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<package>, C<use>
151 =item Miscellaneous functions
153 C<defined>, C<dump>, C<eval>, C<formline>, C<local>, C<my>, 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<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<prototype>, C<qx>,
204 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<socket>, C<socketpair>, C<stat>, C<symlink>, C<syscall>,
238 C<sysopen>, C<system>, C<times>, C<truncate>, C<umask>, C<unlink>,
239 C<utime>, C<wait>, C<waitpid>
241 For more information about the portability of these functions, see
242 L<perlport> and other available platform-specific documentation.
244 =head2 Alphabetical Listing of Perl Functions
248 =item I<-X> FILEHANDLE
254 A file test, where X is one of the letters listed below. This unary
255 operator takes one argument, either a filename or a filehandle, and
256 tests the associated file to see if something is true about it. If the
257 argument is omitted, tests C<$_>, except for C<-t>, which tests STDIN.
258 Unless otherwise documented, it returns C<1> for true and C<''> for false, or
259 the undefined value if the file doesn't exist. Despite the funny
260 names, precedence is the same as any other named unary operator, and
261 the argument may be parenthesized like any other unary operator. The
262 operator may be any of:
263 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>
264 X<-S>X<-b>X<-c>X<-t>X<-u>X<-g>X<-k>X<-T>X<-B>X<-M>X<-A>X<-C>
266 -r File is readable by effective uid/gid.
267 -w File is writable by effective uid/gid.
268 -x File is executable by effective uid/gid.
269 -o File is owned by effective uid.
271 -R File is readable by real uid/gid.
272 -W File is writable by real uid/gid.
273 -X File is executable by real uid/gid.
274 -O File is owned by real uid.
277 -z File has zero size.
278 -s File has nonzero size (returns size).
280 -f File is a plain file.
281 -d File is a directory.
282 -l File is a symbolic link.
283 -p File is a named pipe (FIFO), or Filehandle is a pipe.
285 -b File is a block special file.
286 -c File is a character special file.
287 -t Filehandle is opened to a tty.
289 -u File has setuid bit set.
290 -g File has setgid bit set.
291 -k File has sticky bit set.
293 -T File is an ASCII text file.
294 -B File is a "binary" file (opposite of -T).
296 -M Age of file in days when script started.
297 -A Same for access time.
298 -C Same for inode change time.
304 next unless -f $_; # ignore specials
308 The interpretation of the file permission operators C<-r>, C<-R>,
309 C<-w>, C<-W>, C<-x>, and C<-X> is by default based solely on the mode
310 of the file and the uids and gids of the user. There may be other
311 reasons you can't actually read, write, or execute the file. Such
312 reasons may be for example network filesystem access controls, ACLs
313 (access control lists), read-only filesystems, and unrecognized
316 Also note that, for the superuser on the local filesystems, the C<-r>,
317 C<-R>, C<-w>, and C<-W> tests always return 1, and C<-x> and C<-X> return 1
318 if any execute bit is set in the mode. Scripts run by the superuser
319 may thus need to do a stat() to determine the actual mode of the file,
320 or temporarily set their effective uid to something else.
322 If you are using ACLs, there is a pragma called C<filetest> that may
323 produce more accurate results than the bare stat() mode bits.
324 When under the C<use filetest 'access'> the above-mentioned filetests
325 will test whether the permission can (not) be granted using the
326 access() family of system calls. Also note that the C<-x> and C<-X> may
327 under this pragma return true even if there are no execute permission
328 bits set (nor any extra execute permission ACLs). This strangeness is
329 due to the underlying system calls' definitions. Read the
330 documentation for the C<filetest> pragma for more information.
332 Note that C<-s/a/b/> does not do a negated substitution. Saying
333 C<-exp($foo)> still works as expected, however--only single letters
334 following a minus are interpreted as file tests.
336 The C<-T> and C<-B> switches work as follows. The first block or so of the
337 file is examined for odd characters such as strange control codes or
338 characters with the high bit set. If too many strange characters (>30%)
339 are found, it's a C<-B> file, otherwise it's a C<-T> file. Also, any file
340 containing null in the first block is considered a binary file. If C<-T>
341 or C<-B> is used on a filehandle, the current stdio buffer is examined
342 rather than the first block. Both C<-T> and C<-B> return true on a null
343 file, or a file at EOF when testing a filehandle. Because you have to
344 read a file to do the C<-T> test, on most occasions you want to use a C<-f>
345 against the file first, as in C<next unless -f $file && -T $file>.
347 If any of the file tests (or either the C<stat> or C<lstat> operators) are given
348 the special filehandle consisting of a solitary underline, then the stat
349 structure of the previous file test (or stat operator) is used, saving
350 a system call. (This doesn't work with C<-t>, and you need to remember
351 that lstat() and C<-l> will leave values in the stat structure for the
352 symbolic link, not the real file.) Example:
354 print "Can do.\n" if -r $a || -w _ || -x _;
357 print "Readable\n" if -r _;
358 print "Writable\n" if -w _;
359 print "Executable\n" if -x _;
360 print "Setuid\n" if -u _;
361 print "Setgid\n" if -g _;
362 print "Sticky\n" if -k _;
363 print "Text\n" if -T _;
364 print "Binary\n" if -B _;
370 Returns the absolute value of its argument.
371 If VALUE is omitted, uses C<$_>.
373 =item accept NEWSOCKET,GENERICSOCKET
375 Accepts an incoming socket connect, just as the accept(2) system call
376 does. Returns the packed address if it succeeded, false otherwise.
377 See the example in L<perlipc/"Sockets: Client/Server Communication">.
379 On systems that support a close-on-exec flag on files, the flag will
380 be set for the newly opened file descriptor, as determined by the
381 value of $^F. See L<perlvar/$^F>.
387 Arranges to have a SIGALRM delivered to this process after the
388 specified number of seconds have elapsed. If SECONDS is not specified,
389 the value stored in C<$_> is used. (On some machines,
390 unfortunately, the elapsed time may be up to one second less than you
391 specified because of how seconds are counted.) Only one timer may be
392 counting at once. Each call disables the previous timer, and an
393 argument of C<0> may be supplied to cancel the previous timer without
394 starting a new one. The returned value is the amount of time remaining
395 on the previous timer.
397 For delays of finer granularity than one second, you may use Perl's
398 four-argument version of select() leaving the first three arguments
399 undefined, or you might be able to use the C<syscall> interface to
400 access setitimer(2) if your system supports it. The Time::HiRes module
401 from CPAN may also prove useful.
403 It is usually a mistake to intermix C<alarm> and C<sleep> calls.
404 (C<sleep> may be internally implemented in your system with C<alarm>)
406 If you want to use C<alarm> to time out a system call you need to use an
407 C<eval>/C<die> pair. You can't rely on the alarm causing the system call to
408 fail with C<$!> set to C<EINTR> because Perl sets up signal handlers to
409 restart system calls on some systems. Using C<eval>/C<die> always works,
410 modulo the caveats given in L<perlipc/"Signals">.
413 local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
415 $nread = sysread SOCKET, $buffer, $size;
419 die unless $@ eq "alarm\n"; # propagate unexpected errors
428 Returns the arctangent of Y/X in the range -PI to PI.
430 For the tangent operation, you may use the C<Math::Trig::tan>
431 function, or use the familiar relation:
433 sub tan { sin($_[0]) / cos($_[0]) }
435 =item bind SOCKET,NAME
437 Binds a network address to a socket, just as the bind system call
438 does. Returns true if it succeeded, false otherwise. NAME should be a
439 packed address of the appropriate type for the socket. See the examples in
440 L<perlipc/"Sockets: Client/Server Communication">.
442 =item binmode FILEHANDLE, DISCIPLINE
444 =item binmode FILEHANDLE
446 Arranges for FILEHANDLE to be read or written in "binary" or "text" mode
447 on systems where the run-time libraries distinguish between binary and
448 text files. If FILEHANDLE is an expression, the value is taken as the
449 name of the filehandle. DISCIPLINE can be either of C<":raw"> for
450 binary mode or C<":crlf"> for "text" mode. If the DISCIPLINE is
451 omitted, it defaults to C<":raw">.
453 binmode() should be called after open() but before any I/O is done on
456 On many systems binmode() currently has no effect, but in future, it
457 will be extended to support user-defined input and output disciplines.
458 On some systems binmode() is necessary when you're not working with a
459 text file. For the sake of portability it is a good idea to always use
460 it when appropriate, and to never use it when it isn't appropriate.
462 In other words: Regardless of platform, use binmode() on binary
463 files, and do not use binmode() on text files.
465 The C<open> pragma can be used to establish default disciplines.
468 The operating system, device drivers, C libraries, and Perl run-time
469 system all work together to let the programmer treat a single
470 character (C<\n>) as the line terminator, irrespective of the external
471 representation. On many operating systems, the native text file
472 representation matches the internal representation, but on some
473 platforms the external representation of C<\n> is made up of more than
476 Mac OS and all variants of Unix use a single character to end each line
477 in the external representation of text (even though that single
478 character is not necessarily the same across these platforms).
479 Consequently binmode() has no effect on these operating systems. In
480 other systems like VMS, MS-DOS and the various flavors of MS-Windows
481 your program sees a C<\n> as a simple C<\cJ>, but what's stored in text
482 files are the two characters C<\cM\cJ>. That means that, if you don't
483 use binmode() on these systems, C<\cM\cJ> sequences on disk will be
484 converted to C<\n> on input, and any C<\n> in your program will be
485 converted back to C<\cM\cJ> on output. This is what you want for text
486 files, but it can be disastrous for binary files.
488 Another consequence of using binmode() (on some systems) is that
489 special end-of-file markers will be seen as part of the data stream.
490 For systems from the Microsoft family this means that if your binary
491 data contains C<\cZ>, the I/O subsystem will ragard it as the end of
492 the file, unless you use binmode().
494 binmode() is not only important for readline() and print() operations,
495 but also when using read(), seek(), sysread(), syswrite() and tell()
496 (see L<perlport> for more details). See the C<$/> and C<$\> variables
497 in L<perlvar> for how to manually set your input and output
498 line-termination sequences.
500 =item bless REF,CLASSNAME
504 This function tells the thingy referenced by REF that it is now an object
505 in the CLASSNAME package. If CLASSNAME is omitted, the current package
506 is used. Because a C<bless> is often the last thing in a constructor,
507 it returns the reference for convenience. Always use the two-argument
508 version if the function doing the blessing might be inherited by a
509 derived class. See L<perltoot> and L<perlobj> for more about the blessing
510 (and blessings) of objects.
512 Consider always blessing objects in CLASSNAMEs that are mixed case.
513 Namespaces with all lowercase names are considered reserved for
514 Perl pragmata. Builtin types have all uppercase names, so to prevent
515 confusion, you may wish to avoid such package names as well. Make sure
516 that CLASSNAME is a true value.
518 See L<perlmod/"Perl Modules">.
524 Returns the context of the current subroutine call. In scalar context,
525 returns the caller's package name if there is a caller, that is, if
526 we're in a subroutine or C<eval> or C<require>, and the undefined value
527 otherwise. In list context, returns
529 ($package, $filename, $line) = caller;
531 With EXPR, it returns some extra information that the debugger uses to
532 print a stack trace. The value of EXPR indicates how many call frames
533 to go back before the current one.
535 ($package, $filename, $line, $subroutine, $hasargs,
536 $wantarray, $evaltext, $is_require, $hints, $bitmask) = caller($i);
538 Here $subroutine may be C<(eval)> if the frame is not a subroutine
539 call, but an C<eval>. In such a case additional elements $evaltext and
540 C<$is_require> are set: C<$is_require> is true if the frame is created by a
541 C<require> or C<use> statement, $evaltext contains the text of the
542 C<eval EXPR> statement. In particular, for a C<eval BLOCK> statement,
543 $filename is C<(eval)>, but $evaltext is undefined. (Note also that
544 each C<use> statement creates a C<require> frame inside an C<eval EXPR>)
545 frame. C<$hints> and C<$bitmask> contain pragmatic hints that the caller
546 was compiled with. The C<$hints> and C<$bitmask> values are subject to
547 change between versions of Perl, and are not meant for external use.
549 Furthermore, when called from within the DB package, caller returns more
550 detailed information: it sets the list variable C<@DB::args> to be the
551 arguments with which the subroutine was invoked.
553 Be aware that the optimizer might have optimized call frames away before
554 C<caller> had a chance to get the information. That means that C<caller(N)>
555 might not return information about the call frame you expect it do, for
556 C<< N > 1 >>. In particular, C<@DB::args> might have information from the
557 previous time C<caller> was called.
561 Changes the working directory to EXPR, if possible. If EXPR is omitted,
562 changes to the directory specified by C<$ENV{HOME}>, if set; if not,
563 changes to the directory specified by C<$ENV{LOGDIR}>. If neither is
564 set, C<chdir> does nothing. It returns true upon success, false
565 otherwise. See the example under C<die>.
569 Changes the permissions of a list of files. The first element of the
570 list must be the numerical mode, which should probably be an octal
571 number, and which definitely should I<not> a string of octal digits:
572 C<0644> is okay, C<'0644'> is not. Returns the number of files
573 successfully changed. See also L</oct>, if all you have is a string.
575 $cnt = chmod 0755, 'foo', 'bar';
576 chmod 0755, @executables;
577 $mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to
579 $mode = '0644'; chmod oct($mode), 'foo'; # this is better
580 $mode = 0644; chmod $mode, 'foo'; # this is best
582 You can also import the symbolic C<S_I*> constants from the Fcntl
587 chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
588 # This is identical to the chmod 0755 of the above example.
596 This safer version of L</chop> removes any trailing string
597 that corresponds to the current value of C<$/> (also known as
598 $INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
599 number of characters removed from all its arguments. It's often used to
600 remove the newline from the end of an input record when you're worried
601 that the final record may be missing its newline. When in paragraph
602 mode (C<$/ = "">), it removes all trailing newlines from the string.
603 When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is
604 a reference to an integer or the like, see L<perlvar>) chomp() won't
606 If VARIABLE is omitted, it chomps C<$_>. Example:
609 chomp; # avoid \n on last field
614 You can actually chomp anything that's an lvalue, including an assignment:
617 chomp($answer = <STDIN>);
619 If you chomp a list, each element is chomped, and the total number of
620 characters removed is returned.
628 Chops off the last character of a string and returns the character
629 chopped. It's used primarily to remove the newline from the end of an
630 input record, but is much more efficient than C<s/\n//> because it neither
631 scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
635 chop; # avoid \n on last field
640 You can actually chop anything that's an lvalue, including an assignment:
643 chop($answer = <STDIN>);
645 If you chop a list, each element is chopped. Only the value of the
646 last C<chop> is returned.
648 Note that C<chop> returns the last character. To return all but the last
649 character, use C<substr($string, 0, -1)>.
653 Changes the owner (and group) of a list of files. The first two
654 elements of the list must be the I<numeric> uid and gid, in that
655 order. A value of -1 in either position is interpreted by most
656 systems to leave that value unchanged. Returns the number of files
657 successfully changed.
659 $cnt = chown $uid, $gid, 'foo', 'bar';
660 chown $uid, $gid, @filenames;
662 Here's an example that looks up nonnumeric uids in the passwd file:
665 chomp($user = <STDIN>);
667 chomp($pattern = <STDIN>);
669 ($login,$pass,$uid,$gid) = getpwnam($user)
670 or die "$user not in passwd file";
672 @ary = glob($pattern); # expand filenames
673 chown $uid, $gid, @ary;
675 On most systems, you are not allowed to change the ownership of the
676 file unless you're the superuser, although you should be able to change
677 the group to any of your secondary groups. On insecure systems, these
678 restrictions may be relaxed, but this is not a portable assumption.
679 On POSIX systems, you can detect this condition this way:
681 use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
682 $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
688 Returns the character represented by that NUMBER in the character set.
689 For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
690 chr(0x263a) is a Unicode smiley face (but only within the scope of
691 a C<use utf8>). For the reverse, use L</ord>.
692 See L<utf8> for more about Unicode.
694 If NUMBER is omitted, uses C<$_>.
696 =item chroot FILENAME
700 This function works like the system call by the same name: it makes the
701 named directory the new root directory for all further pathnames that
702 begin with a C</> by your process and all its children. (It doesn't
703 change your current working directory, which is unaffected.) For security
704 reasons, this call is restricted to the superuser. If FILENAME is
705 omitted, does a C<chroot> to C<$_>.
707 =item close FILEHANDLE
711 Closes the file or pipe associated with the file handle, returning true
712 only if stdio successfully flushes buffers and closes the system file
713 descriptor. Closes the currently selected filehandle if the argument
716 You don't have to close FILEHANDLE if you are immediately going to do
717 another C<open> on it, because C<open> will close it for you. (See
718 C<open>.) However, an explicit C<close> on an input file resets the line
719 counter (C<$.>), while the implicit close done by C<open> does not.
721 If the file handle came from a piped open C<close> will additionally
722 return false if one of the other system calls involved fails or if the
723 program exits with non-zero status. (If the only problem was that the
724 program exited non-zero C<$!> will be set to C<0>.) Closing a pipe
725 also waits for the process executing on the pipe to complete, in case you
726 want to look at the output of the pipe afterwards, and
727 implicitly puts the exit status value of that command into C<$?>.
729 Prematurely closing the read end of a pipe (i.e. before the process
730 writing to it at the other end has closed it) will result in a
731 SIGPIPE being delivered to the writer. If the other end can't
732 handle that, be sure to read all the data before closing the pipe.
736 open(OUTPUT, '|sort >foo') # pipe to sort
737 or die "Can't start sort: $!";
738 #... # print stuff to output
739 close OUTPUT # wait for sort to finish
740 or warn $! ? "Error closing sort pipe: $!"
741 : "Exit status $? from sort";
742 open(INPUT, 'foo') # get sort's results
743 or die "Can't open 'foo' for input: $!";
745 FILEHANDLE may be an expression whose value can be used as an indirect
746 filehandle, usually the real filehandle name.
748 =item closedir DIRHANDLE
750 Closes a directory opened by C<opendir> and returns the success of that
753 DIRHANDLE may be an expression whose value can be used as an indirect
754 dirhandle, usually the real dirhandle name.
756 =item connect SOCKET,NAME
758 Attempts to connect to a remote socket, just as the connect system call
759 does. Returns true if it succeeded, false otherwise. NAME should be a
760 packed address of the appropriate type for the socket. See the examples in
761 L<perlipc/"Sockets: Client/Server Communication">.
765 Actually a flow control statement rather than a function. If there is a
766 C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
767 C<foreach>), it is always executed just before the conditional is about to
768 be evaluated again, just like the third part of a C<for> loop in C. Thus
769 it can be used to increment a loop variable, even when the loop has been
770 continued via the C<next> statement (which is similar to the C C<continue>
773 C<last>, C<next>, or C<redo> may appear within a C<continue>
774 block. C<last> and C<redo> will behave as if they had been executed within
775 the main block. So will C<next>, but since it will execute a C<continue>
776 block, it may be more entertaining.
779 ### redo always comes here
782 ### next always comes here
784 # then back the top to re-check EXPR
786 ### last always comes here
788 Omitting the C<continue> section is semantically equivalent to using an
789 empty one, logically enough. In that case, C<next> goes directly back
790 to check the condition at the top of the loop.
794 Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
795 takes cosine of C<$_>.
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>.
918 Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
919 used to report whether memory for that aggregate has ever been
920 allocated. This behavior may disappear in future versions of Perl.
921 You should instead use a simple test for size:
923 if (@an_array) { print "has array elements\n" }
924 if (%a_hash) { print "has hash members\n" }
926 When used on a hash element, it tells you whether the value is defined,
927 not whether the key exists in the hash. Use L</exists> for the latter
932 print if defined $switch{'D'};
933 print "$val\n" while defined($val = pop(@ary));
934 die "Can't readlink $sym: $!"
935 unless defined($value = readlink $sym);
936 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
937 $debugging = 0 unless defined $debugging;
939 Note: Many folks tend to overuse C<defined>, and then are surprised to
940 discover that the number C<0> and C<""> (the zero-length string) are, in fact,
941 defined values. For example, if you say
945 The pattern match succeeds, and C<$1> is defined, despite the fact that it
946 matched "nothing". But it didn't really match nothing--rather, it
947 matched something that happened to be zero characters long. This is all
948 very above-board and honest. When a function returns an undefined value,
949 it's an admission that it couldn't give you an honest answer. So you
950 should use C<defined> only when you're questioning the integrity of what
951 you're trying to do. At other times, a simple comparison to C<0> or C<""> is
954 See also L</undef>, L</exists>, L</ref>.
958 Given an expression that specifies a hash element, array element, hash slice,
959 or array slice, deletes the specified element(s) from the hash or array.
960 In the case of an array, if the array elements happen to be at the end,
961 the size of the array will shrink to the highest element that tests
962 true for exists() (or 0 if no such element exists).
964 Returns each element so deleted or the undefined value if there was no such
965 element. Deleting from C<$ENV{}> modifies the environment. Deleting from
966 a hash tied to a DBM file deletes the entry from the DBM file. Deleting
967 from a C<tie>d hash or array may not necessarily return anything.
969 Deleting an array element effectively returns that position of the array
970 to its initial, uninitialized state. Subsequently testing for the same
971 element with exists() will return false. Note that deleting array
972 elements in the middle of an array will not shift the index of the ones
973 after them down--use splice() for that. See L</exists>.
975 The following (inefficiently) deletes all the values of %HASH and @ARRAY:
977 foreach $key (keys %HASH) {
981 foreach $index (0 .. $#ARRAY) {
982 delete $ARRAY[$index];
987 delete @HASH{keys %HASH};
989 delete @ARRAY[0 .. $#ARRAY];
991 But both of these are slower than just assigning the empty list
992 or undefining %HASH or @ARRAY:
994 %HASH = (); # completely empty %HASH
995 undef %HASH; # forget %HASH ever existed
997 @ARRAY = (); # completely empty @ARRAY
998 undef @ARRAY; # forget @ARRAY ever existed
1000 Note that the EXPR can be arbitrarily complicated as long as the final
1001 operation is a hash element, array element, hash slice, or array slice
1004 delete $ref->[$x][$y]{$key};
1005 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
1007 delete $ref->[$x][$y][$index];
1008 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
1012 Outside an C<eval>, prints the value of LIST to C<STDERR> and
1013 exits with the current value of C<$!> (errno). If C<$!> is C<0>,
1014 exits with the value of C<<< ($? >> 8) >>> (backtick `command`
1015 status). If C<<< ($? >> 8) >>> is C<0>, exits with C<255>. Inside
1016 an C<eval(),> the error message is stuffed into C<$@> and the
1017 C<eval> is terminated with the undefined value. This makes
1018 C<die> the way to raise an exception.
1020 Equivalent examples:
1022 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
1023 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
1025 If the value of EXPR does not end in a newline, the current script line
1026 number and input line number (if any) are also printed, and a newline
1027 is supplied. Note that the "input line number" (also known as "chunk")
1028 is subject to whatever notion of "line" happens to be currently in
1029 effect, and is also available as the special variable C<$.>.
1030 See L<perlvar/"$/"> and L<perlvar/"$.">.
1032 Hint: sometimes appending C<", stopped"> to your message
1033 will cause it to make better sense when the string C<"at foo line 123"> is
1034 appended. Suppose you are running script "canasta".
1036 die "/etc/games is no good";
1037 die "/etc/games is no good, stopped";
1039 produce, respectively
1041 /etc/games is no good at canasta line 123.
1042 /etc/games is no good, stopped at canasta line 123.
1044 See also exit(), warn(), and the Carp module.
1046 If LIST is empty and C<$@> already contains a value (typically from a
1047 previous eval) that value is reused after appending C<"\t...propagated">.
1048 This is useful for propagating exceptions:
1051 die unless $@ =~ /Expected exception/;
1053 If C<$@> is empty then the string C<"Died"> is used.
1055 die() can also be called with a reference argument. If this happens to be
1056 trapped within an eval(), $@ contains the reference. This behavior permits
1057 a more elaborate exception handling implementation using objects that
1058 maintain arbitary state about the nature of the exception. Such a scheme
1059 is sometimes preferable to matching particular string values of $@ using
1060 regular expressions. Here's an example:
1062 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
1064 if (ref($@) && UNIVERSAL::isa($@,"Some::Module::Exception")) {
1065 # handle Some::Module::Exception
1068 # handle all other possible exceptions
1072 Because perl will stringify uncaught exception messages before displaying
1073 them, you may want to overload stringification operations on such custom
1074 exception objects. See L<overload> for details about that.
1076 You can arrange for a callback to be run just before the C<die>
1077 does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
1078 handler will be called with the error text and can change the error
1079 message, if it sees fit, by calling C<die> again. See
1080 L<perlvar/$SIG{expr}> for details on setting C<%SIG> entries, and
1081 L<"eval BLOCK"> for some examples. Although this feature was meant
1082 to be run only right before your program was to exit, this is not
1083 currently the case--the C<$SIG{__DIE__}> hook is currently called
1084 even inside eval()ed blocks/strings! If one wants the hook to do
1085 nothing in such situations, put
1089 as the first line of the handler (see L<perlvar/$^S>). Because
1090 this promotes strange action at a distance, this counterintuitive
1091 behavior may be fixed in a future release.
1095 Not really a function. Returns the value of the last command in the
1096 sequence of commands indicated by BLOCK. When modified by a loop
1097 modifier, executes the BLOCK once before testing the loop condition.
1098 (On other statements the loop modifiers test the conditional first.)
1100 C<do BLOCK> does I<not> count as a loop, so the loop control statements
1101 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1102 See L<perlsyn> for alternative strategies.
1104 =item do SUBROUTINE(LIST)
1106 A deprecated form of subroutine call. See L<perlsub>.
1110 Uses the value of EXPR as a filename and executes the contents of the
1111 file as a Perl script. Its primary use is to include subroutines
1112 from a Perl subroutine library.
1118 scalar eval `cat stat.pl`;
1120 except that it's more efficient and concise, keeps track of the current
1121 filename for error messages, searches the @INC libraries, and updates
1122 C<%INC> if the file is found. See L<perlvar/Predefined Names> for these
1123 variables. It also differs in that code evaluated with C<do FILENAME>
1124 cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1125 same, however, in that it does reparse the file every time you call it,
1126 so you probably don't want to do this inside a loop.
1128 If C<do> cannot read the file, it returns undef and sets C<$!> to the
1129 error. If C<do> can read the file but cannot compile it, it
1130 returns undef and sets an error message in C<$@>. If the file is
1131 successfully compiled, C<do> returns the value of the last expression
1134 Note that inclusion of library modules is better done with the
1135 C<use> and C<require> operators, which also do automatic error checking
1136 and raise an exception if there's a problem.
1138 You might like to use C<do> to read in a program configuration
1139 file. Manual error checking can be done this way:
1141 # read in config files: system first, then user
1142 for $file ("/share/prog/defaults.rc",
1143 "$ENV{HOME}/.someprogrc")
1145 unless ($return = do $file) {
1146 warn "couldn't parse $file: $@" if $@;
1147 warn "couldn't do $file: $!" unless defined $return;
1148 warn "couldn't run $file" unless $return;
1156 This function causes an immediate core dump. See also the B<-u>
1157 command-line switch in L<perlrun>, which does the same thing.
1158 Primarily this is so that you can use the B<undump> program (not
1159 supplied) to turn your core dump into an executable binary after
1160 having initialized all your variables at the beginning of the
1161 program. When the new binary is executed it will begin by executing
1162 a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1163 Think of it as a goto with an intervening core dump and reincarnation.
1164 If C<LABEL> is omitted, restarts the program from the top.
1166 B<WARNING>: Any files opened at the time of the dump will I<not>
1167 be open any more when the program is reincarnated, with possible
1168 resulting confusion on the part of Perl.
1170 This function is now largely obsolete, partly because it's very
1171 hard to convert a core file into an executable, and because the
1172 real compiler backends for generating portable bytecode and compilable
1173 C code have superseded it.
1175 If you're looking to use L<dump> to speed up your program, consider
1176 generating bytecode or native C code as described in L<perlcc>. If
1177 you're just trying to accelerate a CGI script, consider using the
1178 C<mod_perl> extension to B<Apache>, or the CPAN module, Fast::CGI.
1179 You might also consider autoloading or selfloading, which at least
1180 make your program I<appear> to run faster.
1184 When called in list context, returns a 2-element list consisting of the
1185 key and value for the next element of a hash, so that you can iterate over
1186 it. When called in scalar context, returns the key for only the "next"
1187 element in the hash.
1189 Entries are returned in an apparently random order. The actual random
1190 order is subject to change in future versions of perl, but it is guaranteed
1191 to be in the same order as either the C<keys> or C<values> function
1192 would produce on the same (unmodified) hash.
1194 When the hash is entirely read, a null array is returned in list context
1195 (which when assigned produces a false (C<0>) value), and C<undef> in
1196 scalar context. The next call to C<each> after that will start iterating
1197 again. There is a single iterator for each hash, shared by all C<each>,
1198 C<keys>, and C<values> function calls in the program; it can be reset by
1199 reading all the elements from the hash, or by evaluating C<keys HASH> or
1200 C<values HASH>. If you add or delete elements of a hash while you're
1201 iterating over it, you may get entries skipped or duplicated, so don't.
1203 The following prints out your environment like the printenv(1) program,
1204 only in a different order:
1206 while (($key,$value) = each %ENV) {
1207 print "$key=$value\n";
1210 See also C<keys>, C<values> and C<sort>.
1212 =item eof FILEHANDLE
1218 Returns 1 if the next read on FILEHANDLE will return end of file, or if
1219 FILEHANDLE is not open. FILEHANDLE may be an expression whose value
1220 gives the real filehandle. (Note that this function actually
1221 reads a character and then C<ungetc>s it, so isn't very useful in an
1222 interactive context.) Do not read from a terminal file (or call
1223 C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
1224 as terminals may lose the end-of-file condition if you do.
1226 An C<eof> without an argument uses the last file read. Using C<eof()>
1227 with empty parentheses is very different. It refers to the pseudo file
1228 formed from the files listed on the command line and accessed via the
1229 C<< <> >> operator. Since C<< <> >> isn't explicitly opened,
1230 as a normal filehandle is, an C<eof()> before C<< <> >> has been
1231 used will cause C<@ARGV> to be examined to determine if input is
1234 In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to
1235 detect the end of each file, C<eof()> will only detect the end of the
1236 last file. Examples:
1238 # reset line numbering on each input file
1240 next if /^\s*#/; # skip comments
1243 close ARGV if eof; # Not eof()!
1246 # insert dashes just before last line of last file
1248 if (eof()) { # check for end of current file
1249 print "--------------\n";
1250 close(ARGV); # close or last; is needed if we
1251 # are reading from the terminal
1256 Practical hint: you almost never need to use C<eof> in Perl, because the
1257 input operators typically return C<undef> when they run out of data, or if
1264 In the first form, the return value of EXPR is parsed and executed as if it
1265 were a little Perl program. The value of the expression (which is itself
1266 determined within scalar context) is first parsed, and if there weren't any
1267 errors, executed in the context of the current Perl program, so that any
1268 variable settings or subroutine and format definitions remain afterwards.
1269 Note that the value is parsed every time the eval executes. If EXPR is
1270 omitted, evaluates C<$_>. This form is typically used to delay parsing
1271 and subsequent execution of the text of EXPR until run time.
1273 In the second form, the code within the BLOCK is parsed only once--at the
1274 same time the code surrounding the eval itself was parsed--and executed
1275 within the context of the current Perl program. This form is typically
1276 used to trap exceptions more efficiently than the first (see below), while
1277 also providing the benefit of checking the code within BLOCK at compile
1280 The final semicolon, if any, may be omitted from the value of EXPR or within
1283 In both forms, the value returned is the value of the last expression
1284 evaluated inside the mini-program; a return statement may be also used, just
1285 as with subroutines. The expression providing the return value is evaluated
1286 in void, scalar, or list context, depending on the context of the eval itself.
1287 See L</wantarray> for more on how the evaluation context can be determined.
1289 If there is a syntax error or runtime error, or a C<die> statement is
1290 executed, an undefined value is returned by C<eval>, and C<$@> is set to the
1291 error message. If there was no error, C<$@> is guaranteed to be a null
1292 string. Beware that using C<eval> neither silences perl from printing
1293 warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
1294 To do either of those, you have to use the C<$SIG{__WARN__}> facility. See
1295 L</warn> and L<perlvar>.
1297 Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1298 determining whether a particular feature (such as C<socket> or C<symlink>)
1299 is implemented. It is also Perl's exception trapping mechanism, where
1300 the die operator is used to raise exceptions.
1302 If the code to be executed doesn't vary, you may use the eval-BLOCK
1303 form to trap run-time errors without incurring the penalty of
1304 recompiling each time. The error, if any, is still returned in C<$@>.
1307 # make divide-by-zero nonfatal
1308 eval { $answer = $a / $b; }; warn $@ if $@;
1310 # same thing, but less efficient
1311 eval '$answer = $a / $b'; warn $@ if $@;
1313 # a compile-time error
1314 eval { $answer = }; # WRONG
1317 eval '$answer ='; # sets $@
1319 Due to the current arguably broken state of C<__DIE__> hooks, when using
1320 the C<eval{}> form as an exception trap in libraries, you may wish not
1321 to trigger any C<__DIE__> hooks that user code may have installed.
1322 You can use the C<local $SIG{__DIE__}> construct for this purpose,
1323 as shown in this example:
1325 # a very private exception trap for divide-by-zero
1326 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1329 This is especially significant, given that C<__DIE__> hooks can call
1330 C<die> again, which has the effect of changing their error messages:
1332 # __DIE__ hooks may modify error messages
1334 local $SIG{'__DIE__'} =
1335 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
1336 eval { die "foo lives here" };
1337 print $@ if $@; # prints "bar lives here"
1340 Because this promotes action at a distance, this counterintuitive behavior
1341 may be fixed in a future release.
1343 With an C<eval>, you should be especially careful to remember what's
1344 being looked at when:
1350 eval { $x }; # CASE 4
1352 eval "\$$x++"; # CASE 5
1355 Cases 1 and 2 above behave identically: they run the code contained in
1356 the variable $x. (Although case 2 has misleading double quotes making
1357 the reader wonder what else might be happening (nothing is).) Cases 3
1358 and 4 likewise behave in the same way: they run the code C<'$x'>, which
1359 does nothing but return the value of $x. (Case 4 is preferred for
1360 purely visual reasons, but it also has the advantage of compiling at
1361 compile-time instead of at run-time.) Case 5 is a place where
1362 normally you I<would> like to use double quotes, except that in this
1363 particular situation, you can just use symbolic references instead, as
1366 C<eval BLOCK> does I<not> count as a loop, so the loop control statements
1367 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1371 =item exec PROGRAM LIST
1373 The C<exec> function executes a system command I<and never returns>--
1374 use C<system> instead of C<exec> if you want it to return. It fails and
1375 returns false only if the command does not exist I<and> it is executed
1376 directly instead of via your system's command shell (see below).
1378 Since it's a common mistake to use C<exec> instead of C<system>, Perl
1379 warns you if there is a following statement which isn't C<die>, C<warn>,
1380 or C<exit> (if C<-w> is set - but you always do that). If you
1381 I<really> want to follow an C<exec> with some other statement, you
1382 can use one of these styles to avoid the warning:
1384 exec ('foo') or print STDERR "couldn't exec foo: $!";
1385 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
1387 If there is more than one argument in LIST, or if LIST is an array
1388 with more than one value, calls execvp(3) with the arguments in LIST.
1389 If there is only one scalar argument or an array with one element in it,
1390 the argument is checked for shell metacharacters, and if there are any,
1391 the entire argument is passed to the system's command shell for parsing
1392 (this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1393 If there are no shell metacharacters in the argument, it is split into
1394 words and passed directly to C<execvp>, which is more efficient.
1397 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1398 exec "sort $outfile | uniq";
1400 If you don't really want to execute the first argument, but want to lie
1401 to the program you are executing about its own name, you can specify
1402 the program you actually want to run as an "indirect object" (without a
1403 comma) in front of the LIST. (This always forces interpretation of the
1404 LIST as a multivalued list, even if there is only a single scalar in
1407 $shell = '/bin/csh';
1408 exec $shell '-sh'; # pretend it's a login shell
1412 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1414 When the arguments get executed via the system shell, results will
1415 be subject to its quirks and capabilities. See L<perlop/"`STRING`">
1418 Using an indirect object with C<exec> or C<system> is also more
1419 secure. This usage (which also works fine with system()) forces
1420 interpretation of the arguments as a multivalued list, even if the
1421 list had just one argument. That way you're safe from the shell
1422 expanding wildcards or splitting up words with whitespace in them.
1424 @args = ( "echo surprise" );
1426 exec @args; # subject to shell escapes
1428 exec { $args[0] } @args; # safe even with one-arg list
1430 The first version, the one without the indirect object, ran the I<echo>
1431 program, passing it C<"surprise"> an argument. The second version
1432 didn't--it tried to run a program literally called I<"echo surprise">,
1433 didn't find it, and set C<$?> to a non-zero value indicating failure.
1435 Beginning with v5.6.0, Perl will attempt to flush all files opened for
1436 output before the exec, but this may not be supported on some platforms
1437 (see L<perlport>). To be safe, you may need to set C<$|> ($AUTOFLUSH
1438 in English) or call the C<autoflush()> method of C<IO::Handle> on any
1439 open handles in order to avoid lost output.
1441 Note that C<exec> will not call your C<END> blocks, nor will it call
1442 any C<DESTROY> methods in your objects.
1446 Given an expression that specifies a hash element or array element,
1447 returns true if the specified element in the hash or array has ever
1448 been initialized, even if the corresponding value is undefined. The
1449 element is not autovivified if it doesn't exist.
1451 print "Exists\n" if exists $hash{$key};
1452 print "Defined\n" if defined $hash{$key};
1453 print "True\n" if $hash{$key};
1455 print "Exists\n" if exists $array[$index];
1456 print "Defined\n" if defined $array[$index];
1457 print "True\n" if $array[$index];
1459 A hash or array element can be true only if it's defined, and defined if
1460 it exists, but the reverse doesn't necessarily hold true.
1462 Given an expression that specifies the name of a subroutine,
1463 returns true if the specified subroutine has ever been declared, even
1464 if it is undefined. Mentioning a subroutine name for exists or defined
1465 does not count as declaring it.
1467 print "Exists\n" if exists &subroutine;
1468 print "Defined\n" if defined &subroutine;
1470 Note that the EXPR can be arbitrarily complicated as long as the final
1471 operation is a hash or array key lookup or subroutine name:
1473 if (exists $ref->{A}->{B}->{$key}) { }
1474 if (exists $hash{A}{B}{$key}) { }
1476 if (exists $ref->{A}->{B}->[$ix]) { }
1477 if (exists $hash{A}{B}[$ix]) { }
1479 if (exists &{$ref->{A}{B}{$key}}) { }
1481 Although the deepest nested array or hash will not spring into existence
1482 just because its existence was tested, any intervening ones will.
1483 Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
1484 into existence due to the existence test for the $key element above.
1485 This happens anywhere the arrow operator is used, including even:
1488 if (exists $ref->{"Some key"}) { }
1489 print $ref; # prints HASH(0x80d3d5c)
1491 This surprising autovivification in what does not at first--or even
1492 second--glance appear to be an lvalue context may be fixed in a future
1495 See L<perlref/"Pseudo-hashes: Using an array as a hash"> for specifics
1496 on how exists() acts when used on a pseudo-hash.
1498 Use of a subroutine call, rather than a subroutine name, as an argument
1499 to exists() is an error.
1502 exists &sub(); # Error
1506 Evaluates EXPR and exits immediately with that value. Example:
1509 exit 0 if $ans =~ /^[Xx]/;
1511 See also C<die>. If EXPR is omitted, exits with C<0> status. The only
1512 universally recognized values for EXPR are C<0> for success and C<1>
1513 for error; other values are subject to interpretation depending on the
1514 environment in which the Perl program is running. For example, exiting
1515 69 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
1516 the mailer to return the item undelivered, but that's not true everywhere.
1518 Don't use C<exit> to abort a subroutine if there's any chance that
1519 someone might want to trap whatever error happened. Use C<die> instead,
1520 which can be trapped by an C<eval>.
1522 The exit() function does not always exit immediately. It calls any
1523 defined C<END> routines first, but these C<END> routines may not
1524 themselves abort the exit. Likewise any object destructors that need to
1525 be called are called before the real exit. If this is a problem, you
1526 can call C<POSIX:_exit($status)> to avoid END and destructor processing.
1527 See L<perlmod> for details.
1533 Returns I<e> (the natural logarithm base) to the power of EXPR.
1534 If EXPR is omitted, gives C<exp($_)>.
1536 =item fcntl FILEHANDLE,FUNCTION,SCALAR
1538 Implements the fcntl(2) function. You'll probably have to say
1542 first to get the correct constant definitions. Argument processing and
1543 value return works just like C<ioctl> below.
1547 fcntl($filehandle, F_GETFL, $packed_return_buffer)
1548 or die "can't fcntl F_GETFL: $!";
1550 You don't have to check for C<defined> on the return from C<fnctl>.
1551 Like C<ioctl>, it maps a C<0> return from the system call into
1552 C<"0 but true"> in Perl. This string is true in boolean context and C<0>
1553 in numeric context. It is also exempt from the normal B<-w> warnings
1554 on improper numeric conversions.
1556 Note that C<fcntl> will produce a fatal error if used on a machine that
1557 doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
1558 manpage to learn what functions are available on your system.
1560 =item fileno FILEHANDLE
1562 Returns the file descriptor for a filehandle, or undefined if the
1563 filehandle is not open. This is mainly useful for constructing
1564 bitmaps for C<select> and low-level POSIX tty-handling operations.
1565 If FILEHANDLE is an expression, the value is taken as an indirect
1566 filehandle, generally its name.
1568 You can use this to find out whether two handles refer to the
1569 same underlying descriptor:
1571 if (fileno(THIS) == fileno(THAT)) {
1572 print "THIS and THAT are dups\n";
1575 =item flock FILEHANDLE,OPERATION
1577 Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
1578 for success, false on failure. Produces a fatal error if used on a
1579 machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
1580 C<flock> is Perl's portable file locking interface, although it locks
1581 only entire files, not records.
1583 Two potentially non-obvious but traditional C<flock> semantics are
1584 that it waits indefinitely until the lock is granted, and that its locks
1585 B<merely advisory>. Such discretionary locks are more flexible, but offer
1586 fewer guarantees. This means that files locked with C<flock> may be
1587 modified by programs that do not also use C<flock>. See L<perlport>,
1588 your port's specific documentation, or your system-specific local manpages
1589 for details. It's best to assume traditional behavior if you're writing
1590 portable programs. (But if you're not, you should as always feel perfectly
1591 free to write for your own system's idiosyncrasies (sometimes called
1592 "features"). Slavish adherence to portability concerns shouldn't get
1593 in the way of your getting your job done.)
1595 OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1596 LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
1597 you can use the symbolic names if you import them from the Fcntl module,
1598 either individually, or as a group using the ':flock' tag. LOCK_SH
1599 requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
1600 releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
1601 LOCK_SH or LOCK_EX then C<flock> will return immediately rather than blocking
1602 waiting for the lock (check the return status to see if you got it).
1604 To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
1605 before locking or unlocking it.
1607 Note that the emulation built with lockf(3) doesn't provide shared
1608 locks, and it requires that FILEHANDLE be open with write intent. These
1609 are the semantics that lockf(3) implements. Most if not all systems
1610 implement lockf(3) in terms of fcntl(2) locking, though, so the
1611 differing semantics shouldn't bite too many people.
1613 Note also that some versions of C<flock> cannot lock things over the
1614 network; you would need to use the more system-specific C<fcntl> for
1615 that. If you like you can force Perl to ignore your system's flock(2)
1616 function, and so provide its own fcntl(2)-based emulation, by passing
1617 the switch C<-Ud_flock> to the F<Configure> program when you configure
1620 Here's a mailbox appender for BSD systems.
1622 use Fcntl ':flock'; # import LOCK_* constants
1625 flock(MBOX,LOCK_EX);
1626 # and, in case someone appended
1627 # while we were waiting...
1632 flock(MBOX,LOCK_UN);
1635 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1636 or die "Can't open mailbox: $!";
1639 print MBOX $msg,"\n\n";
1642 On systems that support a real flock(), locks are inherited across fork()
1643 calls, whereas those that must resort to the more capricious fcntl()
1644 function lose the locks, making it harder to write servers.
1646 See also L<DB_File> for other flock() examples.
1650 Does a fork(2) system call to create a new process running the
1651 same program at the same point. It returns the child pid to the
1652 parent process, C<0> to the child process, or C<undef> if the fork is
1653 unsuccessful. File descriptors (and sometimes locks on those descriptors)
1654 are shared, while everything else is copied. On most systems supporting
1655 fork(), great care has gone into making it extremely efficient (for
1656 example, using copy-on-write technology on data pages), making it the
1657 dominant paradigm for multitasking over the last few decades.
1659 Beginning with v5.6.0, Perl will attempt to flush all files opened for
1660 output before forking the child process, but this may not be supported
1661 on some platforms (see L<perlport>). To be safe, you may need to set
1662 C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
1663 C<IO::Handle> on any open handles in order to avoid duplicate output.
1665 If you C<fork> without ever waiting on your children, you will
1666 accumulate zombies. On some systems, you can avoid this by setting
1667 C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
1668 forking and reaping moribund children.
1670 Note that if your forked child inherits system file descriptors like
1671 STDIN and STDOUT that are actually connected by a pipe or socket, even
1672 if you exit, then the remote server (such as, say, a CGI script or a
1673 backgrounded job launched from a remote shell) won't think you're done.
1674 You should reopen those to F</dev/null> if it's any issue.
1678 Declare a picture format for use by the C<write> function. For
1682 Test: @<<<<<<<< @||||| @>>>>>
1683 $str, $%, '$' . int($num)
1687 $num = $cost/$quantity;
1691 See L<perlform> for many details and examples.
1693 =item formline PICTURE,LIST
1695 This is an internal function used by C<format>s, though you may call it,
1696 too. It formats (see L<perlform>) a list of values according to the
1697 contents of PICTURE, placing the output into the format output
1698 accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
1699 Eventually, when a C<write> is done, the contents of
1700 C<$^A> are written to some filehandle, but you could also read C<$^A>
1701 yourself and then set C<$^A> back to C<"">. Note that a format typically
1702 does one C<formline> per line of form, but the C<formline> function itself
1703 doesn't care how many newlines are embedded in the PICTURE. This means
1704 that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
1705 You may therefore need to use multiple formlines to implement a single
1706 record format, just like the format compiler.
1708 Be careful if you put double quotes around the picture, because an C<@>
1709 character may be taken to mean the beginning of an array name.
1710 C<formline> always returns true. See L<perlform> for other examples.
1712 =item getc FILEHANDLE
1716 Returns the next character from the input file attached to FILEHANDLE,
1717 or the undefined value at end of file, or if there was an error.
1718 If FILEHANDLE is omitted, reads from STDIN. This is not particularly
1719 efficient. However, it cannot be used by itself to fetch single
1720 characters without waiting for the user to hit enter. For that, try
1721 something more like:
1724 system "stty cbreak </dev/tty >/dev/tty 2>&1";
1727 system "stty", '-icanon', 'eol', "\001";
1733 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
1736 system "stty", 'icanon', 'eol', '^@'; # ASCII null
1740 Determination of whether $BSD_STYLE should be set
1741 is left as an exercise to the reader.
1743 The C<POSIX::getattr> function can do this more portably on
1744 systems purporting POSIX compliance. See also the C<Term::ReadKey>
1745 module from your nearest CPAN site; details on CPAN can be found on
1750 Implements the C library function of the same name, which on most
1751 systems returns the current login from F</etc/utmp>, if any. If null,
1754 $login = getlogin || getpwuid($<) || "Kilroy";
1756 Do not consider C<getlogin> for authentication: it is not as
1757 secure as C<getpwuid>.
1759 =item getpeername SOCKET
1761 Returns the packed sockaddr address of other end of the SOCKET connection.
1764 $hersockaddr = getpeername(SOCK);
1765 ($port, $iaddr) = sockaddr_in($hersockaddr);
1766 $herhostname = gethostbyaddr($iaddr, AF_INET);
1767 $herstraddr = inet_ntoa($iaddr);
1771 Returns the current process group for the specified PID. Use
1772 a PID of C<0> to get the current process group for the
1773 current process. Will raise an exception if used on a machine that
1774 doesn't implement getpgrp(2). If PID is omitted, returns process
1775 group of current process. Note that the POSIX version of C<getpgrp>
1776 does not accept a PID argument, so only C<PID==0> is truly portable.
1780 Returns the process id of the parent process.
1782 =item getpriority WHICH,WHO
1784 Returns the current priority for a process, a process group, or a user.
1785 (See L<getpriority(2)>.) Will raise a fatal exception if used on a
1786 machine that doesn't implement getpriority(2).
1792 =item gethostbyname NAME
1794 =item getnetbyname NAME
1796 =item getprotobyname NAME
1802 =item getservbyname NAME,PROTO
1804 =item gethostbyaddr ADDR,ADDRTYPE
1806 =item getnetbyaddr ADDR,ADDRTYPE
1808 =item getprotobynumber NUMBER
1810 =item getservbyport PORT,PROTO
1828 =item sethostent STAYOPEN
1830 =item setnetent STAYOPEN
1832 =item setprotoent STAYOPEN
1834 =item setservent STAYOPEN
1848 These routines perform the same functions as their counterparts in the
1849 system library. In list context, the return values from the
1850 various get routines are as follows:
1852 ($name,$passwd,$uid,$gid,
1853 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
1854 ($name,$passwd,$gid,$members) = getgr*
1855 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
1856 ($name,$aliases,$addrtype,$net) = getnet*
1857 ($name,$aliases,$proto) = getproto*
1858 ($name,$aliases,$port,$proto) = getserv*
1860 (If the entry doesn't exist you get a null list.)
1862 The exact meaning of the $gcos field varies but it usually contains
1863 the real name of the user (as opposed to the login name) and other
1864 information pertaining to the user. Beware, however, that in many
1865 system users are able to change this information and therefore it
1866 cannot be trusted and therefore the $gcos is is tainted (see
1867 L<perlsec>). The $passwd and $shell, user's encrypted password and
1868 login shell, are also tainted, because of the same reason.
1870 In scalar context, you get the name, unless the function was a
1871 lookup by name, in which case you get the other thing, whatever it is.
1872 (If the entry doesn't exist you get the undefined value.) For example:
1874 $uid = getpwnam($name);
1875 $name = getpwuid($num);
1877 $gid = getgrnam($name);
1878 $name = getgrgid($num;
1882 In I<getpw*()> the fields $quota, $comment, and $expire are special
1883 cases in the sense that in many systems they are unsupported. If the
1884 $quota is unsupported, it is an empty scalar. If it is supported, it
1885 usually encodes the disk quota. If the $comment field is unsupported,
1886 it is an empty scalar. If it is supported it usually encodes some
1887 administrative comment about the user. In some systems the $quota
1888 field may be $change or $age, fields that have to do with password
1889 aging. In some systems the $comment field may be $class. The $expire
1890 field, if present, encodes the expiration period of the account or the
1891 password. For the availability and the exact meaning of these fields
1892 in your system, please consult your getpwnam(3) documentation and your
1893 F<pwd.h> file. You can also find out from within Perl what your
1894 $quota and $comment fields mean and whether you have the $expire field
1895 by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
1896 C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password
1897 files are only supported if your vendor has implemented them in the
1898 intuitive fashion that calling the regular C library routines gets the
1899 shadow versions if you're running under privilege. Those that
1900 incorrectly implement a separate library call are not supported.
1902 The $members value returned by I<getgr*()> is a space separated list of
1903 the login names of the members of the group.
1905 For the I<gethost*()> functions, if the C<h_errno> variable is supported in
1906 C, it will be returned to you via C<$?> if the function call fails. The
1907 C<@addrs> value returned by a successful call is a list of the raw
1908 addresses returned by the corresponding system library call. In the
1909 Internet domain, each address is four bytes long and you can unpack it
1910 by saying something like:
1912 ($a,$b,$c,$d) = unpack('C4',$addr[0]);
1914 The Socket library makes this slightly easier:
1917 $iaddr = inet_aton("127.1"); # or whatever address
1918 $name = gethostbyaddr($iaddr, AF_INET);
1920 # or going the other way
1921 $straddr = inet_ntoa($iaddr);
1923 If you get tired of remembering which element of the return list
1924 contains which return value, by-name interfaces are provided
1925 in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
1926 C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
1927 and C<User::grent>. These override the normal built-ins, supplying
1928 versions that return objects with the appropriate names
1929 for each field. For example:
1933 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
1935 Even though it looks like they're the same method calls (uid),
1936 they aren't, because a C<File::stat> object is different from
1937 a C<User::pwent> object.
1939 =item getsockname SOCKET
1941 Returns the packed sockaddr address of this end of the SOCKET connection,
1942 in case you don't know the address because you have several different
1943 IPs that the connection might have come in on.
1946 $mysockaddr = getsockname(SOCK);
1947 ($port, $myaddr) = sockaddr_in($mysockaddr);
1948 printf "Connect to %s [%s]\n",
1949 scalar gethostbyaddr($myaddr, AF_INET),
1952 =item getsockopt SOCKET,LEVEL,OPTNAME
1954 Returns the socket option requested, or undef if there is an error.
1960 Returns the value of EXPR with filename expansions such as the
1961 standard Unix shell F</bin/csh> would do. This is the internal function
1962 implementing the C<< <*.c> >> operator, but you can use it directly.
1963 If EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is
1964 discussed in more detail in L<perlop/"I/O Operators">.
1966 Beginning with v5.6.0, this operator is implemented using the standard
1967 C<File::Glob> extension. See L<File::Glob> for details.
1971 Converts a time as returned by the time function to a 8-element list
1972 with the time localized for the standard Greenwich time zone.
1973 Typically used as follows:
1976 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday) =
1979 All list elements are numeric, and come straight out of the C `struct
1980 tm'. $sec, $min, and $hour are the seconds, minutes, and hours of the
1981 specified time. $mday is the day of the month, and $mon is the month
1982 itself, in the range C<0..11> with 0 indicating January and 11
1983 indicating December. $year is the number of years since 1900. That
1984 is, $year is C<123> in year 2023. $wday is the day of the week, with
1985 0 indicating Sunday and 3 indicating Wednesday. $yday is the day of
1986 the year, in the range C<1..365> (or C<1..366> in leap years.)
1988 Note that the $year element is I<not> simply the last two digits of
1989 the year. If you assume it is, then you create non-Y2K-compliant
1990 programs--and you wouldn't want to do that, would you?
1992 The proper way to get a complete 4-digit year is simply:
1996 And to get the last two digits of the year (e.g., '01' in 2001) do:
1998 $year = sprintf("%02d", $year % 100);
2000 If EXPR is omitted, C<gmtime()> uses the current time (C<gmtime(time)>).
2002 In scalar context, C<gmtime()> returns the ctime(3) value:
2004 $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
2006 Also see the C<timegm> function provided by the C<Time::Local> module,
2007 and the strftime(3) function available via the POSIX module.
2009 This scalar value is B<not> locale dependent (see L<perllocale>), but
2010 is instead a Perl builtin. Also see the C<Time::Local> module, and the
2011 strftime(3) and mktime(3) functions available via the POSIX module. To
2012 get somewhat similar but locale dependent date strings, set up your
2013 locale environment variables appropriately (please see L<perllocale>)
2014 and try for example:
2016 use POSIX qw(strftime);
2017 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
2019 Note that the C<%a> and C<%b> escapes, which represent the short forms
2020 of the day of the week and the month of the year, may not necessarily
2021 be three characters wide in all locales.
2029 The C<goto-LABEL> form finds the statement labeled with LABEL and resumes
2030 execution there. It may not be used to go into any construct that
2031 requires initialization, such as a subroutine or a C<foreach> loop. It
2032 also can't be used to go into a construct that is optimized away,
2033 or to get out of a block or subroutine given to C<sort>.
2034 It can be used to go almost anywhere else within the dynamic scope,
2035 including out of subroutines, but it's usually better to use some other
2036 construct such as C<last> or C<die>. The author of Perl has never felt the
2037 need to use this form of C<goto> (in Perl, that is--C is another matter).
2039 The C<goto-EXPR> form expects a label name, whose scope will be resolved
2040 dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
2041 necessarily recommended if you're optimizing for maintainability:
2043 goto ("FOO", "BAR", "GLARCH")[$i];
2045 The C<goto-&NAME> form is quite different from the other forms of C<goto>.
2046 In fact, it isn't a goto in the normal sense at all, and doesn't have
2047 the stigma associated with other gotos. Instead, it
2048 substitutes a call to the named subroutine for the currently running
2049 subroutine. This is used by C<AUTOLOAD> subroutines that wish to load
2050 another subroutine and then pretend that the other subroutine had been
2051 called in the first place (except that any modifications to C<@_>
2052 in the current subroutine are propagated to the other subroutine.)
2053 After the C<goto>, not even C<caller> will be able to tell that this
2054 routine was called first.
2056 NAME needn't be the name of a subroutine; it can be a scalar variable
2057 containing a code reference, or a block which evaluates to a code
2060 =item grep BLOCK LIST
2062 =item grep EXPR,LIST
2064 This is similar in spirit to, but not the same as, grep(1) and its
2065 relatives. In particular, it is not limited to using regular expressions.
2067 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2068 C<$_> to each element) and returns the list value consisting of those
2069 elements for which the expression evaluated to true. In scalar
2070 context, returns the number of times the expression was true.
2072 @foo = grep(!/^#/, @bar); # weed out comments
2076 @foo = grep {!/^#/} @bar; # weed out comments
2078 Note that, because C<$_> is a reference into the list value, it can
2079 be used to modify the elements of the array. While this is useful and
2080 supported, it can cause bizarre results if the LIST is not a named array.
2081 Similarly, grep returns aliases into the original list, much as a for
2082 loop's index variable aliases the list elements. That is, modifying an
2083 element of a list returned by grep (for example, in a C<foreach>, C<map>
2084 or another C<grep>) actually modifies the element in the original list.
2085 This is usually something to be avoided when writing clear code.
2087 See also L</map> for a list composed of the results of the BLOCK or EXPR.
2093 Interprets EXPR as a hex string and returns the corresponding value.
2094 (To convert strings that might start with either 0, 0x, or 0b, see
2095 L</oct>.) If EXPR is omitted, uses C<$_>.
2097 print hex '0xAf'; # prints '175'
2098 print hex 'aF'; # same
2100 Hex strings may only represent integers. Strings that would cause
2101 integer overflow trigger a warning.
2105 There is no builtin C<import> function. It is just an ordinary
2106 method (subroutine) defined (or inherited) by modules that wish to export
2107 names to another module. The C<use> function calls the C<import> method
2108 for the package used. See also L</use()>, L<perlmod>, and L<Exporter>.
2110 =item index STR,SUBSTR,POSITION
2112 =item index STR,SUBSTR
2114 The index function searches for one string within another, but without
2115 the wildcard-like behavior of a full regular-expression pattern match.
2116 It returns the position of the first occurrence of SUBSTR in STR at
2117 or after POSITION. If POSITION is omitted, starts searching from the
2118 beginning of the string. The return value is based at C<0> (or whatever
2119 you've set the C<$[> variable to--but don't do that). If the substring
2120 is not found, returns one less than the base, ordinarily C<-1>.
2126 Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
2127 You should not use this function for rounding: one because it truncates
2128 towards C<0>, and two because machine representations of floating point
2129 numbers can sometimes produce counterintuitive results. For example,
2130 C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
2131 because it's really more like -268.99999999999994315658 instead. Usually,
2132 the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
2133 functions will serve you better than will int().
2135 =item ioctl FILEHANDLE,FUNCTION,SCALAR
2137 Implements the ioctl(2) function. You'll probably first have to say
2139 require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph
2141 to get the correct function definitions. If F<ioctl.ph> doesn't
2142 exist or doesn't have the correct definitions you'll have to roll your
2143 own, based on your C header files such as F<< <sys/ioctl.h> >>.
2144 (There is a Perl script called B<h2ph> that comes with the Perl kit that
2145 may help you in this, but it's nontrivial.) SCALAR will be read and/or
2146 written depending on the FUNCTION--a pointer to the string value of SCALAR
2147 will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
2148 has no string value but does have a numeric value, that value will be
2149 passed rather than a pointer to the string value. To guarantee this to be
2150 true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
2151 functions may be needed to manipulate the values of structures used by
2154 The return value of C<ioctl> (and C<fcntl>) is as follows:
2156 if OS returns: then Perl returns:
2158 0 string "0 but true"
2159 anything else that number
2161 Thus Perl returns true on success and false on failure, yet you can
2162 still easily determine the actual value returned by the operating
2165 $retval = ioctl(...) || -1;
2166 printf "System returned %d\n", $retval;
2168 The special string "C<0> but true" is exempt from B<-w> complaints
2169 about improper numeric conversions.
2171 Here's an example of setting a filehandle named C<REMOTE> to be
2172 non-blocking at the system level. You'll have to negotiate C<$|>
2173 on your own, though.
2175 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
2177 $flags = fcntl(REMOTE, F_GETFL, 0)
2178 or die "Can't get flags for the socket: $!\n";
2180 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
2181 or die "Can't set flags for the socket: $!\n";
2183 =item join EXPR,LIST
2185 Joins the separate strings of LIST into a single string with fields
2186 separated by the value of EXPR, and returns that new string. Example:
2188 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
2190 Beware that unlike C<split>, C<join> doesn't take a pattern as its
2191 first argument. Compare L</split>.
2195 Returns a list consisting of all the keys of the named hash. (In
2196 scalar context, returns the number of keys.) The keys are returned in
2197 an apparently random order. The actual random order is subject to
2198 change in future versions of perl, but it is guaranteed to be the same
2199 order as either the C<values> or C<each> function produces (given
2200 that the hash has not been modified). As a side effect, it resets
2203 Here is yet another way to print your environment:
2206 @values = values %ENV;
2208 print pop(@keys), '=', pop(@values), "\n";
2211 or how about sorted by key:
2213 foreach $key (sort(keys %ENV)) {
2214 print $key, '=', $ENV{$key}, "\n";
2217 To sort a hash by value, you'll need to use a C<sort> function.
2218 Here's a descending numeric sort of a hash by its values:
2220 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
2221 printf "%4d %s\n", $hash{$key}, $key;
2224 As an lvalue C<keys> allows you to increase the number of hash buckets
2225 allocated for the given hash. This can gain you a measure of efficiency if
2226 you know the hash is going to get big. (This is similar to pre-extending
2227 an array by assigning a larger number to $#array.) If you say
2231 then C<%hash> will have at least 200 buckets allocated for it--256 of them,
2232 in fact, since it rounds up to the next power of two. These
2233 buckets will be retained even if you do C<%hash = ()>, use C<undef
2234 %hash> if you want to free the storage while C<%hash> is still in scope.
2235 You can't shrink the number of buckets allocated for the hash using
2236 C<keys> in this way (but you needn't worry about doing this by accident,
2237 as trying has no effect).
2239 See also C<each>, C<values> and C<sort>.
2241 =item kill SIGNAL, LIST
2243 Sends a signal to a list of processes. Returns the number of
2244 processes successfully signaled (which is not necessarily the
2245 same as the number actually killed).
2247 $cnt = kill 1, $child1, $child2;
2250 If SIGNAL is zero, no signal is sent to the process. This is a
2251 useful way to check that the process is alive and hasn't changed
2252 its UID. See L<perlport> for notes on the portability of this
2255 Unlike in the shell, if SIGNAL is negative, it kills
2256 process groups instead of processes. (On System V, a negative I<PROCESS>
2257 number will also kill process groups, but that's not portable.) That
2258 means you usually want to use positive not negative signals. You may also
2259 use a signal name in quotes. See L<perlipc/"Signals"> for details.
2265 The C<last> command is like the C<break> statement in C (as used in
2266 loops); it immediately exits the loop in question. If the LABEL is
2267 omitted, the command refers to the innermost enclosing loop. The
2268 C<continue> block, if any, is not executed:
2270 LINE: while (<STDIN>) {
2271 last LINE if /^$/; # exit when done with header
2275 C<last> cannot be used to exit a block which returns a value such as
2276 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2277 a grep() or map() operation.
2279 Note that a block by itself is semantically identical to a loop
2280 that executes once. Thus C<last> can be used to effect an early
2281 exit out of such a block.
2283 See also L</continue> for an illustration of how C<last>, C<next>, and
2290 Returns an lowercased version of EXPR. This is the internal function
2291 implementing the C<\L> escape in double-quoted strings.
2292 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
2295 If EXPR is omitted, uses C<$_>.
2301 Returns the value of EXPR with the first character lowercased. This is
2302 the internal function implementing the C<\l> escape in double-quoted strings.
2303 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
2305 If EXPR is omitted, uses C<$_>.
2311 Returns the length in characters of the value of EXPR. If EXPR is
2312 omitted, returns length of C<$_>. Note that this cannot be used on
2313 an entire array or hash to find out how many elements these have.
2314 For that, use C<scalar @array> and C<scalar keys %hash> respectively.
2316 =item link OLDFILE,NEWFILE
2318 Creates a new filename linked to the old filename. Returns true for
2319 success, false otherwise.
2321 =item listen SOCKET,QUEUESIZE
2323 Does the same thing that the listen system call does. Returns true if
2324 it succeeded, false otherwise. See the example in L<perlipc/"Sockets: Client/Server Communication">.
2328 You really probably want to be using C<my> instead, because C<local> isn't
2329 what most people think of as "local". See L<perlsub/"Private Variables
2330 via my()"> for details.
2332 A local modifies the listed variables to be local to the enclosing
2333 block, file, or eval. If more than one value is listed, the list must
2334 be placed in parentheses. See L<perlsub/"Temporary Values via local()">
2335 for details, including issues with tied arrays and hashes.
2337 =item localtime EXPR
2339 Converts a time as returned by the time function to a 9-element list
2340 with the time analyzed for the local time zone. Typically used as
2344 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
2347 All list elements are numeric, and come straight out of the C `struct
2348 tm'. $sec, $min, and $hour are the seconds, minutes, and hours of the
2349 specified time. $mday is the day of the month, and $mon is the month
2350 itself, in the range C<0..11> with 0 indicating January and 11
2351 indicating December. $year is the number of years since 1900. That
2352 is, $year is C<123> in year 2023. $wday is the day of the week, with
2353 0 indicating Sunday and 3 indicating Wednesday. $yday is the day of
2354 the year, in the range C<1..365> (or C<1..366> in leap years.) $isdst
2355 is true if the specified time occurs during daylight savings time,
2358 Note that the $year element is I<not> simply the last two digits of
2359 the year. If you assume it is, then you create non-Y2K-compliant
2360 programs--and you wouldn't want to do that, would you?
2362 The proper way to get a complete 4-digit year is simply:
2366 And to get the last two digits of the year (e.g., '01' in 2001) do:
2368 $year = sprintf("%02d", $year % 100);
2370 If EXPR is omitted, C<localtime()> uses the current time (C<localtime(time)>).
2372 In scalar context, C<localtime()> returns the ctime(3) value:
2374 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
2376 This scalar value is B<not> locale dependent, see L<perllocale>, but
2377 instead a Perl builtin. Also see the C<Time::Local> module
2378 (to convert the second, minutes, hours, ... back to seconds since the
2379 stroke of midnight the 1st of January 1970, the value returned by
2380 time()), and the strftime(3) and mktime(3) functions available via the
2381 POSIX module. To get somewhat similar but locale dependent date
2382 strings, set up your locale environment variables appropriately
2383 (please see L<perllocale>) and try for example:
2385 use POSIX qw(strftime);
2386 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
2388 Note that the C<%a> and C<%b>, the short forms of the day of the week
2389 and the month of the year, may not necessarily be three characters wide.
2395 This function places an advisory lock on a variable, subroutine,
2396 or referenced object contained in I<THING> until the lock goes out
2397 of scope. This is a built-in function only if your version of Perl
2398 was built with threading enabled, and if you've said C<use Threads>.
2399 Otherwise a user-defined function by this name will be called. See
2406 Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
2407 returns log of C<$_>. To get the log of another base, use basic algebra:
2408 The base-N log of a number is equal to the natural log of that number
2409 divided by the natural log of N. For example:
2413 return log($n)/log(10);
2416 See also L</exp> for the inverse operation.
2418 =item lstat FILEHANDLE
2424 Does the same thing as the C<stat> function (including setting the
2425 special C<_> filehandle) but stats a symbolic link instead of the file
2426 the symbolic link points to. If symbolic links are unimplemented on
2427 your system, a normal C<stat> is done.
2429 If EXPR is omitted, stats C<$_>.
2433 The match operator. See L<perlop>.
2435 =item map BLOCK LIST
2439 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2440 C<$_> to each element) and returns the list value composed of the
2441 results of each such evaluation. In scalar context, returns the
2442 total number of elements so generated. Evaluates BLOCK or EXPR in
2443 list context, so each element of LIST may produce zero, one, or
2444 more elements in the returned value.
2446 @chars = map(chr, @nums);
2448 translates a list of numbers to the corresponding characters. And
2450 %hash = map { getkey($_) => $_ } @array;
2452 is just a funny way to write
2455 foreach $_ (@array) {
2456 $hash{getkey($_)} = $_;
2459 Note that, because C<$_> is a reference into the list value, it can
2460 be used to modify the elements of the array. While this is useful and
2461 supported, it can cause bizarre results if the LIST is not a named array.
2462 Using a regular C<foreach> loop for this purpose would be clearer in
2463 most cases. See also L</grep> for an array composed of those items of
2464 the original list for which the BLOCK or EXPR evaluates to true.
2466 =item mkdir FILENAME,MASK
2468 =item mkdir FILENAME
2470 Creates the directory specified by FILENAME, with permissions
2471 specified by MASK (as modified by C<umask>). If it succeeds it
2472 returns true, otherwise it returns false and sets C<$!> (errno).
2473 If omitted, MASK defaults to 0777.
2475 In general, it is better to create directories with permissive MASK,
2476 and let the user modify that with their C<umask>, than it is to supply
2477 a restrictive MASK and give the user no way to be more permissive.
2478 The exceptions to this rule are when the file or directory should be
2479 kept private (mail files, for instance). The perlfunc(1) entry on
2480 C<umask> discusses the choice of MASK in more detail.
2482 =item msgctl ID,CMD,ARG
2484 Calls the System V IPC function msgctl(2). You'll probably have to say
2488 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
2489 then ARG must be a variable which will hold the returned C<msqid_ds>
2490 structure. Returns like C<ioctl>: the undefined value for error,
2491 C<"0 but true"> for zero, or the actual return value otherwise. See also
2492 C<IPC::SysV> and C<IPC::Semaphore> documentation.
2494 =item msgget KEY,FLAGS
2496 Calls the System V IPC function msgget(2). Returns the message queue
2497 id, or the undefined value if there is an error. See also C<IPC::SysV>
2498 and C<IPC::Msg> documentation.
2500 =item msgsnd ID,MSG,FLAGS
2502 Calls the System V IPC function msgsnd to send the message MSG to the
2503 message queue ID. MSG must begin with the native long integer message
2504 type, which may be created with C<pack("l!", $type)>. Returns true if
2505 successful, or false if there is an error. See also C<IPC::SysV> and
2506 C<IPC::SysV::Msg> documentation.
2508 =item msgrcv ID,VAR,SIZE,TYPE,FLAGS
2510 Calls the System V IPC function msgrcv to receive a message from
2511 message queue ID into variable VAR with a maximum message size of
2512 SIZE. Note that if a message is received, the message type will be
2513 the first thing in VAR, and the maximum length of VAR is SIZE plus the
2514 size of the message type. Returns true if successful, or false if
2515 there is an error. See also C<IPC::SysV> and C<IPC::SysV::Msg> documentation.
2519 =item my EXPR : ATTRIBUTES
2521 A C<my> declares the listed variables to be local (lexically) to the
2522 enclosing block, file, or C<eval>. If
2523 more than one value is listed, the list must be placed in parentheses. See
2524 L<perlsub/"Private Variables via my()"> for details.
2530 The C<next> command is like the C<continue> statement in C; it starts
2531 the next iteration of the loop:
2533 LINE: while (<STDIN>) {
2534 next LINE if /^#/; # discard comments
2538 Note that if there were a C<continue> block on the above, it would get
2539 executed even on discarded lines. If the LABEL is omitted, the command
2540 refers to the innermost enclosing loop.
2542 C<next> cannot be used to exit a block which returns a value such as
2543 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2544 a grep() or map() operation.
2546 Note that a block by itself is semantically identical to a loop
2547 that executes once. Thus C<next> will exit such a block early.
2549 See also L</continue> for an illustration of how C<last>, C<next>, and
2552 =item no Module LIST
2554 See the L</use> function, which C<no> is the opposite of.
2560 Interprets EXPR as an octal string and returns the corresponding
2561 value. (If EXPR happens to start off with C<0x>, interprets it as a
2562 hex string. If EXPR starts off with C<0b>, it is interpreted as a
2563 binary string.) The following will handle decimal, binary, octal, and
2564 hex in the standard Perl or C notation:
2566 $val = oct($val) if $val =~ /^0/;
2568 If EXPR is omitted, uses C<$_>. To go the other way (produce a number
2569 in octal), use sprintf() or printf():
2571 $perms = (stat("filename"))[2] & 07777;
2572 $oct_perms = sprintf "%lo", $perms;
2574 The oct() function is commonly used when a string such as C<644> needs
2575 to be converted into a file mode, for example. (Although perl will
2576 automatically convert strings into numbers as needed, this automatic
2577 conversion assumes base 10.)
2579 =item open FILEHANDLE,MODE,LIST
2581 =item open FILEHANDLE,EXPR
2583 =item open FILEHANDLE
2585 Opens the file whose filename is given by EXPR, and associates it with
2586 FILEHANDLE. If FILEHANDLE is an expression, its value is used as the
2587 name of the real filehandle wanted. If EXPR is omitted, the scalar
2588 variable of the same name as the FILEHANDLE contains the filename.
2589 (Note that lexical variables--those declared with C<my>--will not work
2590 for this purpose; so if you're using C<my>, specify EXPR in your call
2591 to open.) See L<perlopentut> for a kinder, gentler explanation of opening
2594 If MODE is C<< '<' >> or nothing, the file is opened for input.
2595 If MODE is C<< '>' >>, the file is truncated and opened for
2596 output, being created if necessary. If MODE is C<<< '>>' >>>,
2597 the file is opened for appending, again being created if necessary.
2598 You can put a C<'+'> in front of the C<< '>' >> or C<< '<' >> to indicate that
2599 you want both read and write access to the file; thus C<< '+<' >> is almost
2600 always preferred for read/write updates--the C<< '+>' >> mode would clobber the
2601 file first. You can't usually use either read-write mode for updating
2602 textfiles, since they have variable length records. See the B<-i>
2603 switch in L<perlrun> for a better approach. The file is created with
2604 permissions of C<0666> modified by the process' C<umask> value.
2606 These various prefixes correspond to the fopen(3) modes of C<'r'>, C<'r+'>,
2607 C<'w'>, C<'w+'>, C<'a'>, and C<'a+'>.
2609 In the 2-arguments (and 1-argument) form of the call the mode and
2610 filename should be concatenated (in this order), possibly separated by
2611 spaces. It is possible to omit the mode if the mode is C<< '<' >>.
2613 If the filename begins with C<'|'>, the filename is interpreted as a
2614 command to which output is to be piped, and if the filename ends with a
2615 C<'|'>, the filename is interpreted as a command which pipes output to
2616 us. See L<perlipc/"Using open() for IPC">
2617 for more examples of this. (You are not allowed to C<open> to a command
2618 that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
2619 and L<perlipc/"Bidirectional Communication with Another Process">
2622 If MODE is C<'|-'>, the filename is interpreted as a
2623 command to which output is to be piped, and if MODE is
2624 C<'-|'>, the filename is interpreted as a command which pipes output to
2625 us. In the 2-arguments (and 1-argument) form one should replace dash
2626 (C<'-'>) with the command. See L<perlipc/"Using open() for IPC">
2627 for more examples of this. (You are not allowed to C<open> to a command
2628 that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
2629 and L<perlipc/"Bidirectional Communication"> for alternatives.)
2631 In the 2-arguments (and 1-argument) form opening C<'-'> opens STDIN
2632 and opening C<< '>-' >> opens STDOUT.
2635 nonzero upon success, the undefined value otherwise. If the C<open>
2636 involved a pipe, the return value happens to be the pid of the
2639 If you're unfortunate enough to be running Perl on a system that
2640 distinguishes between text files and binary files (modern operating
2641 systems don't care), then you should check out L</binmode> for tips for
2642 dealing with this. The key distinction between systems that need C<binmode>
2643 and those that don't is their text file formats. Systems like Unix, MacOS, and
2644 Plan9, which delimit lines with a single character, and which encode that
2645 character in C as C<"\n">, do not need C<binmode>. The rest need it.
2647 When opening a file, it's usually a bad idea to continue normal execution
2648 if the request failed, so C<open> is frequently used in connection with
2649 C<die>. Even if C<die> won't do what you want (say, in a CGI script,
2650 where you want to make a nicely formatted error message (but there are
2651 modules that can help with that problem)) you should always check
2652 the return value from opening a file. The infrequent exception is when
2653 working with an unopened filehandle is actually what you want to do.
2658 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
2659 while (<ARTICLE>) {...
2661 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
2662 # if the open fails, output is discarded
2664 open(DBASE, '+<', 'dbase.mine') # open for update
2665 or die "Can't open 'dbase.mine' for update: $!";
2667 open(DBASE, '+<dbase.mine') # ditto
2668 or die "Can't open 'dbase.mine' for update: $!";
2670 open(ARTICLE, '-|', "caesar <$article") # decrypt article
2671 or die "Can't start caesar: $!";
2673 open(ARTICLE, "caesar <$article |") # ditto
2674 or die "Can't start caesar: $!";
2676 open(EXTRACT, "|sort >/tmp/Tmp$$") # $$ is our process id
2677 or die "Can't start sort: $!";
2679 # process argument list of files along with any includes
2681 foreach $file (@ARGV) {
2682 process($file, 'fh00');
2686 my($filename, $input) = @_;
2687 $input++; # this is a string increment
2688 unless (open($input, $filename)) {
2689 print STDERR "Can't open $filename: $!\n";
2694 while (<$input>) { # note use of indirection
2695 if (/^#include "(.*)"/) {
2696 process($1, $input);
2703 You may also, in the Bourne shell tradition, specify an EXPR beginning
2704 with C<< '>&' >>, in which case the rest of the string is interpreted as the
2705 name of a filehandle (or file descriptor, if numeric) to be
2706 duped and opened. You may use C<&> after C<< > >>, C<<< >> >>>,
2707 C<< < >>, C<< +> >>, C<<< +>> >>>, and C<< +< >>. The
2708 mode you specify should match the mode of the original filehandle.
2709 (Duping a filehandle does not take into account any existing contents of
2710 stdio buffers.) Duping file handles is not yet supported for 3-argument
2713 Here is a script that saves, redirects, and restores STDOUT and
2717 open(OLDOUT, ">&STDOUT");
2718 open(OLDERR, ">&STDERR");
2720 open(STDOUT, '>', "foo.out") || die "Can't redirect stdout";
2721 open(STDERR, ">&STDOUT") || die "Can't dup stdout";
2723 select(STDERR); $| = 1; # make unbuffered
2724 select(STDOUT); $| = 1; # make unbuffered
2726 print STDOUT "stdout 1\n"; # this works for
2727 print STDERR "stderr 1\n"; # subprocesses too
2732 open(STDOUT, ">&OLDOUT");
2733 open(STDERR, ">&OLDERR");
2735 print STDOUT "stdout 2\n";
2736 print STDERR "stderr 2\n";
2738 If you specify C<< '<&=N' >>, where C<N> is a number, then Perl will do an
2739 equivalent of C's C<fdopen> of that file descriptor; this is more
2740 parsimonious of file descriptors. For example:
2742 open(FILEHANDLE, "<&=$fd")
2744 Note that this feature depends on the fdopen() C library function.
2745 On many UNIX systems, fdopen() is known to fail when file descriptors
2746 exceed a certain value, typically 255. If you need more file
2747 descriptors than that, consider rebuilding Perl to use the C<sfio>
2750 If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'>
2751 with 2-arguments (or 1-argument) form of open(), then
2752 there is an implicit fork done, and the return value of open is the pid
2753 of the child within the parent process, and C<0> within the child
2754 process. (Use C<defined($pid)> to determine whether the open was successful.)
2755 The filehandle behaves normally for the parent, but i/o to that
2756 filehandle is piped from/to the STDOUT/STDIN of the child process.
2757 In the child process the filehandle isn't opened--i/o happens from/to
2758 the new STDOUT or STDIN. Typically this is used like the normal
2759 piped open when you want to exercise more control over just how the
2760 pipe command gets executed, such as when you are running setuid, and
2761 don't want to have to scan shell commands for metacharacters.
2762 The following triples are more or less equivalent:
2764 open(FOO, "|tr '[a-z]' '[A-Z]'");
2765 open(FOO, '|-', "tr '[a-z]' '[A-Z]'");
2766 open(FOO, '|-') || exec 'tr', '[a-z]', '[A-Z]';
2768 open(FOO, "cat -n '$file'|");
2769 open(FOO, '-|', "cat -n '$file'");
2770 open(FOO, '-|') || exec 'cat', '-n', $file;
2772 See L<perlipc/"Safe Pipe Opens"> for more examples of this.
2774 Beginning with v5.6.0, Perl will attempt to flush all files opened for
2775 output before any operation that may do a fork, but this may not be
2776 supported on some platforms (see L<perlport>). To be safe, you may need
2777 to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
2778 of C<IO::Handle> on any open handles.
2780 On systems that support a
2781 close-on-exec flag on files, the flag will be set for the newly opened
2782 file descriptor as determined by the value of $^F. See L<perlvar/$^F>.
2784 Closing any piped filehandle causes the parent process to wait for the
2785 child to finish, and returns the status value in C<$?>.
2787 The filename passed to 2-argument (or 1-argument) form of open()
2788 will have leading and trailing
2789 whitespace deleted, and the normal redirection characters
2790 honored. This property, known as "magic open",
2791 can often be used to good effect. A user could specify a filename of
2792 F<"rsh cat file |">, or you could change certain filenames as needed:
2794 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
2795 open(FH, $filename) or die "Can't open $filename: $!";
2797 Use 3-argument form to open a file with arbitrary weird characters in it,
2799 open(FOO, '<', $file);
2801 otherwise it's necessary to protect any leading and trailing whitespace:
2803 $file =~ s#^(\s)#./$1#;
2804 open(FOO, "< $file\0");
2806 (this may not work on some bizzare filesystems). One should
2807 conscientiously choose between the the I<magic> and 3-arguments form
2812 will allow the user to specify an argument of the form C<"rsh cat file |">,
2813 but will not work on a filename which happens to have a trailing space, while
2815 open IN, '<', $ARGV[0];
2817 will have exactly the opposite restrictions.
2819 If you want a "real" C C<open> (see L<open(2)> on your system), then you
2820 should use the C<sysopen> function, which involves no such magic (but
2821 may use subtly different filemodes than Perl open(), which is mapped
2822 to C fopen()). This is
2823 another way to protect your filenames from interpretation. For example:
2826 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
2827 or die "sysopen $path: $!";
2828 $oldfh = select(HANDLE); $| = 1; select($oldfh);
2829 print HANDLE "stuff $$\n");
2831 print "File contains: ", <HANDLE>;
2833 Using the constructor from the C<IO::Handle> package (or one of its
2834 subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous
2835 filehandles that have the scope of whatever variables hold references to
2836 them, and automatically close whenever and however you leave that scope:
2840 sub read_myfile_munged {
2842 my $handle = new IO::File;
2843 open($handle, "myfile") or die "myfile: $!";
2845 or return (); # Automatically closed here.
2846 mung $first or die "mung failed"; # Or here.
2847 return $first, <$handle> if $ALL; # Or here.
2851 See L</seek> for some details about mixing reading and writing.
2853 =item opendir DIRHANDLE,EXPR
2855 Opens a directory named EXPR for processing by C<readdir>, C<telldir>,
2856 C<seekdir>, C<rewinddir>, and C<closedir>. Returns true if successful.
2857 DIRHANDLEs have their own namespace separate from FILEHANDLEs.
2863 Returns the numeric (ASCII or Unicode) value of the first character of EXPR. If
2864 EXPR is omitted, uses C<$_>. For the reverse, see L</chr>.
2865 See L<utf8> for more about Unicode.
2869 An C<our> declares the listed variables to be valid globals within
2870 the enclosing block, file, or C<eval>. That is, it has the same
2871 scoping rules as a "my" declaration, but does not create a local
2872 variable. If more than one value is listed, the list must be placed
2873 in parentheses. The C<our> declaration has no semantic effect unless
2874 "use strict vars" is in effect, in which case it lets you use the
2875 declared global variable without qualifying it with a package name.
2876 (But only within the lexical scope of the C<our> declaration. In this
2877 it differs from "use vars", which is package scoped.)
2879 An C<our> declaration declares a global variable that will be visible
2880 across its entire lexical scope, even across package boundaries. The
2881 package in which the variable is entered is determined at the point
2882 of the declaration, not at the point of use. This means the following
2886 our $bar; # declares $Foo::bar for rest of lexical scope
2890 print $bar; # prints 20
2892 Multiple C<our> declarations in the same lexical scope are allowed
2893 if they are in different packages. If they happened to be in the same
2894 package, Perl will emit warnings if you have asked for them.
2898 our $bar; # declares $Foo::bar for rest of lexical scope
2902 our $bar = 30; # declares $Bar::bar for rest of lexical scope
2903 print $bar; # prints 30
2905 our $bar; # emits warning
2907 =item pack TEMPLATE,LIST
2909 Takes a LIST of values and converts it into a string using the rules
2910 given by the TEMPLATE. The resulting string is the concatenation of
2911 the converted values. Typically, each converted value looks
2912 like its machine-level representation. For example, on 32-bit machines
2913 a converted integer may be represented by a sequence of 4 bytes.
2916 sequence of characters that give the order and type of values, as
2919 a A string with arbitrary binary data, will be null padded.
2920 A An ascii string, will be space padded.
2921 Z A null terminated (asciz) string, will be null padded.
2923 b A bit string (ascending bit order inside each byte, like vec()).
2924 B A bit string (descending bit order inside each byte).
2925 h A hex string (low nybble first).
2926 H A hex string (high nybble first).
2928 c A signed char value.
2929 C An unsigned char value. Only does bytes. See U for Unicode.
2931 s A signed short value.
2932 S An unsigned short value.
2933 (This 'short' is _exactly_ 16 bits, which may differ from
2934 what a local C compiler calls 'short'. If you want
2935 native-length shorts, use the '!' suffix.)
2937 i A signed integer value.
2938 I An unsigned integer value.
2939 (This 'integer' is _at_least_ 32 bits wide. Its exact
2940 size depends on what a local C compiler calls 'int',
2941 and may even be larger than the 'long' described in
2944 l A signed long value.
2945 L An unsigned long value.
2946 (This 'long' is _exactly_ 32 bits, which may differ from
2947 what a local C compiler calls 'long'. If you want
2948 native-length longs, use the '!' suffix.)
2950 n An unsigned short in "network" (big-endian) order.
2951 N An unsigned long in "network" (big-endian) order.
2952 v An unsigned short in "VAX" (little-endian) order.
2953 V An unsigned long in "VAX" (little-endian) order.
2954 (These 'shorts' and 'longs' are _exactly_ 16 bits and
2955 _exactly_ 32 bits, respectively.)
2957 q A signed quad (64-bit) value.
2958 Q An unsigned quad value.
2959 (Quads are available only if your system supports 64-bit
2960 integer values _and_ if Perl has been compiled to support those.
2961 Causes a fatal error otherwise.)
2963 f A single-precision float in the native format.
2964 d A double-precision float in the native format.
2966 p A pointer to a null-terminated string.
2967 P A pointer to a structure (fixed-length string).
2969 u A uuencoded string.
2970 U A Unicode character number. Encodes to UTF-8 internally.
2971 Works even if C<use utf8> is not in effect.
2973 w A BER compressed integer. Its bytes represent an unsigned
2974 integer in base 128, most significant digit first, with as
2975 few digits as possible. Bit eight (the high bit) is set
2976 on each byte except the last.
2980 @ Null fill to absolute position.
2982 The following rules apply:
2988 Each letter may optionally be followed by a number giving a repeat
2989 count. With all types except C<a>, C<A>, C<Z>, C<b>, C<B>, C<h>,
2990 C<H>, and C<P> the pack function will gobble up that many values from
2991 the LIST. A C<*> for the repeat count means to use however many items are
2992 left, except for C<@>, C<x>, C<X>, where it is equivalent
2993 to C<0>, and C<u>, where it is equivalent to 1 (or 45, what is the
2996 When used with C<Z>, C<*> results in the addition of a trailing null
2997 byte (so the packed result will be one longer than the byte C<length>
3000 The repeat count for C<u> is interpreted as the maximal number of bytes
3001 to encode per line of output, with 0 and 1 replaced by 45.
3005 The C<a>, C<A>, and C<Z> types gobble just one value, but pack it as a
3006 string of length count, padding with nulls or spaces as necessary. When
3007 unpacking, C<A> strips trailing spaces and nulls, C<Z> strips everything
3008 after the first null, and C<a> returns data verbatim. When packing,
3009 C<a>, and C<Z> are equivalent.
3011 If the value-to-pack is too long, it is truncated. If too long and an
3012 explicit count is provided, C<Z> packs only C<$count-1> bytes, followed
3013 by a null byte. Thus C<Z> always packs a trailing null byte under
3018 Likewise, the C<b> and C<B> fields pack a string that many bits long.
3019 Each byte of the input field of pack() generates 1 bit of the result.
3020 Each result bit is based on the least-significant bit of the corresponding
3021 input byte, i.e., on C<ord($byte)%2>. In particular, bytes C<"0"> and
3022 C<"1"> generate bits 0 and 1, as do bytes C<"\0"> and C<"\1">.
3024 Starting from the beginning of the input string of pack(), each 8-tuple
3025 of bytes is converted to 1 byte of output. With format C<b>
3026 the first byte of the 8-tuple determines the least-significant bit of a
3027 byte, and with format C<B> it determines the most-significant bit of
3030 If the length of the input string is not exactly divisible by 8, the
3031 remainder is packed as if the input string were padded by null bytes
3032 at the end. Similarly, during unpack()ing the "extra" bits are ignored.
3034 If the input string of pack() is longer than needed, extra bytes are ignored.
3035 A C<*> for the repeat count of pack() means to use all the bytes of
3036 the input field. On unpack()ing the bits are converted to a string
3037 of C<"0">s and C<"1">s.
3041 The C<h> and C<H> fields pack a string that many nybbles (4-bit groups,
3042 representable as hexadecimal digits, 0-9a-f) long.
3044 Each byte of the input field of pack() generates 4 bits of the result.
3045 For non-alphabetical bytes the result is based on the 4 least-significant
3046 bits of the input byte, i.e., on C<ord($byte)%16>. In particular,
3047 bytes C<"0"> and C<"1"> generate nybbles 0 and 1, as do bytes
3048 C<"\0"> and C<"\1">. For bytes C<"a".."f"> and C<"A".."F"> the result
3049 is compatible with the usual hexadecimal digits, so that C<"a"> and
3050 C<"A"> both generate the nybble C<0xa==10>. The result for bytes
3051 C<"g".."z"> and C<"G".."Z"> is not well-defined.
3053 Starting from the beginning of the input string of pack(), each pair
3054 of bytes is converted to 1 byte of output. With format C<h> the
3055 first byte of the pair determines the least-significant nybble of the
3056 output byte, and with format C<H> it determines the most-significant
3059 If the length of the input string is not even, it behaves as if padded
3060 by a null byte at the end. Similarly, during unpack()ing the "extra"
3061 nybbles are ignored.
3063 If the input string of pack() is longer than needed, extra bytes are ignored.
3064 A C<*> for the repeat count of pack() means to use all the bytes of
3065 the input field. On unpack()ing the bits are converted to a string
3066 of hexadecimal digits.
3070 The C<p> type packs a pointer to a null-terminated string. You are
3071 responsible for ensuring the string is not a temporary value (which can
3072 potentially get deallocated before you get around to using the packed result).
3073 The C<P> type packs a pointer to a structure of the size indicated by the
3074 length. A NULL pointer is created if the corresponding value for C<p> or
3075 C<P> is C<undef>, similarly for unpack().
3079 The C</> template character allows packing and unpacking of strings where
3080 the packed structure contains a byte count followed by the string itself.
3081 You write I<length-item>C</>I<string-item>.
3083 The I<length-item> can be any C<pack> template letter,
3084 and describes how the length value is packed.
3085 The ones likely to be of most use are integer-packing ones like
3086 C<n> (for Java strings), C<w> (for ASN.1 or SNMP)
3087 and C<N> (for Sun XDR).
3089 The I<string-item> must, at present, be C<"A*">, C<"a*"> or C<"Z*">.
3090 For C<unpack> the length of the string is obtained from the I<length-item>,
3091 but if you put in the '*' it will be ignored.
3093 unpack 'C/a', "\04Gurusamy"; gives 'Guru'
3094 unpack 'a3/A* A*', '007 Bond J '; gives (' Bond','J')
3095 pack 'n/a* w/a*','hello,','world'; gives "\000\006hello,\005world"
3097 The I<length-item> is not returned explicitly from C<unpack>.
3099 Adding a count to the I<length-item> letter is unlikely to do anything
3100 useful, unless that letter is C<A>, C<a> or C<Z>. Packing with a
3101 I<length-item> of C<a> or C<Z> may introduce C<"\000"> characters,
3102 which Perl does not regard as legal in numeric strings.
3106 The integer types C<s>, C<S>, C<l>, and C<L> may be
3107 immediately followed by a C<!> suffix to signify native shorts or
3108 longs--as you can see from above for example a bare C<l> does mean
3109 exactly 32 bits, the native C<long> (as seen by the local C compiler)
3110 may be larger. This is an issue mainly in 64-bit platforms. You can
3111 see whether using C<!> makes any difference by
3113 print length(pack("s")), " ", length(pack("s!")), "\n";
3114 print length(pack("l")), " ", length(pack("l!")), "\n";
3116 C<i!> and C<I!> also work but only because of completeness;
3117 they are identical to C<i> and C<I>.
3119 The actual sizes (in bytes) of native shorts, ints, longs, and long
3120 longs on the platform where Perl was built are also available via
3124 print $Config{shortsize}, "\n";
3125 print $Config{intsize}, "\n";
3126 print $Config{longsize}, "\n";
3127 print $Config{longlongsize}, "\n";
3129 (The C<$Config{longlongsize}> will be undefine if your system does
3130 not support long longs.)
3134 The integer formats C<s>, C<S>, C<i>, C<I>, C<l>, and C<L>
3135 are inherently non-portable between processors and operating systems
3136 because they obey the native byteorder and endianness. For example a
3137 4-byte integer 0x12345678 (305419896 decimal) be ordered natively
3138 (arranged in and handled by the CPU registers) into bytes as
3140 0x12 0x34 0x56 0x78 # little-endian
3141 0x78 0x56 0x34 0x12 # big-endian
3143 Basically, the Intel, Alpha, and VAX CPUs are little-endian, while
3144 everybody else, for example Motorola m68k/88k, PPC, Sparc, HP PA,
3145 Power, and Cray are big-endian. MIPS can be either: Digital used it
3146 in little-endian mode; SGI uses it in big-endian mode.
3148 The names `big-endian' and `little-endian' are comic references to
3149 the classic "Gulliver's Travels" (via the paper "On Holy Wars and a
3150 Plea for Peace" by Danny Cohen, USC/ISI IEN 137, April 1, 1980) and
3151 the egg-eating habits of the Lilliputians.
3153 Some systems may have even weirder byte orders such as
3158 You can see your system's preference with
3160 print join(" ", map { sprintf "%#02x", $_ }
3161 unpack("C*",pack("L",0x12345678))), "\n";
3163 The byteorder on the platform where Perl was built is also available
3167 print $Config{byteorder}, "\n";
3169 Byteorders C<'1234'> and C<'12345678'> are little-endian, C<'4321'>
3170 and C<'87654321'> are big-endian.
3172 If you want portable packed integers use the formats C<n>, C<N>,
3173 C<v>, and C<V>, their byte endianness and size is known.
3174 See also L<perlport>.
3178 Real numbers (floats and doubles) are in the native machine format only;
3179 due to the multiplicity of floating formats around, and the lack of a
3180 standard "network" representation, no facility for interchange has been
3181 made. This means that packed floating point data written on one machine
3182 may not be readable on another - even if both use IEEE floating point
3183 arithmetic (as the endian-ness of the memory representation is not part
3184 of the IEEE spec). See also L<perlport>.
3186 Note that Perl uses doubles internally for all numeric calculation, and
3187 converting from double into float and thence back to double again will
3188 lose precision (i.e., C<unpack("f", pack("f", $foo)>) will not in general
3193 You must yourself do any alignment or padding by inserting for example
3194 enough C<'x'>es while packing. There is no way to pack() and unpack()
3195 could know where the bytes are going to or coming from. Therefore
3196 C<pack> (and C<unpack>) handle their output and input as flat
3201 A comment in a TEMPLATE starts with C<#> and goes to the end of line.
3205 If TEMPLATE requires more arguments to pack() than actually given, pack()
3206 assumes additional C<""> arguments. If TEMPLATE requires less arguments
3207 to pack() than actually given, extra arguments are ignored.
3213 $foo = pack("CCCC",65,66,67,68);
3215 $foo = pack("C4",65,66,67,68);
3217 $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);
3218 # same thing with Unicode circled letters
3220 $foo = pack("ccxxcc",65,66,67,68);
3223 # note: the above examples featuring "C" and "c" are true
3224 # only on ASCII and ASCII-derived systems such as ISO Latin 1
3225 # and UTF-8. In EBCDIC the first example would be
3226 # $foo = pack("CCCC",193,194,195,196);
3228 $foo = pack("s2",1,2);
3229 # "\1\0\2\0" on little-endian
3230 # "\0\1\0\2" on big-endian
3232 $foo = pack("a4","abcd","x","y","z");
3235 $foo = pack("aaaa","abcd","x","y","z");
3238 $foo = pack("a14","abcdefg");
3239 # "abcdefg\0\0\0\0\0\0\0"
3241 $foo = pack("i9pl", gmtime);
3242 # a real struct tm (on my system anyway)
3244 $utmp_template = "Z8 Z8 Z16 L";
3245 $utmp = pack($utmp_template, @utmp1);
3246 # a struct utmp (BSDish)
3248 @utmp2 = unpack($utmp_template, $utmp);
3249 # "@utmp1" eq "@utmp2"
3252 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
3255 $foo = pack('sx2l', 12, 34);
3256 # short 12, two zero bytes padding, long 34
3257 $bar = pack('s@4l', 12, 34);
3258 # short 12, zero fill to position 4, long 34
3261 The same template may generally also be used in unpack().
3265 =item package NAMESPACE
3267 Declares the compilation unit as being in the given namespace. The scope
3268 of the package declaration is from the declaration itself through the end
3269 of the enclosing block, file, or eval (the same as the C<my> operator).
3270 All further unqualified dynamic identifiers will be in this namespace.
3271 A package statement affects only dynamic variables--including those
3272 you've used C<local> on--but I<not> lexical variables, which are created
3273 with C<my>. Typically it would be the first declaration in a file to
3274 be included by the C<require> or C<use> operator. You can switch into a
3275 package in more than one place; it merely influences which symbol table
3276 is used by the compiler for the rest of that block. You can refer to
3277 variables and filehandles in other packages by prefixing the identifier
3278 with the package name and a double colon: C<$Package::Variable>.
3279 If the package name is null, the C<main> package as assumed. That is,
3280 C<$::sail> is equivalent to C<$main::sail> (as well as to C<$main'sail>,
3281 still seen in older code).
3283 If NAMESPACE is omitted, then there is no current package, and all
3284 identifiers must be fully qualified or lexicals. This is stricter
3285 than C<use strict>, since it also extends to function names.
3287 See L<perlmod/"Packages"> for more information about packages, modules,
3288 and classes. See L<perlsub> for other scoping issues.
3290 =item pipe READHANDLE,WRITEHANDLE
3292 Opens a pair of connected pipes like the corresponding system call.
3293 Note that if you set up a loop of piped processes, deadlock can occur
3294 unless you are very careful. In addition, note that Perl's pipes use
3295 stdio buffering, so you may need to set C<$|> to flush your WRITEHANDLE
3296 after each command, depending on the application.
3298 See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
3299 for examples of such things.
3301 On systems that support a close-on-exec flag on files, the flag will be set
3302 for the newly opened file descriptors as determined by the value of $^F.
3309 Pops and returns the last value of the array, shortening the array by
3310 one element. Has an effect similar to
3314 If there are no elements in the array, returns the undefined value
3315 (although this may happen at other times as well). If ARRAY is
3316 omitted, pops the C<@ARGV> array in the main program, and the C<@_>
3317 array in subroutines, just like C<shift>.
3323 Returns the offset of where the last C<m//g> search left off for the variable
3324 is in question (C<$_> is used when the variable is not specified). May be
3325 modified to change that offset. Such modification will also influence
3326 the C<\G> zero-width assertion in regular expressions. See L<perlre> and
3329 =item print FILEHANDLE LIST
3335 Prints a string or a list of strings. Returns true if successful.
3336 FILEHANDLE may be a scalar variable name, in which case the variable
3337 contains the name of or a reference to the filehandle, thus introducing
3338 one level of indirection. (NOTE: If FILEHANDLE is a variable and
3339 the next token is a term, it may be misinterpreted as an operator
3340 unless you interpose a C<+> or put parentheses around the arguments.)
3341 If FILEHANDLE is omitted, prints by default to standard output (or
3342 to the last selected output channel--see L</select>). If LIST is
3343 also omitted, prints C<$_> to the currently selected output channel.
3344 To set the default output channel to something other than STDOUT
3345 use the select operation. The current value of C<$,> (if any) is
3346 printed between each LIST item. The current value of C<$\> (if
3347 any) is printed after the entire LIST has been printed. Because
3348 print takes a LIST, anything in the LIST is evaluated in list
3349 context, and any subroutine that you call will have one or more of
3350 its expressions evaluated in list context. Also be careful not to
3351 follow the print keyword with a left parenthesis unless you want
3352 the corresponding right parenthesis to terminate the arguments to
3353 the print--interpose a C<+> or put parentheses around all the
3356 Note that if you're storing FILEHANDLES in an array or other expression,
3357 you will have to use a block returning its value instead:
3359 print { $files[$i] } "stuff\n";
3360 print { $OK ? STDOUT : STDERR } "stuff\n";
3362 =item printf FILEHANDLE FORMAT, LIST
3364 =item printf FORMAT, LIST
3366 Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that C<$\>
3367 (the output record separator) is not appended. The first argument
3368 of the list will be interpreted as the C<printf> format. If C<use locale> is
3369 in effect, the character used for the decimal point in formatted real numbers
3370 is affected by the LC_NUMERIC locale. See L<perllocale>.
3372 Don't fall into the trap of using a C<printf> when a simple
3373 C<print> would do. The C<print> is more efficient and less
3376 =item prototype FUNCTION
3378 Returns the prototype of a function as a string (or C<undef> if the
3379 function has no prototype). FUNCTION is a reference to, or the name of,
3380 the function whose prototype you want to retrieve.
3382 If FUNCTION is a string starting with C<CORE::>, the rest is taken as a
3383 name for Perl builtin. If the builtin is not I<overridable> (such as
3384 C<qw//>) or its arguments cannot be expressed by a prototype (such as
3385 C<system>) returns C<undef> because the builtin does not really behave
3386 like a Perl function. Otherwise, the string describing the equivalent
3387 prototype is returned.
3389 =item push ARRAY,LIST
3391 Treats ARRAY as a stack, and pushes the values of LIST
3392 onto the end of ARRAY. The length of ARRAY increases by the length of
3393 LIST. Has the same effect as
3396 $ARRAY[++$#ARRAY] = $value;
3399 but is more efficient. Returns the new number of elements in the array.
3411 Generalized quotes. See L<perlop/"Regexp Quote-Like Operators">.
3413 =item quotemeta EXPR
3417 Returns the value of EXPR with all non-alphanumeric
3418 characters backslashed. (That is, all characters not matching
3419 C</[A-Za-z_0-9]/> will be preceded by a backslash in the
3420 returned string, regardless of any locale settings.)
3421 This is the internal function implementing
3422 the C<\Q> escape in double-quoted strings.
3424 If EXPR is omitted, uses C<$_>.
3430 Returns a random fractional number greater than or equal to C<0> and less
3431 than the value of EXPR. (EXPR should be positive.) If EXPR is
3432 omitted, the value C<1> is used. Automatically calls C<srand> unless
3433 C<srand> has already been called. See also C<srand>.
3435 (Note: If your rand function consistently returns numbers that are too
3436 large or too small, then your version of Perl was probably compiled
3437 with the wrong number of RANDBITS.)
3439 =item read FILEHANDLE,SCALAR,LENGTH,OFFSET
3441 =item read FILEHANDLE,SCALAR,LENGTH
3443 Attempts to read LENGTH bytes of data into variable SCALAR from the
3444 specified FILEHANDLE. Returns the number of bytes actually read,
3445 C<0> at end of file, or undef if there was an error. SCALAR will be grown
3446 or shrunk to the length actually read. An OFFSET may be specified to
3447 place the read data at some other place than the beginning of the
3448 string. This call is actually implemented in terms of stdio's fread(3)
3449 call. To get a true read(2) system call, see C<sysread>.
3451 =item readdir DIRHANDLE
3453 Returns the next directory entry for a directory opened by C<opendir>.
3454 If used in list context, returns all the rest of the entries in the
3455 directory. If there are no more entries, returns an undefined value in
3456 scalar context or a null list in list context.
3458 If you're planning to filetest the return values out of a C<readdir>, you'd
3459 better prepend the directory in question. Otherwise, because we didn't
3460 C<chdir> there, it would have been testing the wrong file.
3462 opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!";
3463 @dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR);
3468 Reads from the filehandle whose typeglob is contained in EXPR. In scalar
3469 context, each call reads and returns the next line, until end-of-file is
3470 reached, whereupon the subsequent call returns undef. In list context,
3471 reads until end-of-file is reached and returns a list of lines. Note that
3472 the notion of "line" used here is however you may have defined it
3473 with C<$/> or C<$INPUT_RECORD_SEPARATOR>). See L<perlvar/"$/">.
3475 When C<$/> is set to C<undef>, when readline() is in scalar
3476 context (i.e. file slurp mode), and when an empty file is read, it
3477 returns C<''> the first time, followed by C<undef> subsequently.
3479 This is the internal function implementing the C<< <EXPR> >>
3480 operator, but you can use it directly. The C<< <EXPR> >>
3481 operator is discussed in more detail in L<perlop/"I/O Operators">.
3484 $line = readline(*STDIN); # same thing
3490 Returns the value of a symbolic link, if symbolic links are
3491 implemented. If not, gives a fatal error. If there is some system
3492 error, returns the undefined value and sets C<$!> (errno). If EXPR is
3493 omitted, uses C<$_>.
3497 EXPR is executed as a system command.
3498 The collected standard output of the command is returned.
3499 In scalar context, it comes back as a single (potentially
3500 multi-line) string. In list context, returns a list of lines
3501 (however you've defined lines with C<$/> or C<$INPUT_RECORD_SEPARATOR>).
3502 This is the internal function implementing the C<qx/EXPR/>
3503 operator, but you can use it directly. The C<qx/EXPR/>
3504 operator is discussed in more detail in L<perlop/"I/O Operators">.
3506 =item recv SOCKET,SCALAR,LENGTH,FLAGS
3508 Receives a message on a socket. Attempts to receive LENGTH bytes of
3509 data into variable SCALAR from the specified SOCKET filehandle. SCALAR
3510 will be grown or shrunk to the length actually read. Takes the same
3511 flags as the system call of the same name. Returns the address of the
3512 sender if SOCKET's protocol supports this; returns an empty string
3513 otherwise. If there's an error, returns the undefined value. This call
3514 is actually implemented in terms of recvfrom(2) system call. See
3515 L<perlipc/"UDP: Message Passing"> for examples.
3521 The C<redo> command restarts the loop block without evaluating the
3522 conditional again. The C<continue> block, if any, is not executed. If
3523 the LABEL is omitted, the command refers to the innermost enclosing
3524 loop. This command is normally used by programs that want to lie to
3525 themselves about what was just input:
3527 # a simpleminded Pascal comment stripper
3528 # (warning: assumes no { or } in strings)
3529 LINE: while (<STDIN>) {
3530 while (s|({.*}.*){.*}|$1 |) {}
3535 if (/}/) { # end of comment?
3544 C<redo> cannot be used to retry a block which returns a value such as
3545 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
3546 a grep() or map() operation.
3548 Note that a block by itself is semantically identical to a loop
3549 that executes once. Thus C<redo> inside such a block will effectively
3550 turn it into a looping construct.
3552 See also L</continue> for an illustration of how C<last>, C<next>, and
3559 Returns a true value if EXPR is a reference, false otherwise. If EXPR
3560 is not specified, C<$_> will be used. The value returned depends on the
3561 type of thing the reference is a reference to.
3562 Builtin types include:
3572 If the referenced object has been blessed into a package, then that package
3573 name is returned instead. You can think of C<ref> as a C<typeof> operator.
3575 if (ref($r) eq "HASH") {
3576 print "r is a reference to a hash.\n";
3579 print "r is not a reference at all.\n";
3581 if (UNIVERSAL::isa($r, "HASH")) { # for subclassing
3582 print "r is a reference to something that isa hash.\n";
3585 See also L<perlref>.
3587 =item rename OLDNAME,NEWNAME
3589 Changes the name of a file; an existing file NEWNAME will be
3590 clobbered. Returns true for success, false otherwise.
3592 Behavior of this function varies wildly depending on your system
3593 implementation. For example, it will usually not work across file system
3594 boundaries, even though the system I<mv> command sometimes compensates
3595 for this. Other restrictions include whether it works on directories,
3596 open files, or pre-existing files. Check L<perlport> and either the
3597 rename(2) manpage or equivalent system documentation for details.
3599 =item require VERSION
3605 Demands some semantics specified by EXPR, or by C<$_> if EXPR is not
3608 If a VERSION is specified as a literal of the form v5.6.1,
3609 demands that the current version of Perl (C<$^V> or $PERL_VERSION) be
3610 at least as recent as that version, at run time. (For compatibility
3611 with older versions of Perl, a numeric argument will also be interpreted
3612 as VERSION.) Compare with L</use>, which can do a similar check at
3615 require v5.6.1; # run time version check
3616 require 5.6.1; # ditto
3617 require 5.005_03; # float version allowed for compatibility
3619 Otherwise, demands that a library file be included if it hasn't already
3620 been included. The file is included via the do-FILE mechanism, which is
3621 essentially just a variety of C<eval>. Has semantics similar to the following
3626 return 1 if $INC{$filename};
3627 my($realfilename,$result);
3629 foreach $prefix (@INC) {
3630 $realfilename = "$prefix/$filename";
3631 if (-f $realfilename) {
3632 $INC{$filename} = $realfilename;
3633 $result = do $realfilename;
3637 die "Can't find $filename in \@INC";
3639 delete $INC{$filename} if $@ || !$result;
3641 die "$filename did not return true value" unless $result;
3645 Note that the file will not be included twice under the same specified
3646 name. The file must return true as the last statement to indicate
3647 successful execution of any initialization code, so it's customary to
3648 end such a file with C<1;> unless you're sure it'll return true
3649 otherwise. But it's better just to put the C<1;>, in case you add more
3652 If EXPR is a bareword, the require assumes a "F<.pm>" extension and
3653 replaces "F<::>" with "F</>" in the filename for you,
3654 to make it easy to load standard modules. This form of loading of
3655 modules does not risk altering your namespace.
3657 In other words, if you try this:
3659 require Foo::Bar; # a splendid bareword
3661 The require function will actually look for the "F<Foo/Bar.pm>" file in the
3662 directories specified in the C<@INC> array.
3664 But if you try this:
3666 $class = 'Foo::Bar';
3667 require $class; # $class is not a bareword
3669 require "Foo::Bar"; # not a bareword because of the ""
3671 The require function will look for the "F<Foo::Bar>" file in the @INC array and
3672 will complain about not finding "F<Foo::Bar>" there. In this case you can do:
3674 eval "require $class";
3676 For a yet-more-powerful import facility, see L</use> and L<perlmod>.
3682 Generally used in a C<continue> block at the end of a loop to clear
3683 variables and reset C<??> searches so that they work again. The
3684 expression is interpreted as a list of single characters (hyphens
3685 allowed for ranges). All variables and arrays beginning with one of
3686 those letters are reset to their pristine state. If the expression is
3687 omitted, one-match searches (C<?pattern?>) are reset to match again. Resets
3688 only variables or searches in the current package. Always returns
3691 reset 'X'; # reset all X variables
3692 reset 'a-z'; # reset lower case variables
3693 reset; # just reset ?one-time? searches
3695 Resetting C<"A-Z"> is not recommended because you'll wipe out your
3696 C<@ARGV> and C<@INC> arrays and your C<%ENV> hash. Resets only package
3697 variables--lexical variables are unaffected, but they clean themselves
3698 up on scope exit anyway, so you'll probably want to use them instead.
3705 Returns from a subroutine, C<eval>, or C<do FILE> with the value
3706 given in EXPR. Evaluation of EXPR may be in list, scalar, or void
3707 context, depending on how the return value will be used, and the context
3708 may vary from one execution to the next (see C<wantarray>). If no EXPR
3709 is given, returns an empty list in list context, the undefined value in
3710 scalar context, and (of course) nothing at all in a void context.
3712 (Note that in the absence of a explicit C<return>, a subroutine, eval,
3713 or do FILE will automatically return the value of the last expression
3718 In list context, returns a list value consisting of the elements
3719 of LIST in the opposite order. In scalar context, concatenates the
3720 elements of LIST and returns a string value with all characters
3721 in the opposite order.
3723 print reverse <>; # line tac, last line first
3725 undef $/; # for efficiency of <>
3726 print scalar reverse <>; # character tac, last line tsrif
3728 This operator is also handy for inverting a hash, although there are some
3729 caveats. If a value is duplicated in the original hash, only one of those
3730 can be represented as a key in the inverted hash. Also, this has to
3731 unwind one hash and build a whole new one, which may take some time
3732 on a large hash, such as from a DBM file.
3734 %by_name = reverse %by_address; # Invert the hash
3736 =item rewinddir DIRHANDLE
3738 Sets the current position to the beginning of the directory for the
3739 C<readdir> routine on DIRHANDLE.
3741 =item rindex STR,SUBSTR,POSITION
3743 =item rindex STR,SUBSTR
3745 Works just like index() except that it returns the position of the LAST
3746 occurrence of SUBSTR in STR. If POSITION is specified, returns the
3747 last occurrence at or before that position.
3749 =item rmdir FILENAME
3753 Deletes the directory specified by FILENAME if that directory is empty. If it
3754 succeeds it returns true, otherwise it returns false and sets C<$!> (errno). If
3755 FILENAME is omitted, uses C<$_>.
3759 The substitution operator. See L<perlop>.
3763 Forces EXPR to be interpreted in scalar context and returns the value
3766 @counts = ( scalar @a, scalar @b, scalar @c );
3768 There is no equivalent operator to force an expression to
3769 be interpolated in list context because in practice, this is never
3770 needed. If you really wanted to do so, however, you could use
3771 the construction C<@{[ (some expression) ]}>, but usually a simple
3772 C<(some expression)> suffices.
3774 Because C<scalar> is unary operator, if you accidentally use for EXPR a
3775 parenthesized list, this behaves as a scalar comma expression, evaluating
3776 all but the last element in void context and returning the final element
3777 evaluated in scalar context. This is seldom what you want.
3779 The following single statement:
3781 print uc(scalar(&foo,$bar)),$baz;
3783 is the moral equivalent of these two:
3786 print(uc($bar),$baz);
3788 See L<perlop> for more details on unary operators and the comma operator.
3790 =item seek FILEHANDLE,POSITION,WHENCE
3792 Sets FILEHANDLE's position, just like the C<fseek> call of C<stdio>.
3793 FILEHANDLE may be an expression whose value gives the name of the
3794 filehandle. The values for WHENCE are C<0> to set the new position to
3795 POSITION, C<1> to set it to the current position plus POSITION, and
3796 C<2> to set it to EOF plus POSITION (typically negative). For WHENCE
3797 you may use the constants C<SEEK_SET>, C<SEEK_CUR>, and C<SEEK_END>
3798 (start of the file, current position, end of the file) from the Fcntl
3799 module. Returns C<1> upon success, C<0> otherwise.
3801 If you want to position file for C<sysread> or C<syswrite>, don't use
3802 C<seek>--buffering makes its effect on the file's system position
3803 unpredictable and non-portable. Use C<sysseek> instead.
3805 Due to the rules and rigors of ANSI C, on some systems you have to do a
3806 seek whenever you switch between reading and writing. Amongst other
3807 things, this may have the effect of calling stdio's clearerr(3).
3808 A WHENCE of C<1> (C<SEEK_CUR>) is useful for not moving the file position:
3812 This is also useful for applications emulating C<tail -f>. Once you hit
3813 EOF on your read, and then sleep for a while, you might have to stick in a
3814 seek() to reset things. The C<seek> doesn't change the current position,
3815 but it I<does> clear the end-of-file condition on the handle, so that the
3816 next C<< <FILE> >> makes Perl try again to read something. We hope.
3818 If that doesn't work (some stdios are particularly cantankerous), then
3819 you may need something more like this:
3822 for ($curpos = tell(FILE); $_ = <FILE>;
3823 $curpos = tell(FILE)) {
3824 # search for some stuff and put it into files
3826 sleep($for_a_while);
3827 seek(FILE, $curpos, 0);
3830 =item seekdir DIRHANDLE,POS
3832 Sets the current position for the C<readdir> routine on DIRHANDLE. POS
3833 must be a value returned by C<telldir>. Has the same caveats about
3834 possible directory compaction as the corresponding system library
3837 =item select FILEHANDLE
3841 Returns the currently selected filehandle. Sets the current default
3842 filehandle for output, if FILEHANDLE is supplied. This has two
3843 effects: first, a C<write> or a C<print> without a filehandle will
3844 default to this FILEHANDLE. Second, references to variables related to
3845 output will refer to this output channel. For example, if you have to
3846 set the top of form format for more than one output channel, you might
3854 FILEHANDLE may be an expression whose value gives the name of the
3855 actual filehandle. Thus:
3857 $oldfh = select(STDERR); $| = 1; select($oldfh);
3859 Some programmers may prefer to think of filehandles as objects with
3860 methods, preferring to write the last example as:
3863 STDERR->autoflush(1);
3865 =item select RBITS,WBITS,EBITS,TIMEOUT
3867 This calls the select(2) system call with the bit masks specified, which
3868 can be constructed using C<fileno> and C<vec>, along these lines:
3870 $rin = $win = $ein = '';
3871 vec($rin,fileno(STDIN),1) = 1;
3872 vec($win,fileno(STDOUT),1) = 1;
3875 If you want to select on many filehandles you might wish to write a
3879 my(@fhlist) = split(' ',$_[0]);
3882 vec($bits,fileno($_),1) = 1;
3886 $rin = fhbits('STDIN TTY SOCK');
3890 ($nfound,$timeleft) =
3891 select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
3893 or to block until something becomes ready just do this
3895 $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
3897 Most systems do not bother to return anything useful in $timeleft, so
3898 calling select() in scalar context just returns $nfound.
3900 Any of the bit masks can also be undef. The timeout, if specified, is
3901 in seconds, which may be fractional. Note: not all implementations are
3902 capable of returning the$timeleft. If not, they always return
3903 $timeleft equal to the supplied $timeout.
3905 You can effect a sleep of 250 milliseconds this way:
3907 select(undef, undef, undef, 0.25);
3909 B<WARNING>: One should not attempt to mix buffered I/O (like C<read>
3910 or <FH>) with C<select>, except as permitted by POSIX, and even
3911 then only on POSIX systems. You have to use C<sysread> instead.
3913 =item semctl ID,SEMNUM,CMD,ARG
3915 Calls the System V IPC function C<semctl>. You'll probably have to say
3919 first to get the correct constant definitions. If CMD is IPC_STAT or
3920 GETALL, then ARG must be a variable which will hold the returned
3921 semid_ds structure or semaphore value array. Returns like C<ioctl>:
3922 the undefined value for error, "C<0 but true>" for zero, or the actual
3923 return value otherwise. The ARG must consist of a vector of native
3924 short integers, which may may be created with C<pack("s!",(0)x$nsem)>.
3925 See also C<IPC::SysV> and C<IPC::Semaphore> documentation.
3927 =item semget KEY,NSEMS,FLAGS
3929 Calls the System V IPC function semget. Returns the semaphore id, or
3930 the undefined value if there is an error. See also C<IPC::SysV> and
3931 C<IPC::SysV::Semaphore> documentation.
3933 =item semop KEY,OPSTRING
3935 Calls the System V IPC function semop to perform semaphore operations
3936 such as signaling and waiting. OPSTRING must be a packed array of
3937 semop structures. Each semop structure can be generated with
3938 C<pack("sss", $semnum, $semop, $semflag)>. The number of semaphore
3939 operations is implied by the length of OPSTRING. Returns true if
3940 successful, or false if there is an error. As an example, the
3941 following code waits on semaphore $semnum of semaphore id $semid:
3943 $semop = pack("sss", $semnum, -1, 0);
3944 die "Semaphore trouble: $!\n" unless semop($semid, $semop);
3946 To signal the semaphore, replace C<-1> with C<1>. See also C<IPC::SysV>
3947 and C<IPC::SysV::Semaphore> documentation.
3949 =item send SOCKET,MSG,FLAGS,TO
3951 =item send SOCKET,MSG,FLAGS
3953 Sends a message on a socket. Takes the same flags as the system call
3954 of the same name. On unconnected sockets you must specify a
3955 destination to send TO, in which case it does a C C<sendto>. Returns
3956 the number of characters sent, or the undefined value if there is an
3957 error. The C system call sendmsg(2) is currently unimplemented.
3958 See L<perlipc/"UDP: Message Passing"> for examples.
3960 =item setpgrp PID,PGRP
3962 Sets the current process group for the specified PID, C<0> for the current
3963 process. Will produce a fatal error if used on a machine that doesn't
3964 implement POSIX setpgid(2) or BSD setpgrp(2). If the arguments are omitted,
3965 it defaults to C<0,0>. Note that the BSD 4.2 version of C<setpgrp> does not
3966 accept any arguments, so only C<setpgrp(0,0)> is portable. See also
3969 =item setpriority WHICH,WHO,PRIORITY
3971 Sets the current priority for a process, a process group, or a user.
3972 (See setpriority(2).) Will produce a fatal error if used on a machine
3973 that doesn't implement setpriority(2).
3975 =item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
3977 Sets the socket option requested. Returns undefined if there is an
3978 error. OPTVAL may be specified as C<undef> if you don't want to pass an
3985 Shifts the first value of the array off and returns it, shortening the
3986 array by 1 and moving everything down. If there are no elements in the
3987 array, returns the undefined value. If ARRAY is omitted, shifts the
3988 C<@_> array within the lexical scope of subroutines and formats, and the
3989 C<@ARGV> array at file scopes or within the lexical scopes established by
3990 the C<eval ''>, C<BEGIN {}>, C<INIT {}>, C<CHECK {}>, and C<END {}>
3993 See also C<unshift>, C<push>, and C<pop>. C<Shift()> and C<unshift> do the
3994 same thing to the left end of an array that C<pop> and C<push> do to the
3997 =item shmctl ID,CMD,ARG
3999 Calls the System V IPC function shmctl. You'll probably have to say
4003 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
4004 then ARG must be a variable which will hold the returned C<shmid_ds>
4005 structure. Returns like ioctl: the undefined value for error, "C<0> but
4006 true" for zero, or the actual return value otherwise.
4007 See also C<IPC::SysV> documentation.
4009 =item shmget KEY,SIZE,FLAGS
4011 Calls the System V IPC function shmget. Returns the shared memory
4012 segment id, or the undefined value if there is an error.
4013 See also C<IPC::SysV> documentation.
4015 =item shmread ID,VAR,POS,SIZE
4017 =item shmwrite ID,STRING,POS,SIZE
4019 Reads or writes the System V shared memory segment ID starting at
4020 position POS for size SIZE by attaching to it, copying in/out, and
4021 detaching from it. When reading, VAR must be a variable that will
4022 hold the data read. When writing, if STRING is too long, only SIZE
4023 bytes are used; if STRING is too short, nulls are written to fill out
4024 SIZE bytes. Return true if successful, or false if there is an error.
4025 See also C<IPC::SysV> documentation and the C<IPC::Shareable> module
4028 =item shutdown SOCKET,HOW
4030 Shuts down a socket connection in the manner indicated by HOW, which
4031 has the same interpretation as in the system call of the same name.
4033 shutdown(SOCKET, 0); # I/we have stopped reading data
4034 shutdown(SOCKET, 1); # I/we have stopped writing data
4035 shutdown(SOCKET, 2); # I/we have stopped using this socket
4037 This is useful with sockets when you want to tell the other
4038 side you're done writing but not done reading, or vice versa.
4039 It's also a more insistent form of close because it also
4040 disables the file descriptor in any forked copies in other
4047 Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
4048 returns sine of C<$_>.
4050 For the inverse sine operation, you may use the C<Math::Trig::asin>
4051 function, or use this relation:
4053 sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
4059 Causes the script to sleep for EXPR seconds, or forever if no EXPR.
4060 May be interrupted if the process receives a signal such as C<SIGALRM>.
4061 Returns the number of seconds actually slept. You probably cannot
4062 mix C<alarm> and C<sleep> calls, because C<sleep> is often implemented
4065 On some older systems, it may sleep up to a full second less than what
4066 you requested, depending on how it counts seconds. Most modern systems
4067 always sleep the full amount. They may appear to sleep longer than that,
4068 however, because your process might not be scheduled right away in a
4069 busy multitasking system.
4071 For delays of finer granularity than one second, you may use Perl's
4072 C<syscall> interface to access setitimer(2) if your system supports
4073 it, or else see L</select> above. The Time::HiRes module from CPAN
4076 See also the POSIX module's C<sigpause> function.
4078 =item socket SOCKET,DOMAIN,TYPE,PROTOCOL
4080 Opens a socket of the specified kind and attaches it to filehandle
4081 SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for
4082 the system call of the same name. You should C<use Socket> first
4083 to get the proper definitions imported. See the examples in
4084 L<perlipc/"Sockets: Client/Server Communication">.
4086 On systems that support a close-on-exec flag on files, the flag will
4087 be set for the newly opened file descriptor, as determined by the
4088 value of $^F. See L<perlvar/$^F>.
4090 =item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
4092 Creates an unnamed pair of sockets in the specified domain, of the
4093 specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as
4094 for the system call of the same name. If unimplemented, yields a fatal
4095 error. Returns true if successful.
4097 On systems that support a close-on-exec flag on files, the flag will
4098 be set for the newly opened file descriptors, as determined by the value
4099 of $^F. See L<perlvar/$^F>.
4101 Some systems defined C<pipe> in terms of C<socketpair>, in which a call
4102 to C<pipe(Rdr, Wtr)> is essentially:
4105 socketpair(Rdr, Wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC);
4106 shutdown(Rdr, 1); # no more writing for reader
4107 shutdown(Wtr, 0); # no more reading for writer
4109 See L<perlipc> for an example of socketpair use.
4111 =item sort SUBNAME LIST
4113 =item sort BLOCK LIST
4117 Sorts the LIST and returns the sorted list value. If SUBNAME or BLOCK
4118 is omitted, C<sort>s in standard string comparison order. If SUBNAME is
4119 specified, it gives the name of a subroutine that returns an integer
4120 less than, equal to, or greater than C<0>, depending on how the elements
4121 of the list are to be ordered. (The C<< <=> >> and C<cmp>
4122 operators are extremely useful in such routines.) SUBNAME may be a
4123 scalar variable name (unsubscripted), in which case the value provides
4124 the name of (or a reference to) the actual subroutine to use. In place
4125 of a SUBNAME, you can provide a BLOCK as an anonymous, in-line sort
4128 If the subroutine's prototype is C<($$)>, the elements to be compared
4129 are passed by reference in C<@_>, as for a normal subroutine. This is
4130 slower than unprototyped subroutines, where the elements to be
4131 compared are passed into the subroutine
4132 as the package global variables $a and $b (see example below). Note that
4133 in the latter case, it is usually counter-productive to declare $a and
4136 In either case, the subroutine may not be recursive. The values to be
4137 compared are always passed by reference, so don't modify them.
4139 You also cannot exit out of the sort block or subroutine using any of the
4140 loop control operators described in L<perlsyn> or with C<goto>.
4142 When C<use locale> is in effect, C<sort LIST> sorts LIST according to the
4143 current collation locale. See L<perllocale>.
4148 @articles = sort @files;
4150 # same thing, but with explicit sort routine
4151 @articles = sort {$a cmp $b} @files;
4153 # now case-insensitively
4154 @articles = sort {uc($a) cmp uc($b)} @files;
4156 # same thing in reversed order
4157 @articles = sort {$b cmp $a} @files;
4159 # sort numerically ascending
4160 @articles = sort {$a <=> $b} @files;
4162 # sort numerically descending
4163 @articles = sort {$b <=> $a} @files;
4165 # this sorts the %age hash by value instead of key
4166 # using an in-line function
4167 @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
4169 # sort using explicit subroutine name
4171 $age{$a} <=> $age{$b}; # presuming numeric
4173 @sortedclass = sort byage @class;
4175 sub backwards { $b cmp $a }
4176 @harry = qw(dog cat x Cain Abel);
4177 @george = qw(gone chased yz Punished Axed);
4179 # prints AbelCaincatdogx
4180 print sort backwards @harry;
4181 # prints xdogcatCainAbel
4182 print sort @george, 'to', @harry;
4183 # prints AbelAxedCainPunishedcatchaseddoggonetoxyz
4185 # inefficiently sort by descending numeric compare using
4186 # the first integer after the first = sign, or the
4187 # whole record case-insensitively otherwise
4190 ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
4195 # same thing, but much more efficiently;
4196 # we'll build auxiliary indices instead
4200 push @nums, /=(\d+)/;
4205 $nums[$b] <=> $nums[$a]
4207 $caps[$a] cmp $caps[$b]
4211 # same thing, but without any temps
4212 @new = map { $_->[0] }
4213 sort { $b->[1] <=> $a->[1]
4216 } map { [$_, /=(\d+)/, uc($_)] } @old;
4218 # using a prototype allows you to use any comparison subroutine
4219 # as a sort subroutine (including other package's subroutines)
4221 sub backwards ($$) { $_[1] cmp $_[0]; } # $a and $b are not set here
4224 @new = sort other::backwards @old;
4226 If you're using strict, you I<must not> declare $a
4227 and $b as lexicals. They are package globals. That means
4228 if you're in the C<main> package, it's
4230 @articles = sort {$main::b <=> $main::a} @files;
4234 @articles = sort {$::b <=> $::a} @files;
4236 but if you're in the C<FooPack> package, it's
4238 @articles = sort {$FooPack::b <=> $FooPack::a} @files;
4240 The comparison function is required to behave. If it returns
4241 inconsistent results (sometimes saying C<$x[1]> is less than C<$x[2]> and
4242 sometimes saying the opposite, for example) the results are not
4245 =item splice ARRAY,OFFSET,LENGTH,LIST
4247 =item splice ARRAY,OFFSET,LENGTH
4249 =item splice ARRAY,OFFSET
4253 Removes the elements designated by OFFSET and LENGTH from an array, and
4254 replaces them with the elements of LIST, if any. In list context,
4255 returns the elements removed from the array. In scalar context,
4256 returns the last element removed, or C<undef> if no elements are
4257 removed. The array grows or shrinks as necessary.
4258 If OFFSET is negative then it starts that far from the end of the array.
4259 If LENGTH is omitted, removes everything from OFFSET onward.
4260 If LENGTH is negative, leaves that many elements off the end of the array.
4261 If both OFFSET and LENGTH are omitted, removes everything.
4263 The following equivalences hold (assuming C<$[ == 0>):
4265 push(@a,$x,$y) splice(@a,@a,0,$x,$y)
4266 pop(@a) splice(@a,-1)
4267 shift(@a) splice(@a,0,1)
4268 unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
4269 $a[$x] = $y splice(@a,$x,1,$y)
4271 Example, assuming array lengths are passed before arrays:
4273 sub aeq { # compare two list values
4274 my(@a) = splice(@_,0,shift);
4275 my(@b) = splice(@_,0,shift);
4276 return 0 unless @a == @b; # same len?
4278 return 0 if pop(@a) ne pop(@b);
4282 if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
4284 =item split /PATTERN/,EXPR,LIMIT
4286 =item split /PATTERN/,EXPR
4288 =item split /PATTERN/
4292 Splits a string into a list of strings and returns that list. By default,
4293 empty leading fields are preserved, and empty trailing ones are deleted.
4295 If not in list context, returns the number of fields found and splits into
4296 the C<@_> array. (In list context, you can force the split into C<@_> by
4297 using C<??> as the pattern delimiters, but it still returns the list
4298 value.) The use of implicit split to C<@_> is deprecated, however, because
4299 it clobbers your subroutine arguments.
4301 If EXPR is omitted, splits the C<$_> string. If PATTERN is also omitted,
4302 splits on whitespace (after skipping any leading whitespace). Anything
4303 matching PATTERN is taken to be a delimiter separating the fields. (Note
4304 that the delimiter may be longer than one character.)
4306 If LIMIT is specified and positive, splits into no more than that
4307 many fields (though it may split into fewer). If LIMIT is unspecified
4308 or zero, trailing null fields are stripped (which potential users
4309 of C<pop> would do well to remember). If LIMIT is negative, it is
4310 treated as if an arbitrarily large LIMIT had been specified.
4312 A pattern matching the null string (not to be confused with
4313 a null pattern C<//>, which is just one member of the set of patterns
4314 matching a null string) will split the value of EXPR into separate
4315 characters at each point it matches that way. For example:
4317 print join(':', split(/ */, 'hi there'));
4319 produces the output 'h:i:t:h:e:r:e'.
4321 The LIMIT parameter can be used to split a line partially
4323 ($login, $passwd, $remainder) = split(/:/, $_, 3);
4325 When assigning to a list, if LIMIT is omitted, Perl supplies a LIMIT
4326 one larger than the number of variables in the list, to avoid
4327 unnecessary work. For the list above LIMIT would have been 4 by
4328 default. In time critical applications it behooves you not to split
4329 into more fields than you really need.
4331 If the PATTERN contains parentheses, additional list elements are
4332 created from each matching substring in the delimiter.
4334 split(/([,-])/, "1-10,20", 3);
4336 produces the list value
4338 (1, '-', 10, ',', 20)
4340 If you had the entire header of a normal Unix email message in $header,
4341 you could split it up into fields and their values this way:
4343 $header =~ s/\n\s+/ /g; # fix continuation lines
4344 %hdrs = (UNIX_FROM => split /^(\S*?):\s*/m, $header);
4346 The pattern C</PATTERN/> may be replaced with an expression to specify
4347 patterns that vary at runtime. (To do runtime compilation only once,
4348 use C</$variable/o>.)
4350 As a special case, specifying a PATTERN of space (C<' '>) will split on
4351 white space just as C<split> with no arguments does. Thus, C<split(' ')> can
4352 be used to emulate B<awk>'s default behavior, whereas C<split(/ /)>
4353 will give you as many null initial fields as there are leading spaces.
4354 A C<split> on C</\s+/> is like a C<split(' ')> except that any leading
4355 whitespace produces a null first field. A C<split> with no arguments
4356 really does a C<split(' ', $_)> internally.
4360 open(PASSWD, '/etc/passwd');
4362 ($login, $passwd, $uid, $gid,
4363 $gcos, $home, $shell) = split(/:/);
4367 (Note that $shell above will still have a newline on it. See L</chop>,
4368 L</chomp>, and L</join>.)
4370 =item sprintf FORMAT, LIST
4372 Returns a string formatted by the usual C<printf> conventions of the
4373 C library function C<sprintf>. See L<sprintf(3)> or L<printf(3)>
4374 on your system for an explanation of the general principles.
4376 Perl does its own C<sprintf> formatting--it emulates the C
4377 function C<sprintf>, but it doesn't use it (except for floating-point
4378 numbers, and even then only the standard modifiers are allowed). As a
4379 result, any non-standard extensions in your local C<sprintf> are not
4380 available from Perl.
4382 Perl's C<sprintf> permits the following universally-known conversions:
4385 %c a character with the given number
4387 %d a signed integer, in decimal
4388 %u an unsigned integer, in decimal
4389 %o an unsigned integer, in octal
4390 %x an unsigned integer, in hexadecimal
4391 %e a floating-point number, in scientific notation
4392 %f a floating-point number, in fixed decimal notation
4393 %g a floating-point number, in %e or %f notation
4395 In addition, Perl permits the following widely-supported conversions:
4397 %X like %x, but using upper-case letters
4398 %E like %e, but using an upper-case "E"
4399 %G like %g, but with an upper-case "E" (if applicable)
4400 %b an unsigned integer, in binary
4401 %p a pointer (outputs the Perl value's address in hexadecimal)
4402 %n special: *stores* the number of characters output so far
4403 into the next variable in the parameter list
4405 Finally, for backward (and we do mean "backward") compatibility, Perl
4406 permits these unnecessary but widely-supported conversions:
4409 %D a synonym for %ld
4410 %U a synonym for %lu
4411 %O a synonym for %lo
4414 Perl permits the following universally-known flags between the C<%>
4415 and the conversion letter:
4417 space prefix positive number with a space
4418 + prefix positive number with a plus sign
4419 - left-justify within the field
4420 0 use zeros, not spaces, to right-justify
4421 # prefix non-zero octal with "0", non-zero hex with "0x"
4422 number minimum field width
4423 .number "precision": digits after decimal point for
4424 floating-point, max length for string, minimum length
4426 l interpret integer as C type "long" or "unsigned long"
4427 h interpret integer as C type "short" or "unsigned short"
4428 If no flags, interpret integer as C type "int" or "unsigned"
4430 There are also two Perl-specific flags:
4432 V interpret integer as Perl's standard integer type
4433 v interpret string as a vector of integers, output as
4434 numbers separated either by dots, or by an arbitrary
4435 string received from the argument list when the flag
4438 Where a number would appear in the flags, an asterisk (C<*>) may be
4439 used instead, in which case Perl uses the next item in the parameter
4440 list as the given number (that is, as the field width or precision).
4441 If a field width obtained through C<*> is negative, it has the same
4442 effect as the C<-> flag: left-justification.
4444 The C<v> flag is useful for displaying ordinal values of characters
4445 in arbitrary strings:
4447 printf "version is v%vd\n", $^V; # Perl's version
4448 printf "address is %*vX\n", ":", $addr; # IPv6 address
4449 printf "bits are %*vb\n", " ", $bits; # random bitstring
4451 If C<use locale> is in effect, the character used for the decimal
4452 point in formatted real numbers is affected by the LC_NUMERIC locale.
4455 If Perl understands "quads" (64-bit integers) (this requires
4456 either that the platform natively support quads or that Perl
4457 be specifically compiled to support quads), the characters
4461 print quads, and they may optionally be preceded by
4469 You can find out whether your Perl supports quads via L<Config>:
4472 ($Config{use64bitint} eq 'define' || $Config{longsize} == 8) &&
4475 If Perl understands "long doubles" (this requires that the platform
4476 support long doubles), the flags
4480 may optionally be preceded by
4488 You can find out whether your Perl supports long doubles via L<Config>:
4491 $Config{d_longdbl} eq 'define' && print "long doubles\n";
4497 Return the square root of EXPR. If EXPR is omitted, returns square
4498 root of C<$_>. Only works on non-negative operands, unless you've
4499 loaded the standard Math::Complex module.
4502 print sqrt(-2); # prints 1.4142135623731i
4508 Sets the random number seed for the C<rand> operator. If EXPR is
4509 omitted, uses a semi-random value supplied by the kernel (if it supports
4510 the F</dev/urandom> device) or based on the current time and process
4511 ID, among other things. In versions of Perl prior to 5.004 the default
4512 seed was just the current C<time>. This isn't a particularly good seed,
4513 so many old programs supply their own seed value (often C<time ^ $$> or
4514 C<time ^ ($$ + ($$ << 15))>), but that isn't necessary any more.
4516 In fact, it's usually not necessary to call C<srand> at all, because if
4517 it is not called explicitly, it is called implicitly at the first use of
4518 the C<rand> operator. However, this was not the case in version of Perl
4519 before 5.004, so if your script will run under older Perl versions, it
4520 should call C<srand>.
4522 Note that you need something much more random than the default seed for
4523 cryptographic purposes. Checksumming the compressed output of one or more
4524 rapidly changing operating system status programs is the usual method. For
4527 srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip`);
4529 If you're particularly concerned with this, see the C<Math::TrulyRandom>
4532 Do I<not> call C<srand> multiple times in your program unless you know
4533 exactly what you're doing and why you're doing it. The point of the
4534 function is to "seed" the C<rand> function so that C<rand> can produce
4535 a different sequence each time you run your program. Just do it once at the
4536 top of your program, or you I<won't> get random numbers out of C<rand>!
4538 Frequently called programs (like CGI scripts) that simply use
4542 for a seed can fall prey to the mathematical property that
4546 one-third of the time. So don't do that.
4548 =item stat FILEHANDLE
4554 Returns a 13-element list giving the status info for a file, either
4555 the file opened via FILEHANDLE, or named by EXPR. If EXPR is omitted,
4556 it stats C<$_>. Returns a null list if the stat fails. Typically used
4559 ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
4560 $atime,$mtime,$ctime,$blksize,$blocks)
4563 Not all fields are supported on all filesystem types. Here are the
4564 meaning of the fields:
4566 0 dev device number of filesystem
4568 2 mode file mode (type and permissions)
4569 3 nlink number of (hard) links to the file
4570 4 uid numeric user ID of file's owner
4571 5 gid numeric group ID of file's owner
4572 6 rdev the device identifier (special files only)
4573 7 size total size of file, in bytes
4574 8 atime last access time in seconds since the epoch
4575 9 mtime last modify time in seconds since the epoch
4576 10 ctime inode change time (NOT creation time!) in seconds since the epoch
4577 11 blksize preferred block size for file system I/O
4578 12 blocks actual number of blocks allocated
4580 (The epoch was at 00:00 January 1, 1970 GMT.)
4582 If stat is passed the special filehandle consisting of an underline, no
4583 stat is done, but the current contents of the stat structure from the
4584 last stat or filetest are returned. Example:
4586 if (-x $file && (($d) = stat(_)) && $d < 0) {
4587 print "$file is executable NFS file\n";
4590 (This works on machines only for which the device number is negative
4593 Because the mode contains both the file type and its permissions, you
4594 should mask off the file type portion and (s)printf using a C<"%o">
4595 if you want to see the real permissions.
4597 $mode = (stat($filename))[2];
4598 printf "Permissions are %04o\n", $mode & 07777;
4600 In scalar context, C<stat> returns a boolean value indicating success
4601 or failure, and, if successful, sets the information associated with
4602 the special filehandle C<_>.
4604 The File::stat module provides a convenient, by-name access mechanism:
4607 $sb = stat($filename);
4608 printf "File is %s, size is %s, perm %04o, mtime %s\n",
4609 $filename, $sb->size, $sb->mode & 07777,
4610 scalar localtime $sb->mtime;
4612 You can import symbolic mode constants (C<S_IF*>) and functions
4613 (C<S_IS*>) from the Fcntl module:
4617 $mode = (stat($filename))[2];
4619 $user_rwx = ($mode & S_IRWXU) >> 6;
4620 $group_read = ($mode & S_IRGRP) >> 3;
4621 $other_execute = $mode & S_IXOTH;
4623 printf "Permissions are %04o\n", S_ISMODE($mode), "\n";
4625 $is_setuid = $mode & S_ISUID;
4626 $is_setgid = S_ISDIR($mode);
4628 You could write the last two using the C<-u> and C<-d> operators.
4629 The commonly available S_IF* constants are
4631 # Permissions: read, write, execute, for user, group, others.
4633 S_IRWXU S_IRUSR S_IWUSR S_IXUSR
4634 S_IRWXG S_IRGRP S_IWGRP S_IXGRP
4635 S_IRWXO S_IROTH S_IWOTH S_IXOTH
4637 # Setuid/Setgid/Stickiness.
4639 S_ISUID S_ISGID S_ISVTX S_ISTXT
4641 # File types. Not necessarily all are available on your system.
4643 S_IFREG S_IFDIR S_IFLNK S_IFBLK S_ISCHR S_IFIFO S_IFSOCK S_IFWHT S_ENFMT
4645 # The following are compatibility aliases for S_IRUSR, S_IWUSR, S_IXUSR.
4647 S_IREAD S_IWRITE S_IEXEC
4649 and the S_IF* functions are
4651 S_IFMODE($mode) the part of $mode containg the permission bits
4652 and the setuid/setgid/sticky bits
4654 S_IFMT($mode) the part of $mode containing the file type
4655 which can be bit-anded with e.g. S_IFREG
4656 or with the following functions
4658 # The operators -f, -d, -l, -b, -c, -p, and -s.
4660 S_ISREG($mode) S_ISDIR($mode) S_ISLNK($mode)
4661 S_ISBLK($mode) S_ISCHR($mode) S_ISFIFO($mode) S_ISSOCK($mode)
4663 # No direct -X operator counterpart, but for the first one
4664 # the -g operator is often equivalent. The ENFMT stands for
4665 # record flocking enforcement, a platform-dependent feature.
4667 S_ISENFMT($mode) S_ISWHT($mode)
4669 See your native chmod(2) and stat(2) documentation for more details
4670 about the S_* constants.
4676 Takes extra time to study SCALAR (C<$_> if unspecified) in anticipation of
4677 doing many pattern matches on the string before it is next modified.
4678 This may or may not save time, depending on the nature and number of
4679 patterns you are searching on, and on the distribution of character
4680 frequencies in the string to be searched--you probably want to compare
4681 run times with and without it to see which runs faster. Those loops
4682 which scan for many short constant strings (including the constant
4683 parts of more complex patterns) will benefit most. You may have only
4684 one C<study> active at a time--if you study a different scalar the first
4685 is "unstudied". (The way C<study> works is this: a linked list of every
4686 character in the string to be searched is made, so we know, for
4687 example, where all the C<'k'> characters are. From each search string,
4688 the rarest character is selected, based on some static frequency tables
4689 constructed from some C programs and English text. Only those places
4690 that contain this "rarest" character are examined.)
4692 For example, here is a loop that inserts index producing entries
4693 before any line containing a certain pattern:
4697 print ".IX foo\n" if /\bfoo\b/;
4698 print ".IX bar\n" if /\bbar\b/;
4699 print ".IX blurfl\n" if /\bblurfl\b/;
4704 In searching for C</\bfoo\b/>, only those locations in C<$_> that contain C<f>
4705 will be looked at, because C<f> is rarer than C<o>. In general, this is
4706 a big win except in pathological cases. The only question is whether
4707 it saves you more time than it took to build the linked list in the
4710 Note that if you have to look for strings that you don't know till
4711 runtime, you can build an entire loop as a string and C<eval> that to
4712 avoid recompiling all your patterns all the time. Together with
4713 undefining C<$/> to input entire files as one record, this can be very
4714 fast, often faster than specialized programs like fgrep(1). The following
4715 scans a list of files (C<@files>) for a list of words (C<@words>), and prints
4716 out the names of those files that contain a match:
4718 $search = 'while (<>) { study;';
4719 foreach $word (@words) {
4720 $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
4725 eval $search; # this screams
4726 $/ = "\n"; # put back to normal input delimiter
4727 foreach $file (sort keys(%seen)) {
4735 =item sub NAME BLOCK
4737 This is subroutine definition, not a real function I<per se>. With just a
4738 NAME (and possibly prototypes or attributes), it's just a forward declaration.
4739 Without a NAME, it's an anonymous function declaration, and does actually
4740 return a value: the CODE ref of the closure you just created. See L<perlsub>
4741 and L<perlref> for details.
4743 =item substr EXPR,OFFSET,LENGTH,REPLACEMENT
4745 =item substr EXPR,OFFSET,LENGTH
4747 =item substr EXPR,OFFSET
4749 Extracts a substring out of EXPR and returns it. First character is at
4750 offset C<0>, or whatever you've set C<$[> to (but don't do that).
4751 If OFFSET is negative (or more precisely, less than C<$[>), starts
4752 that far from the end of the string. If LENGTH is omitted, returns
4753 everything to the end of the string. If LENGTH is negative, leaves that
4754 many characters off the end of the string.
4756 You can use the substr() function as an lvalue, in which case EXPR
4757 must itself be an lvalue. If you assign something shorter than LENGTH,
4758 the string will shrink, and if you assign something longer than LENGTH,
4759 the string will grow to accommodate it. To keep the string the same
4760 length you may need to pad or chop your value using C<sprintf>.
4762 If OFFSET and LENGTH specify a substring that is partly outside the
4763 string, only the part within the string is returned. If the substring
4764 is beyond either end of the string, substr() returns the undefined
4765 value and produces a warning. When used as an lvalue, specifying a
4766 substring that is entirely outside the string is a fatal error.
4767 Here's an example showing the behavior for boundary cases:
4770 substr($name, 4) = 'dy'; # $name is now 'freddy'
4771 my $null = substr $name, 6, 2; # returns '' (no warning)
4772 my $oops = substr $name, 7; # returns undef, with warning
4773 substr($name, 7) = 'gap'; # fatal error
4775 An alternative to using substr() as an lvalue is to specify the
4776 replacement string as the 4th argument. This allows you to replace
4777 parts of the EXPR and return what was there before in one operation,
4778 just as you can with splice().
4780 =item symlink OLDFILE,NEWFILE
4782 Creates a new filename symbolically linked to the old filename.
4783 Returns C<1> for success, C<0> otherwise. On systems that don't support
4784 symbolic links, produces a fatal error at run time. To check for that,
4787 $symlink_exists = eval { symlink("",""); 1 };
4791 Calls the system call specified as the first element of the list,
4792 passing the remaining elements as arguments to the system call. If
4793 unimplemented, produces a fatal error. The arguments are interpreted
4794 as follows: if a given argument is numeric, the argument is passed as
4795 an int. If not, the pointer to the string value is passed. You are
4796 responsible to make sure a string is pre-extended long enough to
4797 receive any result that might be written into a string. You can't use a
4798 string literal (or other read-only string) as an argument to C<syscall>
4799 because Perl has to assume that any string pointer might be written
4801 integer arguments are not literals and have never been interpreted in a
4802 numeric context, you may need to add C<0> to them to force them to look
4803 like numbers. This emulates the C<syswrite> function (or vice versa):
4805 require 'syscall.ph'; # may need to run h2ph
4807 syscall(&SYS_write, fileno(STDOUT), $s, length $s);
4809 Note that Perl supports passing of up to only 14 arguments to your system call,
4810 which in practice should usually suffice.
4812 Syscall returns whatever value returned by the system call it calls.
4813 If the system call fails, C<syscall> returns C<-1> and sets C<$!> (errno).
4814 Note that some system calls can legitimately return C<-1>. The proper
4815 way to handle such calls is to assign C<$!=0;> before the call and
4816 check the value of C<$!> if syscall returns C<-1>.
4818 There's a problem with C<syscall(&SYS_pipe)>: it returns the file
4819 number of the read end of the pipe it creates. There is no way
4820 to retrieve the file number of the other end. You can avoid this
4821 problem by using C<pipe> instead.
4823 =item sysopen FILEHANDLE,FILENAME,MODE
4825 =item sysopen FILEHANDLE,FILENAME,MODE,PERMS
4827 Opens the file whose filename is given by FILENAME, and associates it
4828 with FILEHANDLE. If FILEHANDLE is an expression, its value is used as
4829 the name of the real filehandle wanted. This function calls the
4830 underlying operating system's C<open> function with the parameters
4831 FILENAME, MODE, PERMS.
4833 The possible values and flag bits of the MODE parameter are
4834 system-dependent; they are available via the standard module C<Fcntl>.
4835 See the documentation of your operating system's C<open> to see which
4836 values and flag bits are available. You may combine several flags
4837 using the C<|>-operator.
4839 Some of the most common values are C<O_RDONLY> for opening the file in
4840 read-only mode, C<O_WRONLY> for opening the file in write-only mode,
4841 and C<O_RDWR> for opening the file in read-write mode, and.
4843 For historical reasons, some values work on almost every system
4844 supported by perl: zero means read-only, one means write-only, and two
4845 means read/write. We know that these values do I<not> work under
4846 OS/390 & VM/ESA Unix and on the Macintosh; you probably don't want to
4847 use them in new code.
4849 If the file named by FILENAME does not exist and the C<open> call creates
4850 it (typically because MODE includes the C<O_CREAT> flag), then the value of
4851 PERMS specifies the permissions of the newly created file. If you omit
4852 the PERMS argument to C<sysopen>, Perl uses the octal value C<0666>.
4853 These permission values need to be in octal, and are modified by your
4854 process's current C<umask>.
4856 In many systems the C<O_EXCL> flag is available for opening files in
4857 exclusive mode. This is B<not> locking: exclusiveness means here that
4858 if the file already exists, sysopen() fails. The C<O_EXCL> wins
4861 Sometimes you may want to truncate an already-existing file: C<O_TRUNC>.
4863 You should seldom if ever use C<0644> as argument to C<sysopen>, because
4864 that takes away the user's option to have a more permissive umask.
4865 Better to omit it. See the perlfunc(1) entry on C<umask> for more
4868 Note that C<sysopen> depends on the fdopen() C library function.
4869 On many UNIX systems, fdopen() is known to fail when file descriptors
4870 exceed a certain value, typically 255. If you need more file
4871 descriptors than that, consider rebuilding Perl to use the C<sfio>
4872 library, or perhaps using the POSIX::open() function.
4874 See L<perlopentut> for a kinder, gentler explanation of opening files.
4876 =item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
4878 =item sysread FILEHANDLE,SCALAR,LENGTH
4880 Attempts to read LENGTH bytes of data into variable SCALAR from the
4881 specified FILEHANDLE, using the system call read(2). It bypasses stdio,
4882 so mixing this with other kinds of reads, C<print>, C<write>,
4883 C<seek>, C<tell>, or C<eof> can cause confusion because stdio
4884 usually buffers data. Returns the number of bytes actually read, C<0>
4885 at end of file, or undef if there was an error. SCALAR will be grown or
4886 shrunk so that the last byte actually read is the last byte of the
4887 scalar after the read.
4889 An OFFSET may be specified to place the read data at some place in the
4890 string other than the beginning. A negative OFFSET specifies
4891 placement at that many bytes counting backwards from the end of the
4892 string. A positive OFFSET greater than the length of SCALAR results
4893 in the string being padded to the required size with C<"\0"> bytes before
4894 the result of the read is appended.
4896 There is no syseof() function, which is ok, since eof() doesn't work
4897 very well on device files (like ttys) anyway. Use sysread() and check
4898 for a return value for 0 to decide whether you're done.
4900 =item sysseek FILEHANDLE,POSITION,WHENCE
4902 Sets FILEHANDLE's system position using the system call lseek(2). It
4903 bypasses stdio, so mixing this with reads (other than C<sysread>),
4904 C<print>, C<write>, C<seek>, C<tell>, or C<eof> may cause confusion.
4905 FILEHANDLE may be an expression whose value gives the name of the
4906 filehandle. The values for WHENCE are C<0> to set the new position to
4907 POSITION, C<1> to set the it to the current position plus POSITION,
4908 and C<2> to set it to EOF plus POSITION (typically negative). For
4909 WHENCE, you may also use the constants C<SEEK_SET>, C<SEEK_CUR>, and
4910 C<SEEK_END> (start of the file, current position, end of the file)
4911 from the Fcntl module.
4913 Returns the new position, or the undefined value on failure. A position
4914 of zero is returned as the string C<"0 but true">; thus C<sysseek> returns
4915 true on success and false on failure, yet you can still easily determine
4920 =item system PROGRAM LIST
4922 Does exactly the same thing as C<exec LIST>, except that a fork is
4923 done first, and the parent process waits for the child process to
4924 complete. Note that argument processing varies depending on the
4925 number of arguments. If there is more than one argument in LIST,
4926 or if LIST is an array with more than one value, starts the program
4927 given by the first element of the list with arguments given by the
4928 rest of the list. If there is only one scalar argument, the argument
4929 is checked for shell metacharacters, and if there are any, the
4930 entire argument is passed to the system's command shell for parsing
4931 (this is C</bin/sh -c> on Unix platforms, but varies on other
4932 platforms). If there are no shell metacharacters in the argument,
4933 it is split into words and passed directly to C<execvp>, which is
4936 Beginning with v5.6.0, Perl will attempt to flush all files opened for
4937 output before any operation that may do a fork, but this may not be
4938 supported on some platforms (see L<perlport>). To be safe, you may need
4939 to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
4940 of C<IO::Handle> on any open handles.
4942 The return value is the exit status of the program as
4943 returned by the C<wait> call. To get the actual exit value divide by
4944 256. See also L</exec>. This is I<not> what you want to use to capture
4945 the output from a command, for that you should use merely backticks or
4946 C<qx//>, as described in L<perlop/"`STRING`">. Return value of -1
4947 indicates a failure to start the program (inspect $! for the reason).
4949 Like C<exec>, C<system> allows you to lie to a program about its name if
4950 you use the C<system PROGRAM LIST> syntax. Again, see L</exec>.
4952 Because C<system> and backticks block C<SIGINT> and C<SIGQUIT>, killing the
4953 program they're running doesn't actually interrupt your program.
4955 @args = ("command", "arg1", "arg2");
4957 or die "system @args failed: $?"
4959 You can check all the failure possibilities by inspecting
4962 $exit_value = $? >> 8;
4963 $signal_num = $? & 127;
4964 $dumped_core = $? & 128;
4966 When the arguments get executed via the system shell, results
4967 and return codes will be subject to its quirks and capabilities.
4968 See L<perlop/"`STRING`"> and L</exec> for details.
4970 =item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
4972 =item syswrite FILEHANDLE,SCALAR,LENGTH
4974 =item syswrite FILEHANDLE,SCALAR
4976 Attempts to write LENGTH bytes of data from variable SCALAR to the
4977 specified FILEHANDLE, using the system call write(2). If LENGTH
4978 is not specified, writes whole SCALAR. It bypasses stdio, so mixing
4979 this with reads (other than C<sysread())>, C<print>, C<write>,
4980 C<seek>, C<tell>, or C<eof> may cause confusion because stdio
4981 usually buffers data. Returns the number of bytes actually written,
4982 or C<undef> if there was an error. If the LENGTH is greater than
4983 the available data in the SCALAR after the OFFSET, only as much
4984 data as is available will be written.
4986 An OFFSET may be specified to write the data from some part of the
4987 string other than the beginning. A negative OFFSET specifies writing
4988 that many bytes counting backwards from the end of the string. In the
4989 case the SCALAR is empty you can use OFFSET but only zero offset.
4991 =item tell FILEHANDLE
4995 Returns the current position for FILEHANDLE. FILEHANDLE may be an
4996 expression whose value gives the name of the actual filehandle. If
4997 FILEHANDLE is omitted, assumes the file last read.
4999 There is no C<systell> function. Use C<sysseek(FH, 0, 1)> for that.
5001 =item telldir DIRHANDLE
5003 Returns the current position of the C<readdir> routines on DIRHANDLE.
5004 Value may be given to C<seekdir> to access a particular location in a
5005 directory. Has the same caveats about possible directory compaction as
5006 the corresponding system library routine.
5008 =item tie VARIABLE,CLASSNAME,LIST
5010 This function binds a variable to a package class that will provide the
5011 implementation for the variable. VARIABLE is the name of the variable
5012 to be enchanted. CLASSNAME is the name of a class implementing objects
5013 of correct type. Any additional arguments are passed to the C<new>
5014 method of the class (meaning C<TIESCALAR>, C<TIEHANDLE>, C<TIEARRAY>,
5015 or C<TIEHASH>). Typically these are arguments such as might be passed
5016 to the C<dbm_open()> function of C. The object returned by the C<new>
5017 method is also returned by the C<tie> function, which would be useful
5018 if you want to access other methods in CLASSNAME.
5020 Note that functions such as C<keys> and C<values> may return huge lists
5021 when used on large objects, like DBM files. You may prefer to use the
5022 C<each> function to iterate over such. Example:
5024 # print out history file offsets
5026 tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
5027 while (($key,$val) = each %HIST) {
5028 print $key, ' = ', unpack('L',$val), "\n";
5032 A class implementing a hash should have the following methods:
5034 TIEHASH classname, LIST
5036 STORE this, key, value
5041 NEXTKEY this, lastkey
5044 A class implementing an ordinary array should have the following methods:
5046 TIEARRAY classname, LIST
5048 STORE this, key, value
5050 STORESIZE this, count
5056 SPLICE this, offset, length, LIST
5060 A class implementing a file handle should have the following methods:
5062 TIEHANDLE classname, LIST
5063 READ this, scalar, length, offset
5066 WRITE this, scalar, length, offset
5068 PRINTF this, format, LIST
5072 A class implementing a scalar should have the following methods:
5074 TIESCALAR classname, LIST
5079 Not all methods indicated above need be implemented. See L<perltie>,
5080 L<Tie::Hash>, L<Tie::Array>, L<Tie::Scalar>, and L<Tie::Handle>.
5082 Unlike C<dbmopen>, the C<tie> function will not use or require a module
5083 for you--you need to do that explicitly yourself. See L<DB_File>
5084 or the F<Config> module for interesting C<tie> implementations.
5086 For further details see L<perltie>, L<"tied VARIABLE">.
5090 Returns a reference to the object underlying VARIABLE (the same value
5091 that was originally returned by the C<tie> call that bound the variable
5092 to a package.) Returns the undefined value if VARIABLE isn't tied to a
5097 Returns the number of non-leap seconds since whatever time the system
5098 considers to be the epoch (that's 00:00:00, January 1, 1904 for MacOS,
5099 and 00:00:00 UTC, January 1, 1970 for most other systems).
5100 Suitable for feeding to C<gmtime> and C<localtime>.
5102 For measuring time in better granularity than one second,
5103 you may use either the Time::HiRes module from CPAN, or
5104 if you have gettimeofday(2), you may be able to use the
5105 C<syscall> interface of Perl, see L<perlfaq8> for details.
5109 Returns a four-element list giving the user and system times, in
5110 seconds, for this process and the children of this process.
5112 ($user,$system,$cuser,$csystem) = times;
5116 The transliteration operator. Same as C<y///>. See L<perlop>.
5118 =item truncate FILEHANDLE,LENGTH
5120 =item truncate EXPR,LENGTH
5122 Truncates the file opened on FILEHANDLE, or named by EXPR, to the
5123 specified length. Produces a fatal error if truncate isn't implemented
5124 on your system. Returns true if successful, the undefined value
5131 Returns an uppercased version of EXPR. This is the internal function
5132 implementing the C<\U> escape in double-quoted strings.
5133 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
5134 Under Unicode (C<use utf8>) it uses the standard Unicode uppercase mappings. (It
5135 does not attempt to do titlecase mapping on initial letters. See C<ucfirst> for that.)
5137 If EXPR is omitted, uses C<$_>.
5143 Returns the value of EXPR with the first character
5144 in uppercase (titlecase in Unicode). This is
5145 the internal function implementing the C<\u> escape in double-quoted strings.
5146 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
5149 If EXPR is omitted, uses C<$_>.
5155 Sets the umask for the process to EXPR and returns the previous value.
5156 If EXPR is omitted, merely returns the current umask.
5158 The Unix permission C<rwxr-x---> is represented as three sets of three
5159 bits, or three octal digits: C<0750> (the leading 0 indicates octal
5160 and isn't one of the digits). The C<umask> value is such a number
5161 representing disabled permissions bits. The permission (or "mode")
5162 values you pass C<mkdir> or C<sysopen> are modified by your umask, so
5163 even if you tell C<sysopen> to create a file with permissions C<0777>,
5164 if your umask is C<0022> then the file will actually be created with
5165 permissions C<0755>. If your C<umask> were C<0027> (group can't
5166 write; others can't read, write, or execute), then passing
5167 C<sysopen> C<0666> would create a file with mode C<0640> (C<0666 &~
5170 Here's some advice: supply a creation mode of C<0666> for regular
5171 files (in C<sysopen>) and one of C<0777> for directories (in
5172 C<mkdir>) and executable files. This gives users the freedom of
5173 choice: if they want protected files, they might choose process umasks
5174 of C<022>, C<027>, or even the particularly antisocial mask of C<077>.
5175 Programs should rarely if ever make policy decisions better left to
5176 the user. The exception to this is when writing files that should be
5177 kept private: mail files, web browser cookies, I<.rhosts> files, and
5180 If umask(2) is not implemented on your system and you are trying to
5181 restrict access for I<yourself> (i.e., (EXPR & 0700) > 0), produces a
5182 fatal error at run time. If umask(2) is not implemented and you are
5183 not trying to restrict access for yourself, returns C<undef>.
5185 Remember that a umask is a number, usually given in octal; it is I<not> a
5186 string of octal digits. See also L</oct>, if all you have is a string.
5192 Undefines the value of EXPR, which must be an lvalue. Use only on a
5193 scalar value, an array (using C<@>), a hash (using C<%>), a subroutine
5194 (using C<&>), or a typeglob (using <*>). (Saying C<undef $hash{$key}>
5195 will probably not do what you expect on most predefined variables or
5196 DBM list values, so don't do that; see L<delete>.) Always returns the
5197 undefined value. You can omit the EXPR, in which case nothing is
5198 undefined, but you still get an undefined value that you could, for
5199 instance, return from a subroutine, assign to a variable or pass as a
5200 parameter. Examples:
5203 undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'};
5207 undef *xyz; # destroys $xyz, @xyz, %xyz, &xyz, etc.
5208 return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it;
5209 select undef, undef, undef, 0.25;
5210 ($a, $b, undef, $c) = &foo; # Ignore third value returned
5212 Note that this is a unary operator, not a list operator.
5218 Deletes a list of files. Returns the number of files successfully
5221 $cnt = unlink 'a', 'b', 'c';
5225 Note: C<unlink> will not delete directories unless you are superuser and
5226 the B<-U> flag is supplied to Perl. Even if these conditions are
5227 met, be warned that unlinking a directory can inflict damage on your
5228 filesystem. Use C<rmdir> instead.
5230 If LIST is omitted, uses C<$_>.
5232 =item unpack TEMPLATE,EXPR
5234 C<unpack> does the reverse of C<pack>: it takes a string
5235 and expands it out into a list of values.
5236 (In scalar context, it returns merely the first value produced.)
5238 The string is broken into chunks described by the TEMPLATE. Each chunk
5239 is converted separately to a value. Typically, either the string is a result
5240 of C<pack>, or the bytes of the string represent a C structure of some
5243 The TEMPLATE has the same format as in the C<pack> function.
5244 Here's a subroutine that does substring:
5247 my($what,$where,$howmuch) = @_;
5248 unpack("x$where a$howmuch", $what);
5253 sub ordinal { unpack("c",$_[0]); } # same as ord()
5255 In addition to fields allowed in pack(), you may prefix a field with
5256 a %<number> to indicate that
5257 you want a <number>-bit checksum of the items instead of the items
5258 themselves. Default is a 16-bit checksum. Checksum is calculated by
5259 summing numeric values of expanded values (for string fields the sum of
5260 C<ord($char)> is taken, for bit fields the sum of zeroes and ones).
5262 For example, the following
5263 computes the same number as the System V sum program:
5267 unpack("%32C*",<>) % 65535;
5270 The following efficiently counts the number of set bits in a bit vector:
5272 $setbits = unpack("%32b*", $selectmask);
5274 The C<p> and C<P> formats should be used with care. Since Perl
5275 has no way of checking whether the value passed to C<unpack()>
5276 corresponds to a valid memory location, passing a pointer value that's
5277 not known to be valid is likely to have disastrous consequences.
5279 If the repeat count of a field is larger than what the remainder of
5280 the input string allows, repeat count is decreased. If the input string
5281 is longer than one described by the TEMPLATE, the rest is ignored.
5283 See L</pack> for more examples and notes.
5285 =item untie VARIABLE
5287 Breaks the binding between a variable and a package. (See C<tie>.)
5289 =item unshift ARRAY,LIST
5291 Does the opposite of a C<shift>. Or the opposite of a C<push>,
5292 depending on how you look at it. Prepends list to the front of the
5293 array, and returns the new number of elements in the array.
5295 unshift(ARGV, '-e') unless $ARGV[0] =~ /^-/;
5297 Note the LIST is prepended whole, not one element at a time, so the
5298 prepended elements stay in the same order. Use C<reverse> to do the
5301 =item use Module VERSION LIST
5303 =item use Module VERSION
5305 =item use Module LIST
5311 Imports some semantics into the current package from the named module,
5312 generally by aliasing certain subroutine or variable names into your
5313 package. It is exactly equivalent to
5315 BEGIN { require Module; import Module LIST; }
5317 except that Module I<must> be a bareword.
5319 VERSION, which can be specified as a literal of the form v5.6.1, demands
5320 that the current version of Perl (C<$^V> or $PERL_VERSION) be at least
5321 as recent as that version. (For compatibility with older versions of Perl,
5322 a numeric literal will also be interpreted as VERSION.) If the version
5323 of the running Perl interpreter is less than VERSION, then an error
5324 message is printed and Perl exits immediately without attempting to
5325 parse the rest of the file. Compare with L</require>, which can do a
5326 similar check at run time.
5328 use v5.6.1; # compile time version check
5330 use 5.005_03; # float version allowed for compatibility
5332 This is often useful if you need to check the current Perl version before
5333 C<use>ing library modules that have changed in incompatible ways from
5334 older versions of Perl. (We try not to do this more than we have to.)
5336 The C<BEGIN> forces the C<require> and C<import> to happen at compile time. The
5337 C<require> makes sure the module is loaded into memory if it hasn't been
5338 yet. The C<import> is not a builtin--it's just an ordinary static method
5339 call into the C<Module> package to tell the module to import the list of
5340 features back into the current package. The module can implement its
5341 C<import> method any way it likes, though most modules just choose to
5342 derive their C<import> method via inheritance from the C<Exporter> class that
5343 is defined in the C<Exporter> module. See L<Exporter>. If no C<import>
5344 method can be found then the call is skipped.
5346 If you don't want your namespace altered, explicitly supply an empty list:
5350 That is exactly equivalent to
5352 BEGIN { require Module }
5354 If the VERSION argument is present between Module and LIST, then the
5355 C<use> will call the VERSION method in class Module with the given
5356 version as an argument. The default VERSION method, inherited from
5357 the UNIVERSAL class, croaks if the given version is larger than the
5358 value of the variable C<$Module::VERSION>.
5360 Again, there is a distinction between omitting LIST (C<import> called
5361 with no arguments) and an explicit empty LIST C<()> (C<import> not
5362 called). Note that there is no comma after VERSION!
5364 Because this is a wide-open interface, pragmas (compiler directives)
5365 are also implemented this way. Currently implemented pragmas are:
5369 use sigtrap qw(SEGV BUS);
5370 use strict qw(subs vars refs);
5371 use subs qw(afunc blurfl);
5372 use warnings qw(all);
5374 Some of these pseudo-modules import semantics into the current
5375 block scope (like C<strict> or C<integer>, unlike ordinary modules,
5376 which import symbols into the current package (which are effective
5377 through the end of the file).
5379 There's a corresponding C<no> command that unimports meanings imported
5380 by C<use>, i.e., it calls C<unimport Module LIST> instead of C<import>.
5386 If no C<unimport> method can be found the call fails with a fatal error.
5388 See L<perlmod> for a list of standard modules and pragmas.
5392 Changes the access and modification times on each file of a list of
5393 files. The first two elements of the list must be the NUMERICAL access
5394 and modification times, in that order. Returns the number of files
5395 successfully changed. The inode change time of each file is set
5396 to the current time. This code has the same effect as the C<touch>
5397 command if the files already exist:
5401 utime $now, $now, @ARGV;
5405 Returns a list consisting of all the values of the named hash. (In a
5406 scalar context, returns the number of values.) The values are
5407 returned in an apparently random order. The actual random order is
5408 subject to change in future versions of perl, but it is guaranteed to
5409 be the same order as either the C<keys> or C<each> function would
5410 produce on the same (unmodified) hash.
5412 Note that you cannot modify the values of a hash this way, because the
5413 returned list is just a copy. You need to use a hash slice for that,
5414 since it's lvaluable in a way that values() is not.
5416 for (values %hash) { s/foo/bar/g } # FAILS!
5417 for (@hash{keys %hash}) { s/foo/bar/g } # ok
5419 As a side effect, calling values() resets the HASH's internal iterator.
5420 See also C<keys>, C<each>, and C<sort>.
5422 =item vec EXPR,OFFSET,BITS
5424 Treats the string in EXPR as a bit vector made up of elements of
5425 width BITS, and returns the value of the element specified by OFFSET
5426 as an unsigned integer. BITS therefore specifies the number of bits
5427 that are reserved for each element in the bit vector. This must
5428 be a power of two from 1 to 32 (or 64, if your platform supports
5431 If BITS is 8, "elements" coincide with bytes of the input string.
5433 If BITS is 16 or more, bytes of the input string are grouped into chunks
5434 of size BITS/8, and each group is converted to a number as with
5435 pack()/unpack() with big-endian formats C<n>/C<N> (and analoguously
5436 for BITS==64). See L<"pack"> for details.
5438 If bits is 4 or less, the string is broken into bytes, then the bits
5439 of each byte are broken into 8/BITS groups. Bits of a byte are
5440 numbered in a little-endian-ish way, as in C<0x01>, C<0x02>,
5441 C<0x04>, C<0x08>, C<0x10>, C<0x20>, C<0x40>, C<0x80>. For example,
5442 breaking the single input byte C<chr(0x36)> into two groups gives a list
5443 C<(0x6, 0x3)>; breaking it into 4 groups gives C<(0x2, 0x1, 0x3, 0x0)>.
5445 C<vec> may also be assigned to, in which case parentheses are needed
5446 to give the expression the correct precedence as in
5448 vec($image, $max_x * $x + $y, 8) = 3;
5450 If the selected element is off the end of the string, the value 0 is
5451 returned. If an element off the end of the string is written to,
5452 Perl will first extend the string with sufficiently many zero bytes.
5454 Strings created with C<vec> can also be manipulated with the logical
5455 operators C<|>, C<&>, C<^>, and C<~>. These operators will assume a bit
5456 vector operation is desired when both operands are strings.
5457 See L<perlop/"Bitwise String Operators">.
5459 The following code will build up an ASCII string saying C<'PerlPerlPerl'>.
5460 The comments show the string after each step. Note that this code works
5461 in the same way on big-endian or little-endian machines.
5464 vec($foo, 0, 32) = 0x5065726C; # 'Perl'
5466 # $foo eq "Perl" eq "\x50\x65\x72\x6C", 32 bits
5467 print vec($foo, 0, 8); # prints 80 == 0x50 == ord('P')
5469 vec($foo, 2, 16) = 0x5065; # 'PerlPe'
5470 vec($foo, 3, 16) = 0x726C; # 'PerlPerl'
5471 vec($foo, 8, 8) = 0x50; # 'PerlPerlP'
5472 vec($foo, 9, 8) = 0x65; # 'PerlPerlPe'
5473 vec($foo, 20, 4) = 2; # 'PerlPerlPe' . "\x02"
5474 vec($foo, 21, 4) = 7; # 'PerlPerlPer'
5476 vec($foo, 45, 2) = 3; # 'PerlPerlPer' . "\x0c"
5477 vec($foo, 93, 1) = 1; # 'PerlPerlPer' . "\x2c"
5478 vec($foo, 94, 1) = 1; # 'PerlPerlPerl'
5481 To transform a bit vector into a string or list of 0's and 1's, use these:
5483 $bits = unpack("b*", $vector);
5484 @bits = split(//, unpack("b*", $vector));
5486 If you know the exact length in bits, it can be used in place of the C<*>.
5488 Here is an example to illustrate how the bits actually fall in place:
5494 unpack("V",$_) 01234567890123456789012345678901
5495 ------------------------------------------------------------------
5500 for ($shift=0; $shift < $width; ++$shift) {
5501 for ($off=0; $off < 32/$width; ++$off) {
5502 $str = pack("B*", "0"x32);
5503 $bits = (1<<$shift);
5504 vec($str, $off, $width) = $bits;
5505 $res = unpack("b*",$str);
5506 $val = unpack("V", $str);
5513 vec($_,@#,@#) = @<< == @######### @>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
5514 $off, $width, $bits, $val, $res
5518 Regardless of the machine architecture on which it is run, the above
5519 example should print the following table:
5522 unpack("V",$_) 01234567890123456789012345678901
5523 ------------------------------------------------------------------
5524 vec($_, 0, 1) = 1 == 1 10000000000000000000000000000000
5525 vec($_, 1, 1) = 1 == 2 01000000000000000000000000000000
5526 vec($_, 2, 1) = 1 == 4 00100000000000000000000000000000
5527 vec($_, 3, 1) = 1 == 8 00010000000000000000000000000000
5528 vec($_, 4, 1) = 1 == 16 00001000000000000000000000000000
5529 vec($_, 5, 1) = 1 == 32 00000100000000000000000000000000
5530 vec($_, 6, 1) = 1 == 64 00000010000000000000000000000000
5531 vec($_, 7, 1) = 1 == 128 00000001000000000000000000000000
5532 vec($_, 8, 1) = 1 == 256 00000000100000000000000000000000
5533 vec($_, 9, 1) = 1 == 512 00000000010000000000000000000000
5534 vec($_,10, 1) = 1 == 1024 00000000001000000000000000000000
5535 vec($_,11, 1) = 1 == 2048 00000000000100000000000000000000
5536 vec($_,12, 1) = 1 == 4096 00000000000010000000000000000000
5537 vec($_,13, 1) = 1 == 8192 00000000000001000000000000000000
5538 vec($_,14, 1) = 1 == 16384 00000000000000100000000000000000
5539 vec($_,15, 1) = 1 == 32768 00000000000000010000000000000000
5540 vec($_,16, 1) = 1 == 65536 00000000000000001000000000000000
5541 vec($_,17, 1) = 1 == 131072 00000000000000000100000000000000
5542 vec($_,18, 1) = 1 == 262144 00000000000000000010000000000000
5543 vec($_,19, 1) = 1 == 524288 00000000000000000001000000000000
5544 vec($_,20, 1) = 1 == 1048576 00000000000000000000100000000000
5545 vec($_,21, 1) = 1 == 2097152 00000000000000000000010000000000
5546 vec($_,22, 1) = 1 == 4194304 00000000000000000000001000000000
5547 vec($_,23, 1) = 1 == 8388608 00000000000000000000000100000000
5548 vec($_,24, 1) = 1 == 16777216 00000000000000000000000010000000
5549 vec($_,25, 1) = 1 == 33554432 00000000000000000000000001000000
5550 vec($_,26, 1) = 1 == 67108864 00000000000000000000000000100000
5551 vec($_,27, 1) = 1 == 134217728 00000000000000000000000000010000
5552 vec($_,28, 1) = 1 == 268435456 00000000000000000000000000001000
5553 vec($_,29, 1) = 1 == 536870912 00000000000000000000000000000100
5554 vec($_,30, 1) = 1 == 1073741824 00000000000000000000000000000010
5555 vec($_,31, 1) = 1 == 2147483648 00000000000000000000000000000001
5556 vec($_, 0, 2) = 1 == 1 10000000000000000000000000000000
5557 vec($_, 1, 2) = 1 == 4 00100000000000000000000000000000
5558 vec($_, 2, 2) = 1 == 16 00001000000000000000000000000000
5559 vec($_, 3, 2) = 1 == 64 00000010000000000000000000000000
5560 vec($_, 4, 2) = 1 == 256 00000000100000000000000000000000
5561 vec($_, 5, 2) = 1 == 1024 00000000001000000000000000000000
5562 vec($_, 6, 2) = 1 == 4096 00000000000010000000000000000000
5563 vec($_, 7, 2) = 1 == 16384 00000000000000100000000000000000
5564 vec($_, 8, 2) = 1 == 65536 00000000000000001000000000000000
5565 vec($_, 9, 2) = 1 == 262144 00000000000000000010000000000000
5566 vec($_,10, 2) = 1 == 1048576 00000000000000000000100000000000
5567 vec($_,11, 2) = 1 == 4194304 00000000000000000000001000000000
5568 vec($_,12, 2) = 1 == 16777216 00000000000000000000000010000000
5569 vec($_,13, 2) = 1 == 67108864 00000000000000000000000000100000
5570 vec($_,14, 2) = 1 == 268435456 00000000000000000000000000001000
5571 vec($_,15, 2) = 1 == 1073741824 00000000000000000000000000000010
5572 vec($_, 0, 2) = 2 == 2 01000000000000000000000000000000
5573 vec($_, 1, 2) = 2 == 8 00010000000000000000000000000000
5574 vec($_, 2, 2) = 2 == 32 00000100000000000000000000000000
5575 vec($_, 3, 2) = 2 == 128 00000001000000000000000000000000
5576 vec($_, 4, 2) = 2 == 512 00000000010000000000000000000000
5577 vec($_, 5, 2) = 2 == 2048 00000000000100000000000000000000
5578 vec($_, 6, 2) = 2 == 8192 00000000000001000000000000000000
5579 vec($_, 7, 2) = 2 == 32768 00000000000000010000000000000000
5580 vec($_, 8, 2) = 2 == 131072 00000000000000000100000000000000
5581 vec($_, 9, 2) = 2 == 524288 00000000000000000001000000000000
5582 vec($_,10, 2) = 2 == 2097152 00000000000000000000010000000000
5583 vec($_,11, 2) = 2 == 8388608 00000000000000000000000100000000
5584 vec($_,12, 2) = 2 == 33554432 00000000000000000000000001000000
5585 vec($_,13, 2) = 2 == 134217728 00000000000000000000000000010000
5586 vec($_,14, 2) = 2 == 536870912 00000000000000000000000000000100
5587 vec($_,15, 2) = 2 == 2147483648 00000000000000000000000000000001
5588 vec($_, 0, 4) = 1 == 1 10000000000000000000000000000000
5589 vec($_, 1, 4) = 1 == 16 00001000000000000000000000000000
5590 vec($_, 2, 4) = 1 == 256 00000000100000000000000000000000
5591 vec($_, 3, 4) = 1 == 4096 00000000000010000000000000000000
5592 vec($_, 4, 4) = 1 == 65536 00000000000000001000000000000000
5593 vec($_, 5, 4) = 1 == 1048576 00000000000000000000100000000000
5594 vec($_, 6, 4) = 1 == 16777216 00000000000000000000000010000000
5595 vec($_, 7, 4) = 1 == 268435456 00000000000000000000000000001000
5596 vec($_, 0, 4) = 2 == 2 01000000000000000000000000000000
5597 vec($_, 1, 4) = 2 == 32 00000100000000000000000000000000
5598 vec($_, 2, 4) = 2 == 512 00000000010000000000000000000000
5599 vec($_, 3, 4) = 2 == 8192 00000000000001000000000000000000
5600 vec($_, 4, 4) = 2 == 131072 00000000000000000100000000000000
5601 vec($_, 5, 4) = 2 == 2097152 00000000000000000000010000000000
5602 vec($_, 6, 4) = 2 == 33554432 00000000000000000000000001000000
5603 vec($_, 7, 4) = 2 == 536870912 00000000000000000000000000000100
5604 vec($_, 0, 4) = 4 == 4 00100000000000000000000000000000
5605 vec($_, 1, 4) = 4 == 64 00000010000000000000000000000000
5606 vec($_, 2, 4) = 4 == 1024 00000000001000000000000000000000
5607 vec($_, 3, 4) = 4 == 16384 00000000000000100000000000000000
5608 vec($_, 4, 4) = 4 == 262144 00000000000000000010000000000000
5609 vec($_, 5, 4) = 4 == 4194304 00000000000000000000001000000000
5610 vec($_, 6, 4) = 4 == 67108864 00000000000000000000000000100000
5611 vec($_, 7, 4) = 4 == 1073741824 00000000000000000000000000000010
5612 vec($_, 0, 4) = 8 == 8 00010000000000000000000000000000
5613 vec($_, 1, 4) = 8 == 128 00000001000000000000000000000000
5614 vec($_, 2, 4) = 8 == 2048 00000000000100000000000000000000
5615 vec($_, 3, 4) = 8 == 32768 00000000000000010000000000000000
5616 vec($_, 4, 4) = 8 == 524288 00000000000000000001000000000000
5617 vec($_, 5, 4) = 8 == 8388608 00000000000000000000000100000000
5618 vec($_, 6, 4) = 8 == 134217728 00000000000000000000000000010000
5619 vec($_, 7, 4) = 8 == 2147483648 00000000000000000000000000000001
5620 vec($_, 0, 8) = 1 == 1 10000000000000000000000000000000
5621 vec($_, 1, 8) = 1 == 256 00000000100000000000000000000000
5622 vec($_, 2, 8) = 1 == 65536 00000000000000001000000000000000
5623 vec($_, 3, 8) = 1 == 16777216 00000000000000000000000010000000
5624 vec($_, 0, 8) = 2 == 2 01000000000000000000000000000000
5625 vec($_, 1, 8) = 2 == 512 00000000010000000000000000000000
5626 vec($_, 2, 8) = 2 == 131072 00000000000000000100000000000000
5627 vec($_, 3, 8) = 2 == 33554432 00000000000000000000000001000000
5628 vec($_, 0, 8) = 4 == 4 00100000000000000000000000000000
5629 vec($_, 1, 8) = 4 == 1024 00000000001000000000000000000000
5630 vec($_, 2, 8) = 4 == 262144 00000000000000000010000000000000
5631 vec($_, 3, 8) = 4 == 67108864 00000000000000000000000000100000
5632 vec($_, 0, 8) = 8 == 8 00010000000000000000000000000000
5633 vec($_, 1, 8) = 8 == 2048 00000000000100000000000000000000
5634 vec($_, 2, 8) = 8 == 524288 00000000000000000001000000000000
5635 vec($_, 3, 8) = 8 == 134217728 00000000000000000000000000010000
5636 vec($_, 0, 8) = 16 == 16 00001000000000000000000000000000
5637 vec($_, 1, 8) = 16 == 4096 00000000000010000000000000000000
5638 vec($_, 2, 8) = 16 == 1048576 00000000000000000000100000000000
5639 vec($_, 3, 8) = 16 == 268435456 00000000000000000000000000001000
5640 vec($_, 0, 8) = 32 == 32 00000100000000000000000000000000
5641 vec($_, 1, 8) = 32 == 8192 00000000000001000000000000000000
5642 vec($_, 2, 8) = 32 == 2097152 00000000000000000000010000000000
5643 vec($_, 3, 8) = 32 == 536870912 00000000000000000000000000000100
5644 vec($_, 0, 8) = 64 == 64 00000010000000000000000000000000
5645 vec($_, 1, 8) = 64 == 16384 00000000000000100000000000000000
5646 vec($_, 2, 8) = 64 == 4194304 00000000000000000000001000000000
5647 vec($_, 3, 8) = 64 == 1073741824 00000000000000000000000000000010
5648 vec($_, 0, 8) = 128 == 128 00000001000000000000000000000000
5649 vec($_, 1, 8) = 128 == 32768 00000000000000010000000000000000
5650 vec($_, 2, 8) = 128 == 8388608 00000000000000000000000100000000
5651 vec($_, 3, 8) = 128 == 2147483648 00000000000000000000000000000001
5655 Behaves like the wait(2) system call on your system: it waits for a child
5656 process to terminate and returns the pid of the deceased process, or
5657 C<-1> if there are no child processes. The status is returned in C<$?>.
5658 Note that a return value of C<-1> could mean that child processes are
5659 being automatically reaped, as described in L<perlipc>.
5661 =item waitpid PID,FLAGS
5663 Waits for a particular child process to terminate and returns the pid of
5664 the deceased process, or C<-1> if there is no such child process. On some
5665 systems, a value of 0 indicates that there are processes still running.
5666 The status is returned in C<$?>. If you say
5668 use POSIX ":sys_wait_h";
5671 $kid = waitpid(-1,&WNOHANG);
5674 then you can do a non-blocking wait for all pending zombie processes.
5675 Non-blocking wait is available on machines supporting either the
5676 waitpid(2) or wait4(2) system calls. However, waiting for a particular
5677 pid with FLAGS of C<0> is implemented everywhere. (Perl emulates the
5678 system call by remembering the status values of processes that have
5679 exited but have not been harvested by the Perl script yet.)
5681 Note that on some systems, a return value of C<-1> could mean that child
5682 processes are being automatically reaped. See L<perlipc> for details,
5683 and for other examples.
5687 Returns true if the context of the currently executing subroutine is
5688 looking for a list value. Returns false if the context is looking
5689 for a scalar. Returns the undefined value if the context is looking
5690 for no value (void context).
5692 return unless defined wantarray; # don't bother doing more
5693 my @a = complex_calculation();
5694 return wantarray ? @a : "@a";
5696 This function should have been named wantlist() instead.
5700 Produces a message on STDERR just like C<die>, but doesn't exit or throw
5703 If LIST is empty and C<$@> already contains a value (typically from a
5704 previous eval) that value is used after appending C<"\t...caught">
5705 to C<$@>. This is useful for staying almost, but not entirely similar to
5708 If C<$@> is empty then the string C<"Warning: Something's wrong"> is used.
5710 No message is printed if there is a C<$SIG{__WARN__}> handler
5711 installed. It is the handler's responsibility to deal with the message
5712 as it sees fit (like, for instance, converting it into a C<die>). Most
5713 handlers must therefore make arrangements to actually display the
5714 warnings that they are not prepared to deal with, by calling C<warn>
5715 again in the handler. Note that this is quite safe and will not
5716 produce an endless loop, since C<__WARN__> hooks are not called from
5719 You will find this behavior is slightly different from that of
5720 C<$SIG{__DIE__}> handlers (which don't suppress the error text, but can
5721 instead call C<die> again to change it).
5723 Using a C<__WARN__> handler provides a powerful way to silence all
5724 warnings (even the so-called mandatory ones). An example:
5726 # wipe out *all* compile-time warnings
5727 BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } }
5729 my $foo = 20; # no warning about duplicate my $foo,
5730 # but hey, you asked for it!
5731 # no compile-time or run-time warnings before here
5734 # run-time warnings enabled after here
5735 warn "\$foo is alive and $foo!"; # does show up
5737 See L<perlvar> for details on setting C<%SIG> entries, and for more
5738 examples. See the Carp module for other kinds of warnings using its
5739 carp() and cluck() functions.
5741 =item write FILEHANDLE
5747 Writes a formatted record (possibly multi-line) to the specified FILEHANDLE,
5748 using the format associated with that file. By default the format for
5749 a file is the one having the same name as the filehandle, but the
5750 format for the current output channel (see the C<select> function) may be set
5751 explicitly by assigning the name of the format to the C<$~> variable.
5753 Top of form processing is handled automatically: if there is
5754 insufficient room on the current page for the formatted record, the
5755 page is advanced by writing a form feed, a special top-of-page format
5756 is used to format the new page header, and then the record is written.
5757 By default the top-of-page format is the name of the filehandle with
5758 "_TOP" appended, but it may be dynamically set to the format of your
5759 choice by assigning the name to the C<$^> variable while the filehandle is
5760 selected. The number of lines remaining on the current page is in
5761 variable C<$->, which can be set to C<0> to force a new page.
5763 If FILEHANDLE is unspecified, output goes to the current default output
5764 channel, which starts out as STDOUT but may be changed by the
5765 C<select> operator. If the FILEHANDLE is an EXPR, then the expression
5766 is evaluated and the resulting string is used to look up the name of
5767 the FILEHANDLE at run time. For more on formats, see L<perlform>.
5769 Note that write is I<not> the opposite of C<read>. Unfortunately.
5773 The transliteration operator. Same as C<tr///>. See L<perlop>.