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 (E<gt>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">.
383 Arranges to have a SIGALRM delivered to this process after the
384 specified number of seconds have elapsed. If SECONDS is not specified,
385 the value stored in C<$_> is used. (On some machines,
386 unfortunately, the elapsed time may be up to one second less than you
387 specified because of how seconds are counted.) Only one timer may be
388 counting at once. Each call disables the previous timer, and an
389 argument of C<0> may be supplied to cancel the previous timer without
390 starting a new one. The returned value is the amount of time remaining
391 on the previous timer.
393 For delays of finer granularity than one second, you may use Perl's
394 four-argument version of select() leaving the first three arguments
395 undefined, or you might be able to use the C<syscall> interface to
396 access setitimer(2) if your system supports it. The Time::HiRes module
397 from CPAN may also prove useful.
399 It is usually a mistake to intermix C<alarm> and C<sleep> calls.
400 (C<sleep> may be internally implemented in your system with C<alarm>)
402 If you want to use C<alarm> to time out a system call you need to use an
403 C<eval>/C<die> pair. You can't rely on the alarm causing the system call to
404 fail with C<$!> set to C<EINTR> because Perl sets up signal handlers to
405 restart system calls on some systems. Using C<eval>/C<die> always works,
406 modulo the caveats given in L<perlipc/"Signals">.
409 local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
411 $nread = sysread SOCKET, $buffer, $size;
415 die unless $@ eq "alarm\n"; # propagate unexpected errors
424 Returns the arctangent of Y/X in the range -PI to PI.
426 For the tangent operation, you may use the C<POSIX::tan()>
427 function, or use the familiar relation:
429 sub tan { sin($_[0]) / cos($_[0]) }
431 =item bind SOCKET,NAME
433 Binds a network address to a socket, just as the bind system call
434 does. Returns true if it succeeded, false otherwise. NAME should be a
435 packed address of the appropriate type for the socket. See the examples in
436 L<perlipc/"Sockets: Client/Server Communication">.
438 =item binmode FILEHANDLE
440 Arranges for FILEHANDLE to be read or written in "binary" mode on
441 systems whose run-time libraries force the programmer to guess
442 between binary and text files. If FILEHANDLE is an expression, the
443 value is taken as the name of the filehandle. binmode() should be
444 called after the C<open> but before any I/O is done on the filehandle.
445 The only way to reset binary mode on a filehandle is to reopen the
448 The operating system, device drivers, C libraries, and Perl run-time
449 system all conspire to let the programmer conveniently treat a
450 simple, one-byte C<\n> as the line terminator, irrespective of its
451 external representation. On Unix and its brethren, the native file
452 representation exactly matches the internal representation, making
453 everyone's lives unbelievably simpler. Consequently, L<binmode>
454 has no effect under Unix, Plan9, or Mac OS, all of which use C<\n>
455 to end each line. (Unix and Plan9 think C<\n> means C<\cJ> and
456 C<\r> means C<\cM>, whereas the Mac goes the other way--it uses
457 C<\cM> for c<\n> and C<\cJ> to mean C<\r>. But that's ok, because
458 it's only one byte, and the internal and external representations
461 In legacy systems like MS-DOS and its embellishments, your program
462 sees a C<\n> as a simple C<\cJ> (just as in Unix), but oddly enough,
463 that's not what's physically stored on disk. What's worse, these
464 systems refuse to help you with this; it's up to you to remember
465 what to do. And you mustn't go applying binmode() with wild abandon,
466 either, because if your system does care about binmode(), then using
467 it when you shouldn't is just as perilous as failing to use it when
470 That means that on any version of Microsoft WinXX that you might
471 care to name (or not), binmode() causes C<\cM\cJ> sequences on disk
472 to be converted to C<\n> when read into your program, and causes
473 any C<\n> in your program to be converted back to C<\cM\cJ> on
474 output to disk. This sad discrepancy leads to no end of
475 problems in not just the readline operator, but also when using
476 seek(), tell(), and read() calls. See L<perlport> for other painful
477 details. See the C<$/> and C<$\> variables in L<perlvar> for how
478 to manually set your input and output line-termination sequences.
480 =item bless REF,CLASSNAME
484 This function tells the thingy referenced by REF that it is now an object
485 in the CLASSNAME package. If CLASSNAME is omitted, the current package
486 is used. Because a C<bless> is often the last thing in a constructor,
487 it returns the reference for convenience. Always use the two-argument
488 version if the function doing the blessing might be inherited by a
489 derived class. See L<perltoot> and L<perlobj> for more about the blessing
490 (and blessings) of objects.
492 Consider always blessing objects in CLASSNAMEs that are mixed case.
493 Namespaces with all lowercase names are considered reserved for
494 Perl pragmata. Builtin types have all uppercase names, so to prevent
495 confusion, you may wish to avoid such package names as well. Make sure
496 that CLASSNAME is a true value.
498 See L<perlmod/"Perl Modules">.
504 Returns the context of the current subroutine call. In scalar context,
505 returns the caller's package name if there is a caller, that is, if
506 we're in a subroutine or C<eval> or C<require>, and the undefined value
507 otherwise. In list context, returns
509 ($package, $filename, $line) = caller;
511 With EXPR, it returns some extra information that the debugger uses to
512 print a stack trace. The value of EXPR indicates how many call frames
513 to go back before the current one.
515 ($package, $filename, $line, $subroutine, $hasargs,
516 $wantarray, $evaltext, $is_require, $hints) = caller($i);
518 Here $subroutine may be C<"(eval)"> if the frame is not a subroutine
519 call, but an C<eval>. In such a case additional elements $evaltext and
520 C<$is_require> are set: C<$is_require> is true if the frame is created by a
521 C<require> or C<use> statement, $evaltext contains the text of the
522 C<eval EXPR> statement. In particular, for a C<eval BLOCK> statement,
523 $filename is C<"(eval)">, but $evaltext is undefined. (Note also that
524 each C<use> statement creates a C<require> frame inside an C<eval EXPR>)
525 frame. C<$hints> contains pragmatic hints that the caller was
526 compiled with. It currently only reflects the hint corresponding to
529 Furthermore, when called from within the DB package, caller returns more
530 detailed information: it sets the list variable C<@DB::args> to be the
531 arguments with which the subroutine was invoked.
533 Be aware that the optimizer might have optimized call frames away before
534 C<caller> had a chance to get the information. That means that C<caller(N)>
535 might not return information about the call frame you expect it do, for
536 C<N E<gt> 1>. In particular, C<@DB::args> might have information from the
537 previous time C<caller> was called.
541 Changes the working directory to EXPR, if possible. If EXPR is omitted,
542 changes to the user's home directory. Returns true upon success,
543 false otherwise. See the example under C<die>.
547 Changes the permissions of a list of files. The first element of the
548 list must be the numerical mode, which should probably be an octal
549 number, and which definitely should I<not> a string of octal digits:
550 C<0644> is okay, C<'0644'> is not. Returns the number of files
551 successfully changed. See also L</oct>, if all you have is a string.
553 $cnt = chmod 0755, 'foo', 'bar';
554 chmod 0755, @executables;
555 $mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to
557 $mode = '0644'; chmod oct($mode), 'foo'; # this is better
558 $mode = 0644; chmod $mode, 'foo'; # this is best
566 This safer version of L</chop> removes any trailing string
567 that corresponds to the current value of C<$/> (also known as
568 $INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
569 number of characters removed from all its arguments. It's often used to
570 remove the newline from the end of an input record when you're worried
571 that the final record may be missing its newline. When in paragraph
572 mode (C<$/ = "">), it removes all trailing newlines from the string.
573 When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is
574 a reference to an integer or the like, see L<perlvar>) chomp() won't
576 If VARIABLE is omitted, it chomps C<$_>. Example:
579 chomp; # avoid \n on last field
584 You can actually chomp anything that's an lvalue, including an assignment:
587 chomp($answer = <STDIN>);
589 If you chomp a list, each element is chomped, and the total number of
590 characters removed is returned.
598 Chops off the last character of a string and returns the character
599 chopped. It's used primarily to remove the newline from the end of an
600 input record, but is much more efficient than C<s/\n//> because it neither
601 scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
605 chop; # avoid \n on last field
610 You can actually chop anything that's an lvalue, including an assignment:
613 chop($answer = <STDIN>);
615 If you chop a list, each element is chopped. Only the value of the
616 last C<chop> is returned.
618 Note that C<chop> returns the last character. To return all but the last
619 character, use C<substr($string, 0, -1)>.
623 Changes the owner (and group) of a list of files. The first two
624 elements of the list must be the I<numeric> uid and gid, in that
625 order. A value of -1 in either position is interpreted by most
626 systems to leave that value unchanged. Returns the number of files
627 successfully changed.
629 $cnt = chown $uid, $gid, 'foo', 'bar';
630 chown $uid, $gid, @filenames;
632 Here's an example that looks up nonnumeric uids in the passwd file:
635 chomp($user = <STDIN>);
637 chomp($pattern = <STDIN>);
639 ($login,$pass,$uid,$gid) = getpwnam($user)
640 or die "$user not in passwd file";
642 @ary = glob($pattern); # expand filenames
643 chown $uid, $gid, @ary;
645 On most systems, you are not allowed to change the ownership of the
646 file unless you're the superuser, although you should be able to change
647 the group to any of your secondary groups. On insecure systems, these
648 restrictions may be relaxed, but this is not a portable assumption.
649 On POSIX systems, you can detect this condition this way:
651 use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
652 $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
658 Returns the character represented by that NUMBER in the character set.
659 For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
660 chr(0x263a) is a Unicode smiley face (but only within the scope of
661 a C<use utf8>). For the reverse, use L</ord>.
662 See L<utf8> for more about Unicode.
664 If NUMBER is omitted, uses C<$_>.
666 =item chroot FILENAME
670 This function works like the system call by the same name: it makes the
671 named directory the new root directory for all further pathnames that
672 begin with a C<"/"> by your process and all its children. (It doesn't
673 change your current working directory, which is unaffected.) For security
674 reasons, this call is restricted to the superuser. If FILENAME is
675 omitted, does a C<chroot> to C<$_>.
677 =item close FILEHANDLE
681 Closes the file or pipe associated with the file handle, returning true
682 only if stdio successfully flushes buffers and closes the system file
683 descriptor. Closes the currently selected filehandle if the argument
686 You don't have to close FILEHANDLE if you are immediately going to do
687 another C<open> on it, because C<open> will close it for you. (See
688 C<open>.) However, an explicit C<close> on an input file resets the line
689 counter (C<$.>), while the implicit close done by C<open> does not.
691 If the file handle came from a piped open C<close> will additionally
692 return false if one of the other system calls involved fails or if the
693 program exits with non-zero status. (If the only problem was that the
694 program exited non-zero C<$!> will be set to C<0>.) Closing a pipe
695 also waits for the process executing on the pipe to complete, in case you
696 want to look at the output of the pipe afterwards, and
697 implicitly puts the exit status value of that command into C<$?>.
699 Prematurely closing the read end of a pipe (i.e. before the process
700 writing to it at the other end has closed it) will result in a
701 SIGPIPE being delivered to the writer. If the other end can't
702 handle that, be sure to read all the data before closing the pipe.
706 open(OUTPUT, '|sort >foo') # pipe to sort
707 or die "Can't start sort: $!";
708 #... # print stuff to output
709 close OUTPUT # wait for sort to finish
710 or warn $! ? "Error closing sort pipe: $!"
711 : "Exit status $? from sort";
712 open(INPUT, 'foo') # get sort's results
713 or die "Can't open 'foo' for input: $!";
715 FILEHANDLE may be an expression whose value can be used as an indirect
716 filehandle, usually the real filehandle name.
718 =item closedir DIRHANDLE
720 Closes a directory opened by C<opendir> and returns the success of that
723 DIRHANDLE may be an expression whose value can be used as an indirect
724 dirhandle, usually the real dirhandle name.
726 =item connect SOCKET,NAME
728 Attempts to connect to a remote socket, just as the connect system call
729 does. Returns true if it succeeded, false otherwise. NAME should be a
730 packed address of the appropriate type for the socket. See the examples in
731 L<perlipc/"Sockets: Client/Server Communication">.
735 Actually a flow control statement rather than a function. If there is a
736 C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
737 C<foreach>), it is always executed just before the conditional is about to
738 be evaluated again, just like the third part of a C<for> loop in C. Thus
739 it can be used to increment a loop variable, even when the loop has been
740 continued via the C<next> statement (which is similar to the C C<continue>
743 C<last>, C<next>, or C<redo> may appear within a C<continue>
744 block. C<last> and C<redo> will behave as if they had been executed within
745 the main block. So will C<next>, but since it will execute a C<continue>
746 block, it may be more entertaining.
749 ### redo always comes here
752 ### next always comes here
754 # then back the top to re-check EXPR
756 ### last always comes here
758 Omitting the C<continue> section is semantically equivalent to using an
759 empty one, logically enough. In that case, C<next> goes directly back
760 to check the condition at the top of the loop.
764 Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
765 takes cosine of C<$_>.
767 For the inverse cosine operation, you may use the C<POSIX::acos()>
768 function, or use this relation:
770 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
772 =item crypt PLAINTEXT,SALT
774 Encrypts a string exactly like the crypt(3) function in the C library
775 (assuming that you actually have a version there that has not been
776 extirpated as a potential munition). This can prove useful for checking
777 the password file for lousy passwords, amongst other things. Only the
778 guys wearing white hats should do this.
780 Note that C<crypt> is intended to be a one-way function, much like breaking
781 eggs to make an omelette. There is no (known) corresponding decrypt
782 function. As a result, this function isn't all that useful for
783 cryptography. (For that, see your nearby CPAN mirror.)
785 When verifying an existing encrypted string you should use the encrypted
786 text as the salt (like C<crypt($plain, $crypted) eq $crypted>). This
787 allows your code to work with the standard C<crypt> and with more
788 exotic implementations. When choosing a new salt create a random two
789 character string whose characters come from the set C<[./0-9A-Za-z]>
790 (like C<join '', ('.', '/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>).
792 Here's an example that makes sure that whoever runs this program knows
795 $pwd = (getpwuid($<))[1];
799 chomp($word = <STDIN>);
803 if (crypt($word, $pwd) ne $pwd) {
809 Of course, typing in your own password to whoever asks you
812 The L<crypt> function is unsuitable for encrypting large quantities
813 of data, not least of all because you can't get the information
814 back. Look at the F<by-module/Crypt> and F<by-module/PGP> directories
815 on your favorite CPAN mirror for a slew of potentially useful
820 [This function has been largely superseded by the C<untie> function.]
822 Breaks the binding between a DBM file and a hash.
824 =item dbmopen HASH,DBNAME,MASK
826 [This function has been largely superseded by the C<tie> function.]
828 This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
829 hash. HASH is the name of the hash. (Unlike normal C<open>, the first
830 argument is I<not> a filehandle, even though it looks like one). DBNAME
831 is the name of the database (without the F<.dir> or F<.pag> extension if
832 any). If the database does not exist, it is created with protection
833 specified by MASK (as modified by the C<umask>). If your system supports
834 only the older DBM functions, you may perform only one C<dbmopen> in your
835 program. In older versions of Perl, if your system had neither DBM nor
836 ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
839 If you don't have write access to the DBM file, you can only read hash
840 variables, not set them. If you want to test whether you can write,
841 either use file tests or try setting a dummy hash entry inside an C<eval>,
842 which will trap the error.
844 Note that functions such as C<keys> and C<values> may return huge lists
845 when used on large DBM files. You may prefer to use the C<each>
846 function to iterate over large DBM files. Example:
848 # print out history file offsets
849 dbmopen(%HIST,'/usr/lib/news/history',0666);
850 while (($key,$val) = each %HIST) {
851 print $key, ' = ', unpack('L',$val), "\n";
855 See also L<AnyDBM_File> for a more general description of the pros and
856 cons of the various dbm approaches, as well as L<DB_File> for a particularly
859 You can control which DBM library you use by loading that library
860 before you call dbmopen():
863 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
864 or die "Can't open netscape history file: $!";
870 Returns a Boolean value telling whether EXPR has a value other than
871 the undefined value C<undef>. If EXPR is not present, C<$_> will be
874 Many operations return C<undef> to indicate failure, end of file,
875 system error, uninitialized variable, and other exceptional
876 conditions. This function allows you to distinguish C<undef> from
877 other values. (A simple Boolean test will not distinguish among
878 C<undef>, zero, the empty string, and C<"0">, which are all equally
879 false.) Note that since C<undef> is a valid scalar, its presence
880 doesn't I<necessarily> indicate an exceptional condition: C<pop>
881 returns C<undef> when its argument is an empty array, I<or> when the
882 element to return happens to be C<undef>.
884 You may also use C<defined(&func)> to check whether subroutine C<&func>
885 has ever been defined. The return value is unaffected by any forward
886 declarations of C<&foo>.
888 Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
889 used to report whether memory for that aggregate has ever been
890 allocated. This behavior may disappear in future versions of Perl.
891 You should instead use a simple test for size:
893 if (@an_array) { print "has array elements\n" }
894 if (%a_hash) { print "has hash members\n" }
896 When used on a hash element, it tells you whether the value is defined,
897 not whether the key exists in the hash. Use L</exists> for the latter
902 print if defined $switch{'D'};
903 print "$val\n" while defined($val = pop(@ary));
904 die "Can't readlink $sym: $!"
905 unless defined($value = readlink $sym);
906 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
907 $debugging = 0 unless defined $debugging;
909 Note: Many folks tend to overuse C<defined>, and then are surprised to
910 discover that the number C<0> and C<""> (the zero-length string) are, in fact,
911 defined values. For example, if you say
915 The pattern match succeeds, and C<$1> is defined, despite the fact that it
916 matched "nothing". But it didn't really match nothing--rather, it
917 matched something that happened to be zero characters long. This is all
918 very above-board and honest. When a function returns an undefined value,
919 it's an admission that it couldn't give you an honest answer. So you
920 should use C<defined> only when you're questioning the integrity of what
921 you're trying to do. At other times, a simple comparison to C<0> or C<""> is
924 See also L</undef>, L</exists>, L</ref>.
928 Deletes the specified key(s) and their associated values from a hash.
929 For each key, returns the deleted value associated with that key, or
930 the undefined value if there was no such key. Deleting from C<$ENV{}>
931 modifies the environment. Deleting from a hash tied to a DBM file
932 deletes the entry from the DBM file. (But deleting from a C<tie>d hash
933 doesn't necessarily return anything.)
935 The following deletes all the values of a hash:
937 foreach $key (keys %HASH) {
943 delete @HASH{keys %HASH}
945 But both of these are slower than just assigning the empty list
948 %hash = (); # completely empty %hash
949 undef %hash; # forget %hash every existed
951 Note that the EXPR can be arbitrarily complicated as long as the final
952 operation is a hash element lookup or hash slice:
954 delete $ref->[$x][$y]{$key};
955 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
959 Outside an C<eval>, prints the value of LIST to C<STDERR> and
960 exits with the current value of C<$!> (errno). If C<$!> is C<0>,
961 exits with the value of C<($? E<gt>E<gt> 8)> (backtick `command`
962 status). If C<($? E<gt>E<gt> 8)> is C<0>, exits with C<255>. Inside
963 an C<eval(),> the error message is stuffed into C<$@> and the
964 C<eval> is terminated with the undefined value. This makes
965 C<die> the way to raise an exception.
969 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
970 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
972 If the value of EXPR does not end in a newline, the current script line
973 number and input line number (if any) are also printed, and a newline
974 is supplied. Note that the "input line number" (also known as "chunk")
975 is subject to whatever notion of "line" happens to be currently in
976 effect, and is also available as the special variable C<$.>.
977 See L<perlvar/"$/"> and L<perlvar/"$.">.
979 Hint: sometimes appending C<", stopped"> to your message
980 will cause it to make better sense when the string C<"at foo line 123"> is
981 appended. Suppose you are running script "canasta".
983 die "/etc/games is no good";
984 die "/etc/games is no good, stopped";
986 produce, respectively
988 /etc/games is no good at canasta line 123.
989 /etc/games is no good, stopped at canasta line 123.
991 See also exit(), warn(), and the Carp module.
993 If LIST is empty and C<$@> already contains a value (typically from a
994 previous eval) that value is reused after appending C<"\t...propagated">.
995 This is useful for propagating exceptions:
998 die unless $@ =~ /Expected exception/;
1000 If C<$@> is empty then the string C<"Died"> is used.
1002 die() can also be called with a reference argument. If this happens to be
1003 trapped within an eval(), $@ contains the reference. This behavior permits
1004 a more elaborate exception handling implementation using objects that
1005 maintain arbitary state about the nature of the exception. Such a scheme
1006 is sometimes preferable to matching particular string values of $@ using
1007 regular expressions. Here's an example:
1009 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
1011 if (ref($@) && UNIVERSAL::isa($@,"Some::Module::Exception")) {
1012 # handle Some::Module::Exception
1015 # handle all other possible exceptions
1019 Because perl will stringify uncaught exception messages before displaying
1020 them, you may want to overload stringification operations on such custom
1021 exception objects. See L<overload> for details about that.
1023 You can arrange for a callback to be run just before the C<die>
1024 does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
1025 handler will be called with the error text and can change the error
1026 message, if it sees fit, by calling C<die> again. See
1027 L<perlvar/$SIG{expr}> for details on setting C<%SIG> entries, and
1028 L<"eval BLOCK"> for some examples. Although this feature was meant
1029 to be run only right before your program was to exit, this is not
1030 currently the case--the C<$SIG{__DIE__}> hook is currently called
1031 even inside eval()ed blocks/strings! If one wants the hook to do
1032 nothing in such situations, put
1036 as the first line of the handler (see L<perlvar/$^S>). Because
1037 this promotes strange action at a distance, this counterintuitive
1038 behavior may be fixed in a future release.
1042 Not really a function. Returns the value of the last command in the
1043 sequence of commands indicated by BLOCK. When modified by a loop
1044 modifier, executes the BLOCK once before testing the loop condition.
1045 (On other statements the loop modifiers test the conditional first.)
1047 C<do BLOCK> does I<not> count as a loop, so the loop control statements
1048 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1049 See L<perlsyn> for alternative strategies.
1051 =item do SUBROUTINE(LIST)
1053 A deprecated form of subroutine call. See L<perlsub>.
1057 Uses the value of EXPR as a filename and executes the contents of the
1058 file as a Perl script. Its primary use is to include subroutines
1059 from a Perl subroutine library.
1065 scalar eval `cat stat.pl`;
1067 except that it's more efficient and concise, keeps track of the current
1068 filename for error messages, searches the @INC libraries, and updates
1069 C<%INC> if the file is found. See L<perlvar/Predefined Names> for these
1070 variables. It also differs in that code evaluated with C<do FILENAME>
1071 cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1072 same, however, in that it does reparse the file every time you call it,
1073 so you probably don't want to do this inside a loop.
1075 If C<do> cannot read the file, it returns undef and sets C<$!> to the
1076 error. If C<do> can read the file but cannot compile it, it
1077 returns undef and sets an error message in C<$@>. If the file is
1078 successfully compiled, C<do> returns the value of the last expression
1081 Note that inclusion of library modules is better done with the
1082 C<use> and C<require> operators, which also do automatic error checking
1083 and raise an exception if there's a problem.
1085 You might like to use C<do> to read in a program configuration
1086 file. Manual error checking can be done this way:
1088 # read in config files: system first, then user
1089 for $file ("/share/prog/defaults.rc",
1090 "$ENV{HOME}/.someprogrc")
1092 unless ($return = do $file) {
1093 warn "couldn't parse $file: $@" if $@;
1094 warn "couldn't do $file: $!" unless defined $return;
1095 warn "couldn't run $file" unless $return;
1103 This function causes an immediate core dump. See also the B<-u>
1104 command-line switch in L<perlrun>, which does the same thing.
1105 Primarily this is so that you can use the B<undump> program (not
1106 supplied) to turn your core dump into an executable binary after
1107 having initialized all your variables at the beginning of the
1108 program. When the new binary is executed it will begin by executing
1109 a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1110 Think of it as a goto with an intervening core dump and reincarnation.
1111 If C<LABEL> is omitted, restarts the program from the top.
1113 B<WARNING>: Any files opened at the time of the dump will I<not>
1114 be open any more when the program is reincarnated, with possible
1115 resulting confusion on the part of Perl.
1117 This function is now largely obsolete, partly because it's very
1118 hard to convert a core file into an executable, and because the
1119 real compiler backends for generating portable bytecode and compilable
1120 C code have superseded it.
1122 If you're looking to use L<dump> to speed up your program, consider
1123 generating bytecode or native C code as described in L<perlcc>. If
1124 you're just trying to accelerate a CGI script, consider using the
1125 C<mod_perl> extension to B<Apache>, or the CPAN module, Fast::CGI.
1126 You might also consider autoloading or selfloading, which at least
1127 make your program I<appear> to run faster.
1131 When called in list context, returns a 2-element list consisting of the
1132 key and value for the next element of a hash, so that you can iterate over
1133 it. When called in scalar context, returns the key for only the "next"
1134 element in the hash. (Note: Keys may be C<"0"> or C<"">, which are logically
1135 false; you may wish to avoid constructs like C<while ($k = each %foo) {}>
1138 Entries are returned in an apparently random order. The actual random
1139 order is subject to change in future versions of perl, but it is guaranteed
1140 to be in the same order as either the C<keys> or C<values> function
1141 would produce on the same (unmodified) hash.
1143 When the hash is entirely read, a null array is returned in list context
1144 (which when assigned produces a false (C<0>) value), and C<undef> in
1145 scalar context. The next call to C<each> after that will start iterating
1146 again. There is a single iterator for each hash, shared by all C<each>,
1147 C<keys>, and C<values> function calls in the program; it can be reset by
1148 reading all the elements from the hash, or by evaluating C<keys HASH> or
1149 C<values HASH>. If you add or delete elements of a hash while you're
1150 iterating over it, you may get entries skipped or duplicated, so don't.
1152 The following prints out your environment like the printenv(1) program,
1153 only in a different order:
1155 while (($key,$value) = each %ENV) {
1156 print "$key=$value\n";
1159 See also C<keys>, C<values> and C<sort>.
1161 =item eof FILEHANDLE
1167 Returns 1 if the next read on FILEHANDLE will return end of file, or if
1168 FILEHANDLE is not open. FILEHANDLE may be an expression whose value
1169 gives the real filehandle. (Note that this function actually
1170 reads a character and then C<ungetc>s it, so isn't very useful in an
1171 interactive context.) Do not read from a terminal file (or call
1172 C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
1173 as terminals may lose the end-of-file condition if you do.
1175 An C<eof> without an argument uses the last file read as argument.
1176 Using C<eof()> with empty parentheses is very different. It indicates
1177 the pseudo file formed of the files listed on the command line,
1178 i.e., C<eof()> is reasonable to use inside a C<while (E<lt>E<gt>)>
1179 loop to detect the end of only the last file. Use C<eof(ARGV)> or
1180 C<eof> without the parentheses to test I<each> file in a while
1181 (E<lt>E<gt>) loop. Examples:
1183 # reset line numbering on each input file
1185 next if /^\s*#/; # skip comments
1188 close ARGV if eof; # Not eof()!
1191 # insert dashes just before last line of last file
1193 if (eof()) { # check for end of current file
1194 print "--------------\n";
1195 close(ARGV); # close or last; is needed if we
1196 # are reading from the terminal
1201 Practical hint: you almost never need to use C<eof> in Perl, because the
1202 input operators return false values when they run out of data, or if there
1209 In the first form, the return value of EXPR is parsed and executed as if it
1210 were a little Perl program. The value of the expression (which is itself
1211 determined within scalar context) is first parsed, and if there weren't any
1212 errors, executed in the context of the current Perl program, so that any
1213 variable settings or subroutine and format definitions remain afterwards.
1214 Note that the value is parsed every time the eval executes. If EXPR is
1215 omitted, evaluates C<$_>. This form is typically used to delay parsing
1216 and subsequent execution of the text of EXPR until run time.
1218 In the second form, the code within the BLOCK is parsed only once--at the
1219 same time the code surrounding the eval itself was parsed--and executed
1220 within the context of the current Perl program. This form is typically
1221 used to trap exceptions more efficiently than the first (see below), while
1222 also providing the benefit of checking the code within BLOCK at compile
1225 The final semicolon, if any, may be omitted from the value of EXPR or within
1228 In both forms, the value returned is the value of the last expression
1229 evaluated inside the mini-program; a return statement may be also used, just
1230 as with subroutines. The expression providing the return value is evaluated
1231 in void, scalar, or list context, depending on the context of the eval itself.
1232 See L</wantarray> for more on how the evaluation context can be determined.
1234 If there is a syntax error or runtime error, or a C<die> statement is
1235 executed, an undefined value is returned by C<eval>, and C<$@> is set to the
1236 error message. If there was no error, C<$@> is guaranteed to be a null
1237 string. Beware that using C<eval> neither silences perl from printing
1238 warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
1239 To do either of those, you have to use the C<$SIG{__WARN__}> facility. See
1240 L</warn> and L<perlvar>.
1242 Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1243 determining whether a particular feature (such as C<socket> or C<symlink>)
1244 is implemented. It is also Perl's exception trapping mechanism, where
1245 the die operator is used to raise exceptions.
1247 If the code to be executed doesn't vary, you may use the eval-BLOCK
1248 form to trap run-time errors without incurring the penalty of
1249 recompiling each time. The error, if any, is still returned in C<$@>.
1252 # make divide-by-zero nonfatal
1253 eval { $answer = $a / $b; }; warn $@ if $@;
1255 # same thing, but less efficient
1256 eval '$answer = $a / $b'; warn $@ if $@;
1258 # a compile-time error
1259 eval { $answer = }; # WRONG
1262 eval '$answer ='; # sets $@
1264 Due to the current arguably broken state of C<__DIE__> hooks, when using
1265 the C<eval{}> form as an exception trap in libraries, you may wish not
1266 to trigger any C<__DIE__> hooks that user code may have installed.
1267 You can use the C<local $SIG{__DIE__}> construct for this purpose,
1268 as shown in this example:
1270 # a very private exception trap for divide-by-zero
1271 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1274 This is especially significant, given that C<__DIE__> hooks can call
1275 C<die> again, which has the effect of changing their error messages:
1277 # __DIE__ hooks may modify error messages
1279 local $SIG{'__DIE__'} =
1280 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
1281 eval { die "foo lives here" };
1282 print $@ if $@; # prints "bar lives here"
1285 Because this promotes action at a distance, this counterintuitive behavior
1286 may be fixed in a future release.
1288 With an C<eval>, you should be especially careful to remember what's
1289 being looked at when:
1295 eval { $x }; # CASE 4
1297 eval "\$$x++"; # CASE 5
1300 Cases 1 and 2 above behave identically: they run the code contained in
1301 the variable $x. (Although case 2 has misleading double quotes making
1302 the reader wonder what else might be happening (nothing is).) Cases 3
1303 and 4 likewise behave in the same way: they run the code C<'$x'>, which
1304 does nothing but return the value of $x. (Case 4 is preferred for
1305 purely visual reasons, but it also has the advantage of compiling at
1306 compile-time instead of at run-time.) Case 5 is a place where
1307 normally you I<would> like to use double quotes, except that in this
1308 particular situation, you can just use symbolic references instead, as
1311 C<eval BLOCK> does I<not> count as a loop, so the loop control statements
1312 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1316 =item exec PROGRAM LIST
1318 The C<exec> function executes a system command I<and never returns>--
1319 use C<system> instead of C<exec> if you want it to return. It fails and
1320 returns false only if the command does not exist I<and> it is executed
1321 directly instead of via your system's command shell (see below).
1323 Since it's a common mistake to use C<exec> instead of C<system>, Perl
1324 warns you if there is a following statement which isn't C<die>, C<warn>,
1325 or C<exit> (if C<-w> is set - but you always do that). If you
1326 I<really> want to follow an C<exec> with some other statement, you
1327 can use one of these styles to avoid the warning:
1329 exec ('foo') or print STDERR "couldn't exec foo: $!";
1330 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
1332 If there is more than one argument in LIST, or if LIST is an array
1333 with more than one value, calls execvp(3) with the arguments in LIST.
1334 If there is only one scalar argument or an array with one element in it,
1335 the argument is checked for shell metacharacters, and if there are any,
1336 the entire argument is passed to the system's command shell for parsing
1337 (this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1338 If there are no shell metacharacters in the argument, it is split into
1339 words and passed directly to C<execvp>, which is more efficient.
1342 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1343 exec "sort $outfile | uniq";
1345 If you don't really want to execute the first argument, but want to lie
1346 to the program you are executing about its own name, you can specify
1347 the program you actually want to run as an "indirect object" (without a
1348 comma) in front of the LIST. (This always forces interpretation of the
1349 LIST as a multivalued list, even if there is only a single scalar in
1352 $shell = '/bin/csh';
1353 exec $shell '-sh'; # pretend it's a login shell
1357 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1359 When the arguments get executed via the system shell, results will
1360 be subject to its quirks and capabilities. See L<perlop/"`STRING`">
1363 Using an indirect object with C<exec> or C<system> is also more
1364 secure. This usage (which also works fine with system()) forces
1365 interpretation of the arguments as a multivalued list, even if the
1366 list had just one argument. That way you're safe from the shell
1367 expanding wildcards or splitting up words with whitespace in them.
1369 @args = ( "echo surprise" );
1371 exec @args; # subject to shell escapes
1373 exec { $args[0] } @args; # safe even with one-arg list
1375 The first version, the one without the indirect object, ran the I<echo>
1376 program, passing it C<"surprise"> an argument. The second version
1377 didn't--it tried to run a program literally called I<"echo surprise">,
1378 didn't find it, and set C<$?> to a non-zero value indicating failure.
1380 Note that C<exec> will not call your C<END> blocks, nor will it call
1381 any C<DESTROY> methods in your objects.
1385 Returns true if the specified hash key exists in its hash, even
1386 if the corresponding value is undefined.
1388 print "Exists\n" if exists $array{$key};
1389 print "Defined\n" if defined $array{$key};
1390 print "True\n" if $array{$key};
1392 A hash element can be true only if it's defined, and defined if
1393 it exists, but the reverse doesn't necessarily hold true.
1395 Note that the EXPR can be arbitrarily complicated as long as the final
1396 operation is a hash key lookup:
1398 if (exists $ref->{A}->{B}->{$key}) { }
1399 if (exists $hash{A}{B}{$key}) { }
1401 Although the last element will not spring into existence just because
1402 its existence was tested, intervening ones will. Thus C<$ref-E<gt>{"A"}>
1403 and C<$ref-E<gt>{"A"}-E<gt>{"B"}> will spring into existence due to the
1404 existence test for a $key element. This happens anywhere the arrow
1405 operator is used, including even
1408 if (exists $ref->{"Some key"}) { }
1409 print $ref; # prints HASH(0x80d3d5c)
1411 This surprising autovivification in what does not at first--or even
1412 second--glance appear to be an lvalue context may be fixed in a future
1417 Evaluates EXPR and exits immediately with that value. Example:
1420 exit 0 if $ans =~ /^[Xx]/;
1422 See also C<die>. If EXPR is omitted, exits with C<0> status. The only
1423 universally recognized values for EXPR are C<0> for success and C<1>
1424 for error; other values are subject to interpretation depending on the
1425 environment in which the Perl program is running. For example, exiting
1426 69 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
1427 the mailer to return the item undelivered, but that's not true everywhere.
1429 Don't use C<exit> to abort a subroutine if there's any chance that
1430 someone might want to trap whatever error happened. Use C<die> instead,
1431 which can be trapped by an C<eval>.
1433 The exit() function does not always exit immediately. It calls any
1434 defined C<END> routines first, but these C<END> routines may not
1435 themselves abort the exit. Likewise any object destructors that need to
1436 be called are called before the real exit. If this is a problem, you
1437 can call C<POSIX:_exit($status)> to avoid END and destructor processing.
1438 See L<perlmod> for details.
1444 Returns I<e> (the natural logarithm base) to the power of EXPR.
1445 If EXPR is omitted, gives C<exp($_)>.
1447 =item fcntl FILEHANDLE,FUNCTION,SCALAR
1449 Implements the fcntl(2) function. You'll probably have to say
1453 first to get the correct constant definitions. Argument processing and
1454 value return works just like C<ioctl> below.
1458 fcntl($filehandle, F_GETFL, $packed_return_buffer)
1459 or die "can't fcntl F_GETFL: $!";
1461 You don't have to check for C<defined> on the return from C<fnctl>.
1462 Like C<ioctl>, it maps a C<0> return from the system call into C<"0
1463 but true"> in Perl. This string is true in boolean context and C<0>
1464 in numeric context. It is also exempt from the normal B<-w> warnings
1465 on improper numeric conversions.
1467 Note that C<fcntl> will produce a fatal error if used on a machine that
1468 doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
1469 manpage to learn what functions are available on your system.
1471 =item fileno FILEHANDLE
1473 Returns the file descriptor for a filehandle, or undefined if the
1474 filehandle is not open. This is mainly useful for constructing
1475 bitmaps for C<select> and low-level POSIX tty-handling operations.
1476 If FILEHANDLE is an expression, the value is taken as an indirect
1477 filehandle, generally its name.
1479 You can use this to find out whether two handles refer to the
1480 same underlying descriptor:
1482 if (fileno(THIS) == fileno(THAT)) {
1483 print "THIS and THAT are dups\n";
1486 =item flock FILEHANDLE,OPERATION
1488 Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
1489 for success, false on failure. Produces a fatal error if used on a
1490 machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
1491 C<flock> is Perl's portable file locking interface, although it locks
1492 only entire files, not records.
1494 Two potentially non-obvious but traditional C<flock> semantics are
1495 that it waits indefinitely until the lock is granted, and that its locks
1496 B<merely advisory>. Such discretionary locks are more flexible, but offer
1497 fewer guarantees. This means that files locked with C<flock> may be
1498 modified by programs that do not also use C<flock>. See L<perlport>,
1499 your port's specific documentation, or your system-specific local manpages
1500 for details. It's best to assume traditional behavior if you're writing
1501 portable programs. (But if you're not, you should as always feel perfectly
1502 free to write for your own system's idiosyncrasies (sometimes called
1503 "features"). Slavish adherence to portability concerns shouldn't get
1504 in the way of your getting your job done.)
1506 OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1507 LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
1508 you can use the symbolic names if import them from the Fcntl module,
1509 either individually, or as a group using the ':flock' tag. LOCK_SH
1510 requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
1511 releases a previously requested lock. If LOCK_NB is added to LOCK_SH or
1512 LOCK_EX then C<flock> will return immediately rather than blocking
1513 waiting for the lock (check the return status to see if you got it).
1515 To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
1516 before locking or unlocking it.
1518 Note that the emulation built with lockf(3) doesn't provide shared
1519 locks, and it requires that FILEHANDLE be open with write intent. These
1520 are the semantics that lockf(3) implements. Most if not all systems
1521 implement lockf(3) in terms of fcntl(2) locking, though, so the
1522 differing semantics shouldn't bite too many people.
1524 Note also that some versions of C<flock> cannot lock things over the
1525 network; you would need to use the more system-specific C<fcntl> for
1526 that. If you like you can force Perl to ignore your system's flock(2)
1527 function, and so provide its own fcntl(2)-based emulation, by passing
1528 the switch C<-Ud_flock> to the F<Configure> program when you configure
1531 Here's a mailbox appender for BSD systems.
1533 use Fcntl ':flock'; # import LOCK_* constants
1536 flock(MBOX,LOCK_EX);
1537 # and, in case someone appended
1538 # while we were waiting...
1543 flock(MBOX,LOCK_UN);
1546 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1547 or die "Can't open mailbox: $!";
1550 print MBOX $msg,"\n\n";
1553 On systems that support a real flock(), locks are inherited across fork()
1554 calls, whereas those that must resort to the more capricious fcntl()
1555 function lose the locks, making it harder to write servers.
1557 See also L<DB_File> for other flock() examples.
1561 Does a fork(2) system call to create a new process running the
1562 same program at the same point. It returns the child pid to the
1563 parent process, C<0> to the child process, or C<undef> if the fork is
1564 unsuccessful. File descriptors (and sometimes locks on those descriptors)
1565 are shared, while everything else is copied. On most systems supporting
1566 fork(), great care has gone into making it extremely efficient (for
1567 example, using copy-on-write technology on data pages), making it the
1568 dominant paradigm for multitasking over the last few decades.
1570 All files opened for output are flushed before forking the child process.
1572 If you C<fork> without ever waiting on your children, you will
1573 accumulate zombies. On some systems, you can avoid this by setting
1574 C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
1575 forking and reaping moribund children.
1577 Note that if your forked child inherits system file descriptors like
1578 STDIN and STDOUT that are actually connected by a pipe or socket, even
1579 if you exit, then the remote server (such as, say, a CGI script or a
1580 backgrounded job launched from a remote shell) won't think you're done.
1581 You should reopen those to F</dev/null> if it's any issue.
1585 Declare a picture format for use by the C<write> function. For
1589 Test: @<<<<<<<< @||||| @>>>>>
1590 $str, $%, '$' . int($num)
1594 $num = $cost/$quantity;
1598 See L<perlform> for many details and examples.
1600 =item formline PICTURE,LIST
1602 This is an internal function used by C<format>s, though you may call it,
1603 too. It formats (see L<perlform>) a list of values according to the
1604 contents of PICTURE, placing the output into the format output
1605 accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
1606 Eventually, when a C<write> is done, the contents of
1607 C<$^A> are written to some filehandle, but you could also read C<$^A>
1608 yourself and then set C<$^A> back to C<"">. Note that a format typically
1609 does one C<formline> per line of form, but the C<formline> function itself
1610 doesn't care how many newlines are embedded in the PICTURE. This means
1611 that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
1612 You may therefore need to use multiple formlines to implement a single
1613 record format, just like the format compiler.
1615 Be careful if you put double quotes around the picture, because an C<@>
1616 character may be taken to mean the beginning of an array name.
1617 C<formline> always returns true. See L<perlform> for other examples.
1619 =item getc FILEHANDLE
1623 Returns the next character from the input file attached to FILEHANDLE,
1624 or the undefined value at end of file, or if there was an error.
1625 If FILEHANDLE is omitted, reads from STDIN. This is not particularly
1626 efficient. However, it cannot be used by itself to fetch single
1627 characters without waiting for the user to hit enter. For that, try
1628 something more like:
1631 system "stty cbreak </dev/tty >/dev/tty 2>&1";
1634 system "stty", '-icanon', 'eol', "\001";
1640 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
1643 system "stty", 'icanon', 'eol', '^@'; # ASCII null
1647 Determination of whether $BSD_STYLE should be set
1648 is left as an exercise to the reader.
1650 The C<POSIX::getattr> function can do this more portably on
1651 systems purporting POSIX compliance. See also the C<Term::ReadKey>
1652 module from your nearest CPAN site; details on CPAN can be found on
1657 Implements the C library function of the same name, which on most
1658 systems returns the current login from F</etc/utmp>, if any. If null,
1661 $login = getlogin || getpwuid($<) || "Kilroy";
1663 Do not consider C<getlogin> for authentication: it is not as
1664 secure as C<getpwuid>.
1666 =item getpeername SOCKET
1668 Returns the packed sockaddr address of other end of the SOCKET connection.
1671 $hersockaddr = getpeername(SOCK);
1672 ($port, $iaddr) = sockaddr_in($hersockaddr);
1673 $herhostname = gethostbyaddr($iaddr, AF_INET);
1674 $herstraddr = inet_ntoa($iaddr);
1678 Returns the current process group for the specified PID. Use
1679 a PID of C<0> to get the current process group for the
1680 current process. Will raise an exception if used on a machine that
1681 doesn't implement getpgrp(2). If PID is omitted, returns process
1682 group of current process. Note that the POSIX version of C<getpgrp>
1683 does not accept a PID argument, so only C<PID==0> is truly portable.
1687 Returns the process id of the parent process.
1689 =item getpriority WHICH,WHO
1691 Returns the current priority for a process, a process group, or a user.
1692 (See L<getpriority(2)>.) Will raise a fatal exception if used on a
1693 machine that doesn't implement getpriority(2).
1699 =item gethostbyname NAME
1701 =item getnetbyname NAME
1703 =item getprotobyname NAME
1709 =item getservbyname NAME,PROTO
1711 =item gethostbyaddr ADDR,ADDRTYPE
1713 =item getnetbyaddr ADDR,ADDRTYPE
1715 =item getprotobynumber NUMBER
1717 =item getservbyport PORT,PROTO
1735 =item sethostent STAYOPEN
1737 =item setnetent STAYOPEN
1739 =item setprotoent STAYOPEN
1741 =item setservent STAYOPEN
1755 These routines perform the same functions as their counterparts in the
1756 system library. In list context, the return values from the
1757 various get routines are as follows:
1759 ($name,$passwd,$uid,$gid,
1760 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
1761 ($name,$passwd,$gid,$members) = getgr*
1762 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
1763 ($name,$aliases,$addrtype,$net) = getnet*
1764 ($name,$aliases,$proto) = getproto*
1765 ($name,$aliases,$port,$proto) = getserv*
1767 (If the entry doesn't exist you get a null list.)
1769 In scalar context, you get the name, unless the function was a
1770 lookup by name, in which case you get the other thing, whatever it is.
1771 (If the entry doesn't exist you get the undefined value.) For example:
1773 $uid = getpwnam($name);
1774 $name = getpwuid($num);
1776 $gid = getgrnam($name);
1777 $name = getgrgid($num;
1781 In I<getpw*()> the fields $quota, $comment, and $expire are
1782 special cases in the sense that in many systems they are unsupported.
1783 If the $quota is unsupported, it is an empty scalar. If it is
1784 supported, it usually encodes the disk quota. If the $comment
1785 field is unsupported, it is an empty scalar. If it is supported it
1786 usually encodes some administrative comment about the user. In some
1787 systems the $quota field may be $change or $age, fields that have
1788 to do with password aging. In some systems the $comment field may
1789 be $class. The $expire field, if present, encodes the expiration
1790 period of the account or the password. For the availability and the
1791 exact meaning of these fields in your system, please consult your
1792 getpwnam(3) documentation and your F<pwd.h> file. You can also find
1793 out from within Perl what your $quota and $comment fields mean
1794 and whether you have the $expire field by using the C<Config> module
1795 and the values C<d_pwquota>, C<d_pwage>, C<d_pwchange>, C<d_pwcomment>,
1796 and C<d_pwexpire>. Shadow password files are only supported if your
1797 vendor has implemented them in the intuitive fashion that calling the
1798 regular C library routines gets the shadow versions if you're running
1799 under privilege. Those that incorrectly implement a separate library
1800 call are not supported.
1802 The $members value returned by I<getgr*()> is a space separated list of
1803 the login names of the members of the group.
1805 For the I<gethost*()> functions, if the C<h_errno> variable is supported in
1806 C, it will be returned to you via C<$?> if the function call fails. The
1807 C<@addrs> value returned by a successful call is a list of the raw
1808 addresses returned by the corresponding system library call. In the
1809 Internet domain, each address is four bytes long and you can unpack it
1810 by saying something like:
1812 ($a,$b,$c,$d) = unpack('C4',$addr[0]);
1814 The Socket library makes this slightly easier:
1817 $iaddr = inet_aton("127.1"); # or whatever address
1818 $name = gethostbyaddr($iaddr, AF_INET);
1820 # or going the other way
1821 $straddr = inet_ntoa($iaddr);
1823 If you get tired of remembering which element of the return list
1824 contains which return value, by-name interfaces are provided
1825 in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
1826 C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
1827 and C<User::grent>. These override the normal built-ins, supplying
1828 versions that return objects with the appropriate names
1829 for each field. For example:
1833 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
1835 Even though it looks like they're the same method calls (uid),
1836 they aren't, because a C<File::stat> object is different from
1837 a C<User::pwent> object.
1839 =item getsockname SOCKET
1841 Returns the packed sockaddr address of this end of the SOCKET connection,
1842 in case you don't know the address because you have several different
1843 IPs that the connection might have come in on.
1846 $mysockaddr = getsockname(SOCK);
1847 ($port, $myaddr) = sockaddr_in($mysockaddr);
1848 printf "Connect to %s [%s]\n",
1849 scalar gethostbyaddr($myaddr, AF_INET),
1852 =item getsockopt SOCKET,LEVEL,OPTNAME
1854 Returns the socket option requested, or undef if there is an error.
1860 Returns the value of EXPR with filename expansions such as the
1861 standard Unix shell F</bin/csh> would do. This is the internal function
1862 implementing the C<E<lt>*.cE<gt>> operator, but you can use it directly.
1863 If EXPR is omitted, C<$_> is used. The C<E<lt>*.cE<gt>> operator is
1864 discussed in more detail in L<perlop/"I/O Operators">.
1868 Converts a time as returned by the time function to a 9-element list
1869 with the time localized for the standard Greenwich time zone.
1870 Typically used as follows:
1873 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
1876 All list elements are numeric, and come straight out of a struct tm.
1877 In particular this means that $mon has the range C<0..11> and $wday
1878 has the range C<0..6> with sunday as day C<0>. Also, $year is the
1879 number of years since 1900, that is, $year is C<123> in year 2023,
1880 I<not> simply the last two digits of the year. If you assume it is,
1881 then you create non-Y2K-compliant programs--and you wouldn't want to do
1884 If EXPR is omitted, does C<gmtime(time())>.
1886 In scalar context, returns the ctime(3) value:
1888 $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
1890 Also see the C<timegm> function provided by the C<Time::Local> module,
1891 and the strftime(3) function available via the POSIX module.
1893 This scalar value is B<not> locale dependent (see L<perllocale>), but
1894 is instead a Perl builtin. Also see the C<Time::Local> module, and the
1895 strftime(3) and mktime(3) functions available via the POSIX module. To
1896 get somewhat similar but locale dependent date strings, set up your
1897 locale environment variables appropriately (please see L<perllocale>)
1898 and try for example:
1900 use POSIX qw(strftime);
1901 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
1903 Note that the C<%a> and C<%b> escapes, which represent the short forms
1904 of the day of the week and the month of the year, may not necessarily
1905 be three characters wide in all locales.
1913 The C<goto-LABEL> form finds the statement labeled with LABEL and resumes
1914 execution there. It may not be used to go into any construct that
1915 requires initialization, such as a subroutine or a C<foreach> loop. It
1916 also can't be used to go into a construct that is optimized away,
1917 or to get out of a block or subroutine given to C<sort>.
1918 It can be used to go almost anywhere else within the dynamic scope,
1919 including out of subroutines, but it's usually better to use some other
1920 construct such as C<last> or C<die>. The author of Perl has never felt the
1921 need to use this form of C<goto> (in Perl, that is--C is another matter).
1923 The C<goto-EXPR> form expects a label name, whose scope will be resolved
1924 dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
1925 necessarily recommended if you're optimizing for maintainability:
1927 goto ("FOO", "BAR", "GLARCH")[$i];
1929 The C<goto-&NAME> form is highly magical, and substitutes a call to the
1930 named subroutine for the currently running subroutine. This is used by
1931 C<AUTOLOAD> subroutines that wish to load another subroutine and then
1932 pretend that the other subroutine had been called in the first place
1933 (except that any modifications to C<@_> in the current subroutine are
1934 propagated to the other subroutine.) After the C<goto>, not even C<caller>
1935 will be able to tell that this routine was called first.
1937 =item grep BLOCK LIST
1939 =item grep EXPR,LIST
1941 This is similar in spirit to, but not the same as, grep(1) and its
1942 relatives. In particular, it is not limited to using regular expressions.
1944 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
1945 C<$_> to each element) and returns the list value consisting of those
1946 elements for which the expression evaluated to true. In scalar
1947 context, returns the number of times the expression was true.
1949 @foo = grep(!/^#/, @bar); # weed out comments
1953 @foo = grep {!/^#/} @bar; # weed out comments
1955 Note that, because C<$_> is a reference into the list value, it can
1956 be used to modify the elements of the array. While this is useful and
1957 supported, it can cause bizarre results if the LIST is not a named array.
1958 Similarly, grep returns aliases into the original list, much as a for
1959 loop's index variable aliases the list elements. That is, modifying an
1960 element of a list returned by grep (for example, in a C<foreach>, C<map>
1961 or another C<grep>) actually modifies the element in the original list.
1962 This is usually something to be avoided when writing clear code.
1964 See also L</map> for a list composed of the results of the BLOCK or EXPR.
1970 Interprets EXPR as a hex string and returns the corresponding value.
1971 (To convert strings that might start with either 0, 0x, or 0b, see
1972 L</oct>.) If EXPR is omitted, uses C<$_>.
1974 print hex '0xAf'; # prints '175'
1975 print hex 'aF'; # same
1977 Hex strings may only represent integers. Strings that would cause
1978 integer overflow trigger a mandatory error message.
1982 There is no builtin C<import> function. It is just an ordinary
1983 method (subroutine) defined (or inherited) by modules that wish to export
1984 names to another module. The C<use> function calls the C<import> method
1985 for the package used. See also L</use()>, L<perlmod>, and L<Exporter>.
1987 =item index STR,SUBSTR,POSITION
1989 =item index STR,SUBSTR
1991 The index function searches for one string within another, but without
1992 the wildcard-like behavior of a full regular-expression pattern match.
1993 It returns the position of the first occurrence of SUBSTR in STR at
1994 or after POSITION. If POSITION is omitted, starts searching from the
1995 beginning of the string. The return value is based at C<0> (or whatever
1996 you've set the C<$[> variable to--but don't do that). If the substring
1997 is not found, returns one less than the base, ordinarily C<-1>.
2003 Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
2004 You should not use this function for rounding: one because it truncates
2005 towards C<0>, and two because machine representations of floating point
2006 numbers can sometimes produce counterintuitive results. For example,
2007 C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
2008 because it's really more like -268.99999999999994315658 instead. Usually,
2009 the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
2010 functions will serve you better than will int().
2012 =item ioctl FILEHANDLE,FUNCTION,SCALAR
2014 Implements the ioctl(2) function. You'll probably first have to say
2016 require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph
2018 to get the correct function definitions. If F<ioctl.ph> doesn't
2019 exist or doesn't have the correct definitions you'll have to roll your
2020 own, based on your C header files such as F<E<lt>sys/ioctl.hE<gt>>.
2021 (There is a Perl script called B<h2ph> that comes with the Perl kit that
2022 may help you in this, but it's nontrivial.) SCALAR will be read and/or
2023 written depending on the FUNCTION--a pointer to the string value of SCALAR
2024 will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
2025 has no string value but does have a numeric value, that value will be
2026 passed rather than a pointer to the string value. To guarantee this to be
2027 true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
2028 functions may be needed to manipulate the values of structures used by
2031 The return value of C<ioctl> (and C<fcntl>) is as follows:
2033 if OS returns: then Perl returns:
2035 0 string "0 but true"
2036 anything else that number
2038 Thus Perl returns true on success and false on failure, yet you can
2039 still easily determine the actual value returned by the operating
2042 $retval = ioctl(...) || -1;
2043 printf "System returned %d\n", $retval;
2045 The special string "C<0> but true" is exempt from B<-w> complaints
2046 about improper numeric conversions.
2048 Here's an example of setting a filehandle named C<REMOTE> to be
2049 non-blocking at the system level. You'll have to negotiate C<$|>
2050 on your own, though.
2052 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
2054 $flags = fcntl(REMOTE, F_GETFL, 0)
2055 or die "Can't get flags for the socket: $!\n";
2057 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
2058 or die "Can't set flags for the socket: $!\n";
2060 =item join EXPR,LIST
2062 Joins the separate strings of LIST into a single string with fields
2063 separated by the value of EXPR, and returns that new string. Example:
2065 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
2067 Beware that unlike C<split>, C<join> doesn't take a pattern as its
2068 first argument. Compare L</split>.
2072 Returns a list consisting of all the keys of the named hash. (In
2073 scalar context, returns the number of keys.) The keys are returned in
2074 an apparently random order. The actual random order is subject to
2075 change in future versions of perl, but it is guaranteed to be the same
2076 order as either the C<values> or C<each> function produces (given
2077 that the hash has not been modified). As a side effect, it resets
2080 Here is yet another way to print your environment:
2083 @values = values %ENV;
2085 print pop(@keys), '=', pop(@values), "\n";
2088 or how about sorted by key:
2090 foreach $key (sort(keys %ENV)) {
2091 print $key, '=', $ENV{$key}, "\n";
2094 To sort a hash by value, you'll need to use a C<sort> function.
2095 Here's a descending numeric sort of a hash by its values:
2097 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
2098 printf "%4d %s\n", $hash{$key}, $key;
2101 As an lvalue C<keys> allows you to increase the number of hash buckets
2102 allocated for the given hash. This can gain you a measure of efficiency if
2103 you know the hash is going to get big. (This is similar to pre-extending
2104 an array by assigning a larger number to $#array.) If you say
2108 then C<%hash> will have at least 200 buckets allocated for it--256 of them,
2109 in fact, since it rounds up to the next power of two. These
2110 buckets will be retained even if you do C<%hash = ()>, use C<undef
2111 %hash> if you want to free the storage while C<%hash> is still in scope.
2112 You can't shrink the number of buckets allocated for the hash using
2113 C<keys> in this way (but you needn't worry about doing this by accident,
2114 as trying has no effect).
2116 See also C<each>, C<values> and C<sort>.
2120 Sends a signal to a list of processes. The first element of
2121 the list must be the signal to send. Returns the number of
2122 processes successfully signaled (which is not necessarily the
2123 same as the number actually killed).
2125 $cnt = kill 1, $child1, $child2;
2128 Unlike in the shell, in Perl if the I<SIGNAL> is negative, it kills
2129 process groups instead of processes. (On System V, a negative I<PROCESS>
2130 number will also kill process groups, but that's not portable.) That
2131 means you usually want to use positive not negative signals. You may also
2132 use a signal name in quotes. See L<perlipc/"Signals"> for details.
2138 The C<last> command is like the C<break> statement in C (as used in
2139 loops); it immediately exits the loop in question. If the LABEL is
2140 omitted, the command refers to the innermost enclosing loop. The
2141 C<continue> block, if any, is not executed:
2143 LINE: while (<STDIN>) {
2144 last LINE if /^$/; # exit when done with header
2148 C<last> cannot be used to exit a block which returns a value such as
2149 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2150 a grep() or map() operation.
2152 See also L</continue> for an illustration of how C<last>, C<next>, and
2159 Returns an lowercased version of EXPR. This is the internal function
2160 implementing the C<\L> escape in double-quoted strings.
2161 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
2164 If EXPR is omitted, uses C<$_>.
2170 Returns the value of EXPR with the first character lowercased. This is
2171 the internal function implementing the C<\l> escape in double-quoted strings.
2172 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
2174 If EXPR is omitted, uses C<$_>.
2180 Returns the length in characters of the value of EXPR. If EXPR is
2181 omitted, returns length of C<$_>. Note that this cannot be used on
2182 an entire array or hash to find out how many elements these have.
2183 For that, use C<scalar @array> and C<scalar keys %hash> respectively.
2185 =item link OLDFILE,NEWFILE
2187 Creates a new filename linked to the old filename. Returns true for
2188 success, false otherwise.
2190 =item listen SOCKET,QUEUESIZE
2192 Does the same thing that the listen system call does. Returns true if
2193 it succeeded, false otherwise. See the example in L<perlipc/"Sockets: Client/Server Communication">.
2197 You really probably want to be using C<my> instead, because C<local> isn't
2198 what most people think of as "local". See L<perlsub/"Private Variables
2199 via my()"> for details.
2201 A local modifies the listed variables to be local to the enclosing
2202 block, file, or eval. If more than one value is listed, the list must
2203 be placed in parentheses. See L<perlsub/"Temporary Values via local()">
2204 for details, including issues with tied arrays and hashes.
2206 =item localtime EXPR
2208 Converts a time as returned by the time function to a 9-element list
2209 with the time analyzed for the local time zone. Typically used as
2213 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
2216 All list elements are numeric, and come straight out of a struct tm.
2217 In particular this means that $mon has the range C<0..11> and $wday
2218 has the range C<0..6> with sunday as day C<0>. Also, $year is the
2219 number of years since 1900, that is, $year is C<123> in year 2023,
2220 and I<not> simply the last two digits of the year. If you assume it is,
2221 then you create non-Y2K-compliant programs--and you wouldn't want to do
2224 If EXPR is omitted, uses the current time (C<localtime(time)>).
2226 In scalar context, returns the ctime(3) value:
2228 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
2230 This scalar value is B<not> locale dependent, see L<perllocale>, but
2231 instead a Perl builtin. Also see the C<Time::Local> module
2232 (to convert the second, minutes, hours, ... back to seconds since the
2233 stroke of midnight the 1st of January 1970, the value returned by
2234 time()), and the strftime(3) and mktime(3) function available via the
2235 POSIX module. To get somewhat similar but locale dependent date
2236 strings, set up your locale environment variables appropriately
2237 (please see L<perllocale>) and try for example:
2239 use POSIX qw(strftime);
2240 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
2242 Note that the C<%a> and C<%b>, the short forms of the day of the week
2243 and the month of the year, may not necessarily be three characters wide.
2249 This function places an advisory lock on a variable, subroutine,
2250 or referenced object contained in I<THING> until the lock goes out
2251 of scope. This is a built-in function only if your version of Perl
2252 was built with threading enabled, and if you've said C<use Threads>.
2253 Otherwise a user-defined function by this name will be called. See
2260 Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
2261 returns log of C<$_>. To get the log of another base, use basic algebra:
2262 The base-N log of a number is equal to the natural log of that number
2263 divided by the natural log of N. For example:
2267 return log($n)/log(10);
2270 See also L</exp> for the inverse operation.
2272 =item lstat FILEHANDLE
2278 Does the same thing as the C<stat> function (including setting the
2279 special C<_> filehandle) but stats a symbolic link instead of the file
2280 the symbolic link points to. If symbolic links are unimplemented on
2281 your system, a normal C<stat> is done.
2283 If EXPR is omitted, stats C<$_>.
2287 The match operator. See L<perlop>.
2289 =item map BLOCK LIST
2293 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2294 C<$_> to each element) and returns the list value composed of the
2295 results of each such evaluation. In scalar context, returns the
2296 total number of elements so generated. Evaluates BLOCK or EXPR in
2297 list context, so each element of LIST may produce zero, one, or
2298 more elements in the returned value.
2300 @chars = map(chr, @nums);
2302 translates a list of numbers to the corresponding characters. And
2304 %hash = map { getkey($_) => $_ } @array;
2306 is just a funny way to write
2309 foreach $_ (@array) {
2310 $hash{getkey($_)} = $_;
2313 Note that, because C<$_> is a reference into the list value, it can
2314 be used to modify the elements of the array. While this is useful and
2315 supported, it can cause bizarre results if the LIST is not a named array.
2316 Using a regular C<foreach> loop for this purpose would be clearer in
2317 most cases. See also L</grep> for an array composed of those items of
2318 the original list for which the BLOCK or EXPR evaluates to true.
2320 =item mkdir FILENAME,MASK
2322 Creates the directory specified by FILENAME, with permissions
2323 specified by MASK (as modified by C<umask>). If it succeeds it
2324 returns true, otherwise it returns false and sets C<$!> (errno).
2326 In general, it is better to create directories with permissive MASK,
2327 and let the user modify that with their C<umask>, than it is to supply
2328 a restrictive MASK and give the user no way to be more permissive.
2329 The exceptions to this rule are when the file or directory should be
2330 kept private (mail files, for instance). The perlfunc(1) entry on
2331 C<umask> discusses the choice of MASK in more detail.
2333 =item msgctl ID,CMD,ARG
2335 Calls the System V IPC function msgctl(2). You'll probably have to say
2339 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
2340 then ARG must be a variable which will hold the returned C<msqid_ds>
2341 structure. Returns like C<ioctl>: the undefined value for error, C<"0 but
2342 true"> for zero, or the actual return value otherwise. See also
2343 C<IPC::SysV> and C<IPC::Semaphore> documentation.
2345 =item msgget KEY,FLAGS
2347 Calls the System V IPC function msgget(2). Returns the message queue
2348 id, or the undefined value if there is an error. See also C<IPC::SysV>
2349 and C<IPC::Msg> documentation.
2351 =item msgsnd ID,MSG,FLAGS
2353 Calls the System V IPC function msgsnd to send the message MSG to the
2354 message queue ID. MSG must begin with the long integer message type,
2355 which may be created with C<pack("l", $type)>. Returns true if
2356 successful, or false if there is an error. See also C<IPC::SysV>
2357 and C<IPC::SysV::Msg> documentation.
2359 =item msgrcv ID,VAR,SIZE,TYPE,FLAGS
2361 Calls the System V IPC function msgrcv to receive a message from
2362 message queue ID into variable VAR with a maximum message size of
2363 SIZE. Note that if a message is received, the message type will be
2364 the first thing in VAR, and the maximum length of VAR is SIZE plus the
2365 size of the message type. Returns true if successful, or false if
2366 there is an error. See also C<IPC::SysV> and C<IPC::SysV::Msg> documentation.
2370 =item my EXPR : ATTRIBUTES
2372 A C<my> declares the listed variables to be local (lexically) to the
2373 enclosing block, file, or C<eval>. If
2374 more than one value is listed, the list must be placed in parentheses. See
2375 L<perlsub/"Private Variables via my()"> for details.
2381 The C<next> command is like the C<continue> statement in C; it starts
2382 the next iteration of the loop:
2384 LINE: while (<STDIN>) {
2385 next LINE if /^#/; # discard comments
2389 Note that if there were a C<continue> block on the above, it would get
2390 executed even on discarded lines. If the LABEL is omitted, the command
2391 refers to the innermost enclosing loop.
2393 C<next> cannot be used to exit a block which returns a value such as
2394 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2395 a grep() or map() operation.
2397 See also L</continue> for an illustration of how C<last>, C<next>, and
2400 =item no Module LIST
2402 See the L</use> function, which C<no> is the opposite of.
2408 Interprets EXPR as an octal string and returns the corresponding
2409 value. (If EXPR happens to start off with C<0x>, interprets it as a
2410 hex string. If EXPR starts off with C<0b>, it is interpreted as a
2411 binary string.) The following will handle decimal, binary, octal, and
2412 hex in the standard Perl or C notation:
2414 $val = oct($val) if $val =~ /^0/;
2416 If EXPR is omitted, uses C<$_>. To go the other way (produce a number
2417 in octal), use sprintf() or printf():
2419 $perms = (stat("filename"))[2] & 07777;
2420 $oct_perms = sprintf "%lo", $perms;
2422 The oct() function is commonly used when a string such as C<644> needs
2423 to be converted into a file mode, for example. (Although perl will
2424 automatically convert strings into numbers as needed, this automatic
2425 conversion assumes base 10.)
2427 =item open FILEHANDLE,MODE,EXPR
2429 =item open FILEHANDLE,EXPR
2431 =item open FILEHANDLE
2433 Opens the file whose filename is given by EXPR, and associates it with
2434 FILEHANDLE. If FILEHANDLE is an expression, its value is used as the
2435 name of the real filehandle wanted. If EXPR is omitted, the scalar
2436 variable of the same name as the FILEHANDLE contains the filename.
2437 (Note that lexical variables--those declared with C<my>--will not work
2438 for this purpose; so if you're using C<my>, specify EXPR in your call
2439 to open.) See L<perlopentut> for a kinder, gentler explanation of opening
2442 If MODE is C<'E<lt>'> or nothing, the file is opened for input.
2443 If MODE is C<'E<gt>'>, the file is truncated and opened for
2444 output, being created if necessary. If MODE is C<'E<gt>E<gt>'>,
2445 the file is opened for appending, again being created if necessary.
2446 You can put a C<'+'> in front of the C<'E<gt>'> or C<'E<lt>'> to indicate that
2447 you want both read and write access to the file; thus C<'+E<lt>'> is almost
2448 always preferred for read/write updates--the C<'+E<gt>'> mode would clobber the
2449 file first. You can't usually use either read-write mode for updating
2450 textfiles, since they have variable length records. See the B<-i>
2451 switch in L<perlrun> for a better approach. The file is created with
2452 permissions of C<0666> modified by the process' C<umask> value.
2454 These various prefixes correspond to the fopen(3) modes of C<'r'>, C<'r+'>, C<'w'>,
2455 C<'w+'>, C<'a'>, and C<'a+'>.
2457 In the 2-arguments (and 1-argument) form of the call the mode and
2458 filename should be concatenated (in this order), possibly separated by
2459 spaces. It is possible to omit the mode if the mode is C<'E<lt>'>.
2461 If the filename begins with C<'|'>, the filename is interpreted as a
2462 command to which output is to be piped, and if the filename ends with a
2463 C<'|'>, the filename is interpreted as a command which pipes output to
2464 us. See L<perlipc/"Using open() for IPC">
2465 for more examples of this. (You are not allowed to C<open> to a command
2466 that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
2467 and L<perlipc/"Bidirectional Communication"> for alternatives.)
2469 If MODE is C<'|-'>, the filename is interpreted as a
2470 command to which output is to be piped, and if MODE is
2471 C<'-|'>, the filename is interpreted as a command which pipes output to
2472 us. In the 2-arguments (and 1-argument) form one should replace dash
2473 (C<'-'>) with the command. See L<perlipc/"Using open() for IPC">
2474 for more examples of this. (You are not allowed to C<open> to a command
2475 that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
2476 and L<perlipc/"Bidirectional Communication"> for alternatives.)
2478 In the 2-arguments (and 1-argument) form opening C<'-'> opens STDIN
2479 and opening C<'E<gt>-'> opens STDOUT.
2482 nonzero upon success, the undefined value otherwise. If the C<open>
2483 involved a pipe, the return value happens to be the pid of the
2486 If you're unfortunate enough to be running Perl on a system that
2487 distinguishes between text files and binary files (modern operating
2488 systems don't care), then you should check out L</binmode> for tips for
2489 dealing with this. The key distinction between systems that need C<binmode>
2490 and those that don't is their text file formats. Systems like Unix, MacOS, and
2491 Plan9, which delimit lines with a single character, and which encode that
2492 character in C as C<"\n">, do not need C<binmode>. The rest need it.
2494 When opening a file, it's usually a bad idea to continue normal execution
2495 if the request failed, so C<open> is frequently used in connection with
2496 C<die>. Even if C<die> won't do what you want (say, in a CGI script,
2497 where you want to make a nicely formatted error message (but there are
2498 modules that can help with that problem)) you should always check
2499 the return value from opening a file. The infrequent exception is when
2500 working with an unopened filehandle is actually what you want to do.
2505 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
2506 while (<ARTICLE>) {...
2508 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
2509 # if the open fails, output is discarded
2511 open(DBASE, '+<', 'dbase.mine') # open for update
2512 or die "Can't open 'dbase.mine' for update: $!";
2514 open(DBASE, '+<dbase.mine') # ditto
2515 or die "Can't open 'dbase.mine' for update: $!";
2517 open(ARTICLE, '-|', "caesar <$article") # decrypt article
2518 or die "Can't start caesar: $!";
2520 open(ARTICLE, "caesar <$article |") # ditto
2521 or die "Can't start caesar: $!";
2523 open(EXTRACT, "|sort >/tmp/Tmp$$") # $$ is our process id
2524 or die "Can't start sort: $!";
2526 # process argument list of files along with any includes
2528 foreach $file (@ARGV) {
2529 process($file, 'fh00');
2533 my($filename, $input) = @_;
2534 $input++; # this is a string increment
2535 unless (open($input, $filename)) {
2536 print STDERR "Can't open $filename: $!\n";
2541 while (<$input>) { # note use of indirection
2542 if (/^#include "(.*)"/) {
2543 process($1, $input);
2550 You may also, in the Bourne shell tradition, specify an EXPR beginning
2551 with C<'E<gt>&'>, in which case the rest of the string is interpreted as the
2552 name of a filehandle (or file descriptor, if numeric) to be
2553 duped and opened. You may use C<&> after C<E<gt>>, C<E<gt>E<gt>>,
2554 C<E<lt>>, C<+E<gt>>, C<+E<gt>E<gt>>, and C<+E<lt>>. The
2555 mode you specify should match the mode of the original filehandle.
2556 (Duping a filehandle does not take into account any existing contents of
2557 stdio buffers.) Duping file handles is not yet supported for 3-argument
2560 Here is a script that saves, redirects, and restores STDOUT and
2564 open(OLDOUT, ">&STDOUT");
2565 open(OLDERR, ">&STDERR");
2567 open(STDOUT, '>', "foo.out") || die "Can't redirect stdout";
2568 open(STDERR, ">&STDOUT") || die "Can't dup stdout";
2570 select(STDERR); $| = 1; # make unbuffered
2571 select(STDOUT); $| = 1; # make unbuffered
2573 print STDOUT "stdout 1\n"; # this works for
2574 print STDERR "stderr 1\n"; # subprocesses too
2579 open(STDOUT, ">&OLDOUT");
2580 open(STDERR, ">&OLDERR");
2582 print STDOUT "stdout 2\n";
2583 print STDERR "stderr 2\n";
2585 If you specify C<'E<lt>&=N'>, where C<N> is a number, then Perl will do an
2586 equivalent of C's C<fdopen> of that file descriptor; this is more
2587 parsimonious of file descriptors. For example:
2589 open(FILEHANDLE, "<&=$fd")
2591 If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'>
2592 with 2-arguments (or 1-argument) form of open(), then
2593 there is an implicit fork done, and the return value of open is the pid
2594 of the child within the parent process, and C<0> within the child
2595 process. (Use C<defined($pid)> to determine whether the open was successful.)
2596 The filehandle behaves normally for the parent, but i/o to that
2597 filehandle is piped from/to the STDOUT/STDIN of the child process.
2598 In the child process the filehandle isn't opened--i/o happens from/to
2599 the new STDOUT or STDIN. Typically this is used like the normal
2600 piped open when you want to exercise more control over just how the
2601 pipe command gets executed, such as when you are running setuid, and
2602 don't want to have to scan shell commands for metacharacters.
2603 The following triples are more or less equivalent:
2605 open(FOO, "|tr '[a-z]' '[A-Z]'");
2606 open(FOO, '|-', "tr '[a-z]' '[A-Z]'");
2607 open(FOO, '|-') || exec 'tr', '[a-z]', '[A-Z]';
2609 open(FOO, "cat -n '$file'|");
2610 open(FOO, '-|', "cat -n '$file'");
2611 open(FOO, '-|') || exec 'cat', '-n', $file;
2613 See L<perlipc/"Safe Pipe Opens"> for more examples of this.
2615 NOTE: On any operation that may do a fork, all files opened for output
2616 are flushed before the fork is attempted. On systems that support a
2617 close-on-exec flag on files, the flag will be set for the newly opened
2618 file descriptor as determined by the value of $^F. See L<perlvar/$^F>.
2620 Closing any piped filehandle causes the parent process to wait for the
2621 child to finish, and returns the status value in C<$?>.
2623 The filename passed to 2-argument (or 1-argument) form of open()
2624 will have leading and trailing
2625 whitespace deleted, and the normal redirection characters
2626 honored. This property, known as "magic open",
2627 can often be used to good effect. A user could specify a filename of
2628 F<"rsh cat file |">, or you could change certain filenames as needed:
2630 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
2631 open(FH, $filename) or die "Can't open $filename: $!";
2633 Use 3-argument form to open a file with arbitrary weird characters in it,
2635 open(FOO, '<', $file);
2637 otherwise it's necessary to protect any leading and trailing whitespace:
2639 $file =~ s#^(\s)#./$1#;
2640 open(FOO, "< $file\0");
2642 (this may not work on some bizzare filesystems). One should
2643 conscientiously choose between the the I<magic> and 3-arguments form
2648 will allow the user to specify an argument of the form C<"rsh cat file |">,
2649 but will not work on a filename which happens to have a trailing space, while
2651 open IN, '<', $ARGV[0];
2653 will have exactly the opposite restrictions.
2655 If you want a "real" C C<open> (see L<open(2)> on your system), then you
2656 should use the C<sysopen> function, which involves no such magic (but
2657 may use subtly different filemodes than Perl open(), which is mapped
2658 to C fopen()). This is
2659 another way to protect your filenames from interpretation. For example:
2662 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
2663 or die "sysopen $path: $!";
2664 $oldfh = select(HANDLE); $| = 1; select($oldfh);
2665 print HANDLE "stuff $$\n");
2667 print "File contains: ", <HANDLE>;
2669 Using the constructor from the C<IO::Handle> package (or one of its
2670 subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous
2671 filehandles that have the scope of whatever variables hold references to
2672 them, and automatically close whenever and however you leave that scope:
2676 sub read_myfile_munged {
2678 my $handle = new IO::File;
2679 open($handle, "myfile") or die "myfile: $!";
2681 or return (); # Automatically closed here.
2682 mung $first or die "mung failed"; # Or here.
2683 return $first, <$handle> if $ALL; # Or here.
2687 See L</seek> for some details about mixing reading and writing.
2689 =item opendir DIRHANDLE,EXPR
2691 Opens a directory named EXPR for processing by C<readdir>, C<telldir>,
2692 C<seekdir>, C<rewinddir>, and C<closedir>. Returns true if successful.
2693 DIRHANDLEs have their own namespace separate from FILEHANDLEs.
2699 Returns the numeric (ASCII or Unicode) value of the first character of EXPR. If
2700 EXPR is omitted, uses C<$_>. For the reverse, see L</chr>.
2701 See L<utf8> for more about Unicode.
2705 An C<our> declares the listed variables to be valid globals within
2706 the enclosing block, file, or C<eval>. That is, it has the same
2707 scoping rules as a "my" declaration, but does not create a local
2708 variable. If more than one value is listed, the list must be placed
2709 in parentheses. The C<our> declaration has no semantic effect unless
2710 "use strict vars" is in effect, in which case it lets you use the
2711 declared global variable without qualifying it with a package name.
2712 (But only within the lexical scope of the C<our> declaration. In this
2713 it differs from "use vars", which is package scoped.)
2715 =item pack TEMPLATE,LIST
2717 Takes a list of values and packs it into a binary structure,
2718 returning the string containing the structure. The TEMPLATE is a
2719 sequence of characters that give the order and type of values, as
2722 a A string with arbitrary binary data, will be null padded.
2723 A An ascii string, will be space padded.
2724 Z A null terminated (asciz) string, will be null padded.
2726 b A bit string (ascending bit order, like vec()).
2727 B A bit string (descending bit order).
2728 h A hex string (low nybble first).
2729 H A hex string (high nybble first).
2731 c A signed char value.
2732 C An unsigned char value. Only does bytes. See U for Unicode.
2734 s A signed short value.
2735 S An unsigned short value.
2736 (This 'short' is _exactly_ 16 bits, which may differ from
2737 what a local C compiler calls 'short'. If you want
2738 native-length shorts, use the '!' suffix.)
2740 i A signed integer value.
2741 I An unsigned integer value.
2742 (This 'integer' is _at_least_ 32 bits wide. Its exact
2743 size depends on what a local C compiler calls 'int',
2744 and may even be larger than the 'long' described in
2747 l A signed long value.
2748 L An unsigned long value.
2749 (This 'long' is _exactly_ 32 bits, which may differ from
2750 what a local C compiler calls 'long'. If you want
2751 native-length longs, use the '!' suffix.)
2753 n A short in "network" (big-endian) order.
2754 N A long in "network" (big-endian) order.
2755 v A short in "VAX" (little-endian) order.
2756 V A long in "VAX" (little-endian) order.
2757 (These 'shorts' and 'longs' are _exactly_ 16 bits and
2758 _exactly_ 32 bits, respectively.)
2760 q A signed quad (64-bit) value.
2761 Q An unsigned quad value.
2762 (Quads are available only if your system supports 64-bit
2763 integer values _and_ if Perl has been compiled to support those.
2764 Causes a fatal error otherwise.)
2766 f A single-precision float in the native format.
2767 d A double-precision float in the native format.
2769 p A pointer to a null-terminated string.
2770 P A pointer to a structure (fixed-length string).
2772 u A uuencoded string.
2773 U A Unicode character number. Encodes to UTF-8 internally.
2774 Works even if C<use utf8> is not in effect.
2776 w A BER compressed integer. Its bytes represent an unsigned
2777 integer in base 128, most significant digit first, with as
2778 few digits as possible. Bit eight (the high bit) is set
2779 on each byte except the last.
2783 @ Null fill to absolute position.
2785 The following rules apply:
2791 Each letter may optionally be followed by a number giving a repeat
2792 count. With all types except C<"a">, C<"A">, C<"Z">, C<"b">, C<"B">, C<"h">,
2793 C<"H">, and C<"P"> the pack function will gobble up that many values from
2794 the LIST. A C<*> for the repeat count means to use however many items are
2799 The C<"a">, C<"A">, and C<"Z"> types gobble just one value, but pack it as a
2800 string of length count, padding with nulls or spaces as necessary. When
2801 unpacking, C<"A"> strips trailing spaces and nulls, C<"Z"> strips everything
2802 after the first null, and C<"a"> returns data verbatim.
2806 Likewise, the C<"b"> and C<"B"> fields pack a string that many bits long.
2810 The C<"h"> and C<"H"> fields pack a string that many nybbles (4-bit groups,
2811 representable as hexadecimal digits, 0-9a-f) long.
2815 The C<"p"> type packs a pointer to a null-terminated string. You are
2816 responsible for ensuring the string is not a temporary value (which can
2817 potentially get deallocated before you get around to using the packed result).
2818 The C<"P"> type packs a pointer to a structure of the size indicated by the
2819 length. A NULL pointer is created if the corresponding value for C<"p"> or
2824 The C<"/"> character allows packing and unpacking of strings where the
2825 packed structure contains a byte count followed by the string itself.
2826 You write I<length-item>C</>I<string-item>.
2828 The I<length-item> can be any C<pack> template letter,
2829 and describes how the length value is packed.
2830 The ones likely to be of most use are integer-packing ones like
2831 C<"n"> (for Java strings), C<"w"> (for ASN.1 or SNMP)
2832 and C<"N"> (for Sun XDR).
2834 The I<string-item> must, at present, be C<"A*">, C<"a*"> or C<"Z*">.
2835 For C<unpack> the length of the string is obtained from the I<length-item>,
2836 but if you put in the '*' it will be ignored.
2838 unpack 'C/a', "\04Gurusamy"; gives 'Guru'
2839 unpack 'a3/A* A*', '007 Bond J '; gives (' Bond','J')
2840 pack 'n/a* w/a*','hello,','world'; gives "\000\006hello,\005world"
2842 The I<length-item> is not returned explicitly from C<unpack>.
2844 Adding a count to the I<length-item> letter
2845 is unlikely to do anything useful,
2846 unless that letter is C<"A">, C<"a"> or C<"Z">.
2847 Packing with a I<length-item> of C<"a"> or C<"Z">
2848 may introduce C<"\000"> characters,
2849 which Perl does not regard as legal in numeric strings.
2853 The integer types C<"s">, C<"S">, C<"l">, and C<"L"> may be
2854 immediately followed by a C<"!"> suffix to signify native shorts or
2855 longs--as you can see from above for example a bare C<"l"> does mean
2856 exactly 32 bits, the native C<long> (as seen by the local C compiler)
2857 may be larger. This is an issue mainly in 64-bit platforms. You can
2858 see whether using C<"!"> makes any difference by
2860 print length(pack("s")), " ", length(pack("s!")), "\n";
2861 print length(pack("l")), " ", length(pack("l!")), "\n";
2863 C<"i!"> and C<"I!"> also work but only because of completeness;
2864 they are identical to C<"i"> and C<"I">.
2866 The actual sizes (in bytes) of native shorts, ints, longs, and long
2867 longs on the platform where Perl was built are also available via
2871 print $Config{shortsize}, "\n";
2872 print $Config{intsize}, "\n";
2873 print $Config{longsize}, "\n";
2874 print $Config{longlongsize}, "\n";
2876 (The C<$Config{longlongsize}> will be undefine if your system does
2877 not support long longs.)
2881 The integer formats C<"s">, C<"S">, C<"i">, C<"I">, C<"l">, and C<"L">
2882 are inherently non-portable between processors and operating systems
2883 because they obey the native byteorder and endianness. For example a
2884 4-byte integer 0x12345678 (305419896 decimal) be ordered natively
2885 (arranged in and handled by the CPU registers) into bytes as
2887 0x12 0x34 0x56 0x78 # little-endian
2888 0x78 0x56 0x34 0x12 # big-endian
2890 Basically, the Intel, Alpha, and VAX CPUs and little-endian, while
2891 everybody else, for example Motorola m68k/88k, PPC, Sparc, HP PA,
2892 Power, and Cray are big-endian. MIPS can be either: Digital used it
2893 in little-endian mode; SGI uses it in big-endian mode.
2895 The names `big-endian' and `little-endian' are comic references to
2896 the classic "Gulliver's Travels" (via the paper "On Holy Wars and a
2897 Plea for Peace" by Danny Cohen, USC/ISI IEN 137, April 1, 1980) and
2898 the egg-eating habits of the Lilliputians.
2900 Some systems may have even weirder byte orders such as
2905 You can see your system's preference with
2907 print join(" ", map { sprintf "%#02x", $_ }
2908 unpack("C*",pack("L",0x12345678))), "\n";
2910 The byteorder on the platform where Perl was built is also available
2914 print $Config{byteorder}, "\n";
2916 Byteorders C<'1234'> and C<'12345678'> are little-endian, C<'4321'>
2917 and C<'87654321'> are big-endian.
2919 If you want portable packed integers use the formats C<"n">, C<"N">,
2920 C<"v">, and C<"V">, their byte endianness and size is known.
2921 See also L<perlport>.
2925 Real numbers (floats and doubles) are in the native machine format only;
2926 due to the multiplicity of floating formats around, and the lack of a
2927 standard "network" representation, no facility for interchange has been
2928 made. This means that packed floating point data written on one machine
2929 may not be readable on another - even if both use IEEE floating point
2930 arithmetic (as the endian-ness of the memory representation is not part
2931 of the IEEE spec). See also L<perlport>.
2933 Note that Perl uses doubles internally for all numeric calculation, and
2934 converting from double into float and thence back to double again will
2935 lose precision (i.e., C<unpack("f", pack("f", $foo)>) will not in general
2940 You must yourself do any alignment or padding by inserting for example
2941 enough C<'x'>es while packing. There is no way to pack() and unpack()
2942 could know where the bytes are going to or coming from. Therefore
2943 C<pack> (and C<unpack>) handle their output and input as flat
2948 A comment in a TEMPLATE starts with C<#> and goes to the end of line.
2954 $foo = pack("CCCC",65,66,67,68);
2956 $foo = pack("C4",65,66,67,68);
2958 $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);
2959 # same thing with Unicode circled letters
2961 $foo = pack("ccxxcc",65,66,67,68);
2964 # note: the above examples featuring "C" and "c" are true
2965 # only on ASCII and ASCII-derived systems such as ISO Latin 1
2966 # and UTF-8. In EBCDIC the first example would be
2967 # $foo = pack("CCCC",193,194,195,196);
2969 $foo = pack("s2",1,2);
2970 # "\1\0\2\0" on little-endian
2971 # "\0\1\0\2" on big-endian
2973 $foo = pack("a4","abcd","x","y","z");
2976 $foo = pack("aaaa","abcd","x","y","z");
2979 $foo = pack("a14","abcdefg");
2980 # "abcdefg\0\0\0\0\0\0\0"
2982 $foo = pack("i9pl", gmtime);
2983 # a real struct tm (on my system anyway)
2985 $utmp_template = "Z8 Z8 Z16 L";
2986 $utmp = pack($utmp_template, @utmp1);
2987 # a struct utmp (BSDish)
2989 @utmp2 = unpack($utmp_template, $utmp);
2990 # "@utmp1" eq "@utmp2"
2993 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
2996 $foo = pack('sx2l', 12, 34);
2997 # short 12, two zero bytes padding, long 34
2998 $bar = pack('s@4l', 12, 34);
2999 # short 12, zero fill to position 4, long 34
3002 The same template may generally also be used in unpack().
3006 =item package NAMESPACE
3008 Declares the compilation unit as being in the given namespace. The scope
3009 of the package declaration is from the declaration itself through the end
3010 of the enclosing block, file, or eval (the same as the C<my> operator).
3011 All further unqualified dynamic identifiers will be in this namespace.
3012 A package statement affects only dynamic variables--including those
3013 you've used C<local> on--but I<not> lexical variables, which are created
3014 with C<my>. Typically it would be the first declaration in a file to
3015 be included by the C<require> or C<use> operator. You can switch into a
3016 package in more than one place; it merely influences which symbol table
3017 is used by the compiler for the rest of that block. You can refer to
3018 variables and filehandles in other packages by prefixing the identifier
3019 with the package name and a double colon: C<$Package::Variable>.
3020 If the package name is null, the C<main> package as assumed. That is,
3021 C<$::sail> is equivalent to C<$main::sail> (as well as to C<$main'sail>,
3022 still seen in older code).
3024 If NAMESPACE is omitted, then there is no current package, and all
3025 identifiers must be fully qualified or lexicals. This is stricter
3026 than C<use strict>, since it also extends to function names.
3028 See L<perlmod/"Packages"> for more information about packages, modules,
3029 and classes. See L<perlsub> for other scoping issues.
3031 =item pipe READHANDLE,WRITEHANDLE
3033 Opens a pair of connected pipes like the corresponding system call.
3034 Note that if you set up a loop of piped processes, deadlock can occur
3035 unless you are very careful. In addition, note that Perl's pipes use
3036 stdio buffering, so you may need to set C<$|> to flush your WRITEHANDLE
3037 after each command, depending on the application.
3039 See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
3040 for examples of such things.
3042 On systems that support a close-on-exec flag on files, the flag will be set
3043 for the newly opened file descriptors as determined by the value of $^F.
3050 Pops and returns the last value of the array, shortening the array by
3051 one element. Has an effect similar to
3055 If there are no elements in the array, returns the undefined value
3056 (although this may happen at other times as well). If ARRAY is
3057 omitted, pops the C<@ARGV> array in the main program, and the C<@_>
3058 array in subroutines, just like C<shift>.
3064 Returns the offset of where the last C<m//g> search left off for the variable
3065 is in question (C<$_> is used when the variable is not specified). May be
3066 modified to change that offset. Such modification will also influence
3067 the C<\G> zero-width assertion in regular expressions. See L<perlre> and
3070 =item print FILEHANDLE LIST
3076 Prints a string or a list of strings. Returns true if successful.
3077 FILEHANDLE may be a scalar variable name, in which case the variable
3078 contains the name of or a reference to the filehandle, thus introducing
3079 one level of indirection. (NOTE: If FILEHANDLE is a variable and
3080 the next token is a term, it may be misinterpreted as an operator
3081 unless you interpose a C<+> or put parentheses around the arguments.)
3082 If FILEHANDLE is omitted, prints by default to standard output (or
3083 to the last selected output channel--see L</select>). If LIST is
3084 also omitted, prints C<$_> to the currently selected output channel.
3085 To set the default output channel to something other than STDOUT
3086 use the select operation. The current value of C<$,> (if any) is
3087 printed between each LIST item. The current value of C<$\> (if
3088 any) is printed after the entire LIST has been printed. Because
3089 print takes a LIST, anything in the LIST is evaluated in list
3090 context, and any subroutine that you call will have one or more of
3091 its expressions evaluated in list context. Also be careful not to
3092 follow the print keyword with a left parenthesis unless you want
3093 the corresponding right parenthesis to terminate the arguments to
3094 the print--interpose a C<+> or put parentheses around all the
3097 Note that if you're storing FILEHANDLES in an array or other expression,
3098 you will have to use a block returning its value instead:
3100 print { $files[$i] } "stuff\n";
3101 print { $OK ? STDOUT : STDERR } "stuff\n";
3103 =item printf FILEHANDLE FORMAT, LIST
3105 =item printf FORMAT, LIST
3107 Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that C<$\>
3108 (the output record separator) is not appended. The first argument
3109 of the list will be interpreted as the C<printf> format. If C<use locale> is
3110 in effect, the character used for the decimal point in formatted real numbers
3111 is affected by the LC_NUMERIC locale. See L<perllocale>.
3113 Don't fall into the trap of using a C<printf> when a simple
3114 C<print> would do. The C<print> is more efficient and less
3117 =item prototype FUNCTION
3119 Returns the prototype of a function as a string (or C<undef> if the
3120 function has no prototype). FUNCTION is a reference to, or the name of,
3121 the function whose prototype you want to retrieve.
3123 If FUNCTION is a string starting with C<CORE::>, the rest is taken as a
3124 name for Perl builtin. If the builtin is not I<overridable> (such as
3125 C<qw//>) or its arguments cannot be expressed by a prototype (such as
3126 C<system>) returns C<undef> because the builtin does not really behave
3127 like a Perl function. Otherwise, the string describing the equivalent
3128 prototype is returned.
3130 =item push ARRAY,LIST
3132 Treats ARRAY as a stack, and pushes the values of LIST
3133 onto the end of ARRAY. The length of ARRAY increases by the length of
3134 LIST. Has the same effect as
3137 $ARRAY[++$#ARRAY] = $value;
3140 but is more efficient. Returns the new number of elements in the array.
3152 Generalized quotes. See L<perlop/"Regexp Quote-Like Operators">.
3154 =item quotemeta EXPR
3158 Returns the value of EXPR with all non-alphanumeric
3159 characters backslashed. (That is, all characters not matching
3160 C</[A-Za-z_0-9]/> will be preceded by a backslash in the
3161 returned string, regardless of any locale settings.)
3162 This is the internal function implementing
3163 the C<\Q> escape in double-quoted strings.
3165 If EXPR is omitted, uses C<$_>.
3171 Returns a random fractional number greater than or equal to C<0> and less
3172 than the value of EXPR. (EXPR should be positive.) If EXPR is
3173 omitted, the value C<1> is used. Automatically calls C<srand> unless
3174 C<srand> has already been called. See also C<srand>.
3176 (Note: If your rand function consistently returns numbers that are too
3177 large or too small, then your version of Perl was probably compiled
3178 with the wrong number of RANDBITS.)
3180 =item read FILEHANDLE,SCALAR,LENGTH,OFFSET
3182 =item read FILEHANDLE,SCALAR,LENGTH
3184 Attempts to read LENGTH bytes of data into variable SCALAR from the
3185 specified FILEHANDLE. Returns the number of bytes actually read,
3186 C<0> at end of file, or undef if there was an error. SCALAR will be grown
3187 or shrunk to the length actually read. An OFFSET may be specified to
3188 place the read data at some other place than the beginning of the
3189 string. This call is actually implemented in terms of stdio's fread(3)
3190 call. To get a true read(2) system call, see C<sysread>.
3192 =item readdir DIRHANDLE
3194 Returns the next directory entry for a directory opened by C<opendir>.
3195 If used in list context, returns all the rest of the entries in the
3196 directory. If there are no more entries, returns an undefined value in
3197 scalar context or a null list in list context.
3199 If you're planning to filetest the return values out of a C<readdir>, you'd
3200 better prepend the directory in question. Otherwise, because we didn't
3201 C<chdir> there, it would have been testing the wrong file.
3203 opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!";
3204 @dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR);
3209 Reads from the filehandle whose typeglob is contained in EXPR. In scalar
3210 context, each call reads and returns the next line, until end-of-file is
3211 reached, whereupon the subsequent call returns undef. In list context,
3212 reads until end-of-file is reached and returns a list of lines. Note that
3213 the notion of "line" used here is however you may have defined it
3214 with C<$/> or C<$INPUT_RECORD_SEPARATOR>). See L<perlvar/"$/">.
3216 When C<$/> is set to C<undef>, when readline() is in scalar
3217 context (i.e. file slurp mode), and when an empty file is read, it
3218 returns C<''> the first time, followed by C<undef> subsequently.
3220 This is the internal function implementing the C<E<lt>EXPRE<gt>>
3221 operator, but you can use it directly. The C<E<lt>EXPRE<gt>>
3222 operator is discussed in more detail in L<perlop/"I/O Operators">.
3225 $line = readline(*STDIN); # same thing
3231 Returns the value of a symbolic link, if symbolic links are
3232 implemented. If not, gives a fatal error. If there is some system
3233 error, returns the undefined value and sets C<$!> (errno). If EXPR is
3234 omitted, uses C<$_>.
3238 EXPR is executed as a system command.
3239 The collected standard output of the command is returned.
3240 In scalar context, it comes back as a single (potentially
3241 multi-line) string. In list context, returns a list of lines
3242 (however you've defined lines with C<$/> or C<$INPUT_RECORD_SEPARATOR>).
3243 This is the internal function implementing the C<qx/EXPR/>
3244 operator, but you can use it directly. The C<qx/EXPR/>
3245 operator is discussed in more detail in L<perlop/"I/O Operators">.
3247 =item recv SOCKET,SCALAR,LENGTH,FLAGS
3249 Receives a message on a socket. Attempts to receive LENGTH bytes of
3250 data into variable SCALAR from the specified SOCKET filehandle.
3251 Actually does a C C<recvfrom>, so that it can return the address of the
3252 sender. Returns the undefined value if there's an error. SCALAR will
3253 be grown or shrunk to the length actually read. Takes the same flags
3254 as the system call of the same name.
3255 See L<perlipc/"UDP: Message Passing"> for examples.
3261 The C<redo> command restarts the loop block without evaluating the
3262 conditional again. The C<continue> block, if any, is not executed. If
3263 the LABEL is omitted, the command refers to the innermost enclosing
3264 loop. This command is normally used by programs that want to lie to
3265 themselves about what was just input:
3267 # a simpleminded Pascal comment stripper
3268 # (warning: assumes no { or } in strings)
3269 LINE: while (<STDIN>) {
3270 while (s|({.*}.*){.*}|$1 |) {}
3275 if (/}/) { # end of comment?
3284 C<redo> cannot be used to retry a block which returns a value such as
3285 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
3286 a grep() or map() operation.
3288 See also L</continue> for an illustration of how C<last>, C<next>, and
3295 Returns a true value if EXPR is a reference, false otherwise. If EXPR
3296 is not specified, C<$_> will be used. The value returned depends on the
3297 type of thing the reference is a reference to.
3298 Builtin types include:
3308 If the referenced object has been blessed into a package, then that package
3309 name is returned instead. You can think of C<ref> as a C<typeof> operator.
3311 if (ref($r) eq "HASH") {
3312 print "r is a reference to a hash.\n";
3315 print "r is not a reference at all.\n";
3317 if (UNIVERSAL::isa($r, "HASH")) { # for subclassing
3318 print "r is a reference to something that isa hash.\n";
3321 See also L<perlref>.
3323 =item rename OLDNAME,NEWNAME
3325 Changes the name of a file; an existing file NEWNAME will be
3326 clobbered. Returns true for success, false otherwise.
3328 Behavior of this function varies wildly depending on your system
3329 implementation. For example, it will usually not work across file system
3330 boundaries, even though the system I<mv> command sometimes compensates
3331 for this. Other restrictions include whether it works on directories,
3332 open files, or pre-existing files. Check L<perlport> and either the
3333 rename(2) manpage or equivalent system documentation for details.
3339 Demands some semantics specified by EXPR, or by C<$_> if EXPR is not
3340 supplied. If EXPR is numeric, demands that the current version of Perl
3341 (C<$]> or $PERL_VERSION) be equal or greater than EXPR.
3343 Otherwise, demands that a library file be included if it hasn't already
3344 been included. The file is included via the do-FILE mechanism, which is
3345 essentially just a variety of C<eval>. Has semantics similar to the following
3350 return 1 if $INC{$filename};
3351 my($realfilename,$result);
3353 foreach $prefix (@INC) {
3354 $realfilename = "$prefix/$filename";
3355 if (-f $realfilename) {
3356 $result = do $realfilename;
3360 die "Can't find $filename in \@INC";
3363 die "$filename did not return true value" unless $result;
3364 $INC{$filename} = $realfilename;
3368 Note that the file will not be included twice under the same specified
3369 name. The file must return true as the last statement to indicate
3370 successful execution of any initialization code, so it's customary to
3371 end such a file with C<1;> unless you're sure it'll return true
3372 otherwise. But it's better just to put the C<1;>, in case you add more
3375 If EXPR is a bareword, the require assumes a "F<.pm>" extension and
3376 replaces "F<::>" with "F</>" in the filename for you,
3377 to make it easy to load standard modules. This form of loading of
3378 modules does not risk altering your namespace.
3380 In other words, if you try this:
3382 require Foo::Bar; # a splendid bareword
3384 The require function will actually look for the "F<Foo/Bar.pm>" file in the
3385 directories specified in the C<@INC> array.
3387 But if you try this:
3389 $class = 'Foo::Bar';
3390 require $class; # $class is not a bareword
3392 require "Foo::Bar"; # not a bareword because of the ""
3394 The require function will look for the "F<Foo::Bar>" file in the @INC array and
3395 will complain about not finding "F<Foo::Bar>" there. In this case you can do:
3397 eval "require $class";
3399 For a yet-more-powerful import facility, see L</use> and L<perlmod>.
3405 Generally used in a C<continue> block at the end of a loop to clear
3406 variables and reset C<??> searches so that they work again. The
3407 expression is interpreted as a list of single characters (hyphens
3408 allowed for ranges). All variables and arrays beginning with one of
3409 those letters are reset to their pristine state. If the expression is
3410 omitted, one-match searches (C<?pattern?>) are reset to match again. Resets
3411 only variables or searches in the current package. Always returns
3414 reset 'X'; # reset all X variables
3415 reset 'a-z'; # reset lower case variables
3416 reset; # just reset ?one-time? searches
3418 Resetting C<"A-Z"> is not recommended because you'll wipe out your
3419 C<@ARGV> and C<@INC> arrays and your C<%ENV> hash. Resets only package
3420 variables--lexical variables are unaffected, but they clean themselves
3421 up on scope exit anyway, so you'll probably want to use them instead.
3428 Returns from a subroutine, C<eval>, or C<do FILE> with the value
3429 given in EXPR. Evaluation of EXPR may be in list, scalar, or void
3430 context, depending on how the return value will be used, and the context
3431 may vary from one execution to the next (see C<wantarray>). If no EXPR
3432 is given, returns an empty list in list context, the undefined value in
3433 scalar context, and (of course) nothing at all in a void context.
3435 (Note that in the absence of a explicit C<return>, a subroutine, eval,
3436 or do FILE will automatically return the value of the last expression
3441 In list context, returns a list value consisting of the elements
3442 of LIST in the opposite order. In scalar context, concatenates the
3443 elements of LIST and returns a string value with all characters
3444 in the opposite order.
3446 print reverse <>; # line tac, last line first
3448 undef $/; # for efficiency of <>
3449 print scalar reverse <>; # character tac, last line tsrif
3451 This operator is also handy for inverting a hash, although there are some
3452 caveats. If a value is duplicated in the original hash, only one of those
3453 can be represented as a key in the inverted hash. Also, this has to
3454 unwind one hash and build a whole new one, which may take some time
3455 on a large hash, such as from a DBM file.
3457 %by_name = reverse %by_address; # Invert the hash
3459 =item rewinddir DIRHANDLE
3461 Sets the current position to the beginning of the directory for the
3462 C<readdir> routine on DIRHANDLE.
3464 =item rindex STR,SUBSTR,POSITION
3466 =item rindex STR,SUBSTR
3468 Works just like index() except that it returns the position of the LAST
3469 occurrence of SUBSTR in STR. If POSITION is specified, returns the
3470 last occurrence at or before that position.
3472 =item rmdir FILENAME
3476 Deletes the directory specified by FILENAME if that directory is empty. If it
3477 succeeds it returns true, otherwise it returns false and sets C<$!> (errno). If
3478 FILENAME is omitted, uses C<$_>.
3482 The substitution operator. See L<perlop>.
3486 Forces EXPR to be interpreted in scalar context and returns the value
3489 @counts = ( scalar @a, scalar @b, scalar @c );
3491 There is no equivalent operator to force an expression to
3492 be interpolated in list context because in practice, this is never
3493 needed. If you really wanted to do so, however, you could use
3494 the construction C<@{[ (some expression) ]}>, but usually a simple
3495 C<(some expression)> suffices.
3497 Because C<scalar> is unary operator, if you accidentally use for EXPR a
3498 parenthesized list, this behaves as a scalar comma expression, evaluating
3499 all but the last element in void context and returning the final element
3500 evaluated in scalar context. This is seldom what you want.
3502 The following single statement:
3504 print uc(scalar(&foo,$bar)),$baz;
3506 is the moral equivalent of these two:
3509 print(uc($bar),$baz);
3511 See L<perlop> for more details on unary operators and the comma operator.
3513 =item seek FILEHANDLE,POSITION,WHENCE
3515 Sets FILEHANDLE's position, just like the C<fseek> call of C<stdio>.
3516 FILEHANDLE may be an expression whose value gives the name of the
3517 filehandle. The values for WHENCE are C<0> to set the new position to
3518 POSITION, C<1> to set it to the current position plus POSITION, and
3519 C<2> to set it to EOF plus POSITION (typically negative). For WHENCE
3520 you may use the constants C<SEEK_SET>, C<SEEK_CUR>, and C<SEEK_END>
3521 (start of the file, current position, end of the file) from any of the
3522 modules Fcntl, C<IO::Seekable>, or POSIX. Returns C<1> upon success,
3525 If you want to position file for C<sysread> or C<syswrite>, don't use
3526 C<seek>--buffering makes its effect on the file's system position
3527 unpredictable and non-portable. Use C<sysseek> instead.
3529 Due to the rules and rigors of ANSI C, on some systems you have to do a
3530 seek whenever you switch between reading and writing. Amongst other
3531 things, this may have the effect of calling stdio's clearerr(3).
3532 A WHENCE of C<1> (C<SEEK_CUR>) is useful for not moving the file position:
3536 This is also useful for applications emulating C<tail -f>. Once you hit
3537 EOF on your read, and then sleep for a while, you might have to stick in a
3538 seek() to reset things. The C<seek> doesn't change the current position,
3539 but it I<does> clear the end-of-file condition on the handle, so that the
3540 next C<E<lt>FILEE<gt>> makes Perl try again to read something. We hope.
3542 If that doesn't work (some stdios are particularly cantankerous), then
3543 you may need something more like this:
3546 for ($curpos = tell(FILE); $_ = <FILE>;
3547 $curpos = tell(FILE)) {
3548 # search for some stuff and put it into files
3550 sleep($for_a_while);
3551 seek(FILE, $curpos, 0);
3554 =item seekdir DIRHANDLE,POS
3556 Sets the current position for the C<readdir> routine on DIRHANDLE. POS
3557 must be a value returned by C<telldir>. Has the same caveats about
3558 possible directory compaction as the corresponding system library
3561 =item select FILEHANDLE
3565 Returns the currently selected filehandle. Sets the current default
3566 filehandle for output, if FILEHANDLE is supplied. This has two
3567 effects: first, a C<write> or a C<print> without a filehandle will
3568 default to this FILEHANDLE. Second, references to variables related to
3569 output will refer to this output channel. For example, if you have to
3570 set the top of form format for more than one output channel, you might
3578 FILEHANDLE may be an expression whose value gives the name of the
3579 actual filehandle. Thus:
3581 $oldfh = select(STDERR); $| = 1; select($oldfh);
3583 Some programmers may prefer to think of filehandles as objects with
3584 methods, preferring to write the last example as:
3587 STDERR->autoflush(1);
3589 =item select RBITS,WBITS,EBITS,TIMEOUT
3591 This calls the select(2) system call with the bit masks specified, which
3592 can be constructed using C<fileno> and C<vec>, along these lines:
3594 $rin = $win = $ein = '';
3595 vec($rin,fileno(STDIN),1) = 1;
3596 vec($win,fileno(STDOUT),1) = 1;
3599 If you want to select on many filehandles you might wish to write a
3603 my(@fhlist) = split(' ',$_[0]);
3606 vec($bits,fileno($_),1) = 1;
3610 $rin = fhbits('STDIN TTY SOCK');
3614 ($nfound,$timeleft) =
3615 select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
3617 or to block until something becomes ready just do this
3619 $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
3621 Most systems do not bother to return anything useful in $timeleft, so
3622 calling select() in scalar context just returns $nfound.
3624 Any of the bit masks can also be undef. The timeout, if specified, is
3625 in seconds, which may be fractional. Note: not all implementations are
3626 capable of returning the$timeleft. If not, they always return
3627 $timeleft equal to the supplied $timeout.
3629 You can effect a sleep of 250 milliseconds this way:
3631 select(undef, undef, undef, 0.25);
3633 B<WARNING>: One should not attempt to mix buffered I/O (like C<read>
3634 or E<lt>FHE<gt>) with C<select>, except as permitted by POSIX, and even
3635 then only on POSIX systems. You have to use C<sysread> instead.
3637 =item semctl ID,SEMNUM,CMD,ARG
3639 Calls the System V IPC function C<semctl>. You'll probably have to say
3643 first to get the correct constant definitions. If CMD is IPC_STAT or
3644 GETALL, then ARG must be a variable which will hold the returned
3645 semid_ds structure or semaphore value array. Returns like C<ioctl>: the
3646 undefined value for error, "C<0 but true>" for zero, or the actual return
3647 value otherwise. See also C<IPC::SysV> and C<IPC::Semaphore> documentation.
3649 =item semget KEY,NSEMS,FLAGS
3651 Calls the System V IPC function semget. Returns the semaphore id, or
3652 the undefined value if there is an error. See also C<IPC::SysV> and
3653 C<IPC::SysV::Semaphore> documentation.
3655 =item semop KEY,OPSTRING
3657 Calls the System V IPC function semop to perform semaphore operations
3658 such as signaling and waiting. OPSTRING must be a packed array of
3659 semop structures. Each semop structure can be generated with
3660 C<pack("sss", $semnum, $semop, $semflag)>. The number of semaphore
3661 operations is implied by the length of OPSTRING. Returns true if
3662 successful, or false if there is an error. As an example, the
3663 following code waits on semaphore $semnum of semaphore id $semid:
3665 $semop = pack("sss", $semnum, -1, 0);
3666 die "Semaphore trouble: $!\n" unless semop($semid, $semop);
3668 To signal the semaphore, replace C<-1> with C<1>. See also C<IPC::SysV>
3669 and C<IPC::SysV::Semaphore> documentation.
3671 =item send SOCKET,MSG,FLAGS,TO
3673 =item send SOCKET,MSG,FLAGS
3675 Sends a message on a socket. Takes the same flags as the system call
3676 of the same name. On unconnected sockets you must specify a
3677 destination to send TO, in which case it does a C C<sendto>. Returns
3678 the number of characters sent, or the undefined value if there is an
3679 error. The C system call sendmsg(2) is currently unimplemented.
3680 See L<perlipc/"UDP: Message Passing"> for examples.
3682 =item setpgrp PID,PGRP
3684 Sets the current process group for the specified PID, C<0> for the current
3685 process. Will produce a fatal error if used on a machine that doesn't
3686 implement setpgrp(2). If the arguments are omitted, it defaults to
3687 C<0,0>. Note that the POSIX version of C<setpgrp> does not accept any
3688 arguments, so only C<setpgrp(0,0)> is portable. See also C<POSIX::setsid()>.
3690 =item setpriority WHICH,WHO,PRIORITY
3692 Sets the current priority for a process, a process group, or a user.
3693 (See setpriority(2).) Will produce a fatal error if used on a machine
3694 that doesn't implement setpriority(2).
3696 =item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
3698 Sets the socket option requested. Returns undefined if there is an
3699 error. OPTVAL may be specified as C<undef> if you don't want to pass an
3706 Shifts the first value of the array off and returns it, shortening the
3707 array by 1 and moving everything down. If there are no elements in the
3708 array, returns the undefined value. If ARRAY is omitted, shifts the
3709 C<@_> array within the lexical scope of subroutines and formats, and the
3710 C<@ARGV> array at file scopes or within the lexical scopes established by
3711 the C<eval ''>, C<BEGIN {}>, C<END {}>, and C<INIT {}> constructs.
3712 See also C<unshift>, C<push>, and C<pop>. C<Shift()> and C<unshift> do the
3713 same thing to the left end of an array that C<pop> and C<push> do to the
3716 =item shmctl ID,CMD,ARG
3718 Calls the System V IPC function shmctl. You'll probably have to say
3722 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
3723 then ARG must be a variable which will hold the returned C<shmid_ds>
3724 structure. Returns like ioctl: the undefined value for error, "C<0> but
3725 true" for zero, or the actual return value otherwise.
3726 See also C<IPC::SysV> documentation.
3728 =item shmget KEY,SIZE,FLAGS
3730 Calls the System V IPC function shmget. Returns the shared memory
3731 segment id, or the undefined value if there is an error.
3732 See also C<IPC::SysV> documentation.
3734 =item shmread ID,VAR,POS,SIZE
3736 =item shmwrite ID,STRING,POS,SIZE
3738 Reads or writes the System V shared memory segment ID starting at
3739 position POS for size SIZE by attaching to it, copying in/out, and
3740 detaching from it. When reading, VAR must be a variable that will
3741 hold the data read. When writing, if STRING is too long, only SIZE
3742 bytes are used; if STRING is too short, nulls are written to fill out
3743 SIZE bytes. Return true if successful, or false if there is an error.
3744 See also C<IPC::SysV> documentation and the C<IPC::Shareable> module
3747 =item shutdown SOCKET,HOW
3749 Shuts down a socket connection in the manner indicated by HOW, which
3750 has the same interpretation as in the system call of the same name.
3752 shutdown(SOCKET, 0); # I/we have stopped reading data
3753 shutdown(SOCKET, 1); # I/we have stopped writing data
3754 shutdown(SOCKET, 2); # I/we have stopped using this socket
3756 This is useful with sockets when you want to tell the other
3757 side you're done writing but not done reading, or vice versa.
3758 It's also a more insistent form of close because it also
3759 disables the file descriptor in any forked copies in other
3766 Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
3767 returns sine of C<$_>.
3769 For the inverse sine operation, you may use the C<POSIX::asin>
3770 function, or use this relation:
3772 sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
3778 Causes the script to sleep for EXPR seconds, or forever if no EXPR.
3779 May be interrupted if the process receives a signal such as C<SIGALRM>.
3780 Returns the number of seconds actually slept. You probably cannot
3781 mix C<alarm> and C<sleep> calls, because C<sleep> is often implemented
3784 On some older systems, it may sleep up to a full second less than what
3785 you requested, depending on how it counts seconds. Most modern systems
3786 always sleep the full amount. They may appear to sleep longer than that,
3787 however, because your process might not be scheduled right away in a
3788 busy multitasking system.
3790 For delays of finer granularity than one second, you may use Perl's
3791 C<syscall> interface to access setitimer(2) if your system supports
3792 it, or else see L</select> above. The Time::HiRes module from CPAN
3795 See also the POSIX module's C<sigpause> function.
3797 =item socket SOCKET,DOMAIN,TYPE,PROTOCOL
3799 Opens a socket of the specified kind and attaches it to filehandle
3800 SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for
3801 the system call of the same name. You should C<use Socket> first
3802 to get the proper definitions imported. See the examples in
3803 L<perlipc/"Sockets: Client/Server Communication">.
3805 =item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
3807 Creates an unnamed pair of sockets in the specified domain, of the
3808 specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as
3809 for the system call of the same name. If unimplemented, yields a fatal
3810 error. Returns true if successful.
3812 Some systems defined C<pipe> in terms of C<socketpair>, in which a call
3813 to C<pipe(Rdr, Wtr)> is essentially:
3816 socketpair(Rdr, Wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC);
3817 shutdown(Rdr, 1); # no more writing for reader
3818 shutdown(Wtr, 0); # no more reading for writer
3820 See L<perlipc> for an example of socketpair use.
3822 =item sort SUBNAME LIST
3824 =item sort BLOCK LIST
3828 Sorts the LIST and returns the sorted list value. If SUBNAME or BLOCK
3829 is omitted, C<sort>s in standard string comparison order. If SUBNAME is
3830 specified, it gives the name of a subroutine that returns an integer
3831 less than, equal to, or greater than C<0>, depending on how the elements
3832 of the list are to be ordered. (The C<E<lt>=E<gt>> and C<cmp>
3833 operators are extremely useful in such routines.) SUBNAME may be a
3834 scalar variable name (unsubscripted), in which case the value provides
3835 the name of (or a reference to) the actual subroutine to use. In place
3836 of a SUBNAME, you can provide a BLOCK as an anonymous, in-line sort
3839 In the interests of efficiency the normal calling code for subroutines is
3840 bypassed, with the following effects: the subroutine may not be a
3841 recursive subroutine, and the two elements to be compared are passed into
3842 the subroutine not via C<@_> but as the package global variables $a and
3843 $b (see example below). They are passed by reference, so don't
3844 modify $a and $b. And don't try to declare them as lexicals either.
3846 You also cannot exit out of the sort block or subroutine using any of the
3847 loop control operators described in L<perlsyn> or with C<goto>.
3849 When C<use locale> is in effect, C<sort LIST> sorts LIST according to the
3850 current collation locale. See L<perllocale>.
3855 @articles = sort @files;
3857 # same thing, but with explicit sort routine
3858 @articles = sort {$a cmp $b} @files;
3860 # now case-insensitively
3861 @articles = sort {uc($a) cmp uc($b)} @files;
3863 # same thing in reversed order
3864 @articles = sort {$b cmp $a} @files;
3866 # sort numerically ascending
3867 @articles = sort {$a <=> $b} @files;
3869 # sort numerically descending
3870 @articles = sort {$b <=> $a} @files;
3872 # this sorts the %age hash by value instead of key
3873 # using an in-line function
3874 @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
3876 # sort using explicit subroutine name
3878 $age{$a} <=> $age{$b}; # presuming numeric
3880 @sortedclass = sort byage @class;
3882 sub backwards { $b cmp $a }
3883 @harry = qw(dog cat x Cain Abel);
3884 @george = qw(gone chased yz Punished Axed);
3886 # prints AbelCaincatdogx
3887 print sort backwards @harry;
3888 # prints xdogcatCainAbel
3889 print sort @george, 'to', @harry;
3890 # prints AbelAxedCainPunishedcatchaseddoggonetoxyz
3892 # inefficiently sort by descending numeric compare using
3893 # the first integer after the first = sign, or the
3894 # whole record case-insensitively otherwise
3897 ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
3902 # same thing, but much more efficiently;
3903 # we'll build auxiliary indices instead
3907 push @nums, /=(\d+)/;
3912 $nums[$b] <=> $nums[$a]
3914 $caps[$a] cmp $caps[$b]
3918 # same thing, but without any temps
3919 @new = map { $_->[0] }
3920 sort { $b->[1] <=> $a->[1]
3923 } map { [$_, /=(\d+)/, uc($_)] } @old;
3925 If you're using strict, you I<must not> declare $a
3926 and $b as lexicals. They are package globals. That means
3927 if you're in the C<main> package, it's
3929 @articles = sort {$main::b <=> $main::a} @files;
3933 @articles = sort {$::b <=> $::a} @files;
3935 but if you're in the C<FooPack> package, it's
3937 @articles = sort {$FooPack::b <=> $FooPack::a} @files;
3939 The comparison function is required to behave. If it returns
3940 inconsistent results (sometimes saying C<$x[1]> is less than C<$x[2]> and
3941 sometimes saying the opposite, for example) the results are not
3944 =item splice ARRAY,OFFSET,LENGTH,LIST
3946 =item splice ARRAY,OFFSET,LENGTH
3948 =item splice ARRAY,OFFSET
3950 Removes the elements designated by OFFSET and LENGTH from an array, and
3951 replaces them with the elements of LIST, if any. In list context,
3952 returns the elements removed from the array. In scalar context,
3953 returns the last element removed, or C<undef> if no elements are
3954 removed. The array grows or shrinks as necessary.
3955 If OFFSET is negative then it starts that far from the end of the array.
3956 If LENGTH is omitted, removes everything from OFFSET onward.
3957 If LENGTH is negative, leave that many elements off the end of the array.
3958 The following equivalences hold (assuming C<$[ == 0>):
3960 push(@a,$x,$y) splice(@a,@a,0,$x,$y)
3961 pop(@a) splice(@a,-1)
3962 shift(@a) splice(@a,0,1)
3963 unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
3964 $a[$x] = $y splice(@a,$x,1,$y)
3966 Example, assuming array lengths are passed before arrays:
3968 sub aeq { # compare two list values
3969 my(@a) = splice(@_,0,shift);
3970 my(@b) = splice(@_,0,shift);
3971 return 0 unless @a == @b; # same len?
3973 return 0 if pop(@a) ne pop(@b);
3977 if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
3979 =item split /PATTERN/,EXPR,LIMIT
3981 =item split /PATTERN/,EXPR
3983 =item split /PATTERN/
3987 Splits a string into a list of strings and returns that list. By default,
3988 empty leading fields are preserved, and empty trailing ones are deleted.
3990 If not in list context, returns the number of fields found and splits into
3991 the C<@_> array. (In list context, you can force the split into C<@_> by
3992 using C<??> as the pattern delimiters, but it still returns the list
3993 value.) The use of implicit split to C<@_> is deprecated, however, because
3994 it clobbers your subroutine arguments.
3996 If EXPR is omitted, splits the C<$_> string. If PATTERN is also omitted,
3997 splits on whitespace (after skipping any leading whitespace). Anything
3998 matching PATTERN is taken to be a delimiter separating the fields. (Note
3999 that the delimiter may be longer than one character.)
4001 If LIMIT is specified and positive, splits into no more than that
4002 many fields (though it may split into fewer). If LIMIT is unspecified
4003 or zero, trailing null fields are stripped (which potential users
4004 of C<pop> would do well to remember). If LIMIT is negative, it is
4005 treated as if an arbitrarily large LIMIT had been specified.
4007 A pattern matching the null string (not to be confused with
4008 a null pattern C<//>, which is just one member of the set of patterns
4009 matching a null string) will split the value of EXPR into separate
4010 characters at each point it matches that way. For example:
4012 print join(':', split(/ */, 'hi there'));
4014 produces the output 'h:i:t:h:e:r:e'.
4016 The LIMIT parameter can be used to split a line partially
4018 ($login, $passwd, $remainder) = split(/:/, $_, 3);
4020 When assigning to a list, if LIMIT is omitted, Perl supplies a LIMIT
4021 one larger than the number of variables in the list, to avoid
4022 unnecessary work. For the list above LIMIT would have been 4 by
4023 default. In time critical applications it behooves you not to split
4024 into more fields than you really need.
4026 If the PATTERN contains parentheses, additional list elements are
4027 created from each matching substring in the delimiter.
4029 split(/([,-])/, "1-10,20", 3);
4031 produces the list value
4033 (1, '-', 10, ',', 20)
4035 If you had the entire header of a normal Unix email message in $header,
4036 you could split it up into fields and their values this way:
4038 $header =~ s/\n\s+/ /g; # fix continuation lines
4039 %hdrs = (UNIX_FROM => split /^(\S*?):\s*/m, $header);
4041 The pattern C</PATTERN/> may be replaced with an expression to specify
4042 patterns that vary at runtime. (To do runtime compilation only once,
4043 use C</$variable/o>.)
4045 As a special case, specifying a PATTERN of space (C<' '>) will split on
4046 white space just as C<split> with no arguments does. Thus, C<split(' ')> can
4047 be used to emulate B<awk>'s default behavior, whereas C<split(/ /)>
4048 will give you as many null initial fields as there are leading spaces.
4049 A C<split> on C</\s+/> is like a C<split(' ')> except that any leading
4050 whitespace produces a null first field. A C<split> with no arguments
4051 really does a C<split(' ', $_)> internally.
4055 open(PASSWD, '/etc/passwd');
4057 ($login, $passwd, $uid, $gid,
4058 $gcos, $home, $shell) = split(/:/);
4062 (Note that $shell above will still have a newline on it. See L</chop>,
4063 L</chomp>, and L</join>.)
4065 =item sprintf FORMAT, LIST
4067 Returns a string formatted by the usual C<printf> conventions of the
4068 C library function C<sprintf>. See L<sprintf(3)> or L<printf(3)>
4069 on your system for an explanation of the general principles.
4071 Perl does its own C<sprintf> formatting--it emulates the C
4072 function C<sprintf>, but it doesn't use it (except for floating-point
4073 numbers, and even then only the standard modifiers are allowed). As a
4074 result, any non-standard extensions in your local C<sprintf> are not
4075 available from Perl.
4077 Perl's C<sprintf> permits the following universally-known conversions:
4080 %c a character with the given number
4082 %d a signed integer, in decimal
4083 %u an unsigned integer, in decimal
4084 %o an unsigned integer, in octal
4085 %x an unsigned integer, in hexadecimal
4086 %e a floating-point number, in scientific notation
4087 %f a floating-point number, in fixed decimal notation
4088 %g a floating-point number, in %e or %f notation
4090 In addition, Perl permits the following widely-supported conversions:
4092 %X like %x, but using upper-case letters
4093 %E like %e, but using an upper-case "E"
4094 %G like %g, but with an upper-case "E" (if applicable)
4095 %b an unsigned integer, in binary
4096 %p a pointer (outputs the Perl value's address in hexadecimal)
4097 %n special: *stores* the number of characters output so far
4098 into the next variable in the parameter list
4100 Finally, for backward (and we do mean "backward") compatibility, Perl
4101 permits these unnecessary but widely-supported conversions:
4104 %D a synonym for %ld
4105 %U a synonym for %lu
4106 %O a synonym for %lo
4109 Perl permits the following universally-known flags between the C<%>
4110 and the conversion letter:
4112 space prefix positive number with a space
4113 + prefix positive number with a plus sign
4114 - left-justify within the field
4115 0 use zeros, not spaces, to right-justify
4116 # prefix non-zero octal with "0", non-zero hex with "0x"
4117 number minimum field width
4118 .number "precision": digits after decimal point for
4119 floating-point, max length for string, minimum length
4121 l interpret integer as C type "long" or "unsigned long"
4122 h interpret integer as C type "short" or "unsigned short"
4123 If no flags, interpret integer as C type "int" or "unsigned"
4125 There is also one Perl-specific flag:
4127 V interpret integer as Perl's standard integer type
4129 Where a number would appear in the flags, an asterisk (C<*>) may be
4130 used instead, in which case Perl uses the next item in the parameter
4131 list as the given number (that is, as the field width or precision).
4132 If a field width obtained through C<*> is negative, it has the same
4133 effect as the C<-> flag: left-justification.
4135 If C<use locale> is in effect, the character used for the decimal
4136 point in formatted real numbers is affected by the LC_NUMERIC locale.
4139 To cope with broken systems that allow the standard locales to be
4140 overridden by malicious users, the return value may be tainted
4141 if any of the floating point formats are used and the conversion
4142 yields something that doesn't look like a normal C-locale floating
4143 point number. This happens regardless of whether C<use locale> is
4146 If Perl understands "quads" (64-bit integers) (this requires
4147 either that the platform natively supports quads or that Perl
4148 has been specifically compiled to support quads), the characters
4152 print quads, and they may optionally be preceded by
4160 You can find out whether your Perl supports quads via L<Config>:
4163 ($Config{use64bits} eq 'define' || $Config{longsize} == 8) &&
4166 If Perl understands "long doubles" (this requires that the platform
4167 supports long doubles), the flags
4171 may optionally be preceded by
4179 You can find out whether your Perl supports long doubles via L<Config>:
4182 $Config{d_longdbl} eq 'define' && print "long doubles\n";
4188 Return the square root of EXPR. If EXPR is omitted, returns square
4189 root of C<$_>. Only works on non-negative operands, unless you've
4190 loaded the standard Math::Complex module.
4193 print sqrt(-2); # prints 1.4142135623731i
4199 Sets the random number seed for the C<rand> operator. If EXPR is
4200 omitted, uses a semi-random value supplied by the kernel (if it supports
4201 the F</dev/urandom> device) or based on the current time and process
4202 ID, among other things. In versions of Perl prior to 5.004 the default
4203 seed was just the current C<time>. This isn't a particularly good seed,
4204 so many old programs supply their own seed value (often C<time ^ $$> or
4205 C<time ^ ($$ + ($$ E<lt>E<lt> 15))>), but that isn't necessary any more.
4207 In fact, it's usually not necessary to call C<srand> at all, because if
4208 it is not called explicitly, it is called implicitly at the first use of
4209 the C<rand> operator. However, this was not the case in version of Perl
4210 before 5.004, so if your script will run under older Perl versions, it
4211 should call C<srand>.
4213 Note that you need something much more random than the default seed for
4214 cryptographic purposes. Checksumming the compressed output of one or more
4215 rapidly changing operating system status programs is the usual method. For
4218 srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip`);
4220 If you're particularly concerned with this, see the C<Math::TrulyRandom>
4223 Do I<not> call C<srand> multiple times in your program unless you know
4224 exactly what you're doing and why you're doing it. The point of the
4225 function is to "seed" the C<rand> function so that C<rand> can produce
4226 a different sequence each time you run your program. Just do it once at the
4227 top of your program, or you I<won't> get random numbers out of C<rand>!
4229 Frequently called programs (like CGI scripts) that simply use
4233 for a seed can fall prey to the mathematical property that
4237 one-third of the time. So don't do that.
4239 =item stat FILEHANDLE
4245 Returns a 13-element list giving the status info for a file, either
4246 the file opened via FILEHANDLE, or named by EXPR. If EXPR is omitted,
4247 it stats C<$_>. Returns a null list if the stat fails. Typically used
4250 ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
4251 $atime,$mtime,$ctime,$blksize,$blocks)
4254 Not all fields are supported on all filesystem types. Here are the
4255 meaning of the fields:
4257 0 dev device number of filesystem
4259 2 mode file mode (type and permissions)
4260 3 nlink number of (hard) links to the file
4261 4 uid numeric user ID of file's owner
4262 5 gid numeric group ID of file's owner
4263 6 rdev the device identifier (special files only)
4264 7 size total size of file, in bytes
4265 8 atime last access time since the epoch
4266 9 mtime last modify time since the epoch
4267 10 ctime inode change time (NOT creation time!) since the epoch
4268 11 blksize preferred block size for file system I/O
4269 12 blocks actual number of blocks allocated
4271 (The epoch was at 00:00 January 1, 1970 GMT.)
4273 If stat is passed the special filehandle consisting of an underline, no
4274 stat is done, but the current contents of the stat structure from the
4275 last stat or filetest are returned. Example:
4277 if (-x $file && (($d) = stat(_)) && $d < 0) {
4278 print "$file is executable NFS file\n";
4281 (This works on machines only for which the device number is negative under NFS.)
4283 Because the mode contains both the file type and its permissions, you
4284 should mask off the file type portion and (s)printf using a C<"%o">
4285 if you want to see the real permissions.
4287 $mode = (stat($filename))[2];
4288 printf "Permissions are %04o\n", $mode & 07777;
4290 In scalar context, C<stat> returns a boolean value indicating success
4291 or failure, and, if successful, sets the information associated with
4292 the special filehandle C<_>.
4294 The File::stat module provides a convenient, by-name access mechanism:
4297 $sb = stat($filename);
4298 printf "File is %s, size is %s, perm %04o, mtime %s\n",
4299 $filename, $sb->size, $sb->mode & 07777,
4300 scalar localtime $sb->mtime;
4306 Takes extra time to study SCALAR (C<$_> if unspecified) in anticipation of
4307 doing many pattern matches on the string before it is next modified.
4308 This may or may not save time, depending on the nature and number of
4309 patterns you are searching on, and on the distribution of character
4310 frequencies in the string to be searched--you probably want to compare
4311 run times with and without it to see which runs faster. Those loops
4312 which scan for many short constant strings (including the constant
4313 parts of more complex patterns) will benefit most. You may have only
4314 one C<study> active at a time--if you study a different scalar the first
4315 is "unstudied". (The way C<study> works is this: a linked list of every
4316 character in the string to be searched is made, so we know, for
4317 example, where all the C<'k'> characters are. From each search string,
4318 the rarest character is selected, based on some static frequency tables
4319 constructed from some C programs and English text. Only those places
4320 that contain this "rarest" character are examined.)
4322 For example, here is a loop that inserts index producing entries
4323 before any line containing a certain pattern:
4327 print ".IX foo\n" if /\bfoo\b/;
4328 print ".IX bar\n" if /\bbar\b/;
4329 print ".IX blurfl\n" if /\bblurfl\b/;
4334 In searching for C</\bfoo\b/>, only those locations in C<$_> that contain C<"f">
4335 will be looked at, because C<"f"> is rarer than C<"o">. In general, this is
4336 a big win except in pathological cases. The only question is whether
4337 it saves you more time than it took to build the linked list in the
4340 Note that if you have to look for strings that you don't know till
4341 runtime, you can build an entire loop as a string and C<eval> that to
4342 avoid recompiling all your patterns all the time. Together with
4343 undefining C<$/> to input entire files as one record, this can be very
4344 fast, often faster than specialized programs like fgrep(1). The following
4345 scans a list of files (C<@files>) for a list of words (C<@words>), and prints
4346 out the names of those files that contain a match:
4348 $search = 'while (<>) { study;';
4349 foreach $word (@words) {
4350 $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
4355 eval $search; # this screams
4356 $/ = "\n"; # put back to normal input delimiter
4357 foreach $file (sort keys(%seen)) {
4365 =item sub NAME BLOCK
4367 This is subroutine definition, not a real function I<per se>. With just a
4368 NAME (and possibly prototypes or attributes), it's just a forward declaration.
4369 Without a NAME, it's an anonymous function declaration, and does actually
4370 return a value: the CODE ref of the closure you just created. See L<perlsub>
4371 and L<perlref> for details.
4373 =item substr EXPR,OFFSET,LENGTH,REPLACEMENT
4375 =item substr EXPR,OFFSET,LENGTH
4377 =item substr EXPR,OFFSET
4379 Extracts a substring out of EXPR and returns it. First character is at
4380 offset C<0>, or whatever you've set C<$[> to (but don't do that).
4381 If OFFSET is negative (or more precisely, less than C<$[>), starts
4382 that far from the end of the string. If LENGTH is omitted, returns
4383 everything to the end of the string. If LENGTH is negative, leaves that
4384 many characters off the end of the string.
4386 You can use the substr() function as an lvalue, in which case EXPR
4387 must itself be an lvalue. If you assign something shorter than LENGTH,
4388 the string will shrink, and if you assign something longer than LENGTH,
4389 the string will grow to accommodate it. To keep the string the same
4390 length you may need to pad or chop your value using C<sprintf>.
4392 If OFFSET and LENGTH specify a substring that is partly outside the
4393 string, only the part within the string is returned. If the substring
4394 is beyond either end of the string, substr() returns the undefined
4395 value and produces a warning. When used as an lvalue, specifying a
4396 substring that is entirely outside the string is a fatal error.
4397 Here's an example showing the behavior for boundary cases:
4400 substr($name, 4) = 'dy'; # $name is now 'freddy'
4401 my $null = substr $name, 6, 2; # returns '' (no warning)
4402 my $oops = substr $name, 7; # returns undef, with warning
4403 substr($name, 7) = 'gap'; # fatal error
4405 An alternative to using substr() as an lvalue is to specify the
4406 replacement string as the 4th argument. This allows you to replace
4407 parts of the EXPR and return what was there before in one operation,
4408 just as you can with splice().
4410 =item symlink OLDFILE,NEWFILE
4412 Creates a new filename symbolically linked to the old filename.
4413 Returns C<1> for success, C<0> otherwise. On systems that don't support
4414 symbolic links, produces a fatal error at run time. To check for that,
4417 $symlink_exists = eval { symlink("",""); 1 };
4421 Calls the system call specified as the first element of the list,
4422 passing the remaining elements as arguments to the system call. If
4423 unimplemented, produces a fatal error. The arguments are interpreted
4424 as follows: if a given argument is numeric, the argument is passed as
4425 an int. If not, the pointer to the string value is passed. You are
4426 responsible to make sure a string is pre-extended long enough to
4427 receive any result that might be written into a string. You can't use a
4428 string literal (or other read-only string) as an argument to C<syscall>
4429 because Perl has to assume that any string pointer might be written
4431 integer arguments are not literals and have never been interpreted in a
4432 numeric context, you may need to add C<0> to them to force them to look
4433 like numbers. This emulates the C<syswrite> function (or vice versa):
4435 require 'syscall.ph'; # may need to run h2ph
4437 syscall(&SYS_write, fileno(STDOUT), $s, length $s);
4439 Note that Perl supports passing of up to only 14 arguments to your system call,
4440 which in practice should usually suffice.
4442 Syscall returns whatever value returned by the system call it calls.
4443 If the system call fails, C<syscall> returns C<-1> and sets C<$!> (errno).
4444 Note that some system calls can legitimately return C<-1>. The proper
4445 way to handle such calls is to assign C<$!=0;> before the call and
4446 check the value of C<$!> if syscall returns C<-1>.
4448 There's a problem with C<syscall(&SYS_pipe)>: it returns the file
4449 number of the read end of the pipe it creates. There is no way
4450 to retrieve the file number of the other end. You can avoid this
4451 problem by using C<pipe> instead.
4453 =item sysopen FILEHANDLE,FILENAME,MODE
4455 =item sysopen FILEHANDLE,FILENAME,MODE,PERMS
4457 Opens the file whose filename is given by FILENAME, and associates it
4458 with FILEHANDLE. If FILEHANDLE is an expression, its value is used as
4459 the name of the real filehandle wanted. This function calls the
4460 underlying operating system's C<open> function with the parameters
4461 FILENAME, MODE, PERMS.
4463 The possible values and flag bits of the MODE parameter are
4464 system-dependent; they are available via the standard module C<Fcntl>.
4465 See the documentation of your operating system's C<open> to see which
4466 values and flag bits are available. You may combine several flags
4467 using the C<|>-operator.
4469 Some of the most common values are C<O_RDONLY> for opening the file in
4470 read-only mode, C<O_WRONLY> for opening the file in write-only mode,
4471 and C<O_RDWR> for opening the file in read-write mode, and.
4473 For historical reasons, some values work on almost every system
4474 supported by perl: zero means read-only, one means write-only, and two
4475 means read/write. We know that these values do I<not> work under
4476 OS/390 & VM/ESA Unix and on the Macintosh; you probably don't want to
4477 se them in new code, use thhe constants discussed in the preceding
4480 If the file named by FILENAME does not exist and the C<open> call creates
4481 it (typically because MODE includes the C<O_CREAT> flag), then the value of
4482 PERMS specifies the permissions of the newly created file. If you omit
4483 the PERMS argument to C<sysopen>, Perl uses the octal value C<0666>.
4484 These permission values need to be in octal, and are modified by your
4485 process's current C<umask>.
4487 In many systems the C<O_EXCL> flag is available for opening files in
4488 exclusive mode. This is B<not> locking: exclusiveness means here that
4489 if the file already exists, sysopen() fails. The C<O_EXCL> wins
4492 Sometimes you may want to truncate an already-existing file: C<O_TRUNC>.
4494 You should seldom if ever use C<0644> as argument to C<sysopen>, because
4495 that takes away the user's option to have a more permissive umask.
4496 Better to omit it. See the perlfunc(1) entry on C<umask> for more
4499 See L<perlopentut> for a kinder, gentler explanation of opening files.
4501 =item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
4503 =item sysread FILEHANDLE,SCALAR,LENGTH
4505 Attempts to read LENGTH bytes of data into variable SCALAR from the
4506 specified FILEHANDLE, using the system call read(2). It bypasses stdio,
4507 so mixing this with other kinds of reads, C<print>, C<write>,
4508 C<seek>, C<tell>, or C<eof> can cause confusion because stdio
4509 usually buffers data. Returns the number of bytes actually read, C<0>
4510 at end of file, or undef if there was an error. SCALAR will be grown or
4511 shrunk so that the last byte actually read is the last byte of the
4512 scalar after the read.
4514 An OFFSET may be specified to place the read data at some place in the
4515 string other than the beginning. A negative OFFSET specifies
4516 placement at that many bytes counting backwards from the end of the
4517 string. A positive OFFSET greater than the length of SCALAR results
4518 in the string being padded to the required size with C<"\0"> bytes before
4519 the result of the read is appended.
4521 There is no syseof() function, which is ok, since eof() doesn't work
4522 very well on device files (like ttys) anyway. Use sysread() and check
4523 for a return value for 0 to decide whether you're done.
4525 =item sysseek FILEHANDLE,POSITION,WHENCE
4527 Sets FILEHANDLE's system position using the system call lseek(2). It
4528 bypasses stdio, so mixing this with reads (other than C<sysread>),
4529 C<print>, C<write>, C<seek>, C<tell>, or C<eof> may cause confusion.
4530 FILEHANDLE may be an expression whose value gives the name of the
4531 filehandle. The values for WHENCE are C<0> to set the new position to
4532 POSITION, C<1> to set the it to the current position plus POSITION,
4533 and C<2> to set it to EOF plus POSITION (typically negative). For
4534 WHENCE, you may also use the constants C<SEEK_SET>, C<SEEK_CUR>, and
4535 C<SEEK_END> (start of the file, current position, end of the file)
4536 from any of the modules Fcntl, C<IO::Seekable>, or POSIX.
4538 Returns the new position, or the undefined value on failure. A position
4539 of zero is returned as the string C<"0 but true">; thus C<sysseek> returns
4540 true on success and false on failure, yet you can still easily determine
4545 =item system PROGRAM LIST
4547 Does exactly the same thing as C<exec LIST>, except that a fork is
4548 done first, and the parent process waits for the child process to
4549 complete. Note that argument processing varies depending on the
4550 number of arguments. If there is more than one argument in LIST,
4551 or if LIST is an array with more than one value, starts the program
4552 given by the first element of the list with arguments given by the
4553 rest of the list. If there is only one scalar argument, the argument
4554 is checked for shell metacharacters, and if there are any, the
4555 entire argument is passed to the system's command shell for parsing
4556 (this is C</bin/sh -c> on Unix platforms, but varies on other
4557 platforms). If there are no shell metacharacters in the argument,
4558 it is split into words and passed directly to C<execvp>, which is
4561 All files opened for output are flushed before attempting the exec().
4563 The return value is the exit status of the program as
4564 returned by the C<wait> call. To get the actual exit value divide by
4565 256. See also L</exec>. This is I<not> what you want to use to capture
4566 the output from a command, for that you should use merely backticks or
4567 C<qx//>, as described in L<perlop/"`STRING`">. Return value of -1
4568 indicates a failure to start the program (inspect $! for the reason).
4570 Like C<exec>, C<system> allows you to lie to a program about its name if
4571 you use the C<system PROGRAM LIST> syntax. Again, see L</exec>.
4573 Because C<system> and backticks block C<SIGINT> and C<SIGQUIT>, killing the
4574 program they're running doesn't actually interrupt your program.
4576 @args = ("command", "arg1", "arg2");
4578 or die "system @args failed: $?"
4580 You can check all the failure possibilities by inspecting
4583 $exit_value = $? >> 8;
4584 $signal_num = $? & 127;
4585 $dumped_core = $? & 128;
4587 When the arguments get executed via the system shell, results
4588 and return codes will be subject to its quirks and capabilities.
4589 See L<perlop/"`STRING`"> and L</exec> for details.
4591 =item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
4593 =item syswrite FILEHANDLE,SCALAR,LENGTH
4595 =item syswrite FILEHANDLE,SCALAR
4597 Attempts to write LENGTH bytes of data from variable SCALAR to the
4598 specified FILEHANDLE, using the system call write(2). If LENGTH
4599 is not specified, writes whole SCALAR. It bypasses stdio, so mixing
4600 this with reads (other than C<sysread())>, C<print>, C<write>,
4601 C<seek>, C<tell>, or C<eof> may cause confusion because stdio
4602 usually buffers data. Returns the number of bytes actually written,
4603 or C<undef> if there was an error. If the LENGTH is greater than
4604 the available data in the SCALAR after the OFFSET, only as much
4605 data as is available will be written.
4607 An OFFSET may be specified to write the data from some part of the
4608 string other than the beginning. A negative OFFSET specifies writing
4609 that many bytes counting backwards from the end of the string. In the
4610 case the SCALAR is empty you can use OFFSET but only zero offset.
4612 =item tell FILEHANDLE
4616 Returns the current position for FILEHANDLE. FILEHANDLE may be an
4617 expression whose value gives the name of the actual filehandle. If
4618 FILEHANDLE is omitted, assumes the file last read.
4620 There is no C<systell> function. Use C<sysseek(FH, 0, 1)> for that.
4622 =item telldir DIRHANDLE
4624 Returns the current position of the C<readdir> routines on DIRHANDLE.
4625 Value may be given to C<seekdir> to access a particular location in a
4626 directory. Has the same caveats about possible directory compaction as
4627 the corresponding system library routine.
4629 =item tie VARIABLE,CLASSNAME,LIST
4631 This function binds a variable to a package class that will provide the
4632 implementation for the variable. VARIABLE is the name of the variable
4633 to be enchanted. CLASSNAME is the name of a class implementing objects
4634 of correct type. Any additional arguments are passed to the C<new>
4635 method of the class (meaning C<TIESCALAR>, C<TIEHANDLE>, C<TIEARRAY>,
4636 or C<TIEHASH>). Typically these are arguments such as might be passed
4637 to the C<dbm_open()> function of C. The object returned by the C<new>
4638 method is also returned by the C<tie> function, which would be useful
4639 if you want to access other methods in CLASSNAME.
4641 Note that functions such as C<keys> and C<values> may return huge lists
4642 when used on large objects, like DBM files. You may prefer to use the
4643 C<each> function to iterate over such. Example:
4645 # print out history file offsets
4647 tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
4648 while (($key,$val) = each %HIST) {
4649 print $key, ' = ', unpack('L',$val), "\n";
4653 A class implementing a hash should have the following methods:
4655 TIEHASH classname, LIST
4657 STORE this, key, value
4662 NEXTKEY this, lastkey
4665 A class implementing an ordinary array should have the following methods:
4667 TIEARRAY classname, LIST
4669 STORE this, key, value
4671 STORESIZE this, count
4677 SPLICE this, offset, length, LIST
4681 A class implementing a file handle should have the following methods:
4683 TIEHANDLE classname, LIST
4684 READ this, scalar, length, offset
4687 WRITE this, scalar, length, offset
4689 PRINTF this, format, LIST
4693 A class implementing a scalar should have the following methods:
4695 TIESCALAR classname, LIST
4700 Not all methods indicated above need be implemented. See L<perltie>,
4701 L<Tie::Hash>, L<Tie::Array>, L<Tie::Scalar>, and L<Tie::Handle>.
4703 Unlike C<dbmopen>, the C<tie> function will not use or require a module
4704 for you--you need to do that explicitly yourself. See L<DB_File>
4705 or the F<Config> module for interesting C<tie> implementations.
4707 For further details see L<perltie>, L<"tied VARIABLE">.
4711 Returns a reference to the object underlying VARIABLE (the same value
4712 that was originally returned by the C<tie> call that bound the variable
4713 to a package.) Returns the undefined value if VARIABLE isn't tied to a
4718 Returns the number of non-leap seconds since whatever time the system
4719 considers to be the epoch (that's 00:00:00, January 1, 1904 for MacOS,
4720 and 00:00:00 UTC, January 1, 1970 for most other systems).
4721 Suitable for feeding to C<gmtime> and C<localtime>.
4723 For measuring time in better granularity than one second,
4724 you may use either the Time::HiRes module from CPAN, or
4725 if you have gettimeofday(2), you may be able to use the
4726 C<syscall> interface of Perl, see L<perlfaq8> for details.
4730 Returns a four-element list giving the user and system times, in
4731 seconds, for this process and the children of this process.
4733 ($user,$system,$cuser,$csystem) = times;
4737 The transliteration operator. Same as C<y///>. See L<perlop>.
4739 =item truncate FILEHANDLE,LENGTH
4741 =item truncate EXPR,LENGTH
4743 Truncates the file opened on FILEHANDLE, or named by EXPR, to the
4744 specified length. Produces a fatal error if truncate isn't implemented
4745 on your system. Returns true if successful, the undefined value
4752 Returns an uppercased version of EXPR. This is the internal function
4753 implementing the C<\U> escape in double-quoted strings.
4754 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
4755 Under Unicode (C<use utf8>) it uses the standard Unicode uppercase mappings. (It
4756 does not attempt to do titlecase mapping on initial letters. See C<ucfirst> for that.)
4758 If EXPR is omitted, uses C<$_>.
4764 Returns the value of EXPR with the first character
4765 in uppercase (titlecase in Unicode). This is
4766 the internal function implementing the C<\u> escape in double-quoted strings.
4767 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
4770 If EXPR is omitted, uses C<$_>.
4776 Sets the umask for the process to EXPR and returns the previous value.
4777 If EXPR is omitted, merely returns the current umask.
4779 The Unix permission C<rwxr-x---> is represented as three sets of three
4780 bits, or three octal digits: C<0750> (the leading 0 indicates octal
4781 and isn't one of the digits). The C<umask> value is such a number
4782 representing disabled permissions bits. The permission (or "mode")
4783 values you pass C<mkdir> or C<sysopen> are modified by your umask, so
4784 even if you tell C<sysopen> to create a file with permissions C<0777>,
4785 if your umask is C<0022> then the file will actually be created with
4786 permissions C<0755>. If your C<umask> were C<0027> (group can't
4787 write; others can't read, write, or execute), then passing
4788 C<sysopen> C<0666> would create a file with mode C<0640> (C<0666 &~
4791 Here's some advice: supply a creation mode of C<0666> for regular
4792 files (in C<sysopen>) and one of C<0777> for directories (in
4793 C<mkdir>) and executable files. This gives users the freedom of
4794 choice: if they want protected files, they might choose process umasks
4795 of C<022>, C<027>, or even the particularly antisocial mask of C<077>.
4796 Programs should rarely if ever make policy decisions better left to
4797 the user. The exception to this is when writing files that should be
4798 kept private: mail files, web browser cookies, I<.rhosts> files, and
4801 If umask(2) is not implemented on your system and you are trying to
4802 restrict access for I<yourself> (i.e., (EXPR & 0700) > 0), produces a
4803 fatal error at run time. If umask(2) is not implemented and you are
4804 not trying to restrict access for yourself, returns C<undef>.
4806 Remember that a umask is a number, usually given in octal; it is I<not> a
4807 string of octal digits. See also L</oct>, if all you have is a string.
4813 Undefines the value of EXPR, which must be an lvalue. Use only on a
4814 scalar value, an array (using C<@>), a hash (using C<%>), a subroutine
4815 (using C<&>), or a typeglob (using <*>). (Saying C<undef $hash{$key}>
4816 will probably not do what you expect on most predefined variables or
4817 DBM list values, so don't do that; see L<delete>.) Always returns the
4818 undefined value. You can omit the EXPR, in which case nothing is
4819 undefined, but you still get an undefined value that you could, for
4820 instance, return from a subroutine, assign to a variable or pass as a
4821 parameter. Examples:
4824 undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'};
4828 undef *xyz; # destroys $xyz, @xyz, %xyz, &xyz, etc.
4829 return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it;
4830 select undef, undef, undef, 0.25;
4831 ($a, $b, undef, $c) = &foo; # Ignore third value returned
4833 Note that this is a unary operator, not a list operator.
4839 Deletes a list of files. Returns the number of files successfully
4842 $cnt = unlink 'a', 'b', 'c';
4846 Note: C<unlink> will not delete directories unless you are superuser and
4847 the B<-U> flag is supplied to Perl. Even if these conditions are
4848 met, be warned that unlinking a directory can inflict damage on your
4849 filesystem. Use C<rmdir> instead.
4851 If LIST is omitted, uses C<$_>.
4853 =item unpack TEMPLATE,EXPR
4855 C<unpack> does the reverse of C<pack>: it takes a string
4856 representing a structure and expands it out into a list of values.
4857 (In scalar context, it returns merely the first value produced.)
4858 The TEMPLATE has the same format as in the C<pack> function.
4859 Here's a subroutine that does substring:
4862 my($what,$where,$howmuch) = @_;
4863 unpack("x$where a$howmuch", $what);
4868 sub ordinal { unpack("c",$_[0]); } # same as ord()
4870 In addition, you may prefix a field with a %E<lt>numberE<gt> to indicate that
4871 you want a E<lt>numberE<gt>-bit checksum of the items instead of the items
4872 themselves. Default is a 16-bit checksum. For example, the following
4873 computes the same number as the System V sum program:
4877 unpack("%32C*",<>) % 65535;
4880 The following efficiently counts the number of set bits in a bit vector:
4882 $setbits = unpack("%32b*", $selectmask);
4884 The C<"p"> and C<"P"> formats should be used with care. Since Perl
4885 has no way of checking whether the value passed to C<unpack()>
4886 corresponds to a valid memory location, passing a pointer value that's
4887 not known to be valid is likely to have disastrous consequences.
4889 See L</pack> for more examples and notes.
4891 =item untie VARIABLE
4893 Breaks the binding between a variable and a package. (See C<tie>.)
4895 =item unshift ARRAY,LIST
4897 Does the opposite of a C<shift>. Or the opposite of a C<push>,
4898 depending on how you look at it. Prepends list to the front of the
4899 array, and returns the new number of elements in the array.
4901 unshift(ARGV, '-e') unless $ARGV[0] =~ /^-/;
4903 Note the LIST is prepended whole, not one element at a time, so the
4904 prepended elements stay in the same order. Use C<reverse> to do the
4907 =item use Module LIST
4911 =item use Module VERSION LIST
4915 Imports some semantics into the current package from the named module,
4916 generally by aliasing certain subroutine or variable names into your
4917 package. It is exactly equivalent to
4919 BEGIN { require Module; import Module LIST; }
4921 except that Module I<must> be a bareword.
4923 If the first argument to C<use> is a number, it is treated as a version
4924 number instead of a module name. If the version of the Perl interpreter
4925 is less than VERSION, then an error message is printed and Perl exits
4926 immediately. This is often useful if you need to check the current
4927 Perl version before C<use>ing library modules that have changed in
4928 incompatible ways from older versions of Perl. (We try not to do
4929 this more than we have to.)
4931 The C<BEGIN> forces the C<require> and C<import> to happen at compile time. The
4932 C<require> makes sure the module is loaded into memory if it hasn't been
4933 yet. The C<import> is not a builtin--it's just an ordinary static method
4934 call into the C<Module> package to tell the module to import the list of
4935 features back into the current package. The module can implement its
4936 C<import> method any way it likes, though most modules just choose to
4937 derive their C<import> method via inheritance from the C<Exporter> class that
4938 is defined in the C<Exporter> module. See L<Exporter>. If no C<import>
4939 method can be found then the error is currently silently ignored. This
4940 may change to a fatal error in a future version.
4942 If you don't want your namespace altered, explicitly supply an empty list:
4946 That is exactly equivalent to
4948 BEGIN { require Module }
4950 If the VERSION argument is present between Module and LIST, then the
4951 C<use> will call the VERSION method in class Module with the given
4952 version as an argument. The default VERSION method, inherited from
4953 the Universal class, croaks if the given version is larger than the
4954 value of the variable C<$Module::VERSION>. (Note that there is not a
4955 comma after VERSION!)
4957 Because this is a wide-open interface, pragmas (compiler directives)
4958 are also implemented this way. Currently implemented pragmas are:
4962 use sigtrap qw(SEGV BUS);
4963 use strict qw(subs vars refs);
4964 use subs qw(afunc blurfl);
4965 use warnings qw(all);
4967 Some of these pseudo-modules import semantics into the current
4968 block scope (like C<strict> or C<integer>, unlike ordinary modules,
4969 which import symbols into the current package (which are effective
4970 through the end of the file).
4972 There's a corresponding C<no> command that unimports meanings imported
4973 by C<use>, i.e., it calls C<unimport Module LIST> instead of C<import>.
4979 If no C<unimport> method can be found the call fails with a fatal error.
4981 See L<perlmod> for a list of standard modules and pragmas.
4985 Changes the access and modification times on each file of a list of
4986 files. The first two elements of the list must be the NUMERICAL access
4987 and modification times, in that order. Returns the number of files
4988 successfully changed. The inode change time of each file is set
4989 to the current time. This code has the same effect as the C<touch>
4990 command if the files already exist:
4994 utime $now, $now, @ARGV;
4998 Returns a list consisting of all the values of the named hash. (In a
4999 scalar context, returns the number of values.) The values are
5000 returned in an apparently random order. The actual random order is
5001 subject to change in future versions of perl, but it is guaranteed to
5002 be the same order as either the C<keys> or C<each> function would
5003 produce on the same (unmodified) hash.
5005 Note that you cannot modify the values of a hash this way, because the
5006 returned list is just a copy. You need to use a hash slice for that,
5007 since it's lvaluable in a way that values() is not.
5009 for (values %hash) { s/foo/bar/g } # FAILS!
5010 for (@hash{keys %hash}) { s/foo/bar/g } # ok
5012 As a side effect, calling values() resets the HASH's internal iterator.
5013 See also C<keys>, C<each>, and C<sort>.
5015 =item vec EXPR,OFFSET,BITS
5017 Treats the string in EXPR as a vector of unsigned integers, and
5018 returns the value of the bit field specified by OFFSET. BITS
5019 specifies the number of bits that are reserved for each entry in the
5020 bit vector. This must be a power of two from 1 to 32 (or 64, if your
5021 platform supports that).
5023 C<vec> may also be assigned to, in which case parentheses are needed
5024 to give the expression the correct precedence as in
5026 vec($image, $max_x * $x + $y, 8) = 3;
5028 Vectors created with C<vec> can also be manipulated with the logical
5029 operators C<|>, C<&>, and C<^>, which will assume a bit vector
5030 operation is desired when both operands are strings.
5031 See L<perlop/"Bitwise String Operators">.
5033 The following code will build up an ASCII string saying C<'PerlPerlPerl'>.
5034 The comments show the string after each step. Note that this code works
5035 in the same way on big-endian or little-endian machines.
5038 vec($foo, 0, 32) = 0x5065726C; # 'Perl'
5039 vec($foo, 2, 16) = 0x5065; # 'PerlPe'
5040 vec($foo, 3, 16) = 0x726C; # 'PerlPerl'
5041 vec($foo, 8, 8) = 0x50; # 'PerlPerlP'
5042 vec($foo, 9, 8) = 0x65; # 'PerlPerlPe'
5043 vec($foo, 20, 4) = 2; # 'PerlPerlPe' . "\x02"
5044 vec($foo, 21, 4) = 7; # 'PerlPerlPer'
5046 vec($foo, 45, 2) = 3; # 'PerlPerlPer' . "\x0c"
5047 vec($foo, 93, 1) = 1; # 'PerlPerlPer' . "\x2c"
5048 vec($foo, 94, 1) = 1; # 'PerlPerlPerl'
5051 To transform a bit vector into a string or list of 0's and 1's, use these:
5053 $bits = unpack("b*", $vector);
5054 @bits = split(//, unpack("b*", $vector));
5056 If you know the exact length in bits, it can be used in place of the C<*>.
5060 Behaves like the wait(2) system call on your system: it waits for a child
5061 process to terminate and returns the pid of the deceased process, or
5062 C<-1> if there are no child processes. The status is returned in C<$?>.
5063 Note that a return value of C<-1> could mean that child processes are
5064 being automatically reaped, as described in L<perlipc>.
5066 =item waitpid PID,FLAGS
5068 Waits for a particular child process to terminate and returns the pid of
5069 the deceased process, or C<-1> if there is no such child process. On some
5070 systems, a value of 0 indicates that there are processes still running.
5071 The status is returned in C<$?>. If you say
5073 use POSIX ":sys_wait_h";
5076 $kid = waitpid(-1,&WNOHANG);
5079 then you can do a non-blocking wait for all pending zombie processes.
5080 Non-blocking wait is available on machines supporting either the
5081 waitpid(2) or wait4(2) system calls. However, waiting for a particular
5082 pid with FLAGS of C<0> is implemented everywhere. (Perl emulates the
5083 system call by remembering the status values of processes that have
5084 exited but have not been harvested by the Perl script yet.)
5086 Note that on some systems, a return value of C<-1> could mean that child
5087 processes are being automatically reaped. See L<perlipc> for details,
5088 and for other examples.
5092 Returns true if the context of the currently executing subroutine is
5093 looking for a list value. Returns false if the context is looking
5094 for a scalar. Returns the undefined value if the context is looking
5095 for no value (void context).
5097 return unless defined wantarray; # don't bother doing more
5098 my @a = complex_calculation();
5099 return wantarray ? @a : "@a";
5101 This function should have been named wantlist() instead.
5105 Produces a message on STDERR just like C<die>, but doesn't exit or throw
5108 If LIST is empty and C<$@> already contains a value (typically from a
5109 previous eval) that value is used after appending C<"\t...caught">
5110 to C<$@>. This is useful for staying almost, but not entirely similar to
5113 If C<$@> is empty then the string C<"Warning: Something's wrong"> is used.
5115 No message is printed if there is a C<$SIG{__WARN__}> handler
5116 installed. It is the handler's responsibility to deal with the message
5117 as it sees fit (like, for instance, converting it into a C<die>). Most
5118 handlers must therefore make arrangements to actually display the
5119 warnings that they are not prepared to deal with, by calling C<warn>
5120 again in the handler. Note that this is quite safe and will not
5121 produce an endless loop, since C<__WARN__> hooks are not called from
5124 You will find this behavior is slightly different from that of
5125 C<$SIG{__DIE__}> handlers (which don't suppress the error text, but can
5126 instead call C<die> again to change it).
5128 Using a C<__WARN__> handler provides a powerful way to silence all
5129 warnings (even the so-called mandatory ones). An example:
5131 # wipe out *all* compile-time warnings
5132 BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } }
5134 my $foo = 20; # no warning about duplicate my $foo,
5135 # but hey, you asked for it!
5136 # no compile-time or run-time warnings before here
5139 # run-time warnings enabled after here
5140 warn "\$foo is alive and $foo!"; # does show up
5142 See L<perlvar> for details on setting C<%SIG> entries, and for more
5143 examples. See the Carp module for other kinds of warnings using its
5144 carp() and cluck() functions.
5146 =item write FILEHANDLE
5152 Writes a formatted record (possibly multi-line) to the specified FILEHANDLE,
5153 using the format associated with that file. By default the format for
5154 a file is the one having the same name as the filehandle, but the
5155 format for the current output channel (see the C<select> function) may be set
5156 explicitly by assigning the name of the format to the C<$~> variable.
5158 Top of form processing is handled automatically: if there is
5159 insufficient room on the current page for the formatted record, the
5160 page is advanced by writing a form feed, a special top-of-page format
5161 is used to format the new page header, and then the record is written.
5162 By default the top-of-page format is the name of the filehandle with
5163 "_TOP" appended, but it may be dynamically set to the format of your
5164 choice by assigning the name to the C<$^> variable while the filehandle is
5165 selected. The number of lines remaining on the current page is in
5166 variable C<$->, which can be set to C<0> to force a new page.
5168 If FILEHANDLE is unspecified, output goes to the current default output
5169 channel, which starts out as STDOUT but may be changed by the
5170 C<select> operator. If the FILEHANDLE is an EXPR, then the expression
5171 is evaluated and the resulting string is used to look up the name of
5172 the FILEHANDLE at run time. For more on formats, see L<perlform>.
5174 Note that write is I<not> the opposite of C<read>. Unfortunately.
5178 The transliteration operator. Same as C<tr///>. See L<perlop>.