C<local> on--but I<not> lexical variables, which are created
3390 with C<my>. Typically it would be the first declaration in a file to
3391 be included by the C<require> or C<use> operator. You can switch into a
3392 package in more than one place; it merely influences which symbol table
3393 is used by the compiler for the rest of that block. You can refer to
3394 variables and filehandles in other packages by prefixing the identifier
3395 with the package name and a double colon: C<$Package::Variable>.
3396 If the package name is null, the C<main> package as assumed. That is,
3397 C<$::sail> is equivalent to C<$main::sail> (as well as to C<$main'sail>,
3398 still seen in older code).
3400 If NAMESPACE is omitted, then there is no current package, and all
3401 identifiers must be fully qualified or lexicals. This is stricter
3402 than C<use strict>, since it also extends to function names.
3404 See L<perlmod/"Packages"> for more information about packages, modules,
3405 and classes. See L<perlsub> for other scoping issues.
3407 =item pipe READHANDLE,WRITEHANDLE
3409 Opens a pair of connected pipes like the corresponding system call.
3410 Note that if you set up a loop of piped processes, deadlock can occur
3411 unless you are very careful. In addition, note that Perl's pipes use
3412 stdio buffering, so you may need to set C<$|> to flush your WRITEHANDLE
3413 after each command, depending on the application.
3415 See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
3416 for examples of such things.
3418 On systems that support a close-on-exec flag on files, the flag will be set
3419 for the newly opened file descriptors as determined by the value of $^F.
3426 Pops and returns the last value of the array, shortening the array by
3427 one element. Has an effect similar to
3431 If there are no elements in the array, returns the undefined value
3432 (although this may happen at other times as well). If ARRAY is
3433 omitted, pops the C<@ARGV> array in the main program, and the C<@_>
3434 array in subroutines, just like C<shift>.
3440 Returns the offset of where the last C<m//g> search left off for the variable
3441 in question (C<$_> is used when the variable is not specified). May be
3442 modified to change that offset. Such modification will also influence
3443 the C<\G> zero-width assertion in regular expressions. See L<perlre> and
3446 =item print FILEHANDLE LIST
3452 Prints a string or a list of strings. Returns true if successful.
3453 FILEHANDLE may be a scalar variable name, in which case the variable
3454 contains the name of or a reference to the filehandle, thus introducing
3455 one level of indirection. (NOTE: If FILEHANDLE is a variable and
3456 the next token is a term, it may be misinterpreted as an operator
3457 unless you interpose a C<+> or put parentheses around the arguments.)
3458 If FILEHANDLE is omitted, prints by default to standard output (or
3459 to the last selected output channel--see L</select>). If LIST is
3460 also omitted, prints C<$_> to the currently selected output channel.
3461 To set the default output channel to something other than STDOUT
3462 use the select operation. The current value of C<$,> (if any) is
3463 printed between each LIST item. The current value of C<$\> (if
3464 any) is printed after the entire LIST has been printed. Because
3465 print takes a LIST, anything in the LIST is evaluated in list
3466 context, and any subroutine that you call will have one or more of
3467 its expressions evaluated in list context. Also be careful not to
3468 follow the print keyword with a left parenthesis unless you want
3469 the corresponding right parenthesis to terminate the arguments to
3470 the print--interpose a C<+> or put parentheses around all the
3473 Note that if you're storing FILEHANDLES in an array or other expression,
3474 you will have to use a block returning its value instead:
3476 print { $files[$i] } "stuff\n";
3477 print { $OK ? STDOUT : STDERR } "stuff\n";
3479 =item printf FILEHANDLE FORMAT, LIST
3481 =item printf FORMAT, LIST
3483 Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that C<$\>
3484 (the output record separator) is not appended. The first argument
3485 of the list will be interpreted as the C<printf> format. If C<use locale> is
3486 in effect, the character used for the decimal point in formatted real numbers
3487 is affected by the LC_NUMERIC locale. See L<perllocale>.
3489 Don't fall into the trap of using a C<printf> when a simple
3490 C<print> would do. The C<print> is more efficient and less
3493 =item prototype FUNCTION
3495 Returns the prototype of a function as a string (or C<undef> if the
3496 function has no prototype). FUNCTION is a reference to, or the name of,
3497 the function whose prototype you want to retrieve.
3499 If FUNCTION is a string starting with C<CORE::>, the rest is taken as a
3500 name for Perl builtin. If the builtin is not I<overridable> (such as
3501 C<qw//>) or its arguments cannot be expressed by a prototype (such as
3502 C<system>) returns C<undef> because the builtin does not really behave
3503 like a Perl function. Otherwise, the string describing the equivalent
3504 prototype is returned.
3506 =item push ARRAY,LIST
3508 Treats ARRAY as a stack, and pushes the values of LIST
3509 onto the end of ARRAY. The length of ARRAY increases by the length of
3510 LIST. Has the same effect as
3513 $ARRAY[++$#ARRAY] = $value;
3516 but is more efficient. Returns the new number of elements in the array.
3528 Generalized quotes. See L<perlop/"Regexp Quote-Like Operators">.
3530 =item quotemeta EXPR
3534 Returns the value of EXPR with all non-"word"
3535 characters backslashed. (That is, all characters not matching
3536 C</[A-Za-z_0-9]/> will be preceded by a backslash in the
3537 returned string, regardless of any locale settings.)
3538 This is the internal function implementing
3539 the C<\Q> escape in double-quoted strings.
3541 If EXPR is omitted, uses C<$_>.
3547 Returns a random fractional number greater than or equal to C<0> and less
3548 than the value of EXPR. (EXPR should be positive.) If EXPR is
3549 omitted, or a C<0>, the value C<1> is used. Automatically calls C<srand>
3550 unless C<srand> has already been called. See also C<srand>.
3552 Apply C<int()> to the value returned by C<rand()> if you want random
3553 integers instead of random fractional numbers. For example,
3557 returns a random integer between C<0> and C<9>, inclusive.
3559 (Note: If your rand function consistently returns numbers that are too
3560 large or too small, then your version of Perl was probably compiled
3561 with the wrong number of RANDBITS.)
3563 =item read FILEHANDLE,SCALAR,LENGTH,OFFSET
3565 =item read FILEHANDLE,SCALAR,LENGTH
3567 Attempts to read LENGTH bytes of data into variable SCALAR from the
3568 specified FILEHANDLE. Returns the number of bytes actually read, C<0>
3569 at end of file, or undef if there was an error. SCALAR will be grown
3570 or shrunk to the length actually read. If SCALAR needs growing, the
3571 new bytes will be zero bytes. An OFFSET may be specified to place
3572 the read data into some other place in SCALAR than the beginning.
3573 The call is actually implemented in terms of stdio's fread(3) call.
3574 To get a true read(2) system call, see C<sysread>.
3576 =item readdir DIRHANDLE
3578 Returns the next directory entry for a directory opened by C<opendir>.
3579 If used in list context, returns all the rest of the entries in the
3580 directory. If there are no more entries, returns an undefined value in
3581 scalar context or a null list in list context.
3583 If you're planning to filetest the return values out of a C<readdir>, you'd
3584 better prepend the directory in question. Otherwise, because we didn't
3585 C<chdir> there, it would have been testing the wrong file.
3587 opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!";
3588 @dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR);
3593 Reads from the filehandle whose typeglob is contained in EXPR. In scalar
3594 context, each call reads and returns the next line, until end-of-file is
3595 reached, whereupon the subsequent call returns undef. In list context,
3596 reads until end-of-file is reached and returns a list of lines. Note that
3597 the notion of "line" used here is however you may have defined it
3598 with C<$/> or C<$INPUT_RECORD_SEPARATOR>). See L<perlvar/"$/">.
3600 When C<$/> is set to C<undef>, when readline() is in scalar
3601 context (i.e. file slurp mode), and when an empty file is read, it
3602 returns C<''> the first time, followed by C<undef> subsequently.
3604 This is the internal function implementing the C<< <EXPR> >>
3605 operator, but you can use it directly. The C<< <EXPR> >>
3606 operator is discussed in more detail in L<perlop/"I/O Operators">.
3609 $line = readline(*STDIN); # same thing
3615 Returns the value of a symbolic link, if symbolic links are
3616 implemented. If not, gives a fatal error. If there is some system
3617 error, returns the undefined value and sets C<$!> (errno). If EXPR is
3618 omitted, uses C<$_>.
3622 EXPR is executed as a system command.
3623 The collected standard output of the command is returned.
3624 In scalar context, it comes back as a single (potentially
3625 multi-line) string. In list context, returns a list of lines
3626 (however you've defined lines with C<$/> or C<$INPUT_RECORD_SEPARATOR>).
3627 This is the internal function implementing the C<qx/EXPR/>
3628 operator, but you can use it directly. The C<qx/EXPR/>
3629 operator is discussed in more detail in L<perlop/"I/O Operators">.
3631 =item recv SOCKET,SCALAR,LENGTH,FLAGS
3633 Receives a message on a socket. Attempts to receive LENGTH bytes of
3634 data into variable SCALAR from the specified SOCKET filehandle. SCALAR
3635 will be grown or shrunk to the length actually read. Takes the same
3636 flags as the system call of the same name. Returns the address of the
3637 sender if SOCKET's protocol supports this; returns an empty string
3638 otherwise. If there's an error, returns the undefined value. This call
3639 is actually implemented in terms of recvfrom(2) system call. See
3640 L<perlipc/"UDP: Message Passing"> for examples.
3646 The C<redo> command restarts the loop block without evaluating the
3647 conditional again. The C<continue> block, if any, is not executed. If
3648 the LABEL is omitted, the command refers to the innermost enclosing
3649 loop. This command is normally used by programs that want to lie to
3650 themselves about what was just input:
3652 # a simpleminded Pascal comment stripper
3653 # (warning: assumes no { or } in strings)
3654 LINE: while (<STDIN>) {
3655 while (s|({.*}.*){.*}|$1 |) {}
3660 if (/}/) { # end of comment?
3669 C<redo> cannot be used to retry a block which returns a value such as
3670 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
3671 a grep() or map() operation.
3673 Note that a block by itself is semantically identical to a loop
3674 that executes once. Thus C<redo> inside such a block will effectively
3675 turn it into a looping construct.
3677 See also L</continue> for an illustration of how C<last>, C<next>, and
3684 Returns a true value if EXPR is a reference, false otherwise. If EXPR
3685 is not specified, C<$_> will be used. The value returned depends on the
3686 type of thing the reference is a reference to.
3687 Builtin types include:
3697 If the referenced object has been blessed into a package, then that package
3698 name is returned instead. You can think of C<ref> as a C<typeof> operator.
3700 if (ref($r) eq "HASH") {
3701 print "r is a reference to a hash.\n";
3704 print "r is not a reference at all.\n";
3706 if (UNIVERSAL::isa($r, "HASH")) { # for subclassing
3707 print "r is a reference to something that isa hash.\n";
3710 See also L<perlref>.
3712 =item rename OLDNAME,NEWNAME
3714 Changes the name of a file; an existing file NEWNAME will be
3715 clobbered. Returns true for success, false otherwise.
3717 Behavior of this function varies wildly depending on your system
3718 implementation. For example, it will usually not work across file system
3719 boundaries, even though the system I<mv> command sometimes compensates
3720 for this. Other restrictions include whether it works on directories,
3721 open files, or pre-existing files. Check L<perlport> and either the
3722 rename(2) manpage or equivalent system documentation for details.
3724 =item require VERSION
3730 Demands some semantics specified by EXPR, or by C<$_> if EXPR is not
3733 If a VERSION is specified as a literal of the form v5.6.1,
3734 demands that the current version of Perl (C<$^V> or $PERL_VERSION) be
3735 at least as recent as that version, at run time. (For compatibility
3736 with older versions of Perl, a numeric argument will also be interpreted
3737 as VERSION.) Compare with L</use>, which can do a similar check at
3740 require v5.6.1; # run time version check
3741 require 5.6.1; # ditto
3742 require 5.005_03; # float version allowed for compatibility
3744 Otherwise, demands that a library file be included if it hasn't already
3745 been included. The file is included via the do-FILE mechanism, which is
3746 essentially just a variety of C<eval>. Has semantics similar to the following
3751 return 1 if $INC{$filename};
3752 my($realfilename,$result);
3754 foreach $prefix (@INC) {
3755 $realfilename = "$prefix/$filename";
3756 if (-f $realfilename) {
3757 $INC{$filename} = $realfilename;
3758 $result = do $realfilename;
3762 die "Can't find $filename in \@INC";
3764 delete $INC{$filename} if $@ || !$result;
3766 die "$filename did not return true value" unless $result;
3770 Note that the file will not be included twice under the same specified
3771 name. The file must return true as the last statement to indicate
3772 successful execution of any initialization code, so it's customary to
3773 end such a file with C<1;> unless you're sure it'll return true
3774 otherwise. But it's better just to put the C<1;>, in case you add more
3777 If EXPR is a bareword, the require assumes a "F<.pm>" extension and
3778 replaces "F<::>" with "F</>" in the filename for you,
3779 to make it easy to load standard modules. This form of loading of
3780 modules does not risk altering your namespace.
3782 In other words, if you try this:
3784 require Foo::Bar; # a splendid bareword
3786 The require function will actually look for the "F<Foo/Bar.pm>" file in the
3787 directories specified in the C<@INC> array.
3789 But if you try this:
3791 $class = 'Foo::Bar';
3792 require $class; # $class is not a bareword
3794 require "Foo::Bar"; # not a bareword because of the ""
3796 The require function will look for the "F<Foo::Bar>" file in the @INC array and
3797 will complain about not finding "F<Foo::Bar>" there. In this case you can do:
3799 eval "require $class";
3801 For a yet-more-powerful import facility, see L</use> and L<perlmod>.
3807 Generally used in a C<continue> block at the end of a loop to clear
3808 variables and reset C<??> searches so that they work again. The
3809 expression is interpreted as a list of single characters (hyphens
3810 allowed for ranges). All variables and arrays beginning with one of
3811 those letters are reset to their pristine state. If the expression is
3812 omitted, one-match searches (C<?pattern?>) are reset to match again. Resets
3813 only variables or searches in the current package. Always returns
3816 reset 'X'; # reset all X variables
3817 reset 'a-z'; # reset lower case variables
3818 reset; # just reset ?one-time? searches
3820 Resetting C<"A-Z"> is not recommended because you'll wipe out your
3821 C<@ARGV> and C<@INC> arrays and your C<%ENV> hash. Resets only package
3822 variables--lexical variables are unaffected, but they clean themselves
3823 up on scope exit anyway, so you'll probably want to use them instead.
3830 Returns from a subroutine, C<eval>, or C<do FILE> with the value
3831 given in EXPR. Evaluation of EXPR may be in list, scalar, or void
3832 context, depending on how the return value will be used, and the context
3833 may vary from one execution to the next (see C<wantarray>). If no EXPR
3834 is given, returns an empty list in list context, the undefined value in
3835 scalar context, and (of course) nothing at all in a void context.
3837 (Note that in the absence of a explicit C<return>, a subroutine, eval,
3838 or do FILE will automatically return the value of the last expression
3843 In list context, returns a list value consisting of the elements
3844 of LIST in the opposite order. In scalar context, concatenates the
3845 elements of LIST and returns a string value with all characters
3846 in the opposite order.
3848 print reverse <>; # line tac, last line first
3850 undef $/; # for efficiency of <>
3851 print scalar reverse <>; # character tac, last line tsrif
3853 This operator is also handy for inverting a hash, although there are some
3854 caveats. If a value is duplicated in the original hash, only one of those
3855 can be represented as a key in the inverted hash. Also, this has to
3856 unwind one hash and build a whole new one, which may take some time
3857 on a large hash, such as from a DBM file.
3859 %by_name = reverse %by_address; # Invert the hash
3861 =item rewinddir DIRHANDLE
3863 Sets the current position to the beginning of the directory for the
3864 C<readdir> routine on DIRHANDLE.
3866 =item rindex STR,SUBSTR,POSITION
3868 =item rindex STR,SUBSTR
3870 Works just like index() except that it returns the position of the LAST
3871 occurrence of SUBSTR in STR. If POSITION is specified, returns the
3872 last occurrence at or before that position.
3874 =item rmdir FILENAME
3878 Deletes the directory specified by FILENAME if that directory is empty. If it
3879 succeeds it returns true, otherwise it returns false and sets C<$!> (errno). If
3880 FILENAME is omitted, uses C<$_>.
3884 The substitution operator. See L<perlop>.
3888 Forces EXPR to be interpreted in scalar context and returns the value
3891 @counts = ( scalar @a, scalar @b, scalar @c );
3893 There is no equivalent operator to force an expression to
3894 be interpolated in list context because in practice, this is never
3895 needed. If you really wanted to do so, however, you could use
3896 the construction C<@{[ (some expression) ]}>, but usually a simple
3897 C<(some expression)> suffices.
3899 Because C<scalar> is unary operator, if you accidentally use for EXPR a
3900 parenthesized list, this behaves as a scalar comma expression, evaluating
3901 all but the last element in void context and returning the final element
3902 evaluated in scalar context. This is seldom what you want.
3904 The following single statement:
3906 print uc(scalar(&foo,$bar)),$baz;
3908 is the moral equivalent of these two:
3911 print(uc($bar),$baz);
3913 See L<perlop> for more details on unary operators and the comma operator.
3915 =item seek FILEHANDLE,POSITION,WHENCE
3917 Sets FILEHANDLE's position, just like the C<fseek> call of C<stdio>.
3918 FILEHANDLE may be an expression whose value gives the name of the
3919 filehandle. The values for WHENCE are C<0> to set the new position to
3920 POSITION, C<1> to set it to the current position plus POSITION, and
3921 C<2> to set it to EOF plus POSITION (typically negative). For WHENCE
3922 you may use the constants C<SEEK_SET>, C<SEEK_CUR>, and C<SEEK_END>
3923 (start of the file, current position, end of the file) from the Fcntl
3924 module. Returns C<1> upon success, C<0> otherwise.
3926 If you want to position file for C<sysread> or C<syswrite>, don't use
3927 C<seek>--buffering makes its effect on the file's system position
3928 unpredictable and non-portable. Use C<sysseek> instead.
3930 Due to the rules and rigors of ANSI C, on some systems you have to do a
3931 seek whenever you switch between reading and writing. Amongst other
3932 things, this may have the effect of calling stdio's clearerr(3).
3933 A WHENCE of C<1> (C<SEEK_CUR>) is useful for not moving the file position:
3937 This is also useful for applications emulating C<tail -f>. Once you hit
3938 EOF on your read, and then sleep for a while, you might have to stick in a
3939 seek() to reset things. The C<seek> doesn't change the current position,
3940 but it I<does> clear the end-of-file condition on the handle, so that the
3941 next C<< <FILE> >> makes Perl try again to read something. We hope.
3943 If that doesn't work (some stdios are particularly cantankerous), then
3944 you may need something more like this:
3947 for ($curpos = tell(FILE); $_ = <FILE>;
3948 $curpos = tell(FILE)) {
3949 # search for some stuff and put it into files
3951 sleep($for_a_while);
3952 seek(FILE, $curpos, 0);
3955 =item seekdir DIRHANDLE,POS
3957 Sets the current position for the C<readdir> routine on DIRHANDLE. POS
3958 must be a value returned by C<telldir>. Has the same caveats about
3959 possible directory compaction as the corresponding system library
3962 =item select FILEHANDLE
3966 Returns the currently selected filehandle. Sets the current default
3967 filehandle for output, if FILEHANDLE is supplied. This has two
3968 effects: first, a C<write> or a C<print> without a filehandle will
3969 default to this FILEHANDLE. Second, references to variables related to
3970 output will refer to this output channel. For example, if you have to
3971 set the top of form format for more than one output channel, you might
3979 FILEHANDLE may be an expression whose value gives the name of the
3980 actual filehandle. Thus:
3982 $oldfh = select(STDERR); $| = 1; select($oldfh);
3984 Some programmers may prefer to think of filehandles as objects with
3985 methods, preferring to write the last example as:
3988 STDERR->autoflush(1);
3990 =item select RBITS,WBITS,EBITS,TIMEOUT
3992 This calls the select(2) system call with the bit masks specified, which
3993 can be constructed using C<fileno> and C<vec>, along these lines:
3995 $rin = $win = $ein = '';
3996 vec($rin,fileno(STDIN),1) = 1;
3997 vec($win,fileno(STDOUT),1) = 1;
4000 If you want to select on many filehandles you might wish to write a
4004 my(@fhlist) = split(' ',$_[0]);
4007 vec($bits,fileno($_),1) = 1;
4011 $rin = fhbits('STDIN TTY SOCK');
4015 ($nfound,$timeleft) =
4016 select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
4018 or to block until something becomes ready just do this
4020 $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
4022 Most systems do not bother to return anything useful in $timeleft, so
4023 calling select() in scalar context just returns $nfound.
4025 Any of the bit masks can also be undef. The timeout, if specified, is
4026 in seconds, which may be fractional. Note: not all implementations are
4027 capable of returning the$timeleft. If not, they always return
4028 $timeleft equal to the supplied $timeout.
4030 You can effect a sleep of 250 milliseconds this way:
4032 select(undef, undef, undef, 0.25);
4034 B<WARNING>: One should not attempt to mix buffered I/O (like C<read>
4035 or <FH>) with C<select>, except as permitted by POSIX, and even
4036 then only on POSIX systems. You have to use C<sysread> instead.
4038 =item semctl ID,SEMNUM,CMD,ARG
4040 Calls the System V IPC function C<semctl>. You'll probably have to say
4044 first to get the correct constant definitions. If CMD is IPC_STAT or
4045 GETALL, then ARG must be a variable which will hold the returned
4046 semid_ds structure or semaphore value array. Returns like C<ioctl>:
4047 the undefined value for error, "C<0 but true>" for zero, or the actual
4048 return value otherwise. The ARG must consist of a vector of native
4049 short integers, which may be created with C<pack("s!",(0)x$nsem)>.
4050 See also L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::Semaphore>
4053 =item semget KEY,NSEMS,FLAGS
4055 Calls the System V IPC function semget. Returns the semaphore id, or
4056 the undefined value if there is an error. See also
4057 L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::SysV::Semaphore>
4060 =item semop KEY,OPSTRING
4062 Calls the System V IPC function semop to perform semaphore operations
4063 such as signaling and waiting. OPSTRING must be a packed array of
4064 semop structures. Each semop structure can be generated with
4065 C<pack("sss", $semnum, $semop, $semflag)>. The number of semaphore
4066 operations is implied by the length of OPSTRING. Returns true if
4067 successful, or false if there is an error. As an example, the
4068 following code waits on semaphore $semnum of semaphore id $semid:
4070 $semop = pack("sss", $semnum, -1, 0);
4071 die "Semaphore trouble: $!\n" unless semop($semid, $semop);
4073 To signal the semaphore, replace C<-1> with C<1>. See also
4074 L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::SysV::Semaphore>
4077 =item send SOCKET,MSG,FLAGS,TO
4079 =item send SOCKET,MSG,FLAGS
4081 Sends a message on a socket. Takes the same flags as the system call
4082 of the same name. On unconnected sockets you must specify a
4083 destination to send TO, in which case it does a C C<sendto>. Returns
4084 the number of characters sent, or the undefined value if there is an
4085 error. The C system call sendmsg(2) is currently unimplemented.
4086 See L<perlipc/"UDP: Message Passing"> for examples.
4088 =item setpgrp PID,PGRP
4090 Sets the current process group for the specified PID, C<0> for the current
4091 process. Will produce a fatal error if used on a machine that doesn't
4092 implement POSIX setpgid(2) or BSD setpgrp(2). If the arguments are omitted,
4093 it defaults to C<0,0>. Note that the BSD 4.2 version of C<setpgrp> does not
4094 accept any arguments, so only C<setpgrp(0,0)> is portable. See also
4097 =item setpriority WHICH,WHO,PRIORITY
4099 Sets the current priority for a process, a process group, or a user.
4100 (See setpriority(2).) Will produce a fatal error if used on a machine
4101 that doesn't implement setpriority(2).
4103 =item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
4105 Sets the socket option requested. Returns undefined if there is an
4106 error. OPTVAL may be specified as C<undef> if you don't want to pass an
4113 Shifts the first value of the array off and returns it, shortening the
4114 array by 1 and moving everything down. If there are no elements in the
4115 array, returns the undefined value. If ARRAY is omitted, shifts the
4116 C<@_> array within the lexical scope of subroutines and formats, and the
4117 C<@ARGV> array at file scopes or within the lexical scopes established by
4118 the C<eval ''>, C<BEGIN {}>, C<INIT {}>, C<CHECK {}>, and C<END {}>
4121 See also C<unshift>, C<push>, and C<pop>. C<shift> and C<unshift> do the
4122 same thing to the left end of an array that C<pop> and C<push> do to the
4125 =item shmctl ID,CMD,ARG
4127 Calls the System V IPC function shmctl. You'll probably have to say
4131 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
4132 then ARG must be a variable which will hold the returned C<shmid_ds>
4133 structure. Returns like ioctl: the undefined value for error, "C<0> but
4134 true" for zero, or the actual return value otherwise.
4135 See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation.
4137 =item shmget KEY,SIZE,FLAGS
4139 Calls the System V IPC function shmget. Returns the shared memory
4140 segment id, or the undefined value if there is an error.
4141 See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation.
4143 =item shmread ID,VAR,POS,SIZE
4145 =item shmwrite ID,STRING,POS,SIZE
4147 Reads or writes the System V shared memory segment ID starting at
4148 position POS for size SIZE by attaching to it, copying in/out, and
4149 detaching from it. When reading, VAR must be a variable that will
4150 hold the data read. When writing, if STRING is too long, only SIZE
4151 bytes are used; if STRING is too short, nulls are written to fill out
4152 SIZE bytes. Return true if successful, or false if there is an error.
4153 shmread() taints the variable. See also L<perlipc/"SysV IPC">,
4154 C<IPC::SysV> documentation, and the C<IPC::Shareable> module from CPAN.
4156 =item shutdown SOCKET,HOW
4158 Shuts down a socket connection in the manner indicated by HOW, which
4159 has the same interpretation as in the system call of the same name.
4161 shutdown(SOCKET, 0); # I/we have stopped reading data
4162 shutdown(SOCKET, 1); # I/we have stopped writing data
4163 shutdown(SOCKET, 2); # I/we have stopped using this socket
4165 This is useful with sockets when you want to tell the other
4166 side you're done writing but not done reading, or vice versa.
4167 It's also a more insistent form of close because it also
4168 disables the file descriptor in any forked copies in other
4175 Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
4176 returns sine of C<$_>.
4178 For the inverse sine operation, you may use the C<Math::Trig::asin>
4179 function, or use this relation:
4181 sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
4187 Causes the script to sleep for EXPR seconds, or forever if no EXPR.
4188 May be interrupted if the process receives a signal such as C<SIGALRM>.
4189 Returns the number of seconds actually slept. You probably cannot
4190 mix C<alarm> and C<sleep> calls, because C<sleep> is often implemented
4193 On some older systems, it may sleep up to a full second less than what
4194 you requested, depending on how it counts seconds. Most modern systems
4195 always sleep the full amount. They may appear to sleep longer than that,
4196 however, because your process might not be scheduled right away in a
4197 busy multitasking system.
4199 For delays of finer granularity than one second, you may use Perl's
4200 C<syscall> interface to access setitimer(2) if your system supports
4201 it, or else see L</select> above. The Time::HiRes module (from CPAN,
4202 and starting from Perl 5.8 part of the standard distribution) may also
4205 See also the POSIX module's C<pause> function.
4207 =item sockatmark SOCKET
4209 Returns true if the socket is positioned at the out-of-band mark
4210 (also known as the urgent data mark), false otherwise. Use right
4211 after reading from the socket.
4213 Not available directly, one has to import the function from
4214 the IO::Socket extension
4216 use IO::Socket 'sockatmark';
4218 Even this doesn't guarantee that sockatmark() really is available,
4219 though, because sockatmark() is a relatively recent addition to
4220 the family of socket functions. If it is unavailable, attempt to
4223 IO::Socket::atmark not implemented on this architecture ...
4225 See also L<IO::Socket>.
4227 =item socket SOCKET,DOMAIN,TYPE,PROTOCOL
4229 Opens a socket of the specified kind and attaches it to filehandle
4230 SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for
4231 the system call of the same name. You should C<use Socket> first
4232 to get the proper definitions imported. See the examples in
4233 L<perlipc/"Sockets: Client/Server Communication">.
4235 On systems that support a close-on-exec flag on files, the flag will
4236 be set for the newly opened file descriptor, as determined by the
4237 value of $^F. See L<perlvar/$^F>.
4239 =item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
4241 Creates an unnamed pair of sockets in the specified domain, of the
4242 specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as
4243 for the system call of the same name. If unimplemented, yields a fatal
4244 error. Returns true if successful.
4246 On systems that support a close-on-exec flag on files, the flag will
4247 be set for the newly opened file descriptors, as determined by the value
4248 of $^F. See L<perlvar/$^F>.
4250 Some systems defined C<pipe> in terms of C<socketpair>, in which a call
4251 to C<pipe(Rdr, Wtr)> is essentially:
4254 socketpair(Rdr, Wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC);
4255 shutdown(Rdr, 1); # no more writing for reader
4256 shutdown(Wtr, 0); # no more reading for writer
4258 See L<perlipc> for an example of socketpair use.
4260 =item sort SUBNAME LIST
4262 =item sort BLOCK LIST
4266 Sorts the LIST and returns the sorted list value. If SUBNAME or BLOCK
4267 is omitted, C<sort>s in standard string comparison order. If SUBNAME is
4268 specified, it gives the name of a subroutine that returns an integer
4269 less than, equal to, or greater than C<0>, depending on how the elements
4270 of the list are to be ordered. (The C<< <=> >> and C<cmp>
4271 operators are extremely useful in such routines.) SUBNAME may be a
4272 scalar variable name (unsubscripted), in which case the value provides
4273 the name of (or a reference to) the actual subroutine to use. In place
4274 of a SUBNAME, you can provide a BLOCK as an anonymous, in-line sort
4277 If the subroutine's prototype is C<($$)>, the elements to be compared
4278 are passed by reference in C<@_>, as for a normal subroutine. This is
4279 slower than unprototyped subroutines, where the elements to be
4280 compared are passed into the subroutine
4281 as the package global variables $a and $b (see example below). Note that
4282 in the latter case, it is usually counter-productive to declare $a and
4285 In either case, the subroutine may not be recursive. The values to be
4286 compared are always passed by reference, so don't modify them.
4288 You also cannot exit out of the sort block or subroutine using any of the
4289 loop control operators described in L<perlsyn> or with C<goto>.
4291 When C<use locale> is in effect, C<sort LIST> sorts LIST according to the
4292 current collation locale. See L<perllocale>.
4294 Perl does B<not> guarantee that sort is stable. (A I<stable> sort
4295 preserves the input order of elements that compare equal.) 5.7 and
4296 5.8 happen to use a stable mergesort, but 5.6 and earlier used quicksort,
4297 which is not stable. Do not assume that future perls will continue to
4303 @articles = sort @files;
4305 # same thing, but with explicit sort routine
4306 @articles = sort {$a cmp $b} @files;
4308 # now case-insensitively
4309 @articles = sort {uc($a) cmp uc($b)} @files;
4311 # same thing in reversed order
4312 @articles = sort {$b cmp $a} @files;
4314 # sort numerically ascending
4315 @articles = sort {$a <=> $b} @files;
4317 # sort numerically descending
4318 @articles = sort {$b <=> $a} @files;
4320 # this sorts the %age hash by value instead of key
4321 # using an in-line function
4322 @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
4324 # sort using explicit subroutine name
4326 $age{$a} <=> $age{$b}; # presuming numeric
4328 @sortedclass = sort byage @class;
4330 sub backwards { $b cmp $a }
4331 @harry = qw(dog cat x Cain Abel);
4332 @george = qw(gone chased yz Punished Axed);
4334 # prints AbelCaincatdogx
4335 print sort backwards @harry;
4336 # prints xdogcatCainAbel
4337 print sort @george, 'to', @harry;
4338 # prints AbelAxedCainPunishedcatchaseddoggonetoxyz
4340 # inefficiently sort by descending numeric compare using
4341 # the first integer after the first = sign, or the
4342 # whole record case-insensitively otherwise
4345 ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
4350 # same thing, but much more efficiently;
4351 # we'll build auxiliary indices instead
4355 push @nums, /=(\d+)/;
4360 $nums[$b] <=> $nums[$a]
4362 $caps[$a] cmp $caps[$b]
4366 # same thing, but without any temps
4367 @new = map { $_->[0] }
4368 sort { $b->[1] <=> $a->[1]
4371 } map { [$_, /=(\d+)/, uc($_)] } @old;
4373 # using a prototype allows you to use any comparison subroutine
4374 # as a sort subroutine (including other package's subroutines)
4376 sub backwards ($$) { $_[1] cmp $_[0]; } # $a and $b are not set here
4379 @new = sort other::backwards @old;
4381 If you're using strict, you I<must not> declare $a
4382 and $b as lexicals. They are package globals. That means
4383 if you're in the C<main> package and type
4385 @articles = sort {$b <=> $a} @files;
4387 then C<$a> and C<$b> are C<$main::a> and C<$main::b> (or C<$::a> and C<$::b>),
4388 but if you're in the C<FooPack> package, it's the same as typing
4390 @articles = sort {$FooPack::b <=> $FooPack::a} @files;
4392 The comparison function is required to behave. If it returns
4393 inconsistent results (sometimes saying C<$x[1]> is less than C<$x[2]> and
4394 sometimes saying the opposite, for example) the results are not
4397 =item splice ARRAY,OFFSET,LENGTH,LIST
4399 =item splice ARRAY,OFFSET,LENGTH
4401 =item splice ARRAY,OFFSET
4405 Removes the elements designated by OFFSET and LENGTH from an array, and
4406 replaces them with the elements of LIST, if any. In list context,
4407 returns the elements removed from the array. In scalar context,
4408 returns the last element removed, or C<undef> if no elements are
4409 removed. The array grows or shrinks as necessary.
4410 If OFFSET is negative then it starts that far from the end of the array.
4411 If LENGTH is omitted, removes everything from OFFSET onward.
4412 If LENGTH is negative, leaves that many elements off the end of the array.
4413 If both OFFSET and LENGTH are omitted, removes everything.
4415 The following equivalences hold (assuming C<$[ == 0>):
4417 push(@a,$x,$y) splice(@a,@a,0,$x,$y)
4418 pop(@a) splice(@a,-1)
4419 shift(@a) splice(@a,0,1)
4420 unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
4421 $a[$x] = $y splice(@a,$x,1,$y)
4423 Example, assuming array lengths are passed before arrays:
4425 sub aeq { # compare two list values
4426 my(@a) = splice(@_,0,shift);
4427 my(@b) = splice(@_,0,shift);
4428 return 0 unless @a == @b; # same len?
4430 return 0 if pop(@a) ne pop(@b);
4434 if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
4436 =item split /PATTERN/,EXPR,LIMIT
4438 =item split /PATTERN/,EXPR
4440 =item split /PATTERN/
4444 Splits a string into a list of strings and returns that list. By default,
4445 empty leading fields are preserved, and empty trailing ones are deleted.
4447 In scalar context, returns the number of fields found and splits into
4448 the C<@_> array. Use of split in scalar context is deprecated, however,
4449 because it clobbers your subroutine arguments.
4451 If EXPR is omitted, splits the C<$_> string. If PATTERN is also omitted,
4452 splits on whitespace (after skipping any leading whitespace). Anything
4453 matching PATTERN is taken to be a delimiter separating the fields. (Note
4454 that the delimiter may be longer than one character.)
4456 If LIMIT is specified and positive, it represents the maximum number
4457 of fields the EXPR will be split into, though the actual number of
4458 fields returned depends on the number of times PATTERN matches within
4459 EXPR. If LIMIT is unspecified or zero, trailing null fields are
4460 stripped (which potential users of C<pop> would do well to remember).
4461 If LIMIT is negative, it is treated as if an arbitrarily large LIMIT
4462 had been specified. Note that splitting an EXPR that evaluates to the
4463 empty string always returns the empty list, regardless of the LIMIT
4466 A pattern matching the null string (not to be confused with
4467 a null pattern C<//>, which is just one member of the set of patterns
4468 matching a null string) will split the value of EXPR into separate
4469 characters at each point it matches that way. For example:
4471 print join(':', split(/ */, 'hi there'));
4473 produces the output 'h:i:t:h:e:r:e'.
4475 Empty leading (or trailing) fields are produced when there positive width
4476 matches at the beginning (or end) of the string; a zero-width match at the
4477 beginning (or end) of the string does not produce an empty field. For
4480 print join(':', split(/(?=\w)/, 'hi there!'));
4482 produces the output 'h:i :t:h:e:r:e!'.
4484 The LIMIT parameter can be used to split a line partially
4486 ($login, $passwd, $remainder) = split(/:/, $_, 3);
4488 When assigning to a list, if LIMIT is omitted, Perl supplies a LIMIT
4489 one larger than the number of variables in the list, to avoid
4490 unnecessary work. For the list above LIMIT would have been 4 by
4491 default. In time critical applications it behooves you not to split
4492 into more fields than you really need.
4494 If the PATTERN contains parentheses, additional list elements are
4495 created from each matching substring in the delimiter.
4497 split(/([,-])/, "1-10,20", 3);
4499 produces the list value
4501 (1, '-', 10, ',', 20)
4503 If you had the entire header of a normal Unix email message in $header,
4504 you could split it up into fields and their values this way:
4506 $header =~ s/\n\s+/ /g; # fix continuation lines
4507 %hdrs = (UNIX_FROM => split /^(\S*?):\s*/m, $header);
4509 The pattern C</PATTERN/> may be replaced with an expression to specify
4510 patterns that vary at runtime. (To do runtime compilation only once,
4511 use C</$variable/o>.)
4513 As a special case, specifying a PATTERN of space (C<' '>) will split on
4514 white space just as C<split> with no arguments does. Thus, C<split(' ')> can
4515 be used to emulate B<awk>'s default behavior, whereas C<split(/ /)>
4516 will give you as many null initial fields as there are leading spaces.
4517 A C<split> on C</\s+/> is like a C<split(' ')> except that any leading
4518 whitespace produces a null first field. A C<split> with no arguments
4519 really does a C<split(' ', $_)> internally.
4521 A PATTERN of C</^/> is treated as if it were C</^/m>, since it isn't
4526 open(PASSWD, '/etc/passwd');
4529 ($login, $passwd, $uid, $gid,
4530 $gcos, $home, $shell) = split(/:/);
4535 =item sprintf FORMAT, LIST
4537 Returns a string formatted by the usual C<printf> conventions of the C
4538 library function C<sprintf>. See below for more details
4539 and see L<sprintf(3)> or L<printf(3)> on your system for an explanation of
4540 the general principles.
4544 # Format number with up to 8 leading zeroes
4545 $result = sprintf("%08d", $number);
4547 # Round number to 3 digits after decimal point
4548 $rounded = sprintf("%.3f", $number);
4550 Perl does its own C<sprintf> formatting--it emulates the C
4551 function C<sprintf>, but it doesn't use it (except for floating-point
4552 numbers, and even then only the standard modifiers are allowed). As a
4553 result, any non-standard extensions in your local C<sprintf> are not
4554 available from Perl.
4556 Unlike C<printf>, C<sprintf> does not do what you probably mean when you
4557 pass it an array as your first argument. The array is given scalar context,
4558 and instead of using the 0th element of the array as the format, Perl will
4559 use the count of elements in the array as the format, which is almost never
4562 Perl's C<sprintf> permits the following universally-known conversions:
4565 %c a character with the given number
4567 %d a signed integer, in decimal
4568 %u an unsigned integer, in decimal
4569 %o an unsigned integer, in octal
4570 %x an unsigned integer, in hexadecimal
4571 %e a floating-point number, in scientific notation
4572 %f a floating-point number, in fixed decimal notation
4573 %g a floating-point number, in %e or %f notation
4575 In addition, Perl permits the following widely-supported conversions:
4577 %X like %x, but using upper-case letters
4578 %E like %e, but using an upper-case "E"
4579 %G like %g, but with an upper-case "E" (if applicable)
4580 %b an unsigned integer, in binary
4581 %p a pointer (outputs the Perl value's address in hexadecimal)
4582 %n special: *stores* the number of characters output so far
4583 into the next variable in the parameter list
4585 Finally, for backward (and we do mean "backward") compatibility, Perl
4586 permits these unnecessary but widely-supported conversions:
4589 %D a synonym for %ld
4590 %U a synonym for %lu
4591 %O a synonym for %lo
4594 Note that the number of exponent digits in the scientific notation by
4595 C<%e>, C<%E>, C<%g> and C<%G> for numbers with the modulus of the
4596 exponent less than 100 is system-dependent: it may be three or less
4597 (zero-padded as necessary). In other words, 1.23 times ten to the
4598 99th may be either "1.23e99" or "1.23e099".
4600 Perl permits the following universally-known flags between the C<%>
4601 and the conversion letter:
4603 space prefix positive number with a space
4604 + prefix positive number with a plus sign
4605 - left-justify within the field
4606 0 use zeros, not spaces, to right-justify
4607 # prefix non-zero octal with "0", non-zero hex with "0x"
4608 number minimum field width
4609 .number "precision": digits after decimal point for
4610 floating-point, max length for string, minimum length
4612 l interpret integer as C type "long" or "unsigned long"
4613 h interpret integer as C type "short" or "unsigned short"
4614 If no flags, interpret integer as C type "int" or "unsigned"
4616 Perl supports parameter ordering, in other words, fetching the
4617 parameters in some explicitly specified "random" ordering as opposed
4618 to the default implicit sequential ordering. The syntax is, instead
4619 of the C<%> and C<*>, to use C<%>I<digits>C<$> and C<*>I<digits>C<$>,
4620 where the I<digits> is the wanted index, from one upwards. For example:
4622 printf "%2\$d %1\$d\n", 12, 34; # will print "34 12\n"
4623 printf "%*2\$d\n", 12, 3; # will print " 12\n"
4625 Note that using the reordering syntax does not interfere with the usual
4626 implicit sequential fetching of the parameters:
4628 printf "%2\$d %d\n", 12, 34; # will print "34 12\n"
4629 printf "%2\$d %d %d\n", 12, 34; # will print "34 12 34\n"
4630 printf "%3\$d %d %d\n", 12, 34, 56; # will print "56 12 34\n"
4631 printf "%2\$*3\$d %d\n", 12, 34, 3; # will print " 34 12\n"
4632 printf "%*3\$2\$d %d\n", 12, 34, 3; # will print " 34 12\n"
4634 There are also two Perl-specific flags:
4636 V interpret integer as Perl's standard integer type
4637 v interpret string as a vector of integers, output as
4638 numbers separated either by dots, or by an arbitrary
4639 string received from the argument list when the flag
4642 Where a number would appear in the flags, an asterisk (C<*>) may be
4643 used instead, in which case Perl uses the next item in the parameter
4644 list as the given number (that is, as the field width or precision).
4645 If a field width obtained through C<*> is negative, it has the same
4646 effect as the C<-> flag: left-justification.
4648 The C<v> flag is useful for displaying ordinal values of characters
4649 in arbitrary strings:
4651 printf "version is v%vd\n", $^V; # Perl's version
4652 printf "address is %*vX\n", ":", $addr; # IPv6 address
4653 printf "bits are %*vb\n", " ", $bits; # random bitstring
4655 If C<use locale> is in effect, the character used for the decimal
4656 point in formatted real numbers is affected by the LC_NUMERIC locale.
4659 If Perl understands "quads" (64-bit integers) (this requires
4660 either that the platform natively support quads or that Perl
4661 be specifically compiled to support quads), the characters
4665 print quads, and they may optionally be preceded by
4673 You can find out whether your Perl supports quads via L<Config>:
4676 ($Config{use64bitint} eq 'define' || $Config{longsize} == 8) &&
4679 If Perl understands "long doubles" (this requires that the platform
4680 support long doubles), the flags
4684 may optionally be preceded by
4692 You can find out whether your Perl supports long doubles via L<Config>:
4695 $Config{d_longdbl} eq 'define' && print "long doubles\n";
4701 Return the square root of EXPR. If EXPR is omitted, returns square
4702 root of C<$_>. Only works on non-negative operands, unless you've
4703 loaded the standard Math::Complex module.
4706 print sqrt(-2); # prints 1.4142135623731i
4712 Sets the random number seed for the C<rand> operator. If EXPR is
4713 omitted, uses a semi-random value supplied by the kernel (if it supports
4714 the F</dev/urandom> device) or based on the current time and process
4715 ID, among other things. In versions of Perl prior to 5.004 the default
4716 seed was just the current C<time>. This isn't a particularly good seed,
4717 so many old programs supply their own seed value (often C<time ^ $$> or
4718 C<time ^ ($$ + ($$ << 15))>), but that isn't necessary any more.
4720 In fact, it's usually not necessary to call C<srand> at all, because if
4721 it is not called explicitly, it is called implicitly at the first use of
4722 the C<rand> operator. However, this was not the case in version of Perl
4723 before 5.004, so if your script will run under older Perl versions, it
4724 should call C<srand>.
4726 Note that you need something much more random than the default seed for
4727 cryptographic purposes. Checksumming the compressed output of one or more
4728 rapidly changing operating system status programs is the usual method. For
4731 srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip`);
4733 If you're particularly concerned with this, see the C<Math::TrulyRandom>
4736 Do I<not> call C<srand> multiple times in your program unless you know
4737 exactly what you're doing and why you're doing it. The point of the
4738 function is to "seed" the C<rand> function so that C<rand> can produce
4739 a different sequence each time you run your program. Just do it once at the
4740 top of your program, or you I<won't> get random numbers out of C<rand>!
4742 Frequently called programs (like CGI scripts) that simply use
4746 for a seed can fall prey to the mathematical property that
4750 one-third of the time. So don't do that.
4752 =item stat FILEHANDLE
4758 Returns a 13-element list giving the status info for a file, either
4759 the file opened via FILEHANDLE, or named by EXPR. If EXPR is omitted,
4760 it stats C<$_>. Returns a null list if the stat fails. Typically used
4763 ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
4764 $atime,$mtime,$ctime,$blksize,$blocks)
4767 Not all fields are supported on all filesystem types. Here are the
4768 meaning of the fields:
4770 0 dev device number of filesystem
4772 2 mode file mode (type and permissions)
4773 3 nlink number of (hard) links to the file
4774 4 uid numeric user ID of file's owner
4775 5 gid numeric group ID of file's owner
4776 6 rdev the device identifier (special files only)
4777 7 size total size of file, in bytes
4778 8 atime last access time in seconds since the epoch
4779 9 mtime last modify time in seconds since the epoch
4780 10 ctime inode change time (NOT creation time!) in seconds since the epoch
4781 11 blksize preferred block size for file system I/O
4782 12 blocks actual number of blocks allocated
4784 (The epoch was at 00:00 January 1, 1970 GMT.)
4786 If stat is passed the special filehandle consisting of an underline, no
4787 stat is done, but the current contents of the stat structure from the
4788 last stat or filetest are returned. Example:
4790 if (-x $file && (($d) = stat(_)) && $d < 0) {
4791 print "$file is executable NFS file\n";
4794 (This works on machines only for which the device number is negative
4797 Because the mode contains both the file type and its permissions, you
4798 should mask off the file type portion and (s)printf using a C<"%o">
4799 if you want to see the real permissions.
4801 $mode = (stat($filename))[2];
4802 printf "Permissions are %04o\n", $mode & 07777;
4804 In scalar context, C<stat> returns a boolean value indicating success
4805 or failure, and, if successful, sets the information associated with
4806 the special filehandle C<_>.
4808 The File::stat module provides a convenient, by-name access mechanism:
4811 $sb = stat($filename);
4812 printf "File is %s, size is %s, perm %04o, mtime %s\n",
4813 $filename, $sb->size, $sb->mode & 07777,
4814 scalar localtime $sb->mtime;
4816 You can import symbolic mode constants (C<S_IF*>) and functions
4817 (C<S_IS*>) from the Fcntl module:
4821 $mode = (stat($filename))[2];
4823 $user_rwx = ($mode & S_IRWXU) >> 6;
4824 $group_read = ($mode & S_IRGRP) >> 3;
4825 $other_execute = $mode & S_IXOTH;
4827 printf "Permissions are %04o\n", S_ISMODE($mode), "\n";
4829 $is_setuid = $mode & S_ISUID;
4830 $is_setgid = S_ISDIR($mode);
4832 You could write the last two using the C<-u> and C<-d> operators.
4833 The commonly available S_IF* constants are
4835 # Permissions: read, write, execute, for user, group, others.
4837 S_IRWXU S_IRUSR S_IWUSR S_IXUSR
4838 S_IRWXG S_IRGRP S_IWGRP S_IXGRP
4839 S_IRWXO S_IROTH S_IWOTH S_IXOTH
4841 # Setuid/Setgid/Stickiness.
4843 S_ISUID S_ISGID S_ISVTX S_ISTXT
4845 # File types. Not necessarily all are available on your system.
4847 S_IFREG S_IFDIR S_IFLNK S_IFBLK S_ISCHR S_IFIFO S_IFSOCK S_IFWHT S_ENFMT
4849 # The following are compatibility aliases for S_IRUSR, S_IWUSR, S_IXUSR.
4851 S_IREAD S_IWRITE S_IEXEC
4853 and the S_IF* functions are
4855 S_IFMODE($mode) the part of $mode containing the permission bits
4856 and the setuid/setgid/sticky bits
4858 S_IFMT($mode) the part of $mode containing the file type
4859 which can be bit-anded with e.g. S_IFREG
4860 or with the following functions
4862 # The operators -f, -d, -l, -b, -c, -p, and -s.
4864 S_ISREG($mode) S_ISDIR($mode) S_ISLNK($mode)
4865 S_ISBLK($mode) S_ISCHR($mode) S_ISFIFO($mode) S_ISSOCK($mode)
4867 # No direct -X operator counterpart, but for the first one
4868 # the -g operator is often equivalent. The ENFMT stands for
4869 # record flocking enforcement, a platform-dependent feature.
4871 S_ISENFMT($mode) S_ISWHT($mode)
4873 See your native chmod(2) and stat(2) documentation for more details
4874 about the S_* constants.
4880 Takes extra time to study SCALAR (C<$_> if unspecified) in anticipation of
4881 doing many pattern matches on the string before it is next modified.
4882 This may or may not save time, depending on the nature and number of
4883 patterns you are searching on, and on the distribution of character
4884 frequencies in the string to be searched--you probably want to compare
4885 run times with and without it to see which runs faster. Those loops
4886 which scan for many short constant strings (including the constant
4887 parts of more complex patterns) will benefit most. You may have only
4888 one C<study> active at a time--if you study a different scalar the first
4889 is "unstudied". (The way C<study> works is this: a linked list of every
4890 character in the string to be searched is made, so we know, for
4891 example, where all the C<'k'> characters are. From each search string,
4892 the rarest character is selected, based on some static frequency tables
4893 constructed from some C programs and English text. Only those places
4894 that contain this "rarest" character are examined.)
4896 For example, here is a loop that inserts index producing entries
4897 before any line containing a certain pattern:
4901 print ".IX foo\n" if /\bfoo\b/;
4902 print ".IX bar\n" if /\bbar\b/;
4903 print ".IX blurfl\n" if /\bblurfl\b/;
4908 In searching for C</\bfoo\b/>, only those locations in C<$_> that contain C<f>
4909 will be looked at, because C<f> is rarer than C<o>. In general, this is
4910 a big win except in pathological cases. The only question is whether
4911 it saves you more time than it took to build the linked list in the
4914 Note that if you have to look for strings that you don't know till
4915 runtime, you can build an entire loop as a string and C<eval> that to
4916 avoid recompiling all your patterns all the time. Together with
4917 undefining C<$/> to input entire files as one record, this can be very
4918 fast, often faster than specialized programs like fgrep(1). The following
4919 scans a list of files (C<@files>) for a list of words (C<@words>), and prints
4920 out the names of those files that contain a match:
4922 $search = 'while (<>) { study;';
4923 foreach $word (@words) {
4924 $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
4929 eval $search; # this screams
4930 $/ = "\n"; # put back to normal input delimiter
4931 foreach $file (sort keys(%seen)) {
4939 =item sub NAME BLOCK
4941 This is subroutine definition, not a real function I<per se>. With just a
4942 NAME (and possibly prototypes or attributes), it's just a forward declaration.
4943 Without a NAME, it's an anonymous function declaration, and does actually
4944 return a value: the CODE ref of the closure you just created. See L<perlsub>
4945 and L<perlref> for details.
4947 =item substr EXPR,OFFSET,LENGTH,REPLACEMENT
4949 =item substr EXPR,OFFSET,LENGTH
4951 =item substr EXPR,OFFSET
4953 Extracts a substring out of EXPR and returns it. First character is at
4954 offset C<0>, or whatever you've set C<$[> to (but don't do that).
4955 If OFFSET is negative (or more precisely, less than C<$[>), starts
4956 that far from the end of the string. If LENGTH is omitted, returns
4957 everything to the end of the string. If LENGTH is negative, leaves that
4958 many characters off the end of the string.
4960 You can use the substr() function as an lvalue, in which case EXPR
4961 must itself be an lvalue. If you assign something shorter than LENGTH,
4962 the string will shrink, and if you assign something longer than LENGTH,
4963 the string will grow to accommodate it. To keep the string the same
4964 length you may need to pad or chop your value using C<sprintf>.
4966 If OFFSET and LENGTH specify a substring that is partly outside the
4967 string, only the part within the string is returned. If the substring
4968 is beyond either end of the string, substr() returns the undefined
4969 value and produces a warning. When used as an lvalue, specifying a
4970 substring that is entirely outside the string is a fatal error.
4971 Here's an example showing the behavior for boundary cases:
4974 substr($name, 4) = 'dy'; # $name is now 'freddy'
4975 my $null = substr $name, 6, 2; # returns '' (no warning)
4976 my $oops = substr $name, 7; # returns undef, with warning
4977 substr($name, 7) = 'gap'; # fatal error
4979 An alternative to using substr() as an lvalue is to specify the
4980 replacement string as the 4th argument. This allows you to replace
4981 parts of the EXPR and return what was there before in one operation,
4982 just as you can with splice().
4984 =item symlink OLDFILE,NEWFILE
4986 Creates a new filename symbolically linked to the old filename.
4987 Returns C<1> for success, C<0> otherwise. On systems that don't support
4988 symbolic links, produces a fatal error at run time. To check for that,
4991 $symlink_exists = eval { symlink("",""); 1 };
4995 Calls the system call specified as the first element of the list,
4996 passing the remaining elements as arguments to the system call. If
4997 unimplemented, produces a fatal error. The arguments are interpreted
4998 as follows: if a given argument is numeric, the argument is passed as
4999 an int. If not, the pointer to the string value is passed. You are
5000 responsible to make sure a string is pre-extended long enough to
5001 receive any result that might be written into a string. You can't use a
5002 string literal (or other read-only string) as an argument to C<syscall>
5003 because Perl has to assume that any string pointer might be written
5005 integer arguments are not literals and have never been interpreted in a
5006 numeric context, you may need to add C<0> to them to force them to look
5007 like numbers. This emulates the C<syswrite> function (or vice versa):
5009 require 'syscall.ph'; # may need to run h2ph
5011 syscall(&SYS_write, fileno(STDOUT), $s, length $s);
5013 Note that Perl supports passing of up to only 14 arguments to your system call,
5014 which in practice should usually suffice.
5016 Syscall returns whatever value returned by the system call it calls.
5017 If the system call fails, C<syscall> returns C<-1> and sets C<$!> (errno).
5018 Note that some system calls can legitimately return C<-1>. The proper
5019 way to handle such calls is to assign C<$!=0;> before the call and
5020 check the value of C<$!> if syscall returns C<-1>.
5022 There's a problem with C<syscall(&SYS_pipe)>: it returns the file
5023 number of the read end of the pipe it creates. There is no way
5024 to retrieve the file number of the other end. You can avoid this
5025 problem by using C<pipe> instead.
5027 =item sysopen FILEHANDLE,FILENAME,MODE
5029 =item sysopen FILEHANDLE,FILENAME,MODE,PERMS
5031 Opens the file whose filename is given by FILENAME, and associates it
5032 with FILEHANDLE. If FILEHANDLE is an expression, its value is used as
5033 the name of the real filehandle wanted. This function calls the
5034 underlying operating system's C<open> function with the parameters
5035 FILENAME, MODE, PERMS.
5037 The possible values and flag bits of the MODE parameter are
5038 system-dependent; they are available via the standard module C<Fcntl>.
5039 See the documentation of your operating system's C<open> to see which
5040 values and flag bits are available. You may combine several flags
5041 using the C<|>-operator.
5043 Some of the most common values are C<O_RDONLY> for opening the file in
5044 read-only mode, C<O_WRONLY> for opening the file in write-only mode,
5045 and C<O_RDWR> for opening the file in read-write mode, and.
5047 For historical reasons, some values work on almost every system
5048 supported by perl: zero means read-only, one means write-only, and two
5049 means read/write. We know that these values do I<not> work under
5050 OS/390 & VM/ESA Unix and on the Macintosh; you probably don't want to
5051 use them in new code.
5053 If the file named by FILENAME does not exist and the C<open> call creates
5054 it (typically because MODE includes the C<O_CREAT> flag), then the value of
5055 PERMS specifies the permissions of the newly created file. If you omit
5056 the PERMS argument to C<sysopen>, Perl uses the octal value C<0666>.
5057 These permission values need to be in octal, and are modified by your
5058 process's current C<umask>.
5060 In many systems the C<O_EXCL> flag is available for opening files in
5061 exclusive mode. This is B<not> locking: exclusiveness means here that
5062 if the file already exists, sysopen() fails. The C<O_EXCL> wins
5065 Sometimes you may want to truncate an already-existing file: C<O_TRUNC>.
5067 You should seldom if ever use C<0644> as argument to C<sysopen>, because
5068 that takes away the user's option to have a more permissive umask.
5069 Better to omit it. See the perlfunc(1) entry on C<umask> for more
5072 Note that C<sysopen> depends on the fdopen() C library function.
5073 On many UNIX systems, fdopen() is known to fail when file descriptors
5074 exceed a certain value, typically 255. If you need more file
5075 descriptors than that, consider rebuilding Perl to use the C<sfio>
5076 library, or perhaps using the POSIX::open() function.
5078 See L<perlopentut> for a kinder, gentler explanation of opening files.
5080 =item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
5082 =item sysread FILEHANDLE,SCALAR,LENGTH
5084 Attempts to read LENGTH bytes of data into variable SCALAR from the
5085 specified FILEHANDLE, using the system call read(2). It bypasses stdio,
5086 so mixing this with other kinds of reads, C<print>, C<write>,
5087 C<seek>, C<tell>, or C<eof> can cause confusion because stdio
5088 usually buffers data. Returns the number of bytes actually read, C<0>
5089 at end of file, or undef if there was an error. SCALAR will be grown or
5090 shrunk so that the last byte actually read is the last byte of the
5091 scalar after the read.
5093 An OFFSET may be specified to place the read data at some place in the
5094 string other than the beginning. A negative OFFSET specifies
5095 placement at that many bytes counting backwards from the end of the
5096 string. A positive OFFSET greater than the length of SCALAR results
5097 in the string being padded to the required size with C<"\0"> bytes before
5098 the result of the read is appended.
5100 There is no syseof() function, which is ok, since eof() doesn't work
5101 very well on device files (like ttys) anyway. Use sysread() and check
5102 for a return value for 0 to decide whether you're done.
5104 =item sysseek FILEHANDLE,POSITION,WHENCE
5106 Sets FILEHANDLE's system position using the system call lseek(2). It
5107 bypasses stdio, so mixing this with reads (other than C<sysread>),
5108 C<print>, C<write>, C<seek>, C<tell>, or C<eof> may cause confusion.
5109 FILEHANDLE may be an expression whose value gives the name of the
5110 filehandle. The values for WHENCE are C<0> to set the new position to
5111 POSITION, C<1> to set the it to the current position plus POSITION,
5112 and C<2> to set it to EOF plus POSITION (typically negative). For
5113 WHENCE, you may also use the constants C<SEEK_SET>, C<SEEK_CUR>, and
5114 C<SEEK_END> (start of the file, current position, end of the file)
5115 from the Fcntl module.
5117 Returns the new position, or the undefined value on failure. A position
5118 of zero is returned as the string C<"0 but true">; thus C<sysseek> returns
5119 true on success and false on failure, yet you can still easily determine
5124 =item system PROGRAM LIST
5126 Does exactly the same thing as C<exec LIST>, except that a fork is
5127 done first, and the parent process waits for the child process to
5128 complete. Note that argument processing varies depending on the
5129 number of arguments. If there is more than one argument in LIST,
5130 or if LIST is an array with more than one value, starts the program
5131 given by the first element of the list with arguments given by the
5132 rest of the list. If there is only one scalar argument, the argument
5133 is checked for shell metacharacters, and if there are any, the
5134 entire argument is passed to the system's command shell for parsing
5135 (this is C</bin/sh -c> on Unix platforms, but varies on other
5136 platforms). If there are no shell metacharacters in the argument,
5137 it is split into words and passed directly to C<execvp>, which is
5140 Beginning with v5.6.0, Perl will attempt to flush all files opened for
5141 output before any operation that may do a fork, but this may not be
5142 supported on some platforms (see L<perlport>). To be safe, you may need
5143 to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
5144 of C<IO::Handle> on any open handles.
5146 The return value is the exit status of the program as
5147 returned by the C<wait> call. To get the actual exit value divide by
5148 256. See also L</exec>. This is I<not> what you want to use to capture
5149 the output from a command, for that you should use merely backticks or
5150 C<qx//>, as described in L<perlop/"`STRING`">. Return value of -1
5151 indicates a failure to start the program (inspect $! for the reason).
5153 Like C<exec>, C<system> allows you to lie to a program about its name if
5154 you use the C<system PROGRAM LIST> syntax. Again, see L</exec>.
5156 Because C<system> and backticks block C<SIGINT> and C<SIGQUIT>, killing the
5157 program they're running doesn't actually interrupt your program.
5159 @args = ("command", "arg1", "arg2");
5161 or die "system @args failed: $?"
5163 You can check all the failure possibilities by inspecting
5166 $exit_value = $? >> 8;
5167 $signal_num = $? & 127;
5168 $dumped_core = $? & 128;
5170 When the arguments get executed via the system shell, results
5171 and return codes will be subject to its quirks and capabilities.
5172 See L<perlop/"`STRING`"> and L</exec> for details.
5174 =item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
5176 =item syswrite FILEHANDLE,SCALAR,LENGTH
5178 =item syswrite FILEHANDLE,SCALAR
5180 Attempts to write LENGTH bytes of data from variable SCALAR to the
5181 specified FILEHANDLE, using the system call write(2). If LENGTH
5182 is not specified, writes whole SCALAR. It bypasses stdio, so mixing
5183 this with reads (other than C<sysread())>, C<print>, C<write>,
5184 C<seek>, C<tell>, or C<eof> may cause confusion because stdio
5185 usually buffers data. Returns the number of bytes actually written,
5186 or C<undef> if there was an error. If the LENGTH is greater than
5187 the available data in the SCALAR after the OFFSET, only as much
5188 data as is available will be written.
5190 An OFFSET may be specified to write the data from some part of the
5191 string other than the beginning. A negative OFFSET specifies writing
5192 that many bytes counting backwards from the end of the string. In the
5193 case the SCALAR is empty you can use OFFSET but only zero offset.
5195 =item tell FILEHANDLE
5199 Returns the current position for FILEHANDLE, or -1 on error. FILEHANDLE
5200 may be an expression whose value gives the name of the actual filehandle.
5201 If FILEHANDLE is omitted, assumes the file last read.
5203 The return value of tell() for the standard streams like the STDIN
5204 depends on the operating system: it may return -1 or something else.
5205 tell() on pipes, fifos, and sockets usually returns -1.
5207 There is no C<systell> function. Use C<sysseek(FH, 0, 1)> for that.
5209 =item telldir DIRHANDLE
5211 Returns the current position of the C<readdir> routines on DIRHANDLE.
5212 Value may be given to C<seekdir> to access a particular location in a
5213 directory. Has the same caveats about possible directory compaction as
5214 the corresponding system library routine.
5216 =item tie VARIABLE,CLASSNAME,LIST
5218 This function binds a variable to a package class that will provide the
5219 implementation for the variable. VARIABLE is the name of the variable
5220 to be enchanted. CLASSNAME is the name of a class implementing objects
5221 of correct type. Any additional arguments are passed to the C<new>
5222 method of the class (meaning C<TIESCALAR>, C<TIEHANDLE>, C<TIEARRAY>,
5223 or C<TIEHASH>). Typically these are arguments such as might be passed
5224 to the C<dbm_open()> function of C. The object returned by the C<new>
5225 method is also returned by the C<tie> function, which would be useful
5226 if you want to access other methods in CLASSNAME.
5228 Note that functions such as C<keys> and C<values> may return huge lists
5229 when used on large objects, like DBM files. You may prefer to use the
5230 C<each> function to iterate over such. Example:
5232 # print out history file offsets
5234 tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
5235 while (($key,$val) = each %HIST) {
5236 print $key, ' = ', unpack('L',$val), "\n";
5240 A class implementing a hash should have the following methods:
5242 TIEHASH classname, LIST
5244 STORE this, key, value
5249 NEXTKEY this, lastkey
5253 A class implementing an ordinary array should have the following methods:
5255 TIEARRAY classname, LIST
5257 STORE this, key, value
5259 STORESIZE this, count
5265 SPLICE this, offset, length, LIST
5270 A class implementing a file handle should have the following methods:
5272 TIEHANDLE classname, LIST
5273 READ this, scalar, length, offset
5276 WRITE this, scalar, length, offset
5278 PRINTF this, format, LIST
5282 SEEK this, position, whence
5284 OPEN this, mode, LIST
5289 A class implementing a scalar should have the following methods:
5291 TIESCALAR classname, LIST
5297 Not all methods indicated above need be implemented. See L<perltie>,
5298 L<Tie::Hash>, L<Tie::Array>, L<Tie::Scalar>, and L<Tie::Handle>.
5300 Unlike C<dbmopen>, the C<tie> function will not use or require a module
5301 for you--you need to do that explicitly yourself. See L<DB_File>
5302 or the F<Config> module for interesting C<tie> implementations.
5304 For further details see L<perltie>, L<"tied VARIABLE">.
5308 Returns a reference to the object underlying VARIABLE (the same value
5309 that was originally returned by the C<tie> call that bound the variable
5310 to a package.) Returns the undefined value if VARIABLE isn't tied to a
5315 Returns the number of non-leap seconds since whatever time the system
5316 considers to be the epoch (that's 00:00:00, January 1, 1904 for MacOS,
5317 and 00:00:00 UTC, January 1, 1970 for most other systems).
5318 Suitable for feeding to C<gmtime> and C<localtime>.
5320 For measuring time in better granularity than one second,
5321 you may use either the Time::HiRes module from CPAN, or
5322 if you have gettimeofday(2), you may be able to use the
5323 C<syscall> interface of Perl, see L<perlfaq8> for details.
5327 Returns a four-element list giving the user and system times, in
5328 seconds, for this process and the children of this process.
5330 ($user,$system,$cuser,$csystem) = times;
5334 The transliteration operator. Same as C<y///>. See L<perlop>.
5336 =item truncate FILEHANDLE,LENGTH
5338 =item truncate EXPR,LENGTH
5340 Truncates the file opened on FILEHANDLE, or named by EXPR, to the
5341 specified length. Produces a fatal error if truncate isn't implemented
5342 on your system. Returns true if successful, the undefined value
5349 Returns an uppercased version of EXPR. This is the internal function
5350 implementing the C<\U> escape in double-quoted strings.
5351 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
5352 Under Unicode (C<use utf8>) it uses the standard Unicode uppercase mappings. (It
5353 does not attempt to do titlecase mapping on initial letters. See C<ucfirst> for that.)
5355 If EXPR is omitted, uses C<$_>.
5361 Returns the value of EXPR with the first character
5362 in uppercase (titlecase in Unicode). This is
5363 the internal function implementing the C<\u> escape in double-quoted strings.
5364 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
5367 If EXPR is omitted, uses C<$_>.
5373 Sets the umask for the process to EXPR and returns the previous value.
5374 If EXPR is omitted, merely returns the current umask.
5376 The Unix permission C<rwxr-x---> is represented as three sets of three
5377 bits, or three octal digits: C<0750> (the leading 0 indicates octal
5378 and isn't one of the digits). The C<umask> value is such a number
5379 representing disabled permissions bits. The permission (or "mode")
5380 values you pass C<mkdir> or C<sysopen> are modified by your umask, so
5381 even if you tell C<sysopen> to create a file with permissions C<0777>,
5382 if your umask is C<0022> then the file will actually be created with
5383 permissions C<0755>. If your C<umask> were C<0027> (group can't
5384 write; others can't read, write, or execute), then passing
5385 C<sysopen> C<0666> would create a file with mode C<0640> (C<0666 &~
5388 Here's some advice: supply a creation mode of C<0666> for regular
5389 files (in C<sysopen>) and one of C<0777> for directories (in
5390 C<mkdir>) and executable files. This gives users the freedom of
5391 choice: if they want protected files, they might choose process umasks
5392 of C<022>, C<027>, or even the particularly antisocial mask of C<077>.
5393 Programs should rarely if ever make policy decisions better left to
5394 the user. The exception to this is when writing files that should be
5395 kept private: mail files, web browser cookies, I<.rhosts> files, and
5398 If umask(2) is not implemented on your system and you are trying to
5399 restrict access for I<yourself> (i.e., (EXPR & 0700) > 0), produces a
5400 fatal error at run time. If umask(2) is not implemented and you are
5401 not trying to restrict access for yourself, returns C<undef>.
5403 Remember that a umask is a number, usually given in octal; it is I<not> a
5404 string of octal digits. See also L</oct>, if all you have is a string.
5410 Undefines the value of EXPR, which must be an lvalue. Use only on a
5411 scalar value, an array (using C<@>), a hash (using C<%>), a subroutine
5412 (using C<&>), or a typeglob (using <*>). (Saying C<undef $hash{$key}>
5413 will probably not do what you expect on most predefined variables or
5414 DBM list values, so don't do that; see L<delete>.) Always returns the
5415 undefined value. You can omit the EXPR, in which case nothing is
5416 undefined, but you still get an undefined value that you could, for
5417 instance, return from a subroutine, assign to a variable or pass as a
5418 parameter. Examples:
5421 undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'};
5425 undef *xyz; # destroys $xyz, @xyz, %xyz, &xyz, etc.
5426 return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it;
5427 select undef, undef, undef, 0.25;
5428 ($a, $b, undef, $c) = &foo; # Ignore third value returned
5430 Note that this is a unary operator, not a list operator.
5436 Deletes a list of files. Returns the number of files successfully
5439 $cnt = unlink 'a', 'b', 'c';
5443 Note: C<unlink> will not delete directories unless you are superuser and
5444 the B<-U> flag is supplied to Perl. Even if these conditions are
5445 met, be warned that unlinking a directory can inflict damage on your
5446 filesystem. Use C<rmdir> instead.
5448 If LIST is omitted, uses C<$_>.
5450 =item unpack TEMPLATE,EXPR
5452 C<unpack> does the reverse of C<pack>: it takes a string
5453 and expands it out into a list of values.
5454 (In scalar context, it returns merely the first value produced.)
5456 The string is broken into chunks described by the TEMPLATE. Each chunk
5457 is converted separately to a value. Typically, either the string is a result
5458 of C<pack>, or the bytes of the string represent a C structure of some
5461 The TEMPLATE has the same format as in the C<pack> function.
5462 Here's a subroutine that does substring:
5465 my($what,$where,$howmuch) = @_;
5466 unpack("x$where a$howmuch", $what);
5471 sub ordinal { unpack("c",$_[0]); } # same as ord()
5473 In addition to fields allowed in pack(), you may prefix a field with
5474 a %<number> to indicate that
5475 you want a <number>-bit checksum of the items instead of the items
5476 themselves. Default is a 16-bit checksum. Checksum is calculated by
5477 summing numeric values of expanded values (for string fields the sum of
5478 C<ord($char)> is taken, for bit fields the sum of zeroes and ones).
5480 For example, the following
5481 computes the same number as the System V sum program:
5485 unpack("%32C*",<>) % 65535;
5488 The following efficiently counts the number of set bits in a bit vector:
5490 $setbits = unpack("%32b*", $selectmask);
5492 The C<p> and C<P> formats should be used with care. Since Perl
5493 has no way of checking whether the value passed to C<unpack()>
5494 corresponds to a valid memory location, passing a pointer value that's
5495 not known to be valid is likely to have disastrous consequences.
5497 If the repeat count of a field is larger than what the remainder of
5498 the input string allows, repeat count is decreased. If the input string
5499 is longer than one described by the TEMPLATE, the rest is ignored.
5501 See L</pack> for more examples and notes.
5503 =item untie VARIABLE
5505 Breaks the binding between a variable and a package. (See C<tie>.)
5507 =item unshift ARRAY,LIST
5509 Does the opposite of a C<shift>. Or the opposite of a C<push>,
5510 depending on how you look at it. Prepends list to the front of the
5511 array, and returns the new number of elements in the array.
5513 unshift(ARGV, '-e') unless $ARGV[0] =~ /^-/;
5515 Note the LIST is prepended whole, not one element at a time, so the
5516 prepended elements stay in the same order. Use C<reverse> to do the
5519 =item use Module VERSION LIST
5521 =item use Module VERSION
5523 =item use Module LIST
5529 Imports some semantics into the current package from the named module,
5530 generally by aliasing certain subroutine or variable names into your
5531 package. It is exactly equivalent to
5533 BEGIN { require Module; import Module LIST; }
5535 except that Module I<must> be a bareword.
5537 VERSION, which can be specified as a literal of the form v5.6.1, demands
5538 that the current version of Perl (C<$^V> or $PERL_VERSION) be at least
5539 as recent as that version. (For compatibility with older versions of Perl,
5540 a numeric literal will also be interpreted as VERSION.) If the version
5541 of the running Perl interpreter is less than VERSION, then an error
5542 message is printed and Perl exits immediately without attempting to
5543 parse the rest of the file. Compare with L</require>, which can do a
5544 similar check at run time.
5546 use v5.6.1; # compile time version check
5548 use 5.005_03; # float version allowed for compatibility
5550 This is often useful if you need to check the current Perl version before
5551 C<use>ing library modules that have changed in incompatible ways from
5552 older versions of Perl. (We try not to do this more than we have to.)
5554 The C<BEGIN> forces the C<require> and C<import> to happen at compile time. The
5555 C<require> makes sure the module is loaded into memory if it hasn't been
5556 yet. The C<import> is not a builtin--it's just an ordinary static method
5557 call into the C<Module> package to tell the module to import the list of
5558 features back into the current package. The module can implement its
5559 C<import> method any way it likes, though most modules just choose to
5560 derive their C<import> method via inheritance from the C<Exporter> class that
5561 is defined in the C<Exporter> module. See L<Exporter>. If no C<import>
5562 method can be found then the call is skipped.
5564 If you do not want to call the package's C<import> method (for instance,
5565 to stop your namespace from being altered), explicitly supply the empty list:
5569 That is exactly equivalent to
5571 BEGIN { require Module }
5573 If the VERSION argument is present between Module and LIST, then the
5574 C<use> will call the VERSION method in class Module with the given
5575 version as an argument. The default VERSION method, inherited from
5576 the UNIVERSAL class, croaks if the given version is larger than the
5577 value of the variable C<$Module::VERSION>.
5579 Again, there is a distinction between omitting LIST (C<import> called
5580 with no arguments) and an explicit empty LIST C<()> (C<import> not
5581 called). Note that there is no comma after VERSION!
5583 Because this is a wide-open interface, pragmas (compiler directives)
5584 are also implemented this way. Currently implemented pragmas are:
5589 use sigtrap qw(SEGV BUS);
5590 use strict qw(subs vars refs);
5591 use subs qw(afunc blurfl);
5592 use warnings qw(all);
5594 Some of these pseudo-modules import semantics into the current
5595 block scope (like C<strict> or C<integer>, unlike ordinary modules,
5596 which import symbols into the current package (which are effective
5597 through the end of the file).
5599 There's a corresponding C<no> command that unimports meanings imported
5600 by C<use>, i.e., it calls C<unimport Module LIST> instead of C<import>.
5606 If no C<unimport> method can be found the call fails with a fatal error.
5608 See L<perlmodlib> for a list of standard modules and pragmas. See L<perlrun>
5609 for the C<-M> and C<-m> command-line options to perl that give C<use>
5610 functionality from the command-line.
5614 Changes the access and modification times on each file of a list of
5615 files. The first two elements of the list must be the NUMERICAL access
5616 and modification times, in that order. Returns the number of files
5617 successfully changed. The inode change time of each file is set
5618 to the current time. This code has the same effect as the C<touch>
5619 command if the files already exist:
5623 utime $now, $now, @ARGV;
5627 Returns a list consisting of all the values of the named hash. (In a
5628 scalar context, returns the number of values.) The values are
5629 returned in an apparently random order. The actual random order is
5630 subject to change in future versions of perl, but it is guaranteed to
5631 be the same order as either the C<keys> or C<each> function would
5632 produce on the same (unmodified) hash.
5634 Note that the values are not copied, which means modifying them will
5635 modify the contents of the hash:
5637 for (values %hash) { s/foo/bar/g } # modifies %hash values
5638 for (@hash{keys %hash}) { s/foo/bar/g } # same
5640 As a side effect, calling values() resets the HASH's internal iterator.
5641 See also C<keys>, C<each>, and C<sort>.
5643 =item vec EXPR,OFFSET,BITS
5645 Treats the string in EXPR as a bit vector made up of elements of
5646 width BITS, and returns the value of the element specified by OFFSET
5647 as an unsigned integer. BITS therefore specifies the number of bits
5648 that are reserved for each element in the bit vector. This must
5649 be a power of two from 1 to 32 (or 64, if your platform supports
5652 If BITS is 8, "elements" coincide with bytes of the input string.
5654 If BITS is 16 or more, bytes of the input string are grouped into chunks
5655 of size BITS/8, and each group is converted to a number as with
5656 pack()/unpack() with big-endian formats C<n>/C<N> (and analogously
5657 for BITS==64). See L<"pack"> for details.
5659 If bits is 4 or less, the string is broken into bytes, then the bits
5660 of each byte are broken into 8/BITS groups. Bits of a byte are
5661 numbered in a little-endian-ish way, as in C<0x01>, C<0x02>,
5662 C<0x04>, C<0x08>, C<0x10>, C<0x20>, C<0x40>, C<0x80>. For example,
5663 breaking the single input byte C<chr(0x36)> into two groups gives a list
5664 C<(0x6, 0x3)>; breaking it into 4 groups gives C<(0x2, 0x1, 0x3, 0x0)>.
5666 C<vec> may also be assigned to, in which case parentheses are needed
5667 to give the expression the correct precedence as in
5669 vec($image, $max_x * $x + $y, 8) = 3;
5671 If the selected element is outside the string, the value 0 is returned.
5672 If an element off the end of the string is written to, Perl will first
5673 extend the string with sufficiently many zero bytes. It is an error
5674 to try to write off the beginning of the string (i.e. negative OFFSET).
5676 The string should not contain any character with the value > 255 (which
5677 can only happen if you're using UTF8 encoding). If it does, it will be
5678 treated as something which is not UTF8 encoded. When the C<vec> was
5679 assigned to, other parts of your program will also no longer consider the
5680 string to be UTF8 encoded. In other words, if you do have such characters
5681 in your string, vec() will operate on the actual byte string, and not the
5682 conceptual character string.
5684 Strings created with C<vec> can also be manipulated with the logical
5685 operators C<|>, C<&>, C<^>, and C<~>. These operators will assume a bit
5686 vector operation is desired when both operands are strings.
5687 See L<perlop/"Bitwise String Operators">.
5689 The following code will build up an ASCII string saying C<'PerlPerlPerl'>.
5690 The comments show the string after each step. Note that this code works
5691 in the same way on big-endian or little-endian machines.
5694 vec($foo, 0, 32) = 0x5065726C; # 'Perl'
5696 # $foo eq "Perl" eq "\x50\x65\x72\x6C", 32 bits
5697 print vec($foo, 0, 8); # prints 80 == 0x50 == ord('P')
5699 vec($foo, 2, 16) = 0x5065; # 'PerlPe'
5700 vec($foo, 3, 16) = 0x726C; # 'PerlPerl'
5701 vec($foo, 8, 8) = 0x50; # 'PerlPerlP'
5702 vec($foo, 9, 8) = 0x65; # 'PerlPerlPe'
5703 vec($foo, 20, 4) = 2; # 'PerlPerlPe' . "\x02"
5704 vec($foo, 21, 4) = 7; # 'PerlPerlPer'
5706 vec($foo, 45, 2) = 3; # 'PerlPerlPer' . "\x0c"
5707 vec($foo, 93, 1) = 1; # 'PerlPerlPer' . "\x2c"
5708 vec($foo, 94, 1) = 1; # 'PerlPerlPerl'
5711 To transform a bit vector into a string or list of 0's and 1's, use these:
5713 $bits = unpack("b*", $vector);
5714 @bits = split(//, unpack("b*", $vector));
5716 If you know the exact length in bits, it can be used in place of the C<*>.
5718 Here is an example to illustrate how the bits actually fall in place:
5724 unpack("V",$_) 01234567890123456789012345678901
5725 ------------------------------------------------------------------
5730 for ($shift=0; $shift < $width; ++$shift) {
5731 for ($off=0; $off < 32/$width; ++$off) {
5732 $str = pack("B*", "0"x32);
5733 $bits = (1<<$shift);
5734 vec($str, $off, $width) = $bits;
5735 $res = unpack("b*",$str);
5736 $val = unpack("V", $str);
5743 vec($_,@#,@#) = @<< == @######### @>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
5744 $off, $width, $bits, $val, $res
5748 Regardless of the machine architecture on which it is run, the above
5749 example should print the following table:
5752 unpack("V",$_) 01234567890123456789012345678901
5753 ------------------------------------------------------------------
5754 vec($_, 0, 1) = 1 == 1 10000000000000000000000000000000
5755 vec($_, 1, 1) = 1 == 2 01000000000000000000000000000000
5756 vec($_, 2, 1) = 1 == 4 00100000000000000000000000000000
5757 vec($_, 3, 1) = 1 == 8 00010000000000000000000000000000
5758 vec($_, 4, 1) = 1 == 16 00001000000000000000000000000000
5759 vec($_, 5, 1) = 1 == 32 00000100000000000000000000000000
5760 vec($_, 6, 1) = 1 == 64 00000010000000000000000000000000
5761 vec($_, 7, 1) = 1 == 128 00000001000000000000000000000000
5762 vec($_, 8, 1) = 1 == 256 00000000100000000000000000000000
5763 vec($_, 9, 1) = 1 == 512 00000000010000000000000000000000
5764 vec($_,10, 1) = 1 == 1024 00000000001000000000000000000000
5765 vec($_,11, 1) = 1 == 2048 00000000000100000000000000000000
5766 vec($_,12, 1) = 1 == 4096 00000000000010000000000000000000
5767 vec($_,13, 1) = 1 == 8192 00000000000001000000000000000000
5768 vec($_,14, 1) = 1 == 16384 00000000000000100000000000000000
5769 vec($_,15, 1) = 1 == 32768 00000000000000010000000000000000
5770 vec($_,16, 1) = 1 == 65536 00000000000000001000000000000000
5771 vec($_,17, 1) = 1 == 131072 00000000000000000100000000000000
5772 vec($_,18, 1) = 1 == 262144 00000000000000000010000000000000
5773 vec($_,19, 1) = 1 == 524288 00000000000000000001000000000000
5774 vec($_,20, 1) = 1 == 1048576 00000000000000000000100000000000
5775 vec($_,21, 1) = 1 == 2097152 00000000000000000000010000000000
5776 vec($_,22, 1) = 1 == 4194304 00000000000000000000001000000000
5777 vec($_,23, 1) = 1 == 8388608 00000000000000000000000100000000
5778 vec($_,24, 1) = 1 == 16777216 00000000000000000000000010000000
5779 vec($_,25, 1) = 1 == 33554432 00000000000000000000000001000000
5780 vec($_,26, 1) = 1 == 67108864 00000000000000000000000000100000
5781 vec($_,27, 1) = 1 == 134217728 00000000000000000000000000010000
5782 vec($_,28, 1) = 1 == 268435456 00000000000000000000000000001000
5783 vec($_,29, 1) = 1 == 536870912 00000000000000000000000000000100
5784 vec($_,30, 1) = 1 == 1073741824 00000000000000000000000000000010
5785 vec($_,31, 1) = 1 == 2147483648 00000000000000000000000000000001
5786 vec($_, 0, 2) = 1 == 1 10000000000000000000000000000000
5787 vec($_, 1, 2) = 1 == 4 00100000000000000000000000000000
5788 vec($_, 2, 2) = 1 == 16 00001000000000000000000000000000
5789 vec($_, 3, 2) = 1 == 64 00000010000000000000000000000000
5790 vec($_, 4, 2) = 1 == 256 00000000100000000000000000000000
5791 vec($_, 5, 2) = 1 == 1024 00000000001000000000000000000000
5792 vec($_, 6, 2) = 1 == 4096 00000000000010000000000000000000
5793 vec($_, 7, 2) = 1 == 16384 00000000000000100000000000000000
5794 vec($_, 8, 2) = 1 == 65536 00000000000000001000000000000000
5795 vec($_, 9, 2) = 1 == 262144 00000000000000000010000000000000
5796 vec($_,10, 2) = 1 == 1048576 00000000000000000000100000000000
5797 vec($_,11, 2) = 1 == 4194304 00000000000000000000001000000000
5798 vec($_,12, 2) = 1 == 16777216 00000000000000000000000010000000
5799 vec($_,13, 2) = 1 == 67108864 00000000000000000000000000100000
5800 vec($_,14, 2) = 1 == 268435456 00000000000000000000000000001000
5801 vec($_,15, 2) = 1 == 1073741824 00000000000000000000000000000010
5802 vec($_, 0, 2) = 2 == 2 01000000000000000000000000000000
5803 vec($_, 1, 2) = 2 == 8 00010000000000000000000000000000
5804 vec($_, 2, 2) = 2 == 32 00000100000000000000000000000000
5805 vec($_, 3, 2) = 2 == 128 00000001000000000000000000000000
5806 vec($_, 4, 2) = 2 == 512 00000000010000000000000000000000
5807 vec($_, 5, 2) = 2 == 2048 00000000000100000000000000000000
5808 vec($_, 6, 2) = 2 == 8192 00000000000001000000000000000000
5809 vec($_, 7, 2) = 2 == 32768 00000000000000010000000000000000
5810 vec($_, 8, 2) = 2 == 131072 00000000000000000100000000000000
5811 vec($_, 9, 2) = 2 == 524288 00000000000000000001000000000000
5812 vec($_,10, 2) = 2 == 2097152 00000000000000000000010000000000
5813 vec($_,11, 2) = 2 == 8388608 00000000000000000000000100000000
5814 vec($_,12, 2) = 2 == 33554432 00000000000000000000000001000000
5815 vec($_,13, 2) = 2 == 134217728 00000000000000000000000000010000
5816 vec($_,14, 2) = 2 == 536870912 00000000000000000000000000000100
5817 vec($_,15, 2) = 2 == 2147483648 00000000000000000000000000000001
5818 vec($_, 0, 4) = 1 == 1 10000000000000000000000000000000
5819 vec($_, 1, 4) = 1 == 16 00001000000000000000000000000000
5820 vec($_, 2, 4) = 1 == 256 00000000100000000000000000000000
5821 vec($_, 3, 4) = 1 == 4096 00000000000010000000000000000000
5822 vec($_, 4, 4) = 1 == 65536 00000000000000001000000000000000
5823 vec($_, 5, 4) = 1 == 1048576 00000000000000000000100000000000
5824 vec($_, 6, 4) = 1 == 16777216 00000000000000000000000010000000
5825 vec($_, 7, 4) = 1 == 268435456 00000000000000000000000000001000
5826 vec($_, 0, 4) = 2 == 2 01000000000000000000000000000000
5827 vec($_, 1, 4) = 2 == 32 00000100000000000000000000000000
5828 vec($_, 2, 4) = 2 == 512 00000000010000000000000000000000
5829 vec($_, 3, 4) = 2 == 8192 00000000000001000000000000000000
5830 vec($_, 4, 4) = 2 == 131072 00000000000000000100000000000000
5831 vec($_, 5, 4) = 2 == 2097152 00000000000000000000010000000000
5832 vec($_, 6, 4) = 2 == 33554432 00000000000000000000000001000000
5833 vec($_, 7, 4) = 2 == 536870912 00000000000000000000000000000100
5834 vec($_, 0, 4) = 4 == 4 00100000000000000000000000000000
5835 vec($_, 1, 4) = 4 == 64 00000010000000000000000000000000
5836 vec($_, 2, 4) = 4 == 1024 00000000001000000000000000000000
5837 vec($_, 3, 4) = 4 == 16384 00000000000000100000000000000000
5838 vec($_, 4, 4) = 4 == 262144 00000000000000000010000000000000
5839 vec($_, 5, 4) = 4 == 4194304 00000000000000000000001000000000
5840 vec($_, 6, 4) = 4 == 67108864 00000000000000000000000000100000
5841 vec($_, 7, 4) = 4 == 1073741824 00000000000000000000000000000010
5842 vec($_, 0, 4) = 8 == 8 00010000000000000000000000000000
5843 vec($_, 1, 4) = 8 == 128 00000001000000000000000000000000
5844 vec($_, 2, 4) = 8 == 2048 00000000000100000000000000000000
5845 vec($_, 3, 4) = 8 == 32768 00000000000000010000000000000000
5846 vec($_, 4, 4) = 8 == 524288 00000000000000000001000000000000
5847 vec($_, 5, 4) = 8 == 8388608 00000000000000000000000100000000
5848 vec($_, 6, 4) = 8 == 134217728 00000000000000000000000000010000
5849 vec($_, 7, 4) = 8 == 2147483648 00000000000000000000000000000001
5850 vec($_, 0, 8) = 1 == 1 10000000000000000000000000000000
5851 vec($_, 1, 8) = 1 == 256 00000000100000000000000000000000
5852 vec($_, 2, 8) = 1 == 65536 00000000000000001000000000000000
5853 vec($_, 3, 8) = 1 == 16777216 00000000000000000000000010000000
5854 vec($_, 0, 8) = 2 == 2 01000000000000000000000000000000
5855 vec($_, 1, 8) = 2 == 512 00000000010000000000000000000000
5856 vec($_, 2, 8) = 2 == 131072 00000000000000000100000000000000
5857 vec($_, 3, 8) = 2 == 33554432 00000000000000000000000001000000
5858 vec($_, 0, 8) = 4 == 4 00100000000000000000000000000000
5859 vec($_, 1, 8) = 4 == 1024 00000000001000000000000000000000
5860 vec($_, 2, 8) = 4 == 262144 00000000000000000010000000000000
5861 vec($_, 3, 8) = 4 == 67108864 00000000000000000000000000100000
5862 vec($_, 0, 8) = 8 == 8 00010000000000000000000000000000
5863 vec($_, 1, 8) = 8 == 2048 00000000000100000000000000000000
5864 vec($_, 2, 8) = 8 == 524288 00000000000000000001000000000000
5865 vec($_, 3, 8) = 8 == 134217728 00000000000000000000000000010000
5866 vec($_, 0, 8) = 16 == 16 00001000000000000000000000000000
5867 vec($_, 1, 8) = 16 == 4096 00000000000010000000000000000000
5868 vec($_, 2, 8) = 16 == 1048576 00000000000000000000100000000000
5869 vec($_, 3, 8) = 16 == 268435456 00000000000000000000000000001000
5870 vec($_, 0, 8) = 32 == 32 00000100000000000000000000000000
5871 vec($_, 1, 8) = 32 == 8192 00000000000001000000000000000000
5872 vec($_, 2, 8) = 32 == 2097152 00000000000000000000010000000000
5873 vec($_, 3, 8) = 32 == 536870912 00000000000000000000000000000100
5874 vec($_, 0, 8) = 64 == 64 00000010000000000000000000000000
5875 vec($_, 1, 8) = 64 == 16384 00000000000000100000000000000000
5876 vec($_, 2, 8) = 64 == 4194304 00000000000000000000001000000000
5877 vec($_, 3, 8) = 64 == 1073741824 00000000000000000000000000000010
5878 vec($_, 0, 8) = 128 == 128 00000001000000000000000000000000
5879 vec($_, 1, 8) = 128 == 32768 00000000000000010000000000000000
5880 vec($_, 2, 8) = 128 == 8388608 00000000000000000000000100000000
5881 vec($_, 3, 8) = 128 == 2147483648 00000000000000000000000000000001
5885 Behaves like the wait(2) system call on your system: it waits for a child
5886 process to terminate and returns the pid of the deceased process, or
5887 C<-1> if there are no child processes. The status is returned in C<$?>.
5888 Note that a return value of C<-1> could mean that child processes are
5889 being automatically reaped, as described in L<perlipc>.
5891 =item waitpid PID,FLAGS
5893 Waits for a particular child process to terminate and returns the pid of
5894 the deceased process, or C<-1> if there is no such child process. On some
5895 systems, a value of 0 indicates that there are processes still running.
5896 The status is returned in C<$?>. If you say
5898 use POSIX ":sys_wait_h";
5901 $kid = waitpid(-1,&WNOHANG);
5904 then you can do a non-blocking wait for all pending zombie processes.
5905 Non-blocking wait is available on machines supporting either the
5906 waitpid(2) or wait4(2) system calls. However, waiting for a particular
5907 pid with FLAGS of C<0> is implemented everywhere. (Perl emulates the
5908 system call by remembering the status values of processes that have
5909 exited but have not been harvested by the Perl script yet.)
5911 Note that on some systems, a return value of C<-1> could mean that child
5912 processes are being automatically reaped. See L<perlipc> for details,
5913 and for other examples.
5917 Returns true if the context of the currently executing subroutine is
5918 looking for a list value. Returns false if the context is looking
5919 for a scalar. Returns the undefined value if the context is looking
5920 for no value (void context).
5922 return unless defined wantarray; # don't bother doing more
5923 my @a = complex_calculation();
5924 return wantarray ? @a : "@a";
5926 This function should have been named wantlist() instead.
5930 Produces a message on STDERR just like C<die>, but doesn't exit or throw
5933 If LIST is empty and C<$@> already contains a value (typically from a
5934 previous eval) that value is used after appending C<"\t...caught">
5935 to C<$@>. This is useful for staying almost, but not entirely similar to
5938 If C<$@> is empty then the string C<"Warning: Something's wrong"> is used.
5940 No message is printed if there is a C<$SIG{__WARN__}> handler
5941 installed. It is the handler's responsibility to deal with the message
5942 as it sees fit (like, for instance, converting it into a C<die>). Most
5943 handlers must therefore make arrangements to actually display the
5944 warnings that they are not prepared to deal with, by calling C<warn>
5945 again in the handler. Note that this is quite safe and will not
5946 produce an endless loop, since C<__WARN__> hooks are not called from
5949 You will find this behavior is slightly different from that of
5950 C<$SIG{__DIE__}> handlers (which don't suppress the error text, but can
5951 instead call C<die> again to change it).
5953 Using a C<__WARN__> handler provides a powerful way to silence all
5954 warnings (even the so-called mandatory ones). An example:
5956 # wipe out *all* compile-time warnings
5957 BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } }
5959 my $foo = 20; # no warning about duplicate my $foo,
5960 # but hey, you asked for it!
5961 # no compile-time or run-time warnings before here
5964 # run-time warnings enabled after here
5965 warn "\$foo is alive and $foo!"; # does show up
5967 See L<perlvar> for details on setting C<%SIG> entries, and for more
5968 examples. See the Carp module for other kinds of warnings using its
5969 carp() and cluck() functions.
5971 =item write FILEHANDLE
5977 Writes a formatted record (possibly multi-line) to the specified FILEHANDLE,
5978 using the format associated with that file. By default the format for
5979 a file is the one having the same name as the filehandle, but the
5980 format for the current output channel (see the C<select> function) may be set
5981 explicitly by assigning the name of the format to the C<$~> variable.
5983 Top of form processing is handled automatically: if there is
5984 insufficient room on the current page for the formatted record, the
5985 page is advanced by writing a form feed, a special top-of-page format
5986 is used to format the new page header, and then the record is written.
5987 By default the top-of-page format is the name of the filehandle with
5988 "_TOP" appended, but it may be dynamically set to the format of your
5989 choice by assigning the name to the C<$^> variable while the filehandle is
5990 selected. The number of lines remaining on the current page is in
5991 variable C<$->, which can be set to C<0> to force a new page.
5993 If FILEHANDLE is unspecified, output goes to the current default output
5994 channel, which starts out as STDOUT but may be changed by the
5995 C<select> operator. If the FILEHANDLE is an EXPR, then the expression
5996 is evaluated and the resulting string is used to look up the name of
5997 the FILEHANDLE at run time. For more on formats, see L<perlform>.
5999 Note that write is I<not> the opposite of C<read>. Unfortunately.
6003 The transliteration operator. Same as C<tr///>. See L<perlop>.