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<qu/STRING/>,
100 C<reverse>, C<rindex>, C<sprintf>, C<substr>, C<tr///>, C<uc>, C<ucfirst>,
103 =item Regular expressions and pattern matching
105 C<m//>, C<pos>, C<quotemeta>, C<s///>, C<split>, C<study>, C<qr//>
107 =item Numeric functions
109 C<abs>, C<atan2>, C<cos>, C<exp>, C<hex>, C<int>, C<log>, C<oct>, C<rand>,
110 C<sin>, C<sqrt>, C<srand>
112 =item Functions for real @ARRAYs
114 C<pop>, C<push>, C<shift>, C<splice>, C<unshift>
116 =item Functions for list data
118 C<grep>, C<join>, C<map>, C<qw/STRING/>, C<reverse>, C<sort>, C<unpack>
120 =item Functions for real %HASHes
122 C<delete>, C<each>, C<exists>, C<keys>, C<values>
124 =item Input and output functions
126 C<binmode>, C<close>, C<closedir>, C<dbmclose>, C<dbmopen>, C<die>, C<eof>,
127 C<fileno>, C<flock>, C<format>, C<getc>, C<print>, C<printf>, C<read>,
128 C<readdir>, C<rewinddir>, C<seek>, C<seekdir>, C<select>, C<syscall>,
129 C<sysread>, C<sysseek>, C<syswrite>, C<tell>, C<telldir>, C<truncate>,
132 =item Functions for fixed length data or records
134 C<pack>, C<read>, C<syscall>, C<sysread>, C<syswrite>, C<unpack>, C<vec>
136 =item Functions for filehandles, files, or directories
138 C<-I<X>>, C<chdir>, C<chmod>, C<chown>, C<chroot>, C<fcntl>, C<glob>,
139 C<ioctl>, C<link>, C<lstat>, C<mkdir>, C<open>, C<opendir>,
140 C<readlink>, C<rename>, C<rmdir>, C<stat>, C<symlink>, C<umask>,
143 =item Keywords related to the control flow of your perl program
145 C<caller>, C<continue>, C<die>, C<do>, C<dump>, C<eval>, C<exit>,
146 C<goto>, C<last>, C<next>, C<redo>, C<return>, C<sub>, C<wantarray>
148 =item Keywords related to scoping
150 C<caller>, C<import>, C<local>, C<my>, C<our>, C<package>, C<use>
152 =item Miscellaneous functions
154 C<defined>, C<dump>, C<eval>, C<formline>, C<local>, C<my>, C<our>, C<reset>,
155 C<scalar>, C<undef>, C<wantarray>
157 =item Functions for processes and process groups
159 C<alarm>, C<exec>, C<fork>, C<getpgrp>, C<getppid>, C<getpriority>, C<kill>,
160 C<pipe>, C<qx/STRING/>, C<setpgrp>, C<setpriority>, C<sleep>, C<system>,
161 C<times>, C<wait>, C<waitpid>
163 =item Keywords related to perl modules
165 C<do>, C<import>, C<no>, C<package>, C<require>, C<use>
167 =item Keywords related to classes and object-orientedness
169 C<bless>, C<dbmclose>, C<dbmopen>, C<package>, C<ref>, C<tie>, C<tied>,
172 =item Low-level socket functions
174 C<accept>, C<bind>, C<connect>, C<getpeername>, C<getsockname>,
175 C<getsockopt>, C<listen>, C<recv>, C<send>, C<setsockopt>, C<shutdown>,
176 C<socket>, C<socketpair>
178 =item System V interprocess communication functions
180 C<msgctl>, C<msgget>, C<msgrcv>, C<msgsnd>, C<semctl>, C<semget>, C<semop>,
181 C<shmctl>, C<shmget>, C<shmread>, C<shmwrite>
183 =item Fetching user and group info
185 C<endgrent>, C<endhostent>, C<endnetent>, C<endpwent>, C<getgrent>,
186 C<getgrgid>, C<getgrnam>, C<getlogin>, C<getpwent>, C<getpwnam>,
187 C<getpwuid>, C<setgrent>, C<setpwent>
189 =item Fetching network info
191 C<endprotoent>, C<endservent>, C<gethostbyaddr>, C<gethostbyname>,
192 C<gethostent>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
193 C<getprotobyname>, C<getprotobynumber>, C<getprotoent>,
194 C<getservbyname>, C<getservbyport>, C<getservent>, C<sethostent>,
195 C<setnetent>, C<setprotoent>, C<setservent>
197 =item Time-related functions
199 C<gmtime>, C<localtime>, C<time>, C<times>
201 =item Functions new in perl5
203 C<abs>, C<bless>, C<chomp>, C<chr>, C<exists>, C<formline>, C<glob>,
204 C<import>, C<lc>, C<lcfirst>, C<map>, C<my>, C<no>, C<our>, C<prototype>,
205 C<qx>, C<qw>, C<readline>, C<readpipe>, C<ref>, C<sub*>, C<sysopen>, C<tie>,
206 C<tied>, C<uc>, C<ucfirst>, C<untie>, C<use>
208 * - C<sub> was a keyword in perl4, but in perl5 it is an
209 operator, which can be used in expressions.
211 =item Functions obsoleted in perl5
213 C<dbmclose>, C<dbmopen>
219 Perl was born in Unix and can therefore access all common Unix
220 system calls. In non-Unix environments, the functionality of some
221 Unix system calls may not be available, or details of the available
222 functionality may differ slightly. The Perl functions affected
225 C<-X>, C<binmode>, C<chmod>, C<chown>, C<chroot>, C<crypt>,
226 C<dbmclose>, C<dbmopen>, C<dump>, C<endgrent>, C<endhostent>,
227 C<endnetent>, C<endprotoent>, C<endpwent>, C<endservent>, C<exec>,
228 C<fcntl>, C<flock>, C<fork>, C<getgrent>, C<getgrgid>, C<gethostent>,
229 C<getlogin>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
230 C<getppid>, C<getprgp>, C<getpriority>, C<getprotobynumber>,
231 C<getprotoent>, C<getpwent>, C<getpwnam>, C<getpwuid>,
232 C<getservbyport>, C<getservent>, C<getsockopt>, C<glob>, C<ioctl>,
233 C<kill>, C<link>, C<lstat>, C<msgctl>, C<msgget>, C<msgrcv>,
234 C<msgsnd>, C<open>, C<pipe>, C<readlink>, C<rename>, C<select>, C<semctl>,
235 C<semget>, C<semop>, C<setgrent>, C<sethostent>, C<setnetent>,
236 C<setpgrp>, C<setpriority>, C<setprotoent>, C<setpwent>,
237 C<setservent>, C<setsockopt>, C<shmctl>, C<shmget>, C<shmread>,
238 C<shmwrite>, C<socket>, C<socketpair>, C<stat>, C<symlink>, C<syscall>,
239 C<sysopen>, C<system>, C<times>, C<truncate>, C<umask>, C<unlink>,
240 C<utime>, C<wait>, C<waitpid>
242 For more information about the portability of these functions, see
243 L<perlport> and other available platform-specific documentation.
245 =head2 Alphabetical Listing of Perl Functions
249 =item I<-X> FILEHANDLE
255 A file test, where X is one of the letters listed below. This unary
256 operator takes one argument, either a filename or a filehandle, and
257 tests the associated file to see if something is true about it. If the
258 argument is omitted, tests C<$_>, except for C<-t>, which tests STDIN.
259 Unless otherwise documented, it returns C<1> for true and C<''> for false, or
260 the undefined value if the file doesn't exist. Despite the funny
261 names, precedence is the same as any other named unary operator, and
262 the argument may be parenthesized like any other unary operator. The
263 operator may be any of:
264 X<-r>X<-w>X<-x>X<-o>X<-R>X<-W>X<-X>X<-O>X<-e>X<-z>X<-s>X<-f>X<-d>X<-l>X<-p>
265 X<-S>X<-b>X<-c>X<-t>X<-u>X<-g>X<-k>X<-T>X<-B>X<-M>X<-A>X<-C>
267 -r File is readable by effective uid/gid.
268 -w File is writable by effective uid/gid.
269 -x File is executable by effective uid/gid.
270 -o File is owned by effective uid.
272 -R File is readable by real uid/gid.
273 -W File is writable by real uid/gid.
274 -X File is executable by real uid/gid.
275 -O File is owned by real uid.
278 -z File has zero size (is empty).
279 -s File has nonzero size (returns size in bytes).
281 -f File is a plain file.
282 -d File is a directory.
283 -l File is a symbolic link.
284 -p File is a named pipe (FIFO), or Filehandle is a pipe.
286 -b File is a block special file.
287 -c File is a character special file.
288 -t Filehandle is opened to a tty.
290 -u File has setuid bit set.
291 -g File has setgid bit set.
292 -k File has sticky bit set.
294 -T File is an ASCII text file.
295 -B File is a "binary" file (opposite of -T).
297 -M Age of file in days when script started.
298 -A Same for access time.
299 -C Same for inode change time.
305 next unless -f $_; # ignore specials
309 The interpretation of the file permission operators C<-r>, C<-R>,
310 C<-w>, C<-W>, C<-x>, and C<-X> is by default based solely on the mode
311 of the file and the uids and gids of the user. There may be other
312 reasons you can't actually read, write, or execute the file. Such
313 reasons may be for example network filesystem access controls, ACLs
314 (access control lists), read-only filesystems, and unrecognized
317 Also note that, for the superuser on the local filesystems, the C<-r>,
318 C<-R>, C<-w>, and C<-W> tests always return 1, and C<-x> and C<-X> return 1
319 if any execute bit is set in the mode. Scripts run by the superuser
320 may thus need to do a stat() to determine the actual mode of the file,
321 or temporarily set their effective uid to something else.
323 If you are using ACLs, there is a pragma called C<filetest> that may
324 produce more accurate results than the bare stat() mode bits.
325 When under the C<use filetest 'access'> the above-mentioned filetests
326 will test whether the permission can (not) be granted using the
327 access() family of system calls. Also note that the C<-x> and C<-X> may
328 under this pragma return true even if there are no execute permission
329 bits set (nor any extra execute permission ACLs). This strangeness is
330 due to the underlying system calls' definitions. Read the
331 documentation for the C<filetest> pragma for more information.
333 Note that C<-s/a/b/> does not do a negated substitution. Saying
334 C<-exp($foo)> still works as expected, however--only single letters
335 following a minus are interpreted as file tests.
337 The C<-T> and C<-B> switches work as follows. The first block or so of the
338 file is examined for odd characters such as strange control codes or
339 characters with the high bit set. If too many strange characters (>30%)
340 are found, it's a C<-B> file, otherwise it's a C<-T> file. Also, any file
341 containing null in the first block is considered a binary file. If C<-T>
342 or C<-B> is used on a filehandle, the current stdio buffer is examined
343 rather than the first block. Both C<-T> and C<-B> return true on a null
344 file, or a file at EOF when testing a filehandle. Because you have to
345 read a file to do the C<-T> test, on most occasions you want to use a C<-f>
346 against the file first, as in C<next unless -f $file && -T $file>.
348 If any of the file tests (or either the C<stat> or C<lstat> operators) are given
349 the special filehandle consisting of a solitary underline, then the stat
350 structure of the previous file test (or stat operator) is used, saving
351 a system call. (This doesn't work with C<-t>, and you need to remember
352 that lstat() and C<-l> will leave values in the stat structure for the
353 symbolic link, not the real file.) Example:
355 print "Can do.\n" if -r $a || -w _ || -x _;
358 print "Readable\n" if -r _;
359 print "Writable\n" if -w _;
360 print "Executable\n" if -x _;
361 print "Setuid\n" if -u _;
362 print "Setgid\n" if -g _;
363 print "Sticky\n" if -k _;
364 print "Text\n" if -T _;
365 print "Binary\n" if -B _;
371 Returns the absolute value of its argument.
372 If VALUE is omitted, uses C<$_>.
374 =item accept NEWSOCKET,GENERICSOCKET
376 Accepts an incoming socket connect, just as the accept(2) system call
377 does. Returns the packed address if it succeeded, false otherwise.
378 See the example in L<perlipc/"Sockets: Client/Server Communication">.
380 On systems that support a close-on-exec flag on files, the flag will
381 be set for the newly opened file descriptor, as determined by the
382 value of $^F. See L<perlvar/$^F>.
388 Arranges to have a SIGALRM delivered to this process after the
389 specified number of seconds have elapsed. If SECONDS is not specified,
390 the value stored in C<$_> is used. (On some machines,
391 unfortunately, the elapsed time may be up to one second less than you
392 specified because of how seconds are counted.) Only one timer may be
393 counting at once. Each call disables the previous timer, and an
394 argument of C<0> may be supplied to cancel the previous timer without
395 starting a new one. The returned value is the amount of time remaining
396 on the previous timer.
398 For delays of finer granularity than one second, you may use Perl's
399 four-argument version of select() leaving the first three arguments
400 undefined, or you might be able to use the C<syscall> interface to
401 access setitimer(2) if your system supports it. The Time::HiRes module
402 from CPAN may also prove useful.
404 It is usually a mistake to intermix C<alarm> and C<sleep> calls.
405 (C<sleep> may be internally implemented in your system with C<alarm>)
407 If you want to use C<alarm> to time out a system call you need to use an
408 C<eval>/C<die> pair. You can't rely on the alarm causing the system call to
409 fail with C<$!> set to C<EINTR> because Perl sets up signal handlers to
410 restart system calls on some systems. Using C<eval>/C<die> always works,
411 modulo the caveats given in L<perlipc/"Signals">.
414 local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
416 $nread = sysread SOCKET, $buffer, $size;
420 die unless $@ eq "alarm\n"; # propagate unexpected errors
429 Returns the arctangent of Y/X in the range -PI to PI.
431 For the tangent operation, you may use the C<Math::Trig::tan>
432 function, or use the familiar relation:
434 sub tan { sin($_[0]) / cos($_[0]) }
436 =item bind SOCKET,NAME
438 Binds a network address to a socket, just as the bind system call
439 does. Returns true if it succeeded, false otherwise. NAME should be a
440 packed address of the appropriate type for the socket. See the examples in
441 L<perlipc/"Sockets: Client/Server Communication">.
443 =item binmode FILEHANDLE, DISCIPLINE
445 =item binmode FILEHANDLE
447 Arranges for FILEHANDLE to be read or written in "binary" or "text" mode
448 on systems where the run-time libraries distinguish between binary and
449 text files. If FILEHANDLE is an expression, the value is taken as the
450 name of the filehandle. DISCIPLINE can be either of C<":raw"> for
451 binary mode or C<":crlf"> for "text" mode. If the DISCIPLINE is
452 omitted, it defaults to C<":raw">.
454 binmode() should be called after open() but before any I/O is done on
457 On many systems binmode() currently has no effect, but in future, it
458 will be extended to support user-defined input and output disciplines.
459 On some systems binmode() is necessary when you're not working with a
460 text file. For the sake of portability it is a good idea to always use
461 it when appropriate, and to never use it when it isn't appropriate.
463 In other words: Regardless of platform, use binmode() on binary
464 files, and do not use binmode() on text files.
466 The C<open> pragma can be used to establish default disciplines.
469 The operating system, device drivers, C libraries, and Perl run-time
470 system all work together to let the programmer treat a single
471 character (C<\n>) as the line terminator, irrespective of the external
472 representation. On many operating systems, the native text file
473 representation matches the internal representation, but on some
474 platforms the external representation of C<\n> is made up of more than
477 Mac OS and all variants of Unix use a single character to end each line
478 in the external representation of text (even though that single
479 character is not necessarily the same across these platforms).
480 Consequently binmode() has no effect on these operating systems. In
481 other systems like VMS, MS-DOS and the various flavors of MS-Windows
482 your program sees a C<\n> as a simple C<\cJ>, but what's stored in text
483 files are the two characters C<\cM\cJ>. That means that, if you don't
484 use binmode() on these systems, C<\cM\cJ> sequences on disk will be
485 converted to C<\n> on input, and any C<\n> in your program will be
486 converted back to C<\cM\cJ> on output. This is what you want for text
487 files, but it can be disastrous for binary files.
489 Another consequence of using binmode() (on some systems) is that
490 special end-of-file markers will be seen as part of the data stream.
491 For systems from the Microsoft family this means that if your binary
492 data contains C<\cZ>, the I/O subsystem will regard it as the end of
493 the file, unless you use binmode().
495 binmode() is not only important for readline() and print() operations,
496 but also when using read(), seek(), sysread(), syswrite() and tell()
497 (see L<perlport> for more details). See the C<$/> and C<$\> variables
498 in L<perlvar> for how to manually set your input and output
499 line-termination sequences.
501 =item bless REF,CLASSNAME
505 This function tells the thingy referenced by REF that it is now an object
506 in the CLASSNAME package. If CLASSNAME is omitted, the current package
507 is used. Because a C<bless> is often the last thing in a constructor,
508 it returns the reference for convenience. Always use the two-argument
509 version if the function doing the blessing might be inherited by a
510 derived class. See L<perltoot> and L<perlobj> for more about the blessing
511 (and blessings) of objects.
513 Consider always blessing objects in CLASSNAMEs that are mixed case.
514 Namespaces with all lowercase names are considered reserved for
515 Perl pragmata. Builtin types have all uppercase names, so to prevent
516 confusion, you may wish to avoid such package names as well. Make sure
517 that CLASSNAME is a true value.
519 See L<perlmod/"Perl Modules">.
525 Returns the context of the current subroutine call. In scalar context,
526 returns the caller's package name if there is a caller, that is, if
527 we're in a subroutine or C<eval> or C<require>, and the undefined value
528 otherwise. In list context, returns
530 ($package, $filename, $line) = caller;
532 With EXPR, it returns some extra information that the debugger uses to
533 print a stack trace. The value of EXPR indicates how many call frames
534 to go back before the current one.
536 ($package, $filename, $line, $subroutine, $hasargs,
537 $wantarray, $evaltext, $is_require, $hints, $bitmask) = caller($i);
539 Here $subroutine may be C<(eval)> if the frame is not a subroutine
540 call, but an C<eval>. In such a case additional elements $evaltext and
541 C<$is_require> are set: C<$is_require> is true if the frame is created by a
542 C<require> or C<use> statement, $evaltext contains the text of the
543 C<eval EXPR> statement. In particular, for an C<eval BLOCK> statement,
544 $filename is C<(eval)>, but $evaltext is undefined. (Note also that
545 each C<use> statement creates a C<require> frame inside an C<eval EXPR>)
546 frame. C<$hasargs> is true if a new instance of C<@_> was set up for the
547 frame. C<$hints> and C<$bitmask> contain pragmatic hints that the caller
548 was compiled with. The C<$hints> and C<$bitmask> values are subject to
549 change between versions of Perl, and are not meant for external use.
551 Furthermore, when called from within the DB package, caller returns more
552 detailed information: it sets the list variable C<@DB::args> to be the
553 arguments with which the subroutine was invoked.
555 Be aware that the optimizer might have optimized call frames away before
556 C<caller> had a chance to get the information. That means that C<caller(N)>
557 might not return information about the call frame you expect it do, for
558 C<< N > 1 >>. In particular, C<@DB::args> might have information from the
559 previous time C<caller> was called.
563 Changes the working directory to EXPR, if possible. If EXPR is omitted,
564 changes to the directory specified by C<$ENV{HOME}>, if set; if not,
565 changes to the directory specified by C<$ENV{LOGDIR}>. If neither is
566 set, C<chdir> does nothing. It returns true upon success, false
567 otherwise. See the example under C<die>.
571 Changes the permissions of a list of files. The first element of the
572 list must be the numerical mode, which should probably be an octal
573 number, and which definitely should I<not> a string of octal digits:
574 C<0644> is okay, C<'0644'> is not. Returns the number of files
575 successfully changed. See also L</oct>, if all you have is a string.
577 $cnt = chmod 0755, 'foo', 'bar';
578 chmod 0755, @executables;
579 $mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to
581 $mode = '0644'; chmod oct($mode), 'foo'; # this is better
582 $mode = 0644; chmod $mode, 'foo'; # this is best
584 You can also import the symbolic C<S_I*> constants from the Fcntl
589 chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
590 # This is identical to the chmod 0755 of the above example.
598 This safer version of L</chop> removes any trailing string
599 that corresponds to the current value of C<$/> (also known as
600 $INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
601 number of characters removed from all its arguments. It's often used to
602 remove the newline from the end of an input record when you're worried
603 that the final record may be missing its newline. When in paragraph
604 mode (C<$/ = "">), it removes all trailing newlines from the string.
605 When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is
606 a reference to an integer or the like, see L<perlvar>) chomp() won't
608 If VARIABLE is omitted, it chomps C<$_>. Example:
611 chomp; # avoid \n on last field
616 If VARIABLE is a hash, it chomps the hash's values, but not its keys.
618 You can actually chomp anything that's an lvalue, including an assignment:
621 chomp($answer = <STDIN>);
623 If you chomp a list, each element is chomped, and the total number of
624 characters removed is returned.
632 Chops off the last character of a string and returns the character
633 chopped. It is much more efficient than C<s/.$//s> because it neither
634 scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
635 If VARIABLE is a hash, it chops the hash's values, but not its keys.
637 You can actually chop anything that's an lvalue, including an assignment.
639 If you chop a list, each element is chopped. Only the value of the
640 last C<chop> is returned.
642 Note that C<chop> returns the last character. To return all but the last
643 character, use C<substr($string, 0, -1)>.
647 Changes the owner (and group) of a list of files. The first two
648 elements of the list must be the I<numeric> uid and gid, in that
649 order. A value of -1 in either position is interpreted by most
650 systems to leave that value unchanged. Returns the number of files
651 successfully changed.
653 $cnt = chown $uid, $gid, 'foo', 'bar';
654 chown $uid, $gid, @filenames;
656 Here's an example that looks up nonnumeric uids in the passwd file:
659 chomp($user = <STDIN>);
661 chomp($pattern = <STDIN>);
663 ($login,$pass,$uid,$gid) = getpwnam($user)
664 or die "$user not in passwd file";
666 @ary = glob($pattern); # expand filenames
667 chown $uid, $gid, @ary;
669 On most systems, you are not allowed to change the ownership of the
670 file unless you're the superuser, although you should be able to change
671 the group to any of your secondary groups. On insecure systems, these
672 restrictions may be relaxed, but this is not a portable assumption.
673 On POSIX systems, you can detect this condition this way:
675 use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
676 $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
682 Returns the character represented by that NUMBER in the character set.
683 For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
684 chr(0x263a) is a Unicode smiley face. Within the scope of C<use utf8>,
685 characters higher than 127 are encoded in Unicode; if you don't want
686 this, temporarily C<use bytes> or use C<pack("C*",...)>
688 For the reverse, use L</ord>.
689 See L<utf8> for more about Unicode.
691 If NUMBER is omitted, uses C<$_>.
693 =item chroot FILENAME
697 This function works like the system call by the same name: it makes the
698 named directory the new root directory for all further pathnames that
699 begin with a C</> by your process and all its children. (It doesn't
700 change your current working directory, which is unaffected.) For security
701 reasons, this call is restricted to the superuser. If FILENAME is
702 omitted, does a C<chroot> to C<$_>.
704 =item close FILEHANDLE
708 Closes the file or pipe associated with the file handle, returning true
709 only if stdio successfully flushes buffers and closes the system file
710 descriptor. Closes the currently selected filehandle if the argument
713 You don't have to close FILEHANDLE if you are immediately going to do
714 another C<open> on it, because C<open> will close it for you. (See
715 C<open>.) However, an explicit C<close> on an input file resets the line
716 counter (C<$.>), while the implicit close done by C<open> does not.
718 If the file handle came from a piped open C<close> will additionally
719 return false if one of the other system calls involved fails or if the
720 program exits with non-zero status. (If the only problem was that the
721 program exited non-zero C<$!> will be set to C<0>.) Closing a pipe
722 also waits for the process executing on the pipe to complete, in case you
723 want to look at the output of the pipe afterwards, and
724 implicitly puts the exit status value of that command into C<$?>.
726 Prematurely closing the read end of a pipe (i.e. before the process
727 writing to it at the other end has closed it) will result in a
728 SIGPIPE being delivered to the writer. If the other end can't
729 handle that, be sure to read all the data before closing the pipe.
733 open(OUTPUT, '|sort >foo') # pipe to sort
734 or die "Can't start sort: $!";
735 #... # print stuff to output
736 close OUTPUT # wait for sort to finish
737 or warn $! ? "Error closing sort pipe: $!"
738 : "Exit status $? from sort";
739 open(INPUT, 'foo') # get sort's results
740 or die "Can't open 'foo' for input: $!";
742 FILEHANDLE may be an expression whose value can be used as an indirect
743 filehandle, usually the real filehandle name.
745 =item closedir DIRHANDLE
747 Closes a directory opened by C<opendir> and returns the success of that
750 DIRHANDLE may be an expression whose value can be used as an indirect
751 dirhandle, usually the real dirhandle name.
753 =item connect SOCKET,NAME
755 Attempts to connect to a remote socket, just as the connect system call
756 does. Returns true if it succeeded, false otherwise. NAME should be a
757 packed address of the appropriate type for the socket. See the examples in
758 L<perlipc/"Sockets: Client/Server Communication">.
762 Actually a flow control statement rather than a function. If there is a
763 C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
764 C<foreach>), it is always executed just before the conditional is about to
765 be evaluated again, just like the third part of a C<for> loop in C. Thus
766 it can be used to increment a loop variable, even when the loop has been
767 continued via the C<next> statement (which is similar to the C C<continue>
770 C<last>, C<next>, or C<redo> may appear within a C<continue>
771 block. C<last> and C<redo> will behave as if they had been executed within
772 the main block. So will C<next>, but since it will execute a C<continue>
773 block, it may be more entertaining.
776 ### redo always comes here
779 ### next always comes here
781 # then back the top to re-check EXPR
783 ### last always comes here
785 Omitting the C<continue> section is semantically equivalent to using an
786 empty one, logically enough. In that case, C<next> goes directly back
787 to check the condition at the top of the loop.
793 Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
794 takes cosine of C<$_>.
796 For the inverse cosine operation, you may use the C<Math::Trig::acos()>
797 function, or use this relation:
799 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
801 =item crypt PLAINTEXT,SALT
803 Encrypts a string exactly like the crypt(3) function in the C library
804 (assuming that you actually have a version there that has not been
805 extirpated as a potential munition). This can prove useful for checking
806 the password file for lousy passwords, amongst other things. Only the
807 guys wearing white hats should do this.
809 Note that C<crypt> is intended to be a one-way function, much like breaking
810 eggs to make an omelette. There is no (known) corresponding decrypt
811 function. As a result, this function isn't all that useful for
812 cryptography. (For that, see your nearby CPAN mirror.)
814 When verifying an existing encrypted string you should use the encrypted
815 text as the salt (like C<crypt($plain, $crypted) eq $crypted>). This
816 allows your code to work with the standard C<crypt> and with more
817 exotic implementations. When choosing a new salt create a random two
818 character string whose characters come from the set C<[./0-9A-Za-z]>
819 (like C<join '', ('.', '/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>).
821 Here's an example that makes sure that whoever runs this program knows
824 $pwd = (getpwuid($<))[1];
828 chomp($word = <STDIN>);
832 if (crypt($word, $pwd) ne $pwd) {
838 Of course, typing in your own password to whoever asks you
841 The L<crypt> function is unsuitable for encrypting large quantities
842 of data, not least of all because you can't get the information
843 back. Look at the F<by-module/Crypt> and F<by-module/PGP> directories
844 on your favorite CPAN mirror for a slew of potentially useful
849 [This function has been largely superseded by the C<untie> function.]
851 Breaks the binding between a DBM file and a hash.
853 =item dbmopen HASH,DBNAME,MASK
855 [This function has been largely superseded by the C<tie> function.]
857 This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
858 hash. HASH is the name of the hash. (Unlike normal C<open>, the first
859 argument is I<not> a filehandle, even though it looks like one). DBNAME
860 is the name of the database (without the F<.dir> or F<.pag> extension if
861 any). If the database does not exist, it is created with protection
862 specified by MASK (as modified by the C<umask>). If your system supports
863 only the older DBM functions, you may perform only one C<dbmopen> in your
864 program. In older versions of Perl, if your system had neither DBM nor
865 ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
868 If you don't have write access to the DBM file, you can only read hash
869 variables, not set them. If you want to test whether you can write,
870 either use file tests or try setting a dummy hash entry inside an C<eval>,
871 which will trap the error.
873 Note that functions such as C<keys> and C<values> may return huge lists
874 when used on large DBM files. You may prefer to use the C<each>
875 function to iterate over large DBM files. Example:
877 # print out history file offsets
878 dbmopen(%HIST,'/usr/lib/news/history',0666);
879 while (($key,$val) = each %HIST) {
880 print $key, ' = ', unpack('L',$val), "\n";
884 See also L<AnyDBM_File> for a more general description of the pros and
885 cons of the various dbm approaches, as well as L<DB_File> for a particularly
888 You can control which DBM library you use by loading that library
889 before you call dbmopen():
892 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
893 or die "Can't open netscape history file: $!";
899 Returns a Boolean value telling whether EXPR has a value other than
900 the undefined value C<undef>. If EXPR is not present, C<$_> will be
903 Many operations return C<undef> to indicate failure, end of file,
904 system error, uninitialized variable, and other exceptional
905 conditions. This function allows you to distinguish C<undef> from
906 other values. (A simple Boolean test will not distinguish among
907 C<undef>, zero, the empty string, and C<"0">, which are all equally
908 false.) Note that since C<undef> is a valid scalar, its presence
909 doesn't I<necessarily> indicate an exceptional condition: C<pop>
910 returns C<undef> when its argument is an empty array, I<or> when the
911 element to return happens to be C<undef>.
913 You may also use C<defined(&func)> to check whether subroutine C<&func>
914 has ever been defined. The return value is unaffected by any forward
915 declarations of C<&foo>. Note that a subroutine which is not defined
916 may still be callable: its package may have an C<AUTOLOAD> method that
917 makes it spring into existence the first time that it is called -- see
920 Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
921 used to report whether memory for that aggregate has ever been
922 allocated. This behavior may disappear in future versions of Perl.
923 You should instead use a simple test for size:
925 if (@an_array) { print "has array elements\n" }
926 if (%a_hash) { print "has hash members\n" }
928 When used on a hash element, it tells you whether the value is defined,
929 not whether the key exists in the hash. Use L</exists> for the latter
934 print if defined $switch{'D'};
935 print "$val\n" while defined($val = pop(@ary));
936 die "Can't readlink $sym: $!"
937 unless defined($value = readlink $sym);
938 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
939 $debugging = 0 unless defined $debugging;
941 Note: Many folks tend to overuse C<defined>, and then are surprised to
942 discover that the number C<0> and C<""> (the zero-length string) are, in fact,
943 defined values. For example, if you say
947 The pattern match succeeds, and C<$1> is defined, despite the fact that it
948 matched "nothing". But it didn't really match nothing--rather, it
949 matched something that happened to be zero characters long. This is all
950 very above-board and honest. When a function returns an undefined value,
951 it's an admission that it couldn't give you an honest answer. So you
952 should use C<defined> only when you're questioning the integrity of what
953 you're trying to do. At other times, a simple comparison to C<0> or C<""> is
956 See also L</undef>, L</exists>, L</ref>.
960 Given an expression that specifies a hash element, array element, hash slice,
961 or array slice, deletes the specified element(s) from the hash or array.
962 In the case of an array, if the array elements happen to be at the end,
963 the size of the array will shrink to the highest element that tests
964 true for exists() (or 0 if no such element exists).
966 Returns each element so deleted or the undefined value if there was no such
967 element. Deleting from C<$ENV{}> modifies the environment. Deleting from
968 a hash tied to a DBM file deletes the entry from the DBM file. Deleting
969 from a C<tie>d hash or array may not necessarily return anything.
971 Deleting an array element effectively returns that position of the array
972 to its initial, uninitialized state. Subsequently testing for the same
973 element with exists() will return false. Note that deleting array
974 elements in the middle of an array will not shift the index of the ones
975 after them down--use splice() for that. See L</exists>.
977 The following (inefficiently) deletes all the values of %HASH and @ARRAY:
979 foreach $key (keys %HASH) {
983 foreach $index (0 .. $#ARRAY) {
984 delete $ARRAY[$index];
989 delete @HASH{keys %HASH};
991 delete @ARRAY[0 .. $#ARRAY];
993 But both of these are slower than just assigning the empty list
994 or undefining %HASH or @ARRAY:
996 %HASH = (); # completely empty %HASH
997 undef %HASH; # forget %HASH ever existed
999 @ARRAY = (); # completely empty @ARRAY
1000 undef @ARRAY; # forget @ARRAY ever existed
1002 Note that the EXPR can be arbitrarily complicated as long as the final
1003 operation is a hash element, array element, hash slice, or array slice
1006 delete $ref->[$x][$y]{$key};
1007 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
1009 delete $ref->[$x][$y][$index];
1010 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
1014 Outside an C<eval>, prints the value of LIST to C<STDERR> and
1015 exits with the current value of C<$!> (errno). If C<$!> is C<0>,
1016 exits with the value of C<<< ($? >> 8) >>> (backtick `command`
1017 status). If C<<< ($? >> 8) >>> is C<0>, exits with C<255>. Inside
1018 an C<eval(),> the error message is stuffed into C<$@> and the
1019 C<eval> is terminated with the undefined value. This makes
1020 C<die> the way to raise an exception.
1022 Equivalent examples:
1024 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
1025 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
1027 If the value of EXPR does not end in a newline, the current script line
1028 number and input line number (if any) are also printed, and a newline
1029 is supplied. Note that the "input line number" (also known as "chunk")
1030 is subject to whatever notion of "line" happens to be currently in
1031 effect, and is also available as the special variable C<$.>.
1032 See L<perlvar/"$/"> and L<perlvar/"$.">.
1034 Hint: sometimes appending C<", stopped"> to your message
1035 will cause it to make better sense when the string C<"at foo line 123"> is
1036 appended. Suppose you are running script "canasta".
1038 die "/etc/games is no good";
1039 die "/etc/games is no good, stopped";
1041 produce, respectively
1043 /etc/games is no good at canasta line 123.
1044 /etc/games is no good, stopped at canasta line 123.
1046 See also exit(), warn(), and the Carp module.
1048 If LIST is empty and C<$@> already contains a value (typically from a
1049 previous eval) that value is reused after appending C<"\t...propagated">.
1050 This is useful for propagating exceptions:
1053 die unless $@ =~ /Expected exception/;
1055 If C<$@> is empty then the string C<"Died"> is used.
1057 die() can also be called with a reference argument. If this happens to be
1058 trapped within an eval(), $@ contains the reference. This behavior permits
1059 a more elaborate exception handling implementation using objects that
1060 maintain arbitrary state about the nature of the exception. Such a scheme
1061 is sometimes preferable to matching particular string values of $@ using
1062 regular expressions. Here's an example:
1064 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
1066 if (ref($@) && UNIVERSAL::isa($@,"Some::Module::Exception")) {
1067 # handle Some::Module::Exception
1070 # handle all other possible exceptions
1074 Because perl will stringify uncaught exception messages before displaying
1075 them, you may want to overload stringification operations on such custom
1076 exception objects. See L<overload> for details about that.
1078 You can arrange for a callback to be run just before the C<die>
1079 does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
1080 handler will be called with the error text and can change the error
1081 message, if it sees fit, by calling C<die> again. See
1082 L<perlvar/$SIG{expr}> for details on setting C<%SIG> entries, and
1083 L<"eval BLOCK"> for some examples. Although this feature was meant
1084 to be run only right before your program was to exit, this is not
1085 currently the case--the C<$SIG{__DIE__}> hook is currently called
1086 even inside eval()ed blocks/strings! If one wants the hook to do
1087 nothing in such situations, put
1091 as the first line of the handler (see L<perlvar/$^S>). Because
1092 this promotes strange action at a distance, this counterintuitive
1093 behavior may be fixed in a future release.
1097 Not really a function. Returns the value of the last command in the
1098 sequence of commands indicated by BLOCK. When modified by a loop
1099 modifier, executes the BLOCK once before testing the loop condition.
1100 (On other statements the loop modifiers test the conditional first.)
1102 C<do BLOCK> does I<not> count as a loop, so the loop control statements
1103 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1104 See L<perlsyn> for alternative strategies.
1106 =item do SUBROUTINE(LIST)
1108 A deprecated form of subroutine call. See L<perlsub>.
1112 Uses the value of EXPR as a filename and executes the contents of the
1113 file as a Perl script. Its primary use is to include subroutines
1114 from a Perl subroutine library.
1120 scalar eval `cat stat.pl`;
1122 except that it's more efficient and concise, keeps track of the current
1123 filename for error messages, searches the @INC libraries, and updates
1124 C<%INC> if the file is found. See L<perlvar/Predefined Names> for these
1125 variables. It also differs in that code evaluated with C<do FILENAME>
1126 cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1127 same, however, in that it does reparse the file every time you call it,
1128 so you probably don't want to do this inside a loop.
1130 If C<do> cannot read the file, it returns undef and sets C<$!> to the
1131 error. If C<do> can read the file but cannot compile it, it
1132 returns undef and sets an error message in C<$@>. If the file is
1133 successfully compiled, C<do> returns the value of the last expression
1136 Note that inclusion of library modules is better done with the
1137 C<use> and C<require> operators, which also do automatic error checking
1138 and raise an exception if there's a problem.
1140 You might like to use C<do> to read in a program configuration
1141 file. Manual error checking can be done this way:
1143 # read in config files: system first, then user
1144 for $file ("/share/prog/defaults.rc",
1145 "$ENV{HOME}/.someprogrc")
1147 unless ($return = do $file) {
1148 warn "couldn't parse $file: $@" if $@;
1149 warn "couldn't do $file: $!" unless defined $return;
1150 warn "couldn't run $file" unless $return;
1158 This function causes an immediate core dump. See also the B<-u>
1159 command-line switch in L<perlrun>, which does the same thing.
1160 Primarily this is so that you can use the B<undump> program (not
1161 supplied) to turn your core dump into an executable binary after
1162 having initialized all your variables at the beginning of the
1163 program. When the new binary is executed it will begin by executing
1164 a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1165 Think of it as a goto with an intervening core dump and reincarnation.
1166 If C<LABEL> is omitted, restarts the program from the top.
1168 B<WARNING>: Any files opened at the time of the dump will I<not>
1169 be open any more when the program is reincarnated, with possible
1170 resulting confusion on the part of Perl.
1172 This function is now largely obsolete, partly because it's very
1173 hard to convert a core file into an executable, and because the
1174 real compiler backends for generating portable bytecode and compilable
1175 C code have superseded it.
1177 If you're looking to use L<dump> to speed up your program, consider
1178 generating bytecode or native C code as described in L<perlcc>. If
1179 you're just trying to accelerate a CGI script, consider using the
1180 C<mod_perl> extension to B<Apache>, or the CPAN module, Fast::CGI.
1181 You might also consider autoloading or selfloading, which at least
1182 make your program I<appear> to run faster.
1186 When called in list context, returns a 2-element list consisting of the
1187 key and value for the next element of a hash, so that you can iterate over
1188 it. When called in scalar context, returns only the key for the next
1189 element in the hash.
1191 Entries are returned in an apparently random order. The actual random
1192 order is subject to change in future versions of perl, but it is guaranteed
1193 to be in the same order as either the C<keys> or C<values> function
1194 would produce on the same (unmodified) hash.
1196 When the hash is entirely read, a null array is returned in list context
1197 (which when assigned produces a false (C<0>) value), and C<undef> in
1198 scalar context. The next call to C<each> after that will start iterating
1199 again. There is a single iterator for each hash, shared by all C<each>,
1200 C<keys>, and C<values> function calls in the program; it can be reset by
1201 reading all the elements from the hash, or by evaluating C<keys HASH> or
1202 C<values HASH>. If you add or delete elements of a hash while you're
1203 iterating over it, you may get entries skipped or duplicated, so
1204 don't. Exception: It is always safe to delete the item most recently
1205 returned by C<each()>, which means that the following code will work:
1207 while (($key, $value) = each %hash) {
1209 delete $hash{$key}; # This is safe
1212 The following prints out your environment like the printenv(1) program,
1213 only in a different order:
1215 while (($key,$value) = each %ENV) {
1216 print "$key=$value\n";
1219 See also C<keys>, C<values> and C<sort>.
1221 =item eof FILEHANDLE
1227 Returns 1 if the next read on FILEHANDLE will return end of file, or if
1228 FILEHANDLE is not open. FILEHANDLE may be an expression whose value
1229 gives the real filehandle. (Note that this function actually
1230 reads a character and then C<ungetc>s it, so isn't very useful in an
1231 interactive context.) Do not read from a terminal file (or call
1232 C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
1233 as terminals may lose the end-of-file condition if you do.
1235 An C<eof> without an argument uses the last file read. Using C<eof()>
1236 with empty parentheses is very different. It refers to the pseudo file
1237 formed from the files listed on the command line and accessed via the
1238 C<< <> >> operator. Since C<< <> >> isn't explicitly opened,
1239 as a normal filehandle is, an C<eof()> before C<< <> >> has been
1240 used will cause C<@ARGV> to be examined to determine if input is
1243 In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to
1244 detect the end of each file, C<eof()> will only detect the end of the
1245 last file. Examples:
1247 # reset line numbering on each input file
1249 next if /^\s*#/; # skip comments
1252 close ARGV if eof; # Not eof()!
1255 # insert dashes just before last line of last file
1257 if (eof()) { # check for end of current file
1258 print "--------------\n";
1259 close(ARGV); # close or last; is needed if we
1260 # are reading from the terminal
1265 Practical hint: you almost never need to use C<eof> in Perl, because the
1266 input operators typically return C<undef> when they run out of data, or if
1273 In the first form, the return value of EXPR is parsed and executed as if it
1274 were a little Perl program. The value of the expression (which is itself
1275 determined within scalar context) is first parsed, and if there weren't any
1276 errors, executed in the lexical context of the current Perl program, so
1277 that any variable settings or subroutine and format definitions remain
1278 afterwards. Note that the value is parsed every time the eval executes.
1279 If EXPR is omitted, evaluates C<$_>. This form is typically used to
1280 delay parsing and subsequent execution of the text of EXPR until run time.
1282 In the second form, the code within the BLOCK is parsed only once--at the
1283 same time the code surrounding the eval itself was parsed--and executed
1284 within the context of the current Perl program. This form is typically
1285 used to trap exceptions more efficiently than the first (see below), while
1286 also providing the benefit of checking the code within BLOCK at compile
1289 The final semicolon, if any, may be omitted from the value of EXPR or within
1292 In both forms, the value returned is the value of the last expression
1293 evaluated inside the mini-program; a return statement may be also used, just
1294 as with subroutines. The expression providing the return value is evaluated
1295 in void, scalar, or list context, depending on the context of the eval itself.
1296 See L</wantarray> for more on how the evaluation context can be determined.
1298 If there is a syntax error or runtime error, or a C<die> statement is
1299 executed, an undefined value is returned by C<eval>, and C<$@> is set to the
1300 error message. If there was no error, C<$@> is guaranteed to be a null
1301 string. Beware that using C<eval> neither silences perl from printing
1302 warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
1303 To do either of those, you have to use the C<$SIG{__WARN__}> facility. See
1304 L</warn> and L<perlvar>.
1306 Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1307 determining whether a particular feature (such as C<socket> or C<symlink>)
1308 is implemented. It is also Perl's exception trapping mechanism, where
1309 the die operator is used to raise exceptions.
1311 If the code to be executed doesn't vary, you may use the eval-BLOCK
1312 form to trap run-time errors without incurring the penalty of
1313 recompiling each time. The error, if any, is still returned in C<$@>.
1316 # make divide-by-zero nonfatal
1317 eval { $answer = $a / $b; }; warn $@ if $@;
1319 # same thing, but less efficient
1320 eval '$answer = $a / $b'; warn $@ if $@;
1322 # a compile-time error
1323 eval { $answer = }; # WRONG
1326 eval '$answer ='; # sets $@
1328 Due to the current arguably broken state of C<__DIE__> hooks, when using
1329 the C<eval{}> form as an exception trap in libraries, you may wish not
1330 to trigger any C<__DIE__> hooks that user code may have installed.
1331 You can use the C<local $SIG{__DIE__}> construct for this purpose,
1332 as shown in this example:
1334 # a very private exception trap for divide-by-zero
1335 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1338 This is especially significant, given that C<__DIE__> hooks can call
1339 C<die> again, which has the effect of changing their error messages:
1341 # __DIE__ hooks may modify error messages
1343 local $SIG{'__DIE__'} =
1344 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
1345 eval { die "foo lives here" };
1346 print $@ if $@; # prints "bar lives here"
1349 Because this promotes action at a distance, this counterintuitive behavior
1350 may be fixed in a future release.
1352 With an C<eval>, you should be especially careful to remember what's
1353 being looked at when:
1359 eval { $x }; # CASE 4
1361 eval "\$$x++"; # CASE 5
1364 Cases 1 and 2 above behave identically: they run the code contained in
1365 the variable $x. (Although case 2 has misleading double quotes making
1366 the reader wonder what else might be happening (nothing is).) Cases 3
1367 and 4 likewise behave in the same way: they run the code C<'$x'>, which
1368 does nothing but return the value of $x. (Case 4 is preferred for
1369 purely visual reasons, but it also has the advantage of compiling at
1370 compile-time instead of at run-time.) Case 5 is a place where
1371 normally you I<would> like to use double quotes, except that in this
1372 particular situation, you can just use symbolic references instead, as
1375 C<eval BLOCK> does I<not> count as a loop, so the loop control statements
1376 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1380 =item exec PROGRAM LIST
1382 The C<exec> function executes a system command I<and never returns>--
1383 use C<system> instead of C<exec> if you want it to return. It fails and
1384 returns false only if the command does not exist I<and> it is executed
1385 directly instead of via your system's command shell (see below).
1387 Since it's a common mistake to use C<exec> instead of C<system>, Perl
1388 warns you if there is a following statement which isn't C<die>, C<warn>,
1389 or C<exit> (if C<-w> is set - but you always do that). If you
1390 I<really> want to follow an C<exec> with some other statement, you
1391 can use one of these styles to avoid the warning:
1393 exec ('foo') or print STDERR "couldn't exec foo: $!";
1394 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
1396 If there is more than one argument in LIST, or if LIST is an array
1397 with more than one value, calls execvp(3) with the arguments in LIST.
1398 If there is only one scalar argument or an array with one element in it,
1399 the argument is checked for shell metacharacters, and if there are any,
1400 the entire argument is passed to the system's command shell for parsing
1401 (this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1402 If there are no shell metacharacters in the argument, it is split into
1403 words and passed directly to C<execvp>, which is more efficient.
1406 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1407 exec "sort $outfile | uniq";
1409 If you don't really want to execute the first argument, but want to lie
1410 to the program you are executing about its own name, you can specify
1411 the program you actually want to run as an "indirect object" (without a
1412 comma) in front of the LIST. (This always forces interpretation of the
1413 LIST as a multivalued list, even if there is only a single scalar in
1416 $shell = '/bin/csh';
1417 exec $shell '-sh'; # pretend it's a login shell
1421 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1423 When the arguments get executed via the system shell, results will
1424 be subject to its quirks and capabilities. See L<perlop/"`STRING`">
1427 Using an indirect object with C<exec> or C<system> is also more
1428 secure. This usage (which also works fine with system()) forces
1429 interpretation of the arguments as a multivalued list, even if the
1430 list had just one argument. That way you're safe from the shell
1431 expanding wildcards or splitting up words with whitespace in them.
1433 @args = ( "echo surprise" );
1435 exec @args; # subject to shell escapes
1437 exec { $args[0] } @args; # safe even with one-arg list
1439 The first version, the one without the indirect object, ran the I<echo>
1440 program, passing it C<"surprise"> an argument. The second version
1441 didn't--it tried to run a program literally called I<"echo surprise">,
1442 didn't find it, and set C<$?> to a non-zero value indicating failure.
1444 Beginning with v5.6.0, Perl will attempt to flush all files opened for
1445 output before the exec, but this may not be supported on some platforms
1446 (see L<perlport>). To be safe, you may need to set C<$|> ($AUTOFLUSH
1447 in English) or call the C<autoflush()> method of C<IO::Handle> on any
1448 open handles in order to avoid lost output.
1450 Note that C<exec> will not call your C<END> blocks, nor will it call
1451 any C<DESTROY> methods in your objects.
1455 Given an expression that specifies a hash element or array element,
1456 returns true if the specified element in the hash or array has ever
1457 been initialized, even if the corresponding value is undefined. The
1458 element is not autovivified if it doesn't exist.
1460 print "Exists\n" if exists $hash{$key};
1461 print "Defined\n" if defined $hash{$key};
1462 print "True\n" if $hash{$key};
1464 print "Exists\n" if exists $array[$index];
1465 print "Defined\n" if defined $array[$index];
1466 print "True\n" if $array[$index];
1468 A hash or array element can be true only if it's defined, and defined if
1469 it exists, but the reverse doesn't necessarily hold true.
1471 Given an expression that specifies the name of a subroutine,
1472 returns true if the specified subroutine has ever been declared, even
1473 if it is undefined. Mentioning a subroutine name for exists or defined
1474 does not count as declaring it. Note that a subroutine which does not
1475 exist may still be callable: its package may have an C<AUTOLOAD>
1476 method that makes it spring into existence the first time that it is
1477 called -- see L<perlsub>.
1479 print "Exists\n" if exists &subroutine;
1480 print "Defined\n" if defined &subroutine;
1482 Note that the EXPR can be arbitrarily complicated as long as the final
1483 operation is a hash or array key lookup or subroutine name:
1485 if (exists $ref->{A}->{B}->{$key}) { }
1486 if (exists $hash{A}{B}{$key}) { }
1488 if (exists $ref->{A}->{B}->[$ix]) { }
1489 if (exists $hash{A}{B}[$ix]) { }
1491 if (exists &{$ref->{A}{B}{$key}}) { }
1493 Although the deepest nested array or hash will not spring into existence
1494 just because its existence was tested, any intervening ones will.
1495 Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
1496 into existence due to the existence test for the $key element above.
1497 This happens anywhere the arrow operator is used, including even:
1500 if (exists $ref->{"Some key"}) { }
1501 print $ref; # prints HASH(0x80d3d5c)
1503 This surprising autovivification in what does not at first--or even
1504 second--glance appear to be an lvalue context may be fixed in a future
1507 See L<perlref/"Pseudo-hashes: Using an array as a hash"> for specifics
1508 on how exists() acts when used on a pseudo-hash.
1510 Use of a subroutine call, rather than a subroutine name, as an argument
1511 to exists() is an error.
1514 exists &sub(); # Error
1518 Evaluates EXPR and exits immediately with that value. Example:
1521 exit 0 if $ans =~ /^[Xx]/;
1523 See also C<die>. If EXPR is omitted, exits with C<0> status. The only
1524 universally recognized values for EXPR are C<0> for success and C<1>
1525 for error; other values are subject to interpretation depending on the
1526 environment in which the Perl program is running. For example, exiting
1527 69 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
1528 the mailer to return the item undelivered, but that's not true everywhere.
1530 Don't use C<exit> to abort a subroutine if there's any chance that
1531 someone might want to trap whatever error happened. Use C<die> instead,
1532 which can be trapped by an C<eval>.
1534 The exit() function does not always exit immediately. It calls any
1535 defined C<END> routines first, but these C<END> routines may not
1536 themselves abort the exit. Likewise any object destructors that need to
1537 be called are called before the real exit. If this is a problem, you
1538 can call C<POSIX:_exit($status)> to avoid END and destructor processing.
1539 See L<perlmod> for details.
1545 Returns I<e> (the natural logarithm base) to the power of EXPR.
1546 If EXPR is omitted, gives C<exp($_)>.
1548 =item fcntl FILEHANDLE,FUNCTION,SCALAR
1550 Implements the fcntl(2) function. You'll probably have to say
1554 first to get the correct constant definitions. Argument processing and
1555 value return works just like C<ioctl> below.
1559 fcntl($filehandle, F_GETFL, $packed_return_buffer)
1560 or die "can't fcntl F_GETFL: $!";
1562 You don't have to check for C<defined> on the return from C<fnctl>.
1563 Like C<ioctl>, it maps a C<0> return from the system call into
1564 C<"0 but true"> in Perl. This string is true in boolean context and C<0>
1565 in numeric context. It is also exempt from the normal B<-w> warnings
1566 on improper numeric conversions.
1568 Note that C<fcntl> will produce a fatal error if used on a machine that
1569 doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
1570 manpage to learn what functions are available on your system.
1572 =item fileno FILEHANDLE
1574 Returns the file descriptor for a filehandle, or undefined if the
1575 filehandle is not open. This is mainly useful for constructing
1576 bitmaps for C<select> and low-level POSIX tty-handling operations.
1577 If FILEHANDLE is an expression, the value is taken as an indirect
1578 filehandle, generally its name.
1580 You can use this to find out whether two handles refer to the
1581 same underlying descriptor:
1583 if (fileno(THIS) == fileno(THAT)) {
1584 print "THIS and THAT are dups\n";
1587 =item flock FILEHANDLE,OPERATION
1589 Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
1590 for success, false on failure. Produces a fatal error if used on a
1591 machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
1592 C<flock> is Perl's portable file locking interface, although it locks
1593 only entire files, not records.
1595 Two potentially non-obvious but traditional C<flock> semantics are
1596 that it waits indefinitely until the lock is granted, and that its locks
1597 B<merely advisory>. Such discretionary locks are more flexible, but offer
1598 fewer guarantees. This means that files locked with C<flock> may be
1599 modified by programs that do not also use C<flock>. See L<perlport>,
1600 your port's specific documentation, or your system-specific local manpages
1601 for details. It's best to assume traditional behavior if you're writing
1602 portable programs. (But if you're not, you should as always feel perfectly
1603 free to write for your own system's idiosyncrasies (sometimes called
1604 "features"). Slavish adherence to portability concerns shouldn't get
1605 in the way of your getting your job done.)
1607 OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1608 LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
1609 you can use the symbolic names if you import them from the Fcntl module,
1610 either individually, or as a group using the ':flock' tag. LOCK_SH
1611 requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
1612 releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
1613 LOCK_SH or LOCK_EX then C<flock> will return immediately rather than blocking
1614 waiting for the lock (check the return status to see if you got it).
1616 To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
1617 before locking or unlocking it.
1619 Note that the emulation built with lockf(3) doesn't provide shared
1620 locks, and it requires that FILEHANDLE be open with write intent. These
1621 are the semantics that lockf(3) implements. Most if not all systems
1622 implement lockf(3) in terms of fcntl(2) locking, though, so the
1623 differing semantics shouldn't bite too many people.
1625 Note also that some versions of C<flock> cannot lock things over the
1626 network; you would need to use the more system-specific C<fcntl> for
1627 that. If you like you can force Perl to ignore your system's flock(2)
1628 function, and so provide its own fcntl(2)-based emulation, by passing
1629 the switch C<-Ud_flock> to the F<Configure> program when you configure
1632 Here's a mailbox appender for BSD systems.
1634 use Fcntl ':flock'; # import LOCK_* constants
1637 flock(MBOX,LOCK_EX);
1638 # and, in case someone appended
1639 # while we were waiting...
1644 flock(MBOX,LOCK_UN);
1647 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1648 or die "Can't open mailbox: $!";
1651 print MBOX $msg,"\n\n";
1654 On systems that support a real flock(), locks are inherited across fork()
1655 calls, whereas those that must resort to the more capricious fcntl()
1656 function lose the locks, making it harder to write servers.
1658 See also L<DB_File> for other flock() examples.
1662 Does a fork(2) system call to create a new process running the
1663 same program at the same point. It returns the child pid to the
1664 parent process, C<0> to the child process, or C<undef> if the fork is
1665 unsuccessful. File descriptors (and sometimes locks on those descriptors)
1666 are shared, while everything else is copied. On most systems supporting
1667 fork(), great care has gone into making it extremely efficient (for
1668 example, using copy-on-write technology on data pages), making it the
1669 dominant paradigm for multitasking over the last few decades.
1671 Beginning with v5.6.0, Perl will attempt to flush all files opened for
1672 output before forking the child process, but this may not be supported
1673 on some platforms (see L<perlport>). To be safe, you may need to set
1674 C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
1675 C<IO::Handle> on any open handles in order to avoid duplicate output.
1677 If you C<fork> without ever waiting on your children, you will
1678 accumulate zombies. On some systems, you can avoid this by setting
1679 C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
1680 forking and reaping moribund children.
1682 Note that if your forked child inherits system file descriptors like
1683 STDIN and STDOUT that are actually connected by a pipe or socket, even
1684 if you exit, then the remote server (such as, say, a CGI script or a
1685 backgrounded job launched from a remote shell) won't think you're done.
1686 You should reopen those to F</dev/null> if it's any issue.
1690 Declare a picture format for use by the C<write> function. For
1694 Test: @<<<<<<<< @||||| @>>>>>
1695 $str, $%, '$' . int($num)
1699 $num = $cost/$quantity;
1703 See L<perlform> for many details and examples.
1705 =item formline PICTURE,LIST
1707 This is an internal function used by C<format>s, though you may call it,
1708 too. It formats (see L<perlform>) a list of values according to the
1709 contents of PICTURE, placing the output into the format output
1710 accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
1711 Eventually, when a C<write> is done, the contents of
1712 C<$^A> are written to some filehandle, but you could also read C<$^A>
1713 yourself and then set C<$^A> back to C<"">. Note that a format typically
1714 does one C<formline> per line of form, but the C<formline> function itself
1715 doesn't care how many newlines are embedded in the PICTURE. This means
1716 that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
1717 You may therefore need to use multiple formlines to implement a single
1718 record format, just like the format compiler.
1720 Be careful if you put double quotes around the picture, because an C<@>
1721 character may be taken to mean the beginning of an array name.
1722 C<formline> always returns true. See L<perlform> for other examples.
1724 =item getc FILEHANDLE
1728 Returns the next character from the input file attached to FILEHANDLE,
1729 or the undefined value at end of file, or if there was an error.
1730 If FILEHANDLE is omitted, reads from STDIN. This is not particularly
1731 efficient. However, it cannot be used by itself to fetch single
1732 characters without waiting for the user to hit enter. For that, try
1733 something more like:
1736 system "stty cbreak </dev/tty >/dev/tty 2>&1";
1739 system "stty", '-icanon', 'eol', "\001";
1745 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
1748 system "stty", 'icanon', 'eol', '^@'; # ASCII null
1752 Determination of whether $BSD_STYLE should be set
1753 is left as an exercise to the reader.
1755 The C<POSIX::getattr> function can do this more portably on
1756 systems purporting POSIX compliance. See also the C<Term::ReadKey>
1757 module from your nearest CPAN site; details on CPAN can be found on
1762 Implements the C library function of the same name, which on most
1763 systems returns the current login from F</etc/utmp>, if any. If null,
1766 $login = getlogin || getpwuid($<) || "Kilroy";
1768 Do not consider C<getlogin> for authentication: it is not as
1769 secure as C<getpwuid>.
1771 =item getpeername SOCKET
1773 Returns the packed sockaddr address of other end of the SOCKET connection.
1776 $hersockaddr = getpeername(SOCK);
1777 ($port, $iaddr) = sockaddr_in($hersockaddr);
1778 $herhostname = gethostbyaddr($iaddr, AF_INET);
1779 $herstraddr = inet_ntoa($iaddr);
1783 Returns the current process group for the specified PID. Use
1784 a PID of C<0> to get the current process group for the
1785 current process. Will raise an exception if used on a machine that
1786 doesn't implement getpgrp(2). If PID is omitted, returns process
1787 group of current process. Note that the POSIX version of C<getpgrp>
1788 does not accept a PID argument, so only C<PID==0> is truly portable.
1792 Returns the process id of the parent process.
1794 =item getpriority WHICH,WHO
1796 Returns the current priority for a process, a process group, or a user.
1797 (See L<getpriority(2)>.) Will raise a fatal exception if used on a
1798 machine that doesn't implement getpriority(2).
1804 =item gethostbyname NAME
1806 =item getnetbyname NAME
1808 =item getprotobyname NAME
1814 =item getservbyname NAME,PROTO
1816 =item gethostbyaddr ADDR,ADDRTYPE
1818 =item getnetbyaddr ADDR,ADDRTYPE
1820 =item getprotobynumber NUMBER
1822 =item getservbyport PORT,PROTO
1840 =item sethostent STAYOPEN
1842 =item setnetent STAYOPEN
1844 =item setprotoent STAYOPEN
1846 =item setservent STAYOPEN
1860 These routines perform the same functions as their counterparts in the
1861 system library. In list context, the return values from the
1862 various get routines are as follows:
1864 ($name,$passwd,$uid,$gid,
1865 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
1866 ($name,$passwd,$gid,$members) = getgr*
1867 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
1868 ($name,$aliases,$addrtype,$net) = getnet*
1869 ($name,$aliases,$proto) = getproto*
1870 ($name,$aliases,$port,$proto) = getserv*
1872 (If the entry doesn't exist you get a null list.)
1874 The exact meaning of the $gcos field varies but it usually contains
1875 the real name of the user (as opposed to the login name) and other
1876 information pertaining to the user. Beware, however, that in many
1877 system users are able to change this information and therefore it
1878 cannot be trusted and therefore the $gcos is tainted (see
1879 L<perlsec>). The $passwd and $shell, user's encrypted password and
1880 login shell, are also tainted, because of the same reason.
1882 In scalar context, you get the name, unless the function was a
1883 lookup by name, in which case you get the other thing, whatever it is.
1884 (If the entry doesn't exist you get the undefined value.) For example:
1886 $uid = getpwnam($name);
1887 $name = getpwuid($num);
1889 $gid = getgrnam($name);
1890 $name = getgrgid($num;
1894 In I<getpw*()> the fields $quota, $comment, and $expire are special
1895 cases in the sense that in many systems they are unsupported. If the
1896 $quota is unsupported, it is an empty scalar. If it is supported, it
1897 usually encodes the disk quota. If the $comment field is unsupported,
1898 it is an empty scalar. If it is supported it usually encodes some
1899 administrative comment about the user. In some systems the $quota
1900 field may be $change or $age, fields that have to do with password
1901 aging. In some systems the $comment field may be $class. The $expire
1902 field, if present, encodes the expiration period of the account or the
1903 password. For the availability and the exact meaning of these fields
1904 in your system, please consult your getpwnam(3) documentation and your
1905 F<pwd.h> file. You can also find out from within Perl what your
1906 $quota and $comment fields mean and whether you have the $expire field
1907 by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
1908 C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password
1909 files are only supported if your vendor has implemented them in the
1910 intuitive fashion that calling the regular C library routines gets the
1911 shadow versions if you're running under privilege or if there exists
1912 the shadow(3) functions as found in System V ( this includes Solaris
1913 and Linux.) Those systems which implement a proprietary shadow password
1914 facility are unlikely to be supported.
1916 The $members value returned by I<getgr*()> is a space separated list of
1917 the login names of the members of the group.
1919 For the I<gethost*()> functions, if the C<h_errno> variable is supported in
1920 C, it will be returned to you via C<$?> if the function call fails. The
1921 C<@addrs> value returned by a successful call is a list of the raw
1922 addresses returned by the corresponding system library call. In the
1923 Internet domain, each address is four bytes long and you can unpack it
1924 by saying something like:
1926 ($a,$b,$c,$d) = unpack('C4',$addr[0]);
1928 The Socket library makes this slightly easier:
1931 $iaddr = inet_aton("127.1"); # or whatever address
1932 $name = gethostbyaddr($iaddr, AF_INET);
1934 # or going the other way
1935 $straddr = inet_ntoa($iaddr);
1937 If you get tired of remembering which element of the return list
1938 contains which return value, by-name interfaces are provided
1939 in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
1940 C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
1941 and C<User::grent>. These override the normal built-ins, supplying
1942 versions that return objects with the appropriate names
1943 for each field. For example:
1947 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
1949 Even though it looks like they're the same method calls (uid),
1950 they aren't, because a C<File::stat> object is different from
1951 a C<User::pwent> object.
1953 =item getsockname SOCKET
1955 Returns the packed sockaddr address of this end of the SOCKET connection,
1956 in case you don't know the address because you have several different
1957 IPs that the connection might have come in on.
1960 $mysockaddr = getsockname(SOCK);
1961 ($port, $myaddr) = sockaddr_in($mysockaddr);
1962 printf "Connect to %s [%s]\n",
1963 scalar gethostbyaddr($myaddr, AF_INET),
1966 =item getsockopt SOCKET,LEVEL,OPTNAME
1968 Returns the socket option requested, or undef if there is an error.
1974 Returns the value of EXPR with filename expansions such as the
1975 standard Unix shell F</bin/csh> would do. This is the internal function
1976 implementing the C<< <*.c> >> operator, but you can use it directly.
1977 If EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is
1978 discussed in more detail in L<perlop/"I/O Operators">.
1980 Beginning with v5.6.0, this operator is implemented using the standard
1981 C<File::Glob> extension. See L<File::Glob> for details.
1985 Converts a time as returned by the time function to a 8-element list
1986 with the time localized for the standard Greenwich time zone.
1987 Typically used as follows:
1990 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday) =
1993 All list elements are numeric, and come straight out of the C `struct
1994 tm'. $sec, $min, and $hour are the seconds, minutes, and hours of the
1995 specified time. $mday is the day of the month, and $mon is the month
1996 itself, in the range C<0..11> with 0 indicating January and 11
1997 indicating December. $year is the number of years since 1900. That
1998 is, $year is C<123> in year 2023. $wday is the day of the week, with
1999 0 indicating Sunday and 3 indicating Wednesday. $yday is the day of
2000 the year, in the range C<0..364> (or C<0..365> in leap years.)
2002 Note that the $year element is I<not> simply the last two digits of
2003 the year. If you assume it is, then you create non-Y2K-compliant
2004 programs--and you wouldn't want to do that, would you?
2006 The proper way to get a complete 4-digit year is simply:
2010 And to get the last two digits of the year (e.g., '01' in 2001) do:
2012 $year = sprintf("%02d", $year % 100);
2014 If EXPR is omitted, C<gmtime()> uses the current time (C<gmtime(time)>).
2016 In scalar context, C<gmtime()> returns the ctime(3) value:
2018 $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
2020 Also see the C<timegm> function provided by the C<Time::Local> module,
2021 and the strftime(3) function available via the POSIX module.
2023 This scalar value is B<not> locale dependent (see L<perllocale>), but
2024 is instead a Perl builtin. Also see the C<Time::Local> module, and the
2025 strftime(3) and mktime(3) functions available via the POSIX module. To
2026 get somewhat similar but locale dependent date strings, set up your
2027 locale environment variables appropriately (please see L<perllocale>)
2028 and try for example:
2030 use POSIX qw(strftime);
2031 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
2033 Note that the C<%a> and C<%b> escapes, which represent the short forms
2034 of the day of the week and the month of the year, may not necessarily
2035 be three characters wide in all locales.
2043 The C<goto-LABEL> form finds the statement labeled with LABEL and resumes
2044 execution there. It may not be used to go into any construct that
2045 requires initialization, such as a subroutine or a C<foreach> loop. It
2046 also can't be used to go into a construct that is optimized away,
2047 or to get out of a block or subroutine given to C<sort>.
2048 It can be used to go almost anywhere else within the dynamic scope,
2049 including out of subroutines, but it's usually better to use some other
2050 construct such as C<last> or C<die>. The author of Perl has never felt the
2051 need to use this form of C<goto> (in Perl, that is--C is another matter).
2053 The C<goto-EXPR> form expects a label name, whose scope will be resolved
2054 dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
2055 necessarily recommended if you're optimizing for maintainability:
2057 goto ("FOO", "BAR", "GLARCH")[$i];
2059 The C<goto-&NAME> form is quite different from the other forms of C<goto>.
2060 In fact, it isn't a goto in the normal sense at all, and doesn't have
2061 the stigma associated with other gotos. Instead, it
2062 substitutes a call to the named subroutine for the currently running
2063 subroutine. This is used by C<AUTOLOAD> subroutines that wish to load
2064 another subroutine and then pretend that the other subroutine had been
2065 called in the first place (except that any modifications to C<@_>
2066 in the current subroutine are propagated to the other subroutine.)
2067 After the C<goto>, not even C<caller> will be able to tell that this
2068 routine was called first.
2070 NAME needn't be the name of a subroutine; it can be a scalar variable
2071 containing a code reference, or a block which evaluates to a code
2074 =item grep BLOCK LIST
2076 =item grep EXPR,LIST
2078 This is similar in spirit to, but not the same as, grep(1) and its
2079 relatives. In particular, it is not limited to using regular expressions.
2081 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2082 C<$_> to each element) and returns the list value consisting of those
2083 elements for which the expression evaluated to true. In scalar
2084 context, returns the number of times the expression was true.
2086 @foo = grep(!/^#/, @bar); # weed out comments
2090 @foo = grep {!/^#/} @bar; # weed out comments
2092 Note that C<$_> is an alias to the list value, so it can be used to
2093 modify the elements of the LIST. While this is useful and supported,
2094 it can cause bizarre results if the elements of LIST are not variables.
2095 Similarly, grep returns aliases into the original list, much as a for
2096 loop's index variable aliases the list elements. That is, modifying an
2097 element of a list returned by grep (for example, in a C<foreach>, C<map>
2098 or another C<grep>) actually modifies the element in the original list.
2099 This is usually something to be avoided when writing clear code.
2101 See also L</map> for a list composed of the results of the BLOCK or EXPR.
2107 Interprets EXPR as a hex string and returns the corresponding value.
2108 (To convert strings that might start with either 0, 0x, or 0b, see
2109 L</oct>.) If EXPR is omitted, uses C<$_>.
2111 print hex '0xAf'; # prints '175'
2112 print hex 'aF'; # same
2114 Hex strings may only represent integers. Strings that would cause
2115 integer overflow trigger a warning.
2119 There is no builtin C<import> function. It is just an ordinary
2120 method (subroutine) defined (or inherited) by modules that wish to export
2121 names to another module. The C<use> function calls the C<import> method
2122 for the package used. See also L</use()>, L<perlmod>, and L<Exporter>.
2124 =item index STR,SUBSTR,POSITION
2126 =item index STR,SUBSTR
2128 The index function searches for one string within another, but without
2129 the wildcard-like behavior of a full regular-expression pattern match.
2130 It returns the position of the first occurrence of SUBSTR in STR at
2131 or after POSITION. If POSITION is omitted, starts searching from the
2132 beginning of the string. The return value is based at C<0> (or whatever
2133 you've set the C<$[> variable to--but don't do that). If the substring
2134 is not found, returns one less than the base, ordinarily C<-1>.
2140 Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
2141 You should not use this function for rounding: one because it truncates
2142 towards C<0>, and two because machine representations of floating point
2143 numbers can sometimes produce counterintuitive results. For example,
2144 C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
2145 because it's really more like -268.99999999999994315658 instead. Usually,
2146 the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
2147 functions will serve you better than will int().
2149 =item ioctl FILEHANDLE,FUNCTION,SCALAR
2151 Implements the ioctl(2) function. You'll probably first have to say
2153 require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph
2155 to get the correct function definitions. If F<ioctl.ph> doesn't
2156 exist or doesn't have the correct definitions you'll have to roll your
2157 own, based on your C header files such as F<< <sys/ioctl.h> >>.
2158 (There is a Perl script called B<h2ph> that comes with the Perl kit that
2159 may help you in this, but it's nontrivial.) SCALAR will be read and/or
2160 written depending on the FUNCTION--a pointer to the string value of SCALAR
2161 will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
2162 has no string value but does have a numeric value, that value will be
2163 passed rather than a pointer to the string value. To guarantee this to be
2164 true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
2165 functions may be needed to manipulate the values of structures used by
2168 The return value of C<ioctl> (and C<fcntl>) is as follows:
2170 if OS returns: then Perl returns:
2172 0 string "0 but true"
2173 anything else that number
2175 Thus Perl returns true on success and false on failure, yet you can
2176 still easily determine the actual value returned by the operating
2179 $retval = ioctl(...) || -1;
2180 printf "System returned %d\n", $retval;
2182 The special string "C<0> but true" is exempt from B<-w> complaints
2183 about improper numeric conversions.
2185 Here's an example of setting a filehandle named C<REMOTE> to be
2186 non-blocking at the system level. You'll have to negotiate C<$|>
2187 on your own, though.
2189 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
2191 $flags = fcntl(REMOTE, F_GETFL, 0)
2192 or die "Can't get flags for the socket: $!\n";
2194 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
2195 or die "Can't set flags for the socket: $!\n";
2197 =item join EXPR,LIST
2199 Joins the separate strings of LIST into a single string with fields
2200 separated by the value of EXPR, and returns that new string. Example:
2202 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
2204 Beware that unlike C<split>, C<join> doesn't take a pattern as its
2205 first argument. Compare L</split>.
2209 Returns a list consisting of all the keys of the named hash. (In
2210 scalar context, returns the number of keys.) The keys are returned in
2211 an apparently random order. The actual random order is subject to
2212 change in future versions of perl, but it is guaranteed to be the same
2213 order as either the C<values> or C<each> function produces (given
2214 that the hash has not been modified). As a side effect, it resets
2217 Here is yet another way to print your environment:
2220 @values = values %ENV;
2222 print pop(@keys), '=', pop(@values), "\n";
2225 or how about sorted by key:
2227 foreach $key (sort(keys %ENV)) {
2228 print $key, '=', $ENV{$key}, "\n";
2231 The returned values are copies of the original keys in the hash, so
2232 modifying them will not affect the original hash. Compare L</values>.
2234 To sort a hash by value, you'll need to use a C<sort> function.
2235 Here's a descending numeric sort of a hash by its values:
2237 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
2238 printf "%4d %s\n", $hash{$key}, $key;
2241 As an lvalue C<keys> allows you to increase the number of hash buckets
2242 allocated for the given hash. This can gain you a measure of efficiency if
2243 you know the hash is going to get big. (This is similar to pre-extending
2244 an array by assigning a larger number to $#array.) If you say
2248 then C<%hash> will have at least 200 buckets allocated for it--256 of them,
2249 in fact, since it rounds up to the next power of two. These
2250 buckets will be retained even if you do C<%hash = ()>, use C<undef
2251 %hash> if you want to free the storage while C<%hash> is still in scope.
2252 You can't shrink the number of buckets allocated for the hash using
2253 C<keys> in this way (but you needn't worry about doing this by accident,
2254 as trying has no effect).
2256 See also C<each>, C<values> and C<sort>.
2258 =item kill SIGNAL, LIST
2260 Sends a signal to a list of processes. Returns the number of
2261 processes successfully signaled (which is not necessarily the
2262 same as the number actually killed).
2264 $cnt = kill 1, $child1, $child2;
2267 If SIGNAL is zero, no signal is sent to the process. This is a
2268 useful way to check that the process is alive and hasn't changed
2269 its UID. See L<perlport> for notes on the portability of this
2272 Unlike in the shell, if SIGNAL is negative, it kills
2273 process groups instead of processes. (On System V, a negative I<PROCESS>
2274 number will also kill process groups, but that's not portable.) That
2275 means you usually want to use positive not negative signals. You may also
2276 use a signal name in quotes. See L<perlipc/"Signals"> for details.
2282 The C<last> command is like the C<break> statement in C (as used in
2283 loops); it immediately exits the loop in question. If the LABEL is
2284 omitted, the command refers to the innermost enclosing loop. The
2285 C<continue> block, if any, is not executed:
2287 LINE: while (<STDIN>) {
2288 last LINE if /^$/; # exit when done with header
2292 C<last> cannot be used to exit a block which returns a value such as
2293 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2294 a grep() or map() operation.
2296 Note that a block by itself is semantically identical to a loop
2297 that executes once. Thus C<last> can be used to effect an early
2298 exit out of such a block.
2300 See also L</continue> for an illustration of how C<last>, C<next>, and
2307 Returns an lowercased version of EXPR. This is the internal function
2308 implementing the C<\L> escape in double-quoted strings.
2309 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
2312 If EXPR is omitted, uses C<$_>.
2318 Returns the value of EXPR with the first character lowercased. This is
2319 the internal function implementing the C<\l> escape in double-quoted strings.
2320 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
2322 If EXPR is omitted, uses C<$_>.
2328 Returns the length in characters of the value of EXPR. If EXPR is
2329 omitted, returns length of C<$_>. Note that this cannot be used on
2330 an entire array or hash to find out how many elements these have.
2331 For that, use C<scalar @array> and C<scalar keys %hash> respectively.
2333 =item link OLDFILE,NEWFILE
2335 Creates a new filename linked to the old filename. Returns true for
2336 success, false otherwise.
2338 =item listen SOCKET,QUEUESIZE
2340 Does the same thing that the listen system call does. Returns true if
2341 it succeeded, false otherwise. See the example in L<perlipc/"Sockets: Client/Server Communication">.
2345 You really probably want to be using C<my> instead, because C<local> isn't
2346 what most people think of as "local". See
2347 L<perlsub/"Private Variables via my()"> for details.
2349 A local modifies the listed variables to be local to the enclosing
2350 block, file, or eval. If more than one value is listed, the list must
2351 be placed in parentheses. See L<perlsub/"Temporary Values via local()">
2352 for details, including issues with tied arrays and hashes.
2354 =item localtime EXPR
2356 Converts a time as returned by the time function to a 9-element list
2357 with the time analyzed for the local time zone. Typically used as
2361 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
2364 All list elements are numeric, and come straight out of the C `struct
2365 tm'. $sec, $min, and $hour are the seconds, minutes, and hours of the
2366 specified time. $mday is the day of the month, and $mon is the month
2367 itself, in the range C<0..11> with 0 indicating January and 11
2368 indicating December. $year is the number of years since 1900. That
2369 is, $year is C<123> in year 2023. $wday is the day of the week, with
2370 0 indicating Sunday and 3 indicating Wednesday. $yday is the day of
2371 the year, in the range C<0..364> (or C<0..365> in leap years.) $isdst
2372 is true if the specified time occurs during daylight savings time,
2375 Note that the $year element is I<not> simply the last two digits of
2376 the year. If you assume it is, then you create non-Y2K-compliant
2377 programs--and you wouldn't want to do that, would you?
2379 The proper way to get a complete 4-digit year is simply:
2383 And to get the last two digits of the year (e.g., '01' in 2001) do:
2385 $year = sprintf("%02d", $year % 100);
2387 If EXPR is omitted, C<localtime()> uses the current time (C<localtime(time)>).
2389 In scalar context, C<localtime()> returns the ctime(3) value:
2391 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
2393 This scalar value is B<not> locale dependent, see L<perllocale>, but
2394 instead a Perl builtin. Also see the C<Time::Local> module
2395 (to convert the second, minutes, hours, ... back to seconds since the
2396 stroke of midnight the 1st of January 1970, the value returned by
2397 time()), and the strftime(3) and mktime(3) functions available via the
2398 POSIX module. To get somewhat similar but locale dependent date
2399 strings, set up your locale environment variables appropriately
2400 (please see L<perllocale>) and try for example:
2402 use POSIX qw(strftime);
2403 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
2405 Note that the C<%a> and C<%b>, the short forms of the day of the week
2406 and the month of the year, may not necessarily be three characters wide.
2412 This function places an advisory lock on a variable, subroutine,
2413 or referenced object contained in I<THING> until the lock goes out
2414 of scope. This is a built-in function only if your version of Perl
2415 was built with threading enabled, and if you've said C<use Threads>.
2416 Otherwise a user-defined function by this name will be called. See
2423 Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
2424 returns log of C<$_>. To get the log of another base, use basic algebra:
2425 The base-N log of a number is equal to the natural log of that number
2426 divided by the natural log of N. For example:
2430 return log($n)/log(10);
2433 See also L</exp> for the inverse operation.
2439 Does the same thing as the C<stat> function (including setting the
2440 special C<_> filehandle) but stats a symbolic link instead of the file
2441 the symbolic link points to. If symbolic links are unimplemented on
2442 your system, a normal C<stat> is done.
2444 If EXPR is omitted, stats C<$_>.
2448 The match operator. See L<perlop>.
2450 =item map BLOCK LIST
2454 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2455 C<$_> to each element) and returns the list value composed of the
2456 results of each such evaluation. In scalar context, returns the
2457 total number of elements so generated. Evaluates BLOCK or EXPR in
2458 list context, so each element of LIST may produce zero, one, or
2459 more elements in the returned value.
2461 @chars = map(chr, @nums);
2463 translates a list of numbers to the corresponding characters. And
2465 %hash = map { getkey($_) => $_ } @array;
2467 is just a funny way to write
2470 foreach $_ (@array) {
2471 $hash{getkey($_)} = $_;
2474 Note that C<$_> is an alias to the list value, so it can be used to
2475 modify the elements of the LIST. While this is useful and supported,
2476 it can cause bizarre results if the elements of LIST are not variables.
2477 Using a regular C<foreach> loop for this purpose would be clearer in
2478 most cases. See also L</grep> for an array composed of those items of
2479 the original list for which the BLOCK or EXPR evaluates to true.
2481 C<{> starts both hash references and blocks, so C<map { ...> could be either
2482 the start of map BLOCK LIST or map EXPR, LIST. Because perl doesn't look
2483 ahead for the closing C<}> it has to take a guess at which its dealing with
2484 based what it finds just after the C<{>. Usually it gets it right, but if it
2485 doesn't it won't realize something is wrong until it gets to the C<}> and
2486 encounters the missing (or unexpected) comma. The syntax error will be
2487 reported close to the C<}> but you'll need to change something near the C<{>
2488 such as using a unary C<+> to give perl some help:
2490 %hash = map { "\L$_", 1 } @array # perl guesses EXPR. wrong
2491 %hash = map { +"\L$_", 1 } @array # perl guesses BLOCK. right
2492 %hash = map { ("\L$_", 1) } @array # this also works
2493 %hash = map { lc($_), 1 } @array # as does this.
2494 %hash = map +( lc($_), 1 ), @array # this is EXPR and works!
2496 %hash = map ( lc($_), 1 ), @array # evaluates to (1, @array)
2498 or to force an anon hash constructor use C<+{>
2500 @hashes = map +{ lc($_), 1 }, @array # EXPR, so needs , at end
2502 and you get list of anonymous hashes each with only 1 entry.
2504 =item mkdir FILENAME,MASK
2506 =item mkdir FILENAME
2508 Creates the directory specified by FILENAME, with permissions
2509 specified by MASK (as modified by C<umask>). If it succeeds it
2510 returns true, otherwise it returns false and sets C<$!> (errno).
2511 If omitted, MASK defaults to 0777.
2513 In general, it is better to create directories with permissive MASK,
2514 and let the user modify that with their C<umask>, than it is to supply
2515 a restrictive MASK and give the user no way to be more permissive.
2516 The exceptions to this rule are when the file or directory should be
2517 kept private (mail files, for instance). The perlfunc(1) entry on
2518 C<umask> discusses the choice of MASK in more detail.
2520 Note that according to the POSIX 1003.1-1996 the FILENAME may have any
2521 number of trailing slashes. Some operating and filesystems do not get
2522 this right, so Perl automatically removes all trailing slashes to keep
2525 =item msgctl ID,CMD,ARG
2527 Calls the System V IPC function msgctl(2). You'll probably have to say
2531 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
2532 then ARG must be a variable which will hold the returned C<msqid_ds>
2533 structure. Returns like C<ioctl>: the undefined value for error,
2534 C<"0 but true"> for zero, or the actual return value otherwise. See also
2535 L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::Semaphore> documentation.
2537 =item msgget KEY,FLAGS
2539 Calls the System V IPC function msgget(2). Returns the message queue
2540 id, or the undefined value if there is an error. See also
2541 L<perlipc/"SysV IPC"> and C<IPC::SysV> and C<IPC::Msg> documentation.
2543 =item msgrcv ID,VAR,SIZE,TYPE,FLAGS
2545 Calls the System V IPC function msgrcv to receive a message from
2546 message queue ID into variable VAR with a maximum message size of
2547 SIZE. Note that when a message is received, the message type as a
2548 native long integer will be the first thing in VAR, followed by the
2549 actual message. This packing may be opened with C<unpack("l! a*")>.
2550 Taints the variable. Returns true if successful, or false if there is
2551 an error. See also L<perlipc/"SysV IPC">, C<IPC::SysV>, and
2552 C<IPC::SysV::Msg> documentation.
2554 =item msgsnd ID,MSG,FLAGS
2556 Calls the System V IPC function msgsnd to send the message MSG to the
2557 message queue ID. MSG must begin with the native long integer message
2558 type, and be followed by the length of the actual message, and finally
2559 the message itself. This kind of packing can be achieved with
2560 C<pack("l! a*", $type, $message)>. Returns true if successful,
2561 or false if there is an error. See also C<IPC::SysV>
2562 and C<IPC::SysV::Msg> documentation.
2566 =item my EXPR : ATTRIBUTES
2568 A C<my> declares the listed variables to be local (lexically) to the
2569 enclosing block, file, or C<eval>. If
2570 more than one value is listed, the list must be placed in parentheses. See
2571 L<perlsub/"Private Variables via my()"> for details.
2577 The C<next> command is like the C<continue> statement in C; it starts
2578 the next iteration of the loop:
2580 LINE: while (<STDIN>) {
2581 next LINE if /^#/; # discard comments
2585 Note that if there were a C<continue> block on the above, it would get
2586 executed even on discarded lines. If the LABEL is omitted, the command
2587 refers to the innermost enclosing loop.
2589 C<next> cannot be used to exit a block which returns a value such as
2590 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2591 a grep() or map() operation.
2593 Note that a block by itself is semantically identical to a loop
2594 that executes once. Thus C<next> will exit such a block early.
2596 See also L</continue> for an illustration of how C<last>, C<next>, and
2599 =item no Module LIST
2601 See the L</use> function, which C<no> is the opposite of.
2607 Interprets EXPR as an octal string and returns the corresponding
2608 value. (If EXPR happens to start off with C<0x>, interprets it as a
2609 hex string. If EXPR starts off with C<0b>, it is interpreted as a
2610 binary string.) The following will handle decimal, binary, octal, and
2611 hex in the standard Perl or C notation:
2613 $val = oct($val) if $val =~ /^0/;
2615 If EXPR is omitted, uses C<$_>. To go the other way (produce a number
2616 in octal), use sprintf() or printf():
2618 $perms = (stat("filename"))[2] & 07777;
2619 $oct_perms = sprintf "%lo", $perms;
2621 The oct() function is commonly used when a string such as C<644> needs
2622 to be converted into a file mode, for example. (Although perl will
2623 automatically convert strings into numbers as needed, this automatic
2624 conversion assumes base 10.)
2626 =item open FILEHANDLE,MODE,LIST
2628 =item open FILEHANDLE,EXPR
2630 =item open FILEHANDLE
2632 Opens the file whose filename is given by EXPR, and associates it with
2633 FILEHANDLE. If FILEHANDLE is an expression, its value is used as the
2634 name of the real filehandle wanted. (This is considered a symbolic
2635 reference, so C<use strict 'refs'> should I<not> be in effect.)
2637 If EXPR is omitted, the scalar
2638 variable of the same name as the FILEHANDLE contains the filename.
2639 (Note that lexical variables--those declared with C<my>--will not work
2640 for this purpose; so if you're using C<my>, specify EXPR in your call
2641 to open.) See L<perlopentut> for a kinder, gentler explanation of opening
2644 If MODE is C<< '<' >> or nothing, the file is opened for input.
2645 If MODE is C<< '>' >>, the file is truncated and opened for
2646 output, being created if necessary. If MODE is C<<< '>>' >>>,
2647 the file is opened for appending, again being created if necessary.
2648 You can put a C<'+'> in front of the C<< '>' >> or C<< '<' >> to indicate that
2649 you want both read and write access to the file; thus C<< '+<' >> is almost
2650 always preferred for read/write updates--the C<< '+>' >> mode would clobber the
2651 file first. You can't usually use either read-write mode for updating
2652 textfiles, since they have variable length records. See the B<-i>
2653 switch in L<perlrun> for a better approach. The file is created with
2654 permissions of C<0666> modified by the process' C<umask> value.
2656 These various prefixes correspond to the fopen(3) modes of C<'r'>, C<'r+'>,
2657 C<'w'>, C<'w+'>, C<'a'>, and C<'a+'>.
2659 In the 2-arguments (and 1-argument) form of the call the mode and
2660 filename should be concatenated (in this order), possibly separated by
2661 spaces. It is possible to omit the mode if the mode is C<< '<' >>.
2663 If the filename begins with C<'|'>, the filename is interpreted as a
2664 command to which output is to be piped, and if the filename ends with a
2665 C<'|'>, the filename is interpreted as a command which pipes output to
2666 us. See L<perlipc/"Using open() for IPC">
2667 for more examples of this. (You are not allowed to C<open> to a command
2668 that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
2669 and L<perlipc/"Bidirectional Communication with Another Process">
2672 If MODE is C<'|-'>, the filename is interpreted as a
2673 command to which output is to be piped, and if MODE is
2674 C<'-|'>, the filename is interpreted as a command which pipes output to
2675 us. In the 2-arguments (and 1-argument) form one should replace dash
2676 (C<'-'>) with the command. See L<perlipc/"Using open() for IPC">
2677 for more examples of this. (You are not allowed to C<open> to a command
2678 that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
2679 and L<perlipc/"Bidirectional Communication"> for alternatives.)
2681 In the 2-arguments (and 1-argument) form opening C<'-'> opens STDIN
2682 and opening C<< '>-' >> opens STDOUT.
2685 nonzero upon success, the undefined value otherwise. If the C<open>
2686 involved a pipe, the return value happens to be the pid of the
2689 If you're unfortunate enough to be running Perl on a system that
2690 distinguishes between text files and binary files (modern operating
2691 systems don't care), then you should check out L</binmode> for tips for
2692 dealing with this. The key distinction between systems that need C<binmode>
2693 and those that don't is their text file formats. Systems like Unix, MacOS, and
2694 Plan9, which delimit lines with a single character, and which encode that
2695 character in C as C<"\n">, do not need C<binmode>. The rest need it.
2697 When opening a file, it's usually a bad idea to continue normal execution
2698 if the request failed, so C<open> is frequently used in connection with
2699 C<die>. Even if C<die> won't do what you want (say, in a CGI script,
2700 where you want to make a nicely formatted error message (but there are
2701 modules that can help with that problem)) you should always check
2702 the return value from opening a file. The infrequent exception is when
2703 working with an unopened filehandle is actually what you want to do.
2708 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
2709 while (<ARTICLE>) {...
2711 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
2712 # if the open fails, output is discarded
2714 open(DBASE, '+<', 'dbase.mine') # open for update
2715 or die "Can't open 'dbase.mine' for update: $!";
2717 open(DBASE, '+<dbase.mine') # ditto
2718 or die "Can't open 'dbase.mine' for update: $!";
2720 open(ARTICLE, '-|', "caesar <$article") # decrypt article
2721 or die "Can't start caesar: $!";
2723 open(ARTICLE, "caesar <$article |") # ditto
2724 or die "Can't start caesar: $!";
2726 open(EXTRACT, "|sort >/tmp/Tmp$$") # $$ is our process id
2727 or die "Can't start sort: $!";
2729 # process argument list of files along with any includes
2731 foreach $file (@ARGV) {
2732 process($file, 'fh00');
2736 my($filename, $input) = @_;
2737 $input++; # this is a string increment
2738 unless (open($input, $filename)) {
2739 print STDERR "Can't open $filename: $!\n";
2744 while (<$input>) { # note use of indirection
2745 if (/^#include "(.*)"/) {
2746 process($1, $input);
2753 You may also, in the Bourne shell tradition, specify an EXPR beginning
2754 with C<< '>&' >>, in which case the rest of the string is interpreted as the
2755 name of a filehandle (or file descriptor, if numeric) to be
2756 duped and opened. You may use C<&> after C<< > >>, C<<< >> >>>,
2757 C<< < >>, C<< +> >>, C<<< +>> >>>, and C<< +< >>. The
2758 mode you specify should match the mode of the original filehandle.
2759 (Duping a filehandle does not take into account any existing contents of
2760 stdio buffers.) Duping file handles is not yet supported for 3-argument
2763 Here is a script that saves, redirects, and restores STDOUT and
2767 open(OLDOUT, ">&STDOUT");
2768 open(OLDERR, ">&STDERR");
2770 open(STDOUT, '>', "foo.out") || die "Can't redirect stdout";
2771 open(STDERR, ">&STDOUT") || die "Can't dup stdout";
2773 select(STDERR); $| = 1; # make unbuffered
2774 select(STDOUT); $| = 1; # make unbuffered
2776 print STDOUT "stdout 1\n"; # this works for
2777 print STDERR "stderr 1\n"; # subprocesses too
2782 open(STDOUT, ">&OLDOUT");
2783 open(STDERR, ">&OLDERR");
2785 print STDOUT "stdout 2\n";
2786 print STDERR "stderr 2\n";
2788 If you specify C<< '<&=N' >>, where C<N> is a number, then Perl will do an
2789 equivalent of C's C<fdopen> of that file descriptor; this is more
2790 parsimonious of file descriptors. For example:
2792 open(FILEHANDLE, "<&=$fd")
2794 Note that this feature depends on the fdopen() C library function.
2795 On many UNIX systems, fdopen() is known to fail when file descriptors
2796 exceed a certain value, typically 255. If you need more file
2797 descriptors than that, consider rebuilding Perl to use the C<sfio>
2800 If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'>
2801 with 2-arguments (or 1-argument) form of open(), then
2802 there is an implicit fork done, and the return value of open is the pid
2803 of the child within the parent process, and C<0> within the child
2804 process. (Use C<defined($pid)> to determine whether the open was successful.)
2805 The filehandle behaves normally for the parent, but i/o to that
2806 filehandle is piped from/to the STDOUT/STDIN of the child process.
2807 In the child process the filehandle isn't opened--i/o happens from/to
2808 the new STDOUT or STDIN. Typically this is used like the normal
2809 piped open when you want to exercise more control over just how the
2810 pipe command gets executed, such as when you are running setuid, and
2811 don't want to have to scan shell commands for metacharacters.
2812 The following triples are more or less equivalent:
2814 open(FOO, "|tr '[a-z]' '[A-Z]'");
2815 open(FOO, '|-', "tr '[a-z]' '[A-Z]'");
2816 open(FOO, '|-') || exec 'tr', '[a-z]', '[A-Z]';
2818 open(FOO, "cat -n '$file'|");
2819 open(FOO, '-|', "cat -n '$file'");
2820 open(FOO, '-|') || exec 'cat', '-n', $file;
2822 See L<perlipc/"Safe Pipe Opens"> for more examples of this.
2824 Beginning with v5.6.0, Perl will attempt to flush all files opened for
2825 output before any operation that may do a fork, but this may not be
2826 supported on some platforms (see L<perlport>). To be safe, you may need
2827 to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
2828 of C<IO::Handle> on any open handles.
2830 On systems that support a
2831 close-on-exec flag on files, the flag will be set for the newly opened
2832 file descriptor as determined by the value of $^F. See L<perlvar/$^F>.
2834 Closing any piped filehandle causes the parent process to wait for the
2835 child to finish, and returns the status value in C<$?>.
2837 The filename passed to 2-argument (or 1-argument) form of open()
2838 will have leading and trailing
2839 whitespace deleted, and the normal redirection characters
2840 honored. This property, known as "magic open",
2841 can often be used to good effect. A user could specify a filename of
2842 F<"rsh cat file |">, or you could change certain filenames as needed:
2844 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
2845 open(FH, $filename) or die "Can't open $filename: $!";
2847 Use 3-argument form to open a file with arbitrary weird characters in it,
2849 open(FOO, '<', $file);
2851 otherwise it's necessary to protect any leading and trailing whitespace:
2853 $file =~ s#^(\s)#./$1#;
2854 open(FOO, "< $file\0");
2856 (this may not work on some bizarre filesystems). One should
2857 conscientiously choose between the I<magic> and 3-arguments form
2862 will allow the user to specify an argument of the form C<"rsh cat file |">,
2863 but will not work on a filename which happens to have a trailing space, while
2865 open IN, '<', $ARGV[0];
2867 will have exactly the opposite restrictions.
2869 If you want a "real" C C<open> (see L<open(2)> on your system), then you
2870 should use the C<sysopen> function, which involves no such magic (but
2871 may use subtly different filemodes than Perl open(), which is mapped
2872 to C fopen()). This is
2873 another way to protect your filenames from interpretation. For example:
2876 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
2877 or die "sysopen $path: $!";
2878 $oldfh = select(HANDLE); $| = 1; select($oldfh);
2879 print HANDLE "stuff $$\n";
2881 print "File contains: ", <HANDLE>;
2883 Using the constructor from the C<IO::Handle> package (or one of its
2884 subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous
2885 filehandles that have the scope of whatever variables hold references to
2886 them, and automatically close whenever and however you leave that scope:
2890 sub read_myfile_munged {
2892 my $handle = new IO::File;
2893 open($handle, "myfile") or die "myfile: $!";
2895 or return (); # Automatically closed here.
2896 mung $first or die "mung failed"; # Or here.
2897 return $first, <$handle> if $ALL; # Or here.
2901 See L</seek> for some details about mixing reading and writing.
2903 =item opendir DIRHANDLE,EXPR
2905 Opens a directory named EXPR for processing by C<readdir>, C<telldir>,
2906 C<seekdir>, C<rewinddir>, and C<closedir>. Returns true if successful.
2907 DIRHANDLEs have their own namespace separate from FILEHANDLEs.
2913 Returns the numeric (ASCII or Unicode) value of the first character of EXPR. If
2914 EXPR is omitted, uses C<$_>. For the reverse, see L</chr>.
2915 See L<utf8> for more about Unicode.
2919 An C<our> declares the listed variables to be valid globals within
2920 the enclosing block, file, or C<eval>. That is, it has the same
2921 scoping rules as a "my" declaration, but does not create a local
2922 variable. If more than one value is listed, the list must be placed
2923 in parentheses. The C<our> declaration has no semantic effect unless
2924 "use strict vars" is in effect, in which case it lets you use the
2925 declared global variable without qualifying it with a package name.
2926 (But only within the lexical scope of the C<our> declaration. In this
2927 it differs from "use vars", which is package scoped.)
2929 An C<our> declaration declares a global variable that will be visible
2930 across its entire lexical scope, even across package boundaries. The
2931 package in which the variable is entered is determined at the point
2932 of the declaration, not at the point of use. This means the following
2936 our $bar; # declares $Foo::bar for rest of lexical scope
2940 print $bar; # prints 20
2942 Multiple C<our> declarations in the same lexical scope are allowed
2943 if they are in different packages. If they happened to be in the same
2944 package, Perl will emit warnings if you have asked for them.
2948 our $bar; # declares $Foo::bar for rest of lexical scope
2952 our $bar = 30; # declares $Bar::bar for rest of lexical scope
2953 print $bar; # prints 30
2955 our $bar; # emits warning
2957 =item pack TEMPLATE,LIST
2959 Takes a LIST of values and converts it into a string using the rules
2960 given by the TEMPLATE. The resulting string is the concatenation of
2961 the converted values. Typically, each converted value looks
2962 like its machine-level representation. For example, on 32-bit machines
2963 a converted integer may be represented by a sequence of 4 bytes.
2966 sequence of characters that give the order and type of values, as
2969 a A string with arbitrary binary data, will be null padded.
2970 A An ASCII string, will be space padded.
2971 Z A null terminated (asciz) string, will be null padded.
2973 b A bit string (ascending bit order inside each byte, like vec()).
2974 B A bit string (descending bit order inside each byte).
2975 h A hex string (low nybble first).
2976 H A hex string (high nybble first).
2978 c A signed char value.
2979 C An unsigned char value. Only does bytes. See U for Unicode.
2981 s A signed short value.
2982 S An unsigned short value.
2983 (This 'short' is _exactly_ 16 bits, which may differ from
2984 what a local C compiler calls 'short'. If you want
2985 native-length shorts, use the '!' suffix.)
2987 i A signed integer value.
2988 I An unsigned integer value.
2989 (This 'integer' is _at_least_ 32 bits wide. Its exact
2990 size depends on what a local C compiler calls 'int',
2991 and may even be larger than the 'long' described in
2994 l A signed long value.
2995 L An unsigned long value.
2996 (This 'long' is _exactly_ 32 bits, which may differ from
2997 what a local C compiler calls 'long'. If you want
2998 native-length longs, use the '!' suffix.)
3000 n An unsigned short in "network" (big-endian) order.
3001 N An unsigned long in "network" (big-endian) order.
3002 v An unsigned short in "VAX" (little-endian) order.
3003 V An unsigned long in "VAX" (little-endian) order.
3004 (These 'shorts' and 'longs' are _exactly_ 16 bits and
3005 _exactly_ 32 bits, respectively.)
3007 q A signed quad (64-bit) value.
3008 Q An unsigned quad value.
3009 (Quads are available only if your system supports 64-bit
3010 integer values _and_ if Perl has been compiled to support those.
3011 Causes a fatal error otherwise.)
3013 f A single-precision float in the native format.
3014 d A double-precision float in the native format.
3016 p A pointer to a null-terminated string.
3017 P A pointer to a structure (fixed-length string).
3019 u A uuencoded string.
3020 U A Unicode character number. Encodes to UTF-8 internally.
3021 Works even if C<use utf8> is not in effect.
3023 w A BER compressed integer. Its bytes represent an unsigned
3024 integer in base 128, most significant digit first, with as
3025 few digits as possible. Bit eight (the high bit) is set
3026 on each byte except the last.
3030 @ Null fill to absolute position.
3032 The following rules apply:
3038 Each letter may optionally be followed by a number giving a repeat
3039 count. With all types except C<a>, C<A>, C<Z>, C<b>, C<B>, C<h>,
3040 C<H>, and C<P> the pack function will gobble up that many values from
3041 the LIST. A C<*> for the repeat count means to use however many items are
3042 left, except for C<@>, C<x>, C<X>, where it is equivalent
3043 to C<0>, and C<u>, where it is equivalent to 1 (or 45, what is the
3046 When used with C<Z>, C<*> results in the addition of a trailing null
3047 byte (so the packed result will be one longer than the byte C<length>
3050 The repeat count for C<u> is interpreted as the maximal number of bytes
3051 to encode per line of output, with 0 and 1 replaced by 45.
3055 The C<a>, C<A>, and C<Z> types gobble just one value, but pack it as a
3056 string of length count, padding with nulls or spaces as necessary. When
3057 unpacking, C<A> strips trailing spaces and nulls, C<Z> strips everything
3058 after the first null, and C<a> returns data verbatim. When packing,
3059 C<a>, and C<Z> are equivalent.
3061 If the value-to-pack is too long, it is truncated. If too long and an
3062 explicit count is provided, C<Z> packs only C<$count-1> bytes, followed
3063 by a null byte. Thus C<Z> always packs a trailing null byte under
3068 Likewise, the C<b> and C<B> fields pack a string that many bits long.
3069 Each byte of the input field of pack() generates 1 bit of the result.
3070 Each result bit is based on the least-significant bit of the corresponding
3071 input byte, i.e., on C<ord($byte)%2>. In particular, bytes C<"0"> and
3072 C<"1"> generate bits 0 and 1, as do bytes C<"\0"> and C<"\1">.
3074 Starting from the beginning of the input string of pack(), each 8-tuple
3075 of bytes is converted to 1 byte of output. With format C<b>
3076 the first byte of the 8-tuple determines the least-significant bit of a
3077 byte, and with format C<B> it determines the most-significant bit of
3080 If the length of the input string is not exactly divisible by 8, the
3081 remainder is packed as if the input string were padded by null bytes
3082 at the end. Similarly, during unpack()ing the "extra" bits are ignored.
3084 If the input string of pack() is longer than needed, extra bytes are ignored.
3085 A C<*> for the repeat count of pack() means to use all the bytes of
3086 the input field. On unpack()ing the bits are converted to a string
3087 of C<"0">s and C<"1">s.
3091 The C<h> and C<H> fields pack a string that many nybbles (4-bit groups,
3092 representable as hexadecimal digits, 0-9a-f) long.
3094 Each byte of the input field of pack() generates 4 bits of the result.
3095 For non-alphabetical bytes the result is based on the 4 least-significant
3096 bits of the input byte, i.e., on C<ord($byte)%16>. In particular,
3097 bytes C<"0"> and C<"1"> generate nybbles 0 and 1, as do bytes
3098 C<"\0"> and C<"\1">. For bytes C<"a".."f"> and C<"A".."F"> the result
3099 is compatible with the usual hexadecimal digits, so that C<"a"> and
3100 C<"A"> both generate the nybble C<0xa==10>. The result for bytes
3101 C<"g".."z"> and C<"G".."Z"> is not well-defined.
3103 Starting from the beginning of the input string of pack(), each pair
3104 of bytes is converted to 1 byte of output. With format C<h> the
3105 first byte of the pair determines the least-significant nybble of the
3106 output byte, and with format C<H> it determines the most-significant
3109 If the length of the input string is not even, it behaves as if padded
3110 by a null byte at the end. Similarly, during unpack()ing the "extra"
3111 nybbles are ignored.
3113 If the input string of pack() is longer than needed, extra bytes are ignored.
3114 A C<*> for the repeat count of pack() means to use all the bytes of
3115 the input field. On unpack()ing the bits are converted to a string
3116 of hexadecimal digits.
3120 The C<p> type packs a pointer to a null-terminated string. You are
3121 responsible for ensuring the string is not a temporary value (which can
3122 potentially get deallocated before you get around to using the packed result).
3123 The C<P> type packs a pointer to a structure of the size indicated by the
3124 length. A NULL pointer is created if the corresponding value for C<p> or
3125 C<P> is C<undef>, similarly for unpack().
3129 The C</> template character allows packing and unpacking of strings where
3130 the packed structure contains a byte count followed by the string itself.
3131 You write I<length-item>C</>I<string-item>.
3133 The I<length-item> can be any C<pack> template letter,
3134 and describes how the length value is packed.
3135 The ones likely to be of most use are integer-packing ones like
3136 C<n> (for Java strings), C<w> (for ASN.1 or SNMP)
3137 and C<N> (for Sun XDR).
3139 The I<string-item> must, at present, be C<"A*">, C<"a*"> or C<"Z*">.
3140 For C<unpack> the length of the string is obtained from the I<length-item>,
3141 but if you put in the '*' it will be ignored.
3143 unpack 'C/a', "\04Gurusamy"; gives 'Guru'
3144 unpack 'a3/A* A*', '007 Bond J '; gives (' Bond','J')
3145 pack 'n/a* w/a*','hello,','world'; gives "\000\006hello,\005world"
3147 The I<length-item> is not returned explicitly from C<unpack>.
3149 Adding a count to the I<length-item> letter is unlikely to do anything
3150 useful, unless that letter is C<A>, C<a> or C<Z>. Packing with a
3151 I<length-item> of C<a> or C<Z> may introduce C<"\000"> characters,
3152 which Perl does not regard as legal in numeric strings.
3156 The integer types C<s>, C<S>, C<l>, and C<L> may be
3157 immediately followed by a C<!> suffix to signify native shorts or
3158 longs--as you can see from above for example a bare C<l> does mean
3159 exactly 32 bits, the native C<long> (as seen by the local C compiler)
3160 may be larger. This is an issue mainly in 64-bit platforms. You can
3161 see whether using C<!> makes any difference by
3163 print length(pack("s")), " ", length(pack("s!")), "\n";
3164 print length(pack("l")), " ", length(pack("l!")), "\n";
3166 C<i!> and C<I!> also work but only because of completeness;
3167 they are identical to C<i> and C<I>.
3169 The actual sizes (in bytes) of native shorts, ints, longs, and long
3170 longs on the platform where Perl was built are also available via
3174 print $Config{shortsize}, "\n";
3175 print $Config{intsize}, "\n";
3176 print $Config{longsize}, "\n";
3177 print $Config{longlongsize}, "\n";
3179 (The C<$Config{longlongsize}> will be undefine if your system does
3180 not support long longs.)
3184 The integer formats C<s>, C<S>, C<i>, C<I>, C<l>, and C<L>
3185 are inherently non-portable between processors and operating systems
3186 because they obey the native byteorder and endianness. For example a
3187 4-byte integer 0x12345678 (305419896 decimal) be ordered natively
3188 (arranged in and handled by the CPU registers) into bytes as
3190 0x12 0x34 0x56 0x78 # big-endian
3191 0x78 0x56 0x34 0x12 # little-endian
3193 Basically, the Intel and VAX CPUs are little-endian, while everybody
3194 else, for example Motorola m68k/88k, PPC, Sparc, HP PA, Power, and
3195 Cray are big-endian. Alpha and MIPS can be either: Digital/Compaq
3196 used/uses them in little-endian mode; SGI/Cray uses them in big-endian mode.
3198 The names `big-endian' and `little-endian' are comic references to
3199 the classic "Gulliver's Travels" (via the paper "On Holy Wars and a
3200 Plea for Peace" by Danny Cohen, USC/ISI IEN 137, April 1, 1980) and
3201 the egg-eating habits of the Lilliputians.
3203 Some systems may have even weirder byte orders such as
3208 You can see your system's preference with
3210 print join(" ", map { sprintf "%#02x", $_ }
3211 unpack("C*",pack("L",0x12345678))), "\n";
3213 The byteorder on the platform where Perl was built is also available
3217 print $Config{byteorder}, "\n";
3219 Byteorders C<'1234'> and C<'12345678'> are little-endian, C<'4321'>
3220 and C<'87654321'> are big-endian.
3222 If you want portable packed integers use the formats C<n>, C<N>,
3223 C<v>, and C<V>, their byte endianness and size is known.
3224 See also L<perlport>.
3228 Real numbers (floats and doubles) are in the native machine format only;
3229 due to the multiplicity of floating formats around, and the lack of a
3230 standard "network" representation, no facility for interchange has been
3231 made. This means that packed floating point data written on one machine
3232 may not be readable on another - even if both use IEEE floating point
3233 arithmetic (as the endian-ness of the memory representation is not part
3234 of the IEEE spec). See also L<perlport>.
3236 Note that Perl uses doubles internally for all numeric calculation, and
3237 converting from double into float and thence back to double again will
3238 lose precision (i.e., C<unpack("f", pack("f", $foo)>) will not in general
3243 If the pattern begins with a C<U>, the resulting string will be treated
3244 as Unicode-encoded. You can force UTF8 encoding on in a string with an
3245 initial C<U0>, and the bytes that follow will be interpreted as Unicode
3246 characters. If you don't want this to happen, you can begin your pattern
3247 with C<C0> (or anything else) to force Perl not to UTF8 encode your
3248 string, and then follow this with a C<U*> somewhere in your pattern.
3252 You must yourself do any alignment or padding by inserting for example
3253 enough C<'x'>es while packing. There is no way to pack() and unpack()
3254 could know where the bytes are going to or coming from. Therefore
3255 C<pack> (and C<unpack>) handle their output and input as flat
3260 A comment in a TEMPLATE starts with C<#> and goes to the end of line.
3264 If TEMPLATE requires more arguments to pack() than actually given, pack()
3265 assumes additional C<""> arguments. If TEMPLATE requires less arguments
3266 to pack() than actually given, extra arguments are ignored.
3272 $foo = pack("CCCC",65,66,67,68);
3274 $foo = pack("C4",65,66,67,68);
3276 $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);
3277 # same thing with Unicode circled letters
3279 $foo = pack("ccxxcc",65,66,67,68);
3282 # note: the above examples featuring "C" and "c" are true
3283 # only on ASCII and ASCII-derived systems such as ISO Latin 1
3284 # and UTF-8. In EBCDIC the first example would be
3285 # $foo = pack("CCCC",193,194,195,196);
3287 $foo = pack("s2",1,2);
3288 # "\1\0\2\0" on little-endian
3289 # "\0\1\0\2" on big-endian
3291 $foo = pack("a4","abcd","x","y","z");
3294 $foo = pack("aaaa","abcd","x","y","z");
3297 $foo = pack("a14","abcdefg");
3298 # "abcdefg\0\0\0\0\0\0\0"
3300 $foo = pack("i9pl", gmtime);
3301 # a real struct tm (on my system anyway)
3303 $utmp_template = "Z8 Z8 Z16 L";
3304 $utmp = pack($utmp_template, @utmp1);
3305 # a struct utmp (BSDish)
3307 @utmp2 = unpack($utmp_template, $utmp);
3308 # "@utmp1" eq "@utmp2"
3311 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
3314 $foo = pack('sx2l', 12, 34);
3315 # short 12, two zero bytes padding, long 34
3316 $bar = pack('s@4l', 12, 34);
3317 # short 12, zero fill to position 4, long 34
3320 The same template may generally also be used in unpack().
3322 =item package NAMESPACE
3326 Declares the compilation unit as being in the given namespace. The scope
3327 of the package declaration is from the declaration itself through the end
3328 of the enclosing block, file, or eval (the same as the C<my> operator).
3329 All further unqualified dynamic identifiers will be in this namespace.
3330 A package statement affects only dynamic variables--including those
3331 you've used C<local> on--but I<not> lexical variables, which are created
3332 with C<my>. Typically it would be the first declaration in a file to
3333 be included by the C<require> or C<use> operator. You can switch into a
3334 package in more than one place; it merely influences which symbol table
3335 is used by the compiler for the rest of that block. You can refer to
3336 variables and filehandles in other packages by prefixing the identifier
3337 with the package name and a double colon: C<$Package::Variable>.
3338 If the package name is null, the C<main> package as assumed. That is,
3339 C<$::sail> is equivalent to C<$main::sail> (as well as to C<$main'sail>,
3340 still seen in older code).
3342 If NAMESPACE is omitted, then there is no current package, and all
3343 identifiers must be fully qualified or lexicals. This is stricter
3344 than C<use strict>, since it also extends to function names.
3346 See L<perlmod/"Packages"> for more information about packages, modules,
3347 and classes. See L<perlsub> for other scoping issues.
3349 =item pipe READHANDLE,WRITEHANDLE
3351 Opens a pair of connected pipes like the corresponding system call.
3352 Note that if you set up a loop of piped processes, deadlock can occur
3353 unless you are very careful. In addition, note that Perl's pipes use
3354 stdio buffering, so you may need to set C<$|> to flush your WRITEHANDLE
3355 after each command, depending on the application.
3357 See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
3358 for examples of such things.
3360 On systems that support a close-on-exec flag on files, the flag will be set
3361 for the newly opened file descriptors as determined by the value of $^F.
3368 Pops and returns the last value of the array, shortening the array by
3369 one element. Has an effect similar to
3373 If there are no elements in the array, returns the undefined value
3374 (although this may happen at other times as well). If ARRAY is
3375 omitted, pops the C<@ARGV> array in the main program, and the C<@_>
3376 array in subroutines, just like C<shift>.
3382 Returns the offset of where the last C<m//g> search left off for the variable
3383 in question (C<$_> is used when the variable is not specified). May be
3384 modified to change that offset. Such modification will also influence
3385 the C<\G> zero-width assertion in regular expressions. See L<perlre> and
3388 =item print FILEHANDLE LIST
3394 Prints a string or a list of strings. Returns true if successful.
3395 FILEHANDLE may be a scalar variable name, in which case the variable
3396 contains the name of or a reference to the filehandle, thus introducing
3397 one level of indirection. (NOTE: If FILEHANDLE is a variable and
3398 the next token is a term, it may be misinterpreted as an operator
3399 unless you interpose a C<+> or put parentheses around the arguments.)
3400 If FILEHANDLE is omitted, prints by default to standard output (or
3401 to the last selected output channel--see L</select>). If LIST is
3402 also omitted, prints C<$_> to the currently selected output channel.
3403 To set the default output channel to something other than STDOUT
3404 use the select operation. The current value of C<$,> (if any) is
3405 printed between each LIST item. The current value of C<$\> (if
3406 any) is printed after the entire LIST has been printed. Because
3407 print takes a LIST, anything in the LIST is evaluated in list
3408 context, and any subroutine that you call will have one or more of
3409 its expressions evaluated in list context. Also be careful not to
3410 follow the print keyword with a left parenthesis unless you want
3411 the corresponding right parenthesis to terminate the arguments to
3412 the print--interpose a C<+> or put parentheses around all the
3415 Note that if you're storing FILEHANDLES in an array or other expression,
3416 you will have to use a block returning its value instead:
3418 print { $files[$i] } "stuff\n";
3419 print { $OK ? STDOUT : STDERR } "stuff\n";
3421 =item printf FILEHANDLE FORMAT, LIST
3423 =item printf FORMAT, LIST
3425 Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that C<$\>
3426 (the output record separator) is not appended. The first argument
3427 of the list will be interpreted as the C<printf> format. If C<use locale> is
3428 in effect, the character used for the decimal point in formatted real numbers
3429 is affected by the LC_NUMERIC locale. See L<perllocale>.
3431 Don't fall into the trap of using a C<printf> when a simple
3432 C<print> would do. The C<print> is more efficient and less
3435 =item prototype FUNCTION
3437 Returns the prototype of a function as a string (or C<undef> if the
3438 function has no prototype). FUNCTION is a reference to, or the name of,
3439 the function whose prototype you want to retrieve.
3441 If FUNCTION is a string starting with C<CORE::>, the rest is taken as a
3442 name for Perl builtin. If the builtin is not I<overridable> (such as
3443 C<qw//>) or its arguments cannot be expressed by a prototype (such as
3444 C<system>) returns C<undef> because the builtin does not really behave
3445 like a Perl function. Otherwise, the string describing the equivalent
3446 prototype is returned.
3448 =item push ARRAY,LIST
3450 Treats ARRAY as a stack, and pushes the values of LIST
3451 onto the end of ARRAY. The length of ARRAY increases by the length of
3452 LIST. Has the same effect as
3455 $ARRAY[++$#ARRAY] = $value;
3458 but is more efficient. Returns the new number of elements in the array.
3472 Generalized quotes. See L<perlop/"Regexp Quote-Like Operators">.
3474 =item quotemeta EXPR
3478 Returns the value of EXPR with all non-"word"
3479 characters backslashed. (That is, all characters not matching
3480 C</[A-Za-z_0-9]/> will be preceded by a backslash in the
3481 returned string, regardless of any locale settings.)
3482 This is the internal function implementing
3483 the C<\Q> escape in double-quoted strings.
3485 If EXPR is omitted, uses C<$_>.
3491 Returns a random fractional number greater than or equal to C<0> and less
3492 than the value of EXPR. (EXPR should be positive.) If EXPR is
3493 omitted, the value C<1> is used. Automatically calls C<srand> unless
3494 C<srand> has already been called. See also C<srand>.
3496 (Note: If your rand function consistently returns numbers that are too
3497 large or too small, then your version of Perl was probably compiled
3498 with the wrong number of RANDBITS.)
3500 =item read FILEHANDLE,SCALAR,LENGTH,OFFSET
3502 =item read FILEHANDLE,SCALAR,LENGTH
3504 Attempts to read LENGTH bytes of data into variable SCALAR from the
3505 specified FILEHANDLE. Returns the number of bytes actually read, C<0>
3506 at end of file, or undef if there was an error. SCALAR will be grown
3507 or shrunk to the length actually read. If SCALAR needs growing, the
3508 new bytes will be zero bytes. An OFFSET may be specified to place
3509 the read data into some other place in SCALAR than the beginning.
3510 The call is actually implemented in terms of stdio's fread(3) call.
3511 To get a true read(2) system call, see C<sysread>.
3513 =item readdir DIRHANDLE
3515 Returns the next directory entry for a directory opened by C<opendir>.
3516 If used in list context, returns all the rest of the entries in the
3517 directory. If there are no more entries, returns an undefined value in
3518 scalar context or a null list in list context.
3520 If you're planning to filetest the return values out of a C<readdir>, you'd
3521 better prepend the directory in question. Otherwise, because we didn't
3522 C<chdir> there, it would have been testing the wrong file.
3524 opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!";
3525 @dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR);
3530 Reads from the filehandle whose typeglob is contained in EXPR. In scalar
3531 context, each call reads and returns the next line, until end-of-file is
3532 reached, whereupon the subsequent call returns undef. In list context,
3533 reads until end-of-file is reached and returns a list of lines. Note that
3534 the notion of "line" used here is however you may have defined it
3535 with C<$/> or C<$INPUT_RECORD_SEPARATOR>). See L<perlvar/"$/">.
3537 When C<$/> is set to C<undef>, when readline() is in scalar
3538 context (i.e. file slurp mode), and when an empty file is read, it
3539 returns C<''> the first time, followed by C<undef> subsequently.
3541 This is the internal function implementing the C<< <EXPR> >>
3542 operator, but you can use it directly. The C<< <EXPR> >>
3543 operator is discussed in more detail in L<perlop/"I/O Operators">.
3546 $line = readline(*STDIN); # same thing
3552 Returns the value of a symbolic link, if symbolic links are
3553 implemented. If not, gives a fatal error. If there is some system
3554 error, returns the undefined value and sets C<$!> (errno). If EXPR is
3555 omitted, uses C<$_>.
3559 EXPR is executed as a system command.
3560 The collected standard output of the command is returned.
3561 In scalar context, it comes back as a single (potentially
3562 multi-line) string. In list context, returns a list of lines
3563 (however you've defined lines with C<$/> or C<$INPUT_RECORD_SEPARATOR>).
3564 This is the internal function implementing the C<qx/EXPR/>
3565 operator, but you can use it directly. The C<qx/EXPR/>
3566 operator is discussed in more detail in L<perlop/"I/O Operators">.
3568 =item recv SOCKET,SCALAR,LENGTH,FLAGS
3570 Receives a message on a socket. Attempts to receive LENGTH bytes of
3571 data into variable SCALAR from the specified SOCKET filehandle. SCALAR
3572 will be grown or shrunk to the length actually read. Takes the same
3573 flags as the system call of the same name. Returns the address of the
3574 sender if SOCKET's protocol supports this; returns an empty string
3575 otherwise. If there's an error, returns the undefined value. This call
3576 is actually implemented in terms of recvfrom(2) system call. See
3577 L<perlipc/"UDP: Message Passing"> for examples.
3583 The C<redo> command restarts the loop block without evaluating the
3584 conditional again. The C<continue> block, if any, is not executed. If
3585 the LABEL is omitted, the command refers to the innermost enclosing
3586 loop. This command is normally used by programs that want to lie to
3587 themselves about what was just input:
3589 # a simpleminded Pascal comment stripper
3590 # (warning: assumes no { or } in strings)
3591 LINE: while (<STDIN>) {
3592 while (s|({.*}.*){.*}|$1 |) {}
3597 if (/}/) { # end of comment?
3606 C<redo> cannot be used to retry a block which returns a value such as
3607 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
3608 a grep() or map() operation.
3610 Note that a block by itself is semantically identical to a loop
3611 that executes once. Thus C<redo> inside such a block will effectively
3612 turn it into a looping construct.
3614 See also L</continue> for an illustration of how C<last>, C<next>, and
3621 Returns a true value if EXPR is a reference, false otherwise. If EXPR
3622 is not specified, C<$_> will be used. The value returned depends on the
3623 type of thing the reference is a reference to.
3624 Builtin types include:
3634 If the referenced object has been blessed into a package, then that package
3635 name is returned instead. You can think of C<ref> as a C<typeof> operator.
3637 if (ref($r) eq "HASH") {
3638 print "r is a reference to a hash.\n";
3641 print "r is not a reference at all.\n";
3643 if (UNIVERSAL::isa($r, "HASH")) { # for subclassing
3644 print "r is a reference to something that isa hash.\n";
3647 See also L<perlref>.
3649 =item rename OLDNAME,NEWNAME
3651 Changes the name of a file; an existing file NEWNAME will be
3652 clobbered. Returns true for success, false otherwise.
3654 Behavior of this function varies wildly depending on your system
3655 implementation. For example, it will usually not work across file system
3656 boundaries, even though the system I<mv> command sometimes compensates
3657 for this. Other restrictions include whether it works on directories,
3658 open files, or pre-existing files. Check L<perlport> and either the
3659 rename(2) manpage or equivalent system documentation for details.
3661 =item require VERSION
3667 Demands some semantics specified by EXPR, or by C<$_> if EXPR is not
3670 If a VERSION is specified as a literal of the form v5.6.1,
3671 demands that the current version of Perl (C<$^V> or $PERL_VERSION) be
3672 at least as recent as that version, at run time. (For compatibility
3673 with older versions of Perl, a numeric argument will also be interpreted
3674 as VERSION.) Compare with L</use>, which can do a similar check at
3677 require v5.6.1; # run time version check
3678 require 5.6.1; # ditto
3679 require 5.005_03; # float version allowed for compatibility
3681 Otherwise, demands that a library file be included if it hasn't already
3682 been included. The file is included via the do-FILE mechanism, which is
3683 essentially just a variety of C<eval>. Has semantics similar to the following
3688 return 1 if $INC{$filename};
3689 my($realfilename,$result);
3691 foreach $prefix (@INC) {
3692 $realfilename = "$prefix/$filename";
3693 if (-f $realfilename) {
3694 $INC{$filename} = $realfilename;
3695 $result = do $realfilename;
3699 die "Can't find $filename in \@INC";
3701 delete $INC{$filename} if $@ || !$result;
3703 die "$filename did not return true value" unless $result;
3707 Note that the file will not be included twice under the same specified
3708 name. The file must return true as the last statement to indicate
3709 successful execution of any initialization code, so it's customary to
3710 end such a file with C<1;> unless you're sure it'll return true
3711 otherwise. But it's better just to put the C<1;>, in case you add more
3714 If EXPR is a bareword, the require assumes a "F<.pm>" extension and
3715 replaces "F<::>" with "F</>" in the filename for you,
3716 to make it easy to load standard modules. This form of loading of
3717 modules does not risk altering your namespace.
3719 In other words, if you try this:
3721 require Foo::Bar; # a splendid bareword
3723 The require function will actually look for the "F<Foo/Bar.pm>" file in the
3724 directories specified in the C<@INC> array.
3726 But if you try this:
3728 $class = 'Foo::Bar';
3729 require $class; # $class is not a bareword
3731 require "Foo::Bar"; # not a bareword because of the ""
3733 The require function will look for the "F<Foo::Bar>" file in the @INC array and
3734 will complain about not finding "F<Foo::Bar>" there. In this case you can do:
3736 eval "require $class";
3738 For a yet-more-powerful import facility, see L</use> and L<perlmod>.
3744 Generally used in a C<continue> block at the end of a loop to clear
3745 variables and reset C<??> searches so that they work again. The
3746 expression is interpreted as a list of single characters (hyphens
3747 allowed for ranges). All variables and arrays beginning with one of
3748 those letters are reset to their pristine state. If the expression is
3749 omitted, one-match searches (C<?pattern?>) are reset to match again. Resets
3750 only variables or searches in the current package. Always returns
3753 reset 'X'; # reset all X variables
3754 reset 'a-z'; # reset lower case variables
3755 reset; # just reset ?one-time? searches
3757 Resetting C<"A-Z"> is not recommended because you'll wipe out your
3758 C<@ARGV> and C<@INC> arrays and your C<%ENV> hash. Resets only package
3759 variables--lexical variables are unaffected, but they clean themselves
3760 up on scope exit anyway, so you'll probably want to use them instead.
3767 Returns from a subroutine, C<eval>, or C<do FILE> with the value
3768 given in EXPR. Evaluation of EXPR may be in list, scalar, or void
3769 context, depending on how the return value will be used, and the context
3770 may vary from one execution to the next (see C<wantarray>). If no EXPR
3771 is given, returns an empty list in list context, the undefined value in
3772 scalar context, and (of course) nothing at all in a void context.
3774 (Note that in the absence of a explicit C<return>, a subroutine, eval,
3775 or do FILE will automatically return the value of the last expression
3780 In list context, returns a list value consisting of the elements
3781 of LIST in the opposite order. In scalar context, concatenates the
3782 elements of LIST and returns a string value with all characters
3783 in the opposite order.
3785 print reverse <>; # line tac, last line first
3787 undef $/; # for efficiency of <>
3788 print scalar reverse <>; # character tac, last line tsrif
3790 This operator is also handy for inverting a hash, although there are some
3791 caveats. If a value is duplicated in the original hash, only one of those
3792 can be represented as a key in the inverted hash. Also, this has to
3793 unwind one hash and build a whole new one, which may take some time
3794 on a large hash, such as from a DBM file.
3796 %by_name = reverse %by_address; # Invert the hash
3798 =item rewinddir DIRHANDLE
3800 Sets the current position to the beginning of the directory for the
3801 C<readdir> routine on DIRHANDLE.
3803 =item rindex STR,SUBSTR,POSITION
3805 =item rindex STR,SUBSTR
3807 Works just like index() except that it returns the position of the LAST
3808 occurrence of SUBSTR in STR. If POSITION is specified, returns the
3809 last occurrence at or before that position.
3811 =item rmdir FILENAME
3815 Deletes the directory specified by FILENAME if that directory is empty. If it
3816 succeeds it returns true, otherwise it returns false and sets C<$!> (errno). If
3817 FILENAME is omitted, uses C<$_>.
3821 The substitution operator. See L<perlop>.
3825 Forces EXPR to be interpreted in scalar context and returns the value
3828 @counts = ( scalar @a, scalar @b, scalar @c );
3830 There is no equivalent operator to force an expression to
3831 be interpolated in list context because in practice, this is never
3832 needed. If you really wanted to do so, however, you could use
3833 the construction C<@{[ (some expression) ]}>, but usually a simple
3834 C<(some expression)> suffices.
3836 Because C<scalar> is unary operator, if you accidentally use for EXPR a
3837 parenthesized list, this behaves as a scalar comma expression, evaluating
3838 all but the last element in void context and returning the final element
3839 evaluated in scalar context. This is seldom what you want.
3841 The following single statement:
3843 print uc(scalar(&foo,$bar)),$baz;
3845 is the moral equivalent of these two:
3848 print(uc($bar),$baz);
3850 See L<perlop> for more details on unary operators and the comma operator.
3852 =item seek FILEHANDLE,POSITION,WHENCE
3854 Sets FILEHANDLE's position, just like the C<fseek> call of C<stdio>.
3855 FILEHANDLE may be an expression whose value gives the name of the
3856 filehandle. The values for WHENCE are C<0> to set the new position to
3857 POSITION, C<1> to set it to the current position plus POSITION, and
3858 C<2> to set it to EOF plus POSITION (typically negative). For WHENCE
3859 you may use the constants C<SEEK_SET>, C<SEEK_CUR>, and C<SEEK_END>
3860 (start of the file, current position, end of the file) from the Fcntl
3861 module. Returns C<1> upon success, C<0> otherwise.
3863 If you want to position file for C<sysread> or C<syswrite>, don't use
3864 C<seek>--buffering makes its effect on the file's system position
3865 unpredictable and non-portable. Use C<sysseek> instead.
3867 Due to the rules and rigors of ANSI C, on some systems you have to do a
3868 seek whenever you switch between reading and writing. Amongst other
3869 things, this may have the effect of calling stdio's clearerr(3).
3870 A WHENCE of C<1> (C<SEEK_CUR>) is useful for not moving the file position:
3874 This is also useful for applications emulating C<tail -f>. Once you hit
3875 EOF on your read, and then sleep for a while, you might have to stick in a
3876 seek() to reset things. The C<seek> doesn't change the current position,
3877 but it I<does> clear the end-of-file condition on the handle, so that the
3878 next C<< <FILE> >> makes Perl try again to read something. We hope.
3880 If that doesn't work (some stdios are particularly cantankerous), then
3881 you may need something more like this:
3884 for ($curpos = tell(FILE); $_ = <FILE>;
3885 $curpos = tell(FILE)) {
3886 # search for some stuff and put it into files
3888 sleep($for_a_while);
3889 seek(FILE, $curpos, 0);
3892 =item seekdir DIRHANDLE,POS
3894 Sets the current position for the C<readdir> routine on DIRHANDLE. POS
3895 must be a value returned by C<telldir>. Has the same caveats about
3896 possible directory compaction as the corresponding system library
3899 =item select FILEHANDLE
3903 Returns the currently selected filehandle. Sets the current default
3904 filehandle for output, if FILEHANDLE is supplied. This has two
3905 effects: first, a C<write> or a C<print> without a filehandle will
3906 default to this FILEHANDLE. Second, references to variables related to
3907 output will refer to this output channel. For example, if you have to
3908 set the top of form format for more than one output channel, you might
3916 FILEHANDLE may be an expression whose value gives the name of the
3917 actual filehandle. Thus:
3919 $oldfh = select(STDERR); $| = 1; select($oldfh);
3921 Some programmers may prefer to think of filehandles as objects with
3922 methods, preferring to write the last example as:
3925 STDERR->autoflush(1);
3927 =item select RBITS,WBITS,EBITS,TIMEOUT
3929 This calls the select(2) system call with the bit masks specified, which
3930 can be constructed using C<fileno> and C<vec>, along these lines:
3932 $rin = $win = $ein = '';
3933 vec($rin,fileno(STDIN),1) = 1;
3934 vec($win,fileno(STDOUT),1) = 1;
3937 If you want to select on many filehandles you might wish to write a
3941 my(@fhlist) = split(' ',$_[0]);
3944 vec($bits,fileno($_),1) = 1;
3948 $rin = fhbits('STDIN TTY SOCK');
3952 ($nfound,$timeleft) =
3953 select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
3955 or to block until something becomes ready just do this
3957 $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
3959 Most systems do not bother to return anything useful in $timeleft, so
3960 calling select() in scalar context just returns $nfound.
3962 Any of the bit masks can also be undef. The timeout, if specified, is
3963 in seconds, which may be fractional. Note: not all implementations are
3964 capable of returning the$timeleft. If not, they always return
3965 $timeleft equal to the supplied $timeout.
3967 You can effect a sleep of 250 milliseconds this way:
3969 select(undef, undef, undef, 0.25);
3971 B<WARNING>: One should not attempt to mix buffered I/O (like C<read>
3972 or <FH>) with C<select>, except as permitted by POSIX, and even
3973 then only on POSIX systems. You have to use C<sysread> instead.
3975 =item semctl ID,SEMNUM,CMD,ARG
3977 Calls the System V IPC function C<semctl>. You'll probably have to say
3981 first to get the correct constant definitions. If CMD is IPC_STAT or
3982 GETALL, then ARG must be a variable which will hold the returned
3983 semid_ds structure or semaphore value array. Returns like C<ioctl>:
3984 the undefined value for error, "C<0 but true>" for zero, or the actual
3985 return value otherwise. The ARG must consist of a vector of native
3986 short integers, which may be created with C<pack("s!",(0)x$nsem)>.
3987 See also L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::Semaphore>
3990 =item semget KEY,NSEMS,FLAGS
3992 Calls the System V IPC function semget. Returns the semaphore id, or
3993 the undefined value if there is an error. See also
3994 L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::SysV::Semaphore>
3997 =item semop KEY,OPSTRING
3999 Calls the System V IPC function semop to perform semaphore operations
4000 such as signaling and waiting. OPSTRING must be a packed array of
4001 semop structures. Each semop structure can be generated with
4002 C<pack("sss", $semnum, $semop, $semflag)>. The number of semaphore
4003 operations is implied by the length of OPSTRING. Returns true if
4004 successful, or false if there is an error. As an example, the
4005 following code waits on semaphore $semnum of semaphore id $semid:
4007 $semop = pack("sss", $semnum, -1, 0);
4008 die "Semaphore trouble: $!\n" unless semop($semid, $semop);
4010 To signal the semaphore, replace C<-1> with C<1>. See also
4011 L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::SysV::Semaphore>
4014 =item send SOCKET,MSG,FLAGS,TO
4016 =item send SOCKET,MSG,FLAGS
4018 Sends a message on a socket. Takes the same flags as the system call
4019 of the same name. On unconnected sockets you must specify a
4020 destination to send TO, in which case it does a C C<sendto>. Returns
4021 the number of characters sent, or the undefined value if there is an
4022 error. The C system call sendmsg(2) is currently unimplemented.
4023 See L<perlipc/"UDP: Message Passing"> for examples.
4025 =item setpgrp PID,PGRP
4027 Sets the current process group for the specified PID, C<0> for the current
4028 process. Will produce a fatal error if used on a machine that doesn't
4029 implement POSIX setpgid(2) or BSD setpgrp(2). If the arguments are omitted,
4030 it defaults to C<0,0>. Note that the BSD 4.2 version of C<setpgrp> does not
4031 accept any arguments, so only C<setpgrp(0,0)> is portable. See also
4034 =item setpriority WHICH,WHO,PRIORITY
4036 Sets the current priority for a process, a process group, or a user.
4037 (See setpriority(2).) Will produce a fatal error if used on a machine
4038 that doesn't implement setpriority(2).
4040 =item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
4042 Sets the socket option requested. Returns undefined if there is an
4043 error. OPTVAL may be specified as C<undef> if you don't want to pass an
4050 Shifts the first value of the array off and returns it, shortening the
4051 array by 1 and moving everything down. If there are no elements in the
4052 array, returns the undefined value. If ARRAY is omitted, shifts the
4053 C<@_> array within the lexical scope of subroutines and formats, and the
4054 C<@ARGV> array at file scopes or within the lexical scopes established by
4055 the C<eval ''>, C<BEGIN {}>, C<INIT {}>, C<CHECK {}>, and C<END {}>
4058 See also C<unshift>, C<push>, and C<pop>. C<shift> and C<unshift> do the
4059 same thing to the left end of an array that C<pop> and C<push> do to the
4062 =item shmctl ID,CMD,ARG
4064 Calls the System V IPC function shmctl. You'll probably have to say
4068 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
4069 then ARG must be a variable which will hold the returned C<shmid_ds>
4070 structure. Returns like ioctl: the undefined value for error, "C<0> but
4071 true" for zero, or the actual return value otherwise.
4072 See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation.
4074 =item shmget KEY,SIZE,FLAGS
4076 Calls the System V IPC function shmget. Returns the shared memory
4077 segment id, or the undefined value if there is an error.
4078 See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation.
4080 =item shmread ID,VAR,POS,SIZE
4082 =item shmwrite ID,STRING,POS,SIZE
4084 Reads or writes the System V shared memory segment ID starting at
4085 position POS for size SIZE by attaching to it, copying in/out, and
4086 detaching from it. When reading, VAR must be a variable that will
4087 hold the data read. When writing, if STRING is too long, only SIZE
4088 bytes are used; if STRING is too short, nulls are written to fill out
4089 SIZE bytes. Return true if successful, or false if there is an error.
4090 shmread() taints the variable. See also L<perlipc/"SysV IPC">,
4091 C<IPC::SysV> documentation, and the C<IPC::Shareable> module from CPAN.
4093 =item shutdown SOCKET,HOW
4095 Shuts down a socket connection in the manner indicated by HOW, which
4096 has the same interpretation as in the system call of the same name.
4098 shutdown(SOCKET, 0); # I/we have stopped reading data
4099 shutdown(SOCKET, 1); # I/we have stopped writing data
4100 shutdown(SOCKET, 2); # I/we have stopped using this socket
4102 This is useful with sockets when you want to tell the other
4103 side you're done writing but not done reading, or vice versa.
4104 It's also a more insistent form of close because it also
4105 disables the file descriptor in any forked copies in other
4112 Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
4113 returns sine of C<$_>.
4115 For the inverse sine operation, you may use the C<Math::Trig::asin>
4116 function, or use this relation:
4118 sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
4124 Causes the script to sleep for EXPR seconds, or forever if no EXPR.
4125 May be interrupted if the process receives a signal such as C<SIGALRM>.
4126 Returns the number of seconds actually slept. You probably cannot
4127 mix C<alarm> and C<sleep> calls, because C<sleep> is often implemented
4130 On some older systems, it may sleep up to a full second less than what
4131 you requested, depending on how it counts seconds. Most modern systems
4132 always sleep the full amount. They may appear to sleep longer than that,
4133 however, because your process might not be scheduled right away in a
4134 busy multitasking system.
4136 For delays of finer granularity than one second, you may use Perl's
4137 C<syscall> interface to access setitimer(2) if your system supports
4138 it, or else see L</select> above. The Time::HiRes module from CPAN
4141 See also the POSIX module's C<pause> function.
4143 =item socket SOCKET,DOMAIN,TYPE,PROTOCOL
4145 Opens a socket of the specified kind and attaches it to filehandle
4146 SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for
4147 the system call of the same name. You should C<use Socket> first
4148 to get the proper definitions imported. See the examples in
4149 L<perlipc/"Sockets: Client/Server Communication">.
4151 On systems that support a close-on-exec flag on files, the flag will
4152 be set for the newly opened file descriptor, as determined by the
4153 value of $^F. See L<perlvar/$^F>.
4155 =item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
4157 Creates an unnamed pair of sockets in the specified domain, of the
4158 specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as
4159 for the system call of the same name. If unimplemented, yields a fatal
4160 error. Returns true if successful.
4162 On systems that support a close-on-exec flag on files, the flag will
4163 be set for the newly opened file descriptors, as determined by the value
4164 of $^F. See L<perlvar/$^F>.
4166 Some systems defined C<pipe> in terms of C<socketpair>, in which a call
4167 to C<pipe(Rdr, Wtr)> is essentially:
4170 socketpair(Rdr, Wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC);
4171 shutdown(Rdr, 1); # no more writing for reader
4172 shutdown(Wtr, 0); # no more reading for writer
4174 See L<perlipc> for an example of socketpair use.
4176 =item sort SUBNAME LIST
4178 =item sort BLOCK LIST
4182 Sorts the LIST and returns the sorted list value. If SUBNAME or BLOCK
4183 is omitted, C<sort>s in standard string comparison order. If SUBNAME is
4184 specified, it gives the name of a subroutine that returns an integer
4185 less than, equal to, or greater than C<0>, depending on how the elements
4186 of the list are to be ordered. (The C<< <=> >> and C<cmp>
4187 operators are extremely useful in such routines.) SUBNAME may be a
4188 scalar variable name (unsubscripted), in which case the value provides
4189 the name of (or a reference to) the actual subroutine to use. In place
4190 of a SUBNAME, you can provide a BLOCK as an anonymous, in-line sort
4193 If the subroutine's prototype is C<($$)>, the elements to be compared
4194 are passed by reference in C<@_>, as for a normal subroutine. This is
4195 slower than unprototyped subroutines, where the elements to be
4196 compared are passed into the subroutine
4197 as the package global variables $a and $b (see example below). Note that
4198 in the latter case, it is usually counter-productive to declare $a and
4201 In either case, the subroutine may not be recursive. The values to be
4202 compared are always passed by reference, so don't modify them.
4204 You also cannot exit out of the sort block or subroutine using any of the
4205 loop control operators described in L<perlsyn> or with C<goto>.
4207 When C<use locale> is in effect, C<sort LIST> sorts LIST according to the
4208 current collation locale. See L<perllocale>.
4213 @articles = sort @files;
4215 # same thing, but with explicit sort routine
4216 @articles = sort {$a cmp $b} @files;
4218 # now case-insensitively
4219 @articles = sort {uc($a) cmp uc($b)} @files;
4221 # same thing in reversed order
4222 @articles = sort {$b cmp $a} @files;
4224 # sort numerically ascending
4225 @articles = sort {$a <=> $b} @files;
4227 # sort numerically descending
4228 @articles = sort {$b <=> $a} @files;
4230 # this sorts the %age hash by value instead of key
4231 # using an in-line function
4232 @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
4234 # sort using explicit subroutine name
4236 $age{$a} <=> $age{$b}; # presuming numeric
4238 @sortedclass = sort byage @class;
4240 sub backwards { $b cmp $a }
4241 @harry = qw(dog cat x Cain Abel);
4242 @george = qw(gone chased yz Punished Axed);
4244 # prints AbelCaincatdogx
4245 print sort backwards @harry;
4246 # prints xdogcatCainAbel
4247 print sort @george, 'to', @harry;
4248 # prints AbelAxedCainPunishedcatchaseddoggonetoxyz
4250 # inefficiently sort by descending numeric compare using
4251 # the first integer after the first = sign, or the
4252 # whole record case-insensitively otherwise
4255 ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
4260 # same thing, but much more efficiently;
4261 # we'll build auxiliary indices instead
4265 push @nums, /=(\d+)/;
4270 $nums[$b] <=> $nums[$a]
4272 $caps[$a] cmp $caps[$b]
4276 # same thing, but without any temps
4277 @new = map { $_->[0] }
4278 sort { $b->[1] <=> $a->[1]
4281 } map { [$_, /=(\d+)/, uc($_)] } @old;
4283 # using a prototype allows you to use any comparison subroutine
4284 # as a sort subroutine (including other package's subroutines)
4286 sub backwards ($$) { $_[1] cmp $_[0]; } # $a and $b are not set here
4289 @new = sort other::backwards @old;
4291 If you're using strict, you I<must not> declare $a
4292 and $b as lexicals. They are package globals. That means
4293 if you're in the C<main> package and type
4295 @articles = sort {$b <=> $a} @files;
4297 then C<$a> and C<$b> are C<$main::a> and C<$main::b> (or C<$::a> and C<$::b>),
4298 but if you're in the C<FooPack> package, it's the same as typing
4300 @articles = sort {$FooPack::b <=> $FooPack::a} @files;
4302 The comparison function is required to behave. If it returns
4303 inconsistent results (sometimes saying C<$x[1]> is less than C<$x[2]> and
4304 sometimes saying the opposite, for example) the results are not
4307 =item splice ARRAY,OFFSET,LENGTH,LIST
4309 =item splice ARRAY,OFFSET,LENGTH
4311 =item splice ARRAY,OFFSET
4315 Removes the elements designated by OFFSET and LENGTH from an array, and
4316 replaces them with the elements of LIST, if any. In list context,
4317 returns the elements removed from the array. In scalar context,
4318 returns the last element removed, or C<undef> if no elements are
4319 removed. The array grows or shrinks as necessary.
4320 If OFFSET is negative then it starts that far from the end of the array.
4321 If LENGTH is omitted, removes everything from OFFSET onward.
4322 If LENGTH is negative, leaves that many elements off the end of the array.
4323 If both OFFSET and LENGTH are omitted, removes everything.
4325 The following equivalences hold (assuming C<$[ == 0>):
4327 push(@a,$x,$y) splice(@a,@a,0,$x,$y)
4328 pop(@a) splice(@a,-1)
4329 shift(@a) splice(@a,0,1)
4330 unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
4331 $a[$x] = $y splice(@a,$x,1,$y)
4333 Example, assuming array lengths are passed before arrays:
4335 sub aeq { # compare two list values
4336 my(@a) = splice(@_,0,shift);
4337 my(@b) = splice(@_,0,shift);
4338 return 0 unless @a == @b; # same len?
4340 return 0 if pop(@a) ne pop(@b);
4344 if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
4346 =item split /PATTERN/,EXPR,LIMIT
4348 =item split /PATTERN/,EXPR
4350 =item split /PATTERN/
4354 Splits a string into a list of strings and returns that list. By default,
4355 empty leading fields are preserved, and empty trailing ones are deleted.
4357 In scalar context, returns the number of fields found and splits into
4358 the C<@_> array. Use of split in scalar context is deprecated, however,
4359 because it clobbers your subroutine arguments.
4361 If EXPR is omitted, splits the C<$_> string. If PATTERN is also omitted,
4362 splits on whitespace (after skipping any leading whitespace). Anything
4363 matching PATTERN is taken to be a delimiter separating the fields. (Note
4364 that the delimiter may be longer than one character.)
4366 If LIMIT is specified and positive, splits into no more than that
4367 many fields (though it may split into fewer). If LIMIT is unspecified
4368 or zero, trailing null fields are stripped (which potential users
4369 of C<pop> would do well to remember). If LIMIT is negative, it is
4370 treated as if an arbitrarily large LIMIT had been specified.
4372 A pattern matching the null string (not to be confused with
4373 a null pattern C<//>, which is just one member of the set of patterns
4374 matching a null string) will split the value of EXPR into separate
4375 characters at each point it matches that way. For example:
4377 print join(':', split(/ */, 'hi there'));
4379 produces the output 'h:i:t:h:e:r:e'.
4381 Empty leading (or trailing) fields are produced when there positive width
4382 matches at the beginning (or end) of the string; a zero-width match at the
4383 beginning (or end) of the string does not produce an empty field. For
4386 print join(':', split(/(?=\w)/, 'hi there!'));
4388 produces the output 'h:i :t:h:e:r:e!'.
4390 The LIMIT parameter can be used to split a line partially
4392 ($login, $passwd, $remainder) = split(/:/, $_, 3);
4394 When assigning to a list, if LIMIT is omitted, Perl supplies a LIMIT
4395 one larger than the number of variables in the list, to avoid
4396 unnecessary work. For the list above LIMIT would have been 4 by
4397 default. In time critical applications it behooves you not to split
4398 into more fields than you really need.
4400 If the PATTERN contains parentheses, additional list elements are
4401 created from each matching substring in the delimiter.
4403 split(/([,-])/, "1-10,20", 3);
4405 produces the list value
4407 (1, '-', 10, ',', 20)
4409 If you had the entire header of a normal Unix email message in $header,
4410 you could split it up into fields and their values this way:
4412 $header =~ s/\n\s+/ /g; # fix continuation lines
4413 %hdrs = (UNIX_FROM => split /^(\S*?):\s*/m, $header);
4415 The pattern C</PATTERN/> may be replaced with an expression to specify
4416 patterns that vary at runtime. (To do runtime compilation only once,
4417 use C</$variable/o>.)
4419 As a special case, specifying a PATTERN of space (C<' '>) will split on
4420 white space just as C<split> with no arguments does. Thus, C<split(' ')> can
4421 be used to emulate B<awk>'s default behavior, whereas C<split(/ /)>
4422 will give you as many null initial fields as there are leading spaces.
4423 A C<split> on C</\s+/> is like a C<split(' ')> except that any leading
4424 whitespace produces a null first field. A C<split> with no arguments
4425 really does a C<split(' ', $_)> internally.
4427 A PATTERN of C</^/> is treated as if it were C</^/m>, since it isn't
4432 open(PASSWD, '/etc/passwd');
4435 ($login, $passwd, $uid, $gid,
4436 $gcos, $home, $shell) = split(/:/);
4441 =item sprintf FORMAT, LIST
4443 Returns a string formatted by the usual C<printf> conventions of the C
4444 library function C<sprintf>. See below for more details
4445 and see L<sprintf(3)> or L<printf(3)> on your system for an explanation of
4446 the general principles.
4450 # Format number with up to 8 leading zeroes
4451 $result = sprintf("%08d", $number);
4453 # Round number to 3 digits after decimal point
4454 $rounded = sprintf("%.3f", $number);
4456 Perl does its own C<sprintf> formatting--it emulates the C
4457 function C<sprintf>, but it doesn't use it (except for floating-point
4458 numbers, and even then only the standard modifiers are allowed). As a
4459 result, any non-standard extensions in your local C<sprintf> are not
4460 available from Perl.
4462 Unlike C<printf>, C<sprintf> does not do what you probably mean when you
4463 pass it an array as your first argument. The array is given scalar context,
4464 and instead of using the 0th element of the array as the format, Perl will
4465 use the count of elements in the array as the format, which is almost never
4468 Perl's C<sprintf> permits the following universally-known conversions:
4471 %c a character with the given number
4473 %d a signed integer, in decimal
4474 %u an unsigned integer, in decimal
4475 %o an unsigned integer, in octal
4476 %x an unsigned integer, in hexadecimal
4477 %e a floating-point number, in scientific notation
4478 %f a floating-point number, in fixed decimal notation
4479 %g a floating-point number, in %e or %f notation
4481 In addition, Perl permits the following widely-supported conversions:
4483 %X like %x, but using upper-case letters
4484 %E like %e, but using an upper-case "E"
4485 %G like %g, but with an upper-case "E" (if applicable)
4486 %b an unsigned integer, in binary
4487 %p a pointer (outputs the Perl value's address in hexadecimal)
4488 %n special: *stores* the number of characters output so far
4489 into the next variable in the parameter list
4491 Finally, for backward (and we do mean "backward") compatibility, Perl
4492 permits these unnecessary but widely-supported conversions:
4495 %D a synonym for %ld
4496 %U a synonym for %lu
4497 %O a synonym for %lo
4500 Note that the number of exponent digits in the scientific notation by
4501 C<%e>, C<%E>, C<%g> and C<%G> for numbers with the modulus of the
4502 exponent less than 100 is system-dependent: it may be three or less
4503 (zero-padded as necessary). In other words, 1.23 times ten to the
4504 99th may be either "1.23e99" or "1.23e099".
4506 Perl permits the following universally-known flags between the C<%>
4507 and the conversion letter:
4509 space prefix positive number with a space
4510 + prefix positive number with a plus sign
4511 - left-justify within the field
4512 0 use zeros, not spaces, to right-justify
4513 # prefix non-zero octal with "0", non-zero hex with "0x"
4514 number minimum field width
4515 .number "precision": digits after decimal point for
4516 floating-point, max length for string, minimum length
4518 l interpret integer as C type "long" or "unsigned long"
4519 h interpret integer as C type "short" or "unsigned short"
4520 If no flags, interpret integer as C type "int" or "unsigned"
4522 Perl supports parameter ordering, in other words, fetching the
4523 parameters in some explicitly specified "random" ordering as opposed
4524 to the default implicit sequential ordering. The syntax is, instead
4525 of the C<%> and C<*>, to use C<%>I<digits>C<$> and C<*>I<digits>C<$>,
4526 where the I<digits> is the wanted index, from one upwards. For example:
4528 printf "%2\$d %1\$d\n", 12, 34; # will print "34 12\n"
4529 printf "%*2\$d\n", 12, 3; # will print " 12\n"
4531 Note that using the reordering syntax does not interfere with the usual
4532 implicit sequential fetching of the parameters:
4534 printf "%2\$d %d\n", 12, 34; # will print "34 12\n"
4535 printf "%2\$d %d %d\n", 12, 34; # will print "34 12 34\n"
4536 printf "%3\$d %d %d\n", 12, 34, 56; # will print "56 12 34\n"
4537 printf "%2\$*3\$d %d\n", 12, 34, 3; # will print " 34 12\n"
4538 printf "%*3\$2\$d %d\n", 12, 34, 3; # will print " 34 12\n"
4540 There are also two Perl-specific flags:
4542 V interpret integer as Perl's standard integer type
4543 v interpret string as a vector of integers, output as
4544 numbers separated either by dots, or by an arbitrary
4545 string received from the argument list when the flag
4548 Where a number would appear in the flags, an asterisk (C<*>) may be
4549 used instead, in which case Perl uses the next item in the parameter
4550 list as the given number (that is, as the field width or precision).
4551 If a field width obtained through C<*> is negative, it has the same
4552 effect as the C<-> flag: left-justification.
4554 The C<v> flag is useful for displaying ordinal values of characters
4555 in arbitrary strings:
4557 printf "version is v%vd\n", $^V; # Perl's version
4558 printf "address is %*vX\n", ":", $addr; # IPv6 address
4559 printf "bits are %*vb\n", " ", $bits; # random bitstring
4561 If C<use locale> is in effect, the character used for the decimal
4562 point in formatted real numbers is affected by the LC_NUMERIC locale.
4565 If Perl understands "quads" (64-bit integers) (this requires
4566 either that the platform natively support quads or that Perl
4567 be specifically compiled to support quads), the characters
4571 print quads, and they may optionally be preceded by
4579 You can find out whether your Perl supports quads via L<Config>:
4582 ($Config{use64bitint} eq 'define' || $Config{longsize} == 8) &&
4585 If Perl understands "long doubles" (this requires that the platform
4586 support long doubles), the flags
4590 may optionally be preceded by
4598 You can find out whether your Perl supports long doubles via L<Config>:
4601 $Config{d_longdbl} eq 'define' && print "long doubles\n";
4607 Return the square root of EXPR. If EXPR is omitted, returns square
4608 root of C<$_>. Only works on non-negative operands, unless you've
4609 loaded the standard Math::Complex module.
4612 print sqrt(-2); # prints 1.4142135623731i
4618 Sets the random number seed for the C<rand> operator. If EXPR is
4619 omitted, uses a semi-random value supplied by the kernel (if it supports
4620 the F</dev/urandom> device) or based on the current time and process
4621 ID, among other things. In versions of Perl prior to 5.004 the default
4622 seed was just the current C<time>. This isn't a particularly good seed,
4623 so many old programs supply their own seed value (often C<time ^ $$> or
4624 C<time ^ ($$ + ($$ << 15))>), but that isn't necessary any more.
4626 In fact, it's usually not necessary to call C<srand> at all, because if
4627 it is not called explicitly, it is called implicitly at the first use of
4628 the C<rand> operator. However, this was not the case in version of Perl
4629 before 5.004, so if your script will run under older Perl versions, it
4630 should call C<srand>.
4632 Note that you need something much more random than the default seed for
4633 cryptographic purposes. Checksumming the compressed output of one or more
4634 rapidly changing operating system status programs is the usual method. For
4637 srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip`);
4639 If you're particularly concerned with this, see the C<Math::TrulyRandom>
4642 Do I<not> call C<srand> multiple times in your program unless you know
4643 exactly what you're doing and why you're doing it. The point of the
4644 function is to "seed" the C<rand> function so that C<rand> can produce
4645 a different sequence each time you run your program. Just do it once at the
4646 top of your program, or you I<won't> get random numbers out of C<rand>!
4648 Frequently called programs (like CGI scripts) that simply use
4652 for a seed can fall prey to the mathematical property that
4656 one-third of the time. So don't do that.
4658 =item stat FILEHANDLE
4664 Returns a 13-element list giving the status info for a file, either
4665 the file opened via FILEHANDLE, or named by EXPR. If EXPR is omitted,
4666 it stats C<$_>. Returns a null list if the stat fails. Typically used
4669 ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
4670 $atime,$mtime,$ctime,$blksize,$blocks)
4673 Not all fields are supported on all filesystem types. Here are the
4674 meaning of the fields:
4676 0 dev device number of filesystem
4678 2 mode file mode (type and permissions)
4679 3 nlink number of (hard) links to the file
4680 4 uid numeric user ID of file's owner
4681 5 gid numeric group ID of file's owner
4682 6 rdev the device identifier (special files only)
4683 7 size total size of file, in bytes
4684 8 atime last access time in seconds since the epoch
4685 9 mtime last modify time in seconds since the epoch
4686 10 ctime inode change time (NOT creation time!) in seconds since the epoch
4687 11 blksize preferred block size for file system I/O
4688 12 blocks actual number of blocks allocated
4690 (The epoch was at 00:00 January 1, 1970 GMT.)
4692 If stat is passed the special filehandle consisting of an underline, no
4693 stat is done, but the current contents of the stat structure from the
4694 last stat or filetest are returned. Example:
4696 if (-x $file && (($d) = stat(_)) && $d < 0) {
4697 print "$file is executable NFS file\n";
4700 (This works on machines only for which the device number is negative
4703 Because the mode contains both the file type and its permissions, you
4704 should mask off the file type portion and (s)printf using a C<"%o">
4705 if you want to see the real permissions.
4707 $mode = (stat($filename))[2];
4708 printf "Permissions are %04o\n", $mode & 07777;
4710 In scalar context, C<stat> returns a boolean value indicating success
4711 or failure, and, if successful, sets the information associated with
4712 the special filehandle C<_>.
4714 The File::stat module provides a convenient, by-name access mechanism:
4717 $sb = stat($filename);
4718 printf "File is %s, size is %s, perm %04o, mtime %s\n",
4719 $filename, $sb->size, $sb->mode & 07777,
4720 scalar localtime $sb->mtime;
4722 You can import symbolic mode constants (C<S_IF*>) and functions
4723 (C<S_IS*>) from the Fcntl module:
4727 $mode = (stat($filename))[2];
4729 $user_rwx = ($mode & S_IRWXU) >> 6;
4730 $group_read = ($mode & S_IRGRP) >> 3;
4731 $other_execute = $mode & S_IXOTH;
4733 printf "Permissions are %04o\n", S_ISMODE($mode), "\n";
4735 $is_setuid = $mode & S_ISUID;
4736 $is_setgid = S_ISDIR($mode);
4738 You could write the last two using the C<-u> and C<-d> operators.
4739 The commonly available S_IF* constants are
4741 # Permissions: read, write, execute, for user, group, others.
4743 S_IRWXU S_IRUSR S_IWUSR S_IXUSR
4744 S_IRWXG S_IRGRP S_IWGRP S_IXGRP
4745 S_IRWXO S_IROTH S_IWOTH S_IXOTH
4747 # Setuid/Setgid/Stickiness.
4749 S_ISUID S_ISGID S_ISVTX S_ISTXT
4751 # File types. Not necessarily all are available on your system.
4753 S_IFREG S_IFDIR S_IFLNK S_IFBLK S_ISCHR S_IFIFO S_IFSOCK S_IFWHT S_ENFMT
4755 # The following are compatibility aliases for S_IRUSR, S_IWUSR, S_IXUSR.
4757 S_IREAD S_IWRITE S_IEXEC
4759 and the S_IF* functions are
4761 S_IFMODE($mode) the part of $mode containing the permission bits
4762 and the setuid/setgid/sticky bits
4764 S_IFMT($mode) the part of $mode containing the file type
4765 which can be bit-anded with e.g. S_IFREG
4766 or with the following functions
4768 # The operators -f, -d, -l, -b, -c, -p, and -s.
4770 S_ISREG($mode) S_ISDIR($mode) S_ISLNK($mode)
4771 S_ISBLK($mode) S_ISCHR($mode) S_ISFIFO($mode) S_ISSOCK($mode)
4773 # No direct -X operator counterpart, but for the first one
4774 # the -g operator is often equivalent. The ENFMT stands for
4775 # record flocking enforcement, a platform-dependent feature.
4777 S_ISENFMT($mode) S_ISWHT($mode)
4779 See your native chmod(2) and stat(2) documentation for more details
4780 about the S_* constants.
4786 Takes extra time to study SCALAR (C<$_> if unspecified) in anticipation of
4787 doing many pattern matches on the string before it is next modified.
4788 This may or may not save time, depending on the nature and number of
4789 patterns you are searching on, and on the distribution of character
4790 frequencies in the string to be searched--you probably want to compare
4791 run times with and without it to see which runs faster. Those loops
4792 which scan for many short constant strings (including the constant
4793 parts of more complex patterns) will benefit most. You may have only
4794 one C<study> active at a time--if you study a different scalar the first
4795 is "unstudied". (The way C<study> works is this: a linked list of every
4796 character in the string to be searched is made, so we know, for
4797 example, where all the C<'k'> characters are. From each search string,
4798 the rarest character is selected, based on some static frequency tables
4799 constructed from some C programs and English text. Only those places
4800 that contain this "rarest" character are examined.)
4802 For example, here is a loop that inserts index producing entries
4803 before any line containing a certain pattern:
4807 print ".IX foo\n" if /\bfoo\b/;
4808 print ".IX bar\n" if /\bbar\b/;
4809 print ".IX blurfl\n" if /\bblurfl\b/;
4814 In searching for C</\bfoo\b/>, only those locations in C<$_> that contain C<f>
4815 will be looked at, because C<f> is rarer than C<o>. In general, this is
4816 a big win except in pathological cases. The only question is whether
4817 it saves you more time than it took to build the linked list in the
4820 Note that if you have to look for strings that you don't know till
4821 runtime, you can build an entire loop as a string and C<eval> that to
4822 avoid recompiling all your patterns all the time. Together with
4823 undefining C<$/> to input entire files as one record, this can be very
4824 fast, often faster than specialized programs like fgrep(1). The following
4825 scans a list of files (C<@files>) for a list of words (C<@words>), and prints
4826 out the names of those files that contain a match:
4828 $search = 'while (<>) { study;';
4829 foreach $word (@words) {
4830 $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
4835 eval $search; # this screams
4836 $/ = "\n"; # put back to normal input delimiter
4837 foreach $file (sort keys(%seen)) {
4845 =item sub NAME BLOCK
4847 This is subroutine definition, not a real function I<per se>. With just a
4848 NAME (and possibly prototypes or attributes), it's just a forward declaration.
4849 Without a NAME, it's an anonymous function declaration, and does actually
4850 return a value: the CODE ref of the closure you just created. See L<perlsub>
4851 and L<perlref> for details.
4853 =item substr EXPR,OFFSET,LENGTH,REPLACEMENT
4855 =item substr EXPR,OFFSET,LENGTH
4857 =item substr EXPR,OFFSET
4859 Extracts a substring out of EXPR and returns it. First character is at
4860 offset C<0>, or whatever you've set C<$[> to (but don't do that).
4861 If OFFSET is negative (or more precisely, less than C<$[>), starts
4862 that far from the end of the string. If LENGTH is omitted, returns
4863 everything to the end of the string. If LENGTH is negative, leaves that
4864 many characters off the end of the string.
4866 You can use the substr() function as an lvalue, in which case EXPR
4867 must itself be an lvalue. If you assign something shorter than LENGTH,
4868 the string will shrink, and if you assign something longer than LENGTH,
4869 the string will grow to accommodate it. To keep the string the same
4870 length you may need to pad or chop your value using C<sprintf>.
4872 If OFFSET and LENGTH specify a substring that is partly outside the
4873 string, only the part within the string is returned. If the substring
4874 is beyond either end of the string, substr() returns the undefined
4875 value and produces a warning. When used as an lvalue, specifying a
4876 substring that is entirely outside the string is a fatal error.
4877 Here's an example showing the behavior for boundary cases:
4880 substr($name, 4) = 'dy'; # $name is now 'freddy'
4881 my $null = substr $name, 6, 2; # returns '' (no warning)
4882 my $oops = substr $name, 7; # returns undef, with warning
4883 substr($name, 7) = 'gap'; # fatal error
4885 An alternative to using substr() as an lvalue is to specify the
4886 replacement string as the 4th argument. This allows you to replace
4887 parts of the EXPR and return what was there before in one operation,
4888 just as you can with splice().
4890 =item symlink OLDFILE,NEWFILE
4892 Creates a new filename symbolically linked to the old filename.
4893 Returns C<1> for success, C<0> otherwise. On systems that don't support
4894 symbolic links, produces a fatal error at run time. To check for that,
4897 $symlink_exists = eval { symlink("",""); 1 };
4901 Calls the system call specified as the first element of the list,
4902 passing the remaining elements as arguments to the system call. If
4903 unimplemented, produces a fatal error. The arguments are interpreted
4904 as follows: if a given argument is numeric, the argument is passed as
4905 an int. If not, the pointer to the string value is passed. You are
4906 responsible to make sure a string is pre-extended long enough to
4907 receive any result that might be written into a string. You can't use a
4908 string literal (or other read-only string) as an argument to C<syscall>
4909 because Perl has to assume that any string pointer might be written
4911 integer arguments are not literals and have never been interpreted in a
4912 numeric context, you may need to add C<0> to them to force them to look
4913 like numbers. This emulates the C<syswrite> function (or vice versa):
4915 require 'syscall.ph'; # may need to run h2ph
4917 syscall(&SYS_write, fileno(STDOUT), $s, length $s);
4919 Note that Perl supports passing of up to only 14 arguments to your system call,
4920 which in practice should usually suffice.
4922 Syscall returns whatever value returned by the system call it calls.
4923 If the system call fails, C<syscall> returns C<-1> and sets C<$!> (errno).
4924 Note that some system calls can legitimately return C<-1>. The proper
4925 way to handle such calls is to assign C<$!=0;> before the call and
4926 check the value of C<$!> if syscall returns C<-1>.
4928 There's a problem with C<syscall(&SYS_pipe)>: it returns the file
4929 number of the read end of the pipe it creates. There is no way
4930 to retrieve the file number of the other end. You can avoid this
4931 problem by using C<pipe> instead.
4933 =item sysopen FILEHANDLE,FILENAME,MODE
4935 =item sysopen FILEHANDLE,FILENAME,MODE,PERMS
4937 Opens the file whose filename is given by FILENAME, and associates it
4938 with FILEHANDLE. If FILEHANDLE is an expression, its value is used as
4939 the name of the real filehandle wanted. This function calls the
4940 underlying operating system's C<open> function with the parameters
4941 FILENAME, MODE, PERMS.
4943 The possible values and flag bits of the MODE parameter are
4944 system-dependent; they are available via the standard module C<Fcntl>.
4945 See the documentation of your operating system's C<open> to see which
4946 values and flag bits are available. You may combine several flags
4947 using the C<|>-operator.
4949 Some of the most common values are C<O_RDONLY> for opening the file in
4950 read-only mode, C<O_WRONLY> for opening the file in write-only mode,
4951 and C<O_RDWR> for opening the file in read-write mode, and.
4953 For historical reasons, some values work on almost every system
4954 supported by perl: zero means read-only, one means write-only, and two
4955 means read/write. We know that these values do I<not> work under
4956 OS/390 & VM/ESA Unix and on the Macintosh; you probably don't want to
4957 use them in new code.
4959 If the file named by FILENAME does not exist and the C<open> call creates
4960 it (typically because MODE includes the C<O_CREAT> flag), then the value of
4961 PERMS specifies the permissions of the newly created file. If you omit
4962 the PERMS argument to C<sysopen>, Perl uses the octal value C<0666>.
4963 These permission values need to be in octal, and are modified by your
4964 process's current C<umask>.
4966 In many systems the C<O_EXCL> flag is available for opening files in
4967 exclusive mode. This is B<not> locking: exclusiveness means here that
4968 if the file already exists, sysopen() fails. The C<O_EXCL> wins
4971 Sometimes you may want to truncate an already-existing file: C<O_TRUNC>.
4973 You should seldom if ever use C<0644> as argument to C<sysopen>, because
4974 that takes away the user's option to have a more permissive umask.
4975 Better to omit it. See the perlfunc(1) entry on C<umask> for more
4978 Note that C<sysopen> depends on the fdopen() C library function.
4979 On many UNIX systems, fdopen() is known to fail when file descriptors
4980 exceed a certain value, typically 255. If you need more file
4981 descriptors than that, consider rebuilding Perl to use the C<sfio>
4982 library, or perhaps using the POSIX::open() function.
4984 See L<perlopentut> for a kinder, gentler explanation of opening files.
4986 =item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
4988 =item sysread FILEHANDLE,SCALAR,LENGTH
4990 Attempts to read LENGTH bytes of data into variable SCALAR from the
4991 specified FILEHANDLE, using the system call read(2). It bypasses stdio,
4992 so mixing this with other kinds of reads, C<print>, C<write>,
4993 C<seek>, C<tell>, or C<eof> can cause confusion because stdio
4994 usually buffers data. Returns the number of bytes actually read, C<0>
4995 at end of file, or undef if there was an error. SCALAR will be grown or
4996 shrunk so that the last byte actually read is the last byte of the
4997 scalar after the read.
4999 An OFFSET may be specified to place the read data at some place in the
5000 string other than the beginning. A negative OFFSET specifies
5001 placement at that many bytes counting backwards from the end of the
5002 string. A positive OFFSET greater than the length of SCALAR results
5003 in the string being padded to the required size with C<"\0"> bytes before
5004 the result of the read is appended.
5006 There is no syseof() function, which is ok, since eof() doesn't work
5007 very well on device files (like ttys) anyway. Use sysread() and check
5008 for a return value for 0 to decide whether you're done.
5010 =item sysseek FILEHANDLE,POSITION,WHENCE
5012 Sets FILEHANDLE's system position using the system call lseek(2). It
5013 bypasses stdio, so mixing this with reads (other than C<sysread>),
5014 C<print>, C<write>, C<seek>, C<tell>, or C<eof> may cause confusion.
5015 FILEHANDLE may be an expression whose value gives the name of the
5016 filehandle. The values for WHENCE are C<0> to set the new position to
5017 POSITION, C<1> to set the it to the current position plus POSITION,
5018 and C<2> to set it to EOF plus POSITION (typically negative). For
5019 WHENCE, you may also use the constants C<SEEK_SET>, C<SEEK_CUR>, and
5020 C<SEEK_END> (start of the file, current position, end of the file)
5021 from the Fcntl module.
5023 Returns the new position, or the undefined value on failure. A position
5024 of zero is returned as the string C<"0 but true">; thus C<sysseek> returns
5025 true on success and false on failure, yet you can still easily determine
5030 =item system PROGRAM LIST
5032 Does exactly the same thing as C<exec LIST>, except that a fork is
5033 done first, and the parent process waits for the child process to
5034 complete. Note that argument processing varies depending on the
5035 number of arguments. If there is more than one argument in LIST,
5036 or if LIST is an array with more than one value, starts the program
5037 given by the first element of the list with arguments given by the
5038 rest of the list. If there is only one scalar argument, the argument
5039 is checked for shell metacharacters, and if there are any, the
5040 entire argument is passed to the system's command shell for parsing
5041 (this is C</bin/sh -c> on Unix platforms, but varies on other
5042 platforms). If there are no shell metacharacters in the argument,
5043 it is split into words and passed directly to C<execvp>, which is
5046 Beginning with v5.6.0, Perl will attempt to flush all files opened for
5047 output before any operation that may do a fork, but this may not be
5048 supported on some platforms (see L<perlport>). To be safe, you may need
5049 to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
5050 of C<IO::Handle> on any open handles.
5052 The return value is the exit status of the program as
5053 returned by the C<wait> call. To get the actual exit value divide by
5054 256. See also L</exec>. This is I<not> what you want to use to capture
5055 the output from a command, for that you should use merely backticks or
5056 C<qx//>, as described in L<perlop/"`STRING`">. Return value of -1
5057 indicates a failure to start the program (inspect $! for the reason).
5059 Like C<exec>, C<system> allows you to lie to a program about its name if
5060 you use the C<system PROGRAM LIST> syntax. Again, see L</exec>.
5062 Because C<system> and backticks block C<SIGINT> and C<SIGQUIT>, killing the
5063 program they're running doesn't actually interrupt your program.
5065 @args = ("command", "arg1", "arg2");
5067 or die "system @args failed: $?"
5069 You can check all the failure possibilities by inspecting
5072 $exit_value = $? >> 8;
5073 $signal_num = $? & 127;
5074 $dumped_core = $? & 128;
5076 When the arguments get executed via the system shell, results
5077 and return codes will be subject to its quirks and capabilities.
5078 See L<perlop/"`STRING`"> and L</exec> for details.
5080 =item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
5082 =item syswrite FILEHANDLE,SCALAR,LENGTH
5084 =item syswrite FILEHANDLE,SCALAR
5086 Attempts to write LENGTH bytes of data from variable SCALAR to the
5087 specified FILEHANDLE, using the system call write(2). If LENGTH
5088 is not specified, writes whole SCALAR. It bypasses stdio, so mixing
5089 this with reads (other than C<sysread())>, C<print>, C<write>,
5090 C<seek>, C<tell>, or C<eof> may cause confusion because stdio
5091 usually buffers data. Returns the number of bytes actually written,
5092 or C<undef> if there was an error. If the LENGTH is greater than
5093 the available data in the SCALAR after the OFFSET, only as much
5094 data as is available will be written.
5096 An OFFSET may be specified to write the data from some part of the
5097 string other than the beginning. A negative OFFSET specifies writing
5098 that many bytes counting backwards from the end of the string. In the
5099 case the SCALAR is empty you can use OFFSET but only zero offset.
5101 =item tell FILEHANDLE
5105 Returns the current position for FILEHANDLE, or -1 on error. FILEHANDLE
5106 may be an expression whose value gives the name of the actual filehandle.
5107 If FILEHANDLE is omitted, assumes the file last read.
5109 The return value of tell() for the standard streams like the STDIN
5110 depends on the operating system: it may return -1 or something else.
5111 tell() on pipes, fifos, and sockets usually returns -1.
5113 There is no C<systell> function. Use C<sysseek(FH, 0, 1)> for that.
5115 =item telldir DIRHANDLE
5117 Returns the current position of the C<readdir> routines on DIRHANDLE.
5118 Value may be given to C<seekdir> to access a particular location in a
5119 directory. Has the same caveats about possible directory compaction as
5120 the corresponding system library routine.
5122 =item tie VARIABLE,CLASSNAME,LIST
5124 This function binds a variable to a package class that will provide the
5125 implementation for the variable. VARIABLE is the name of the variable
5126 to be enchanted. CLASSNAME is the name of a class implementing objects
5127 of correct type. Any additional arguments are passed to the C<new>
5128 method of the class (meaning C<TIESCALAR>, C<TIEHANDLE>, C<TIEARRAY>,
5129 or C<TIEHASH>). Typically these are arguments such as might be passed
5130 to the C<dbm_open()> function of C. The object returned by the C<new>
5131 method is also returned by the C<tie> function, which would be useful
5132 if you want to access other methods in CLASSNAME.
5134 Note that functions such as C<keys> and C<values> may return huge lists
5135 when used on large objects, like DBM files. You may prefer to use the
5136 C<each> function to iterate over such. Example:
5138 # print out history file offsets
5140 tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
5141 while (($key,$val) = each %HIST) {
5142 print $key, ' = ', unpack('L',$val), "\n";
5146 A class implementing a hash should have the following methods:
5148 TIEHASH classname, LIST
5150 STORE this, key, value
5155 NEXTKEY this, lastkey
5159 A class implementing an ordinary array should have the following methods:
5161 TIEARRAY classname, LIST
5163 STORE this, key, value
5165 STORESIZE this, count
5171 SPLICE this, offset, length, LIST
5176 A class implementing a file handle should have the following methods:
5178 TIEHANDLE classname, LIST
5179 READ this, scalar, length, offset
5182 WRITE this, scalar, length, offset
5184 PRINTF this, format, LIST
5188 SEEK this, position, whence
5190 OPEN this, mode, LIST
5195 A class implementing a scalar should have the following methods:
5197 TIESCALAR classname, LIST
5203 Not all methods indicated above need be implemented. See L<perltie>,
5204 L<Tie::Hash>, L<Tie::Array>, L<Tie::Scalar>, and L<Tie::Handle>.
5206 Unlike C<dbmopen>, the C<tie> function will not use or require a module
5207 for you--you need to do that explicitly yourself. See L<DB_File>
5208 or the F<Config> module for interesting C<tie> implementations.
5210 For further details see L<perltie>, L<"tied VARIABLE">.
5214 Returns a reference to the object underlying VARIABLE (the same value
5215 that was originally returned by the C<tie> call that bound the variable
5216 to a package.) Returns the undefined value if VARIABLE isn't tied to a
5221 Returns the number of non-leap seconds since whatever time the system
5222 considers to be the epoch (that's 00:00:00, January 1, 1904 for MacOS,
5223 and 00:00:00 UTC, January 1, 1970 for most other systems).
5224 Suitable for feeding to C<gmtime> and C<localtime>.
5226 For measuring time in better granularity than one second,
5227 you may use either the Time::HiRes module from CPAN, or
5228 if you have gettimeofday(2), you may be able to use the
5229 C<syscall> interface of Perl, see L<perlfaq8> for details.
5233 Returns a four-element list giving the user and system times, in
5234 seconds, for this process and the children of this process.
5236 ($user,$system,$cuser,$csystem) = times;
5240 The transliteration operator. Same as C<y///>. See L<perlop>.
5242 =item truncate FILEHANDLE,LENGTH
5244 =item truncate EXPR,LENGTH
5246 Truncates the file opened on FILEHANDLE, or named by EXPR, to the
5247 specified length. Produces a fatal error if truncate isn't implemented
5248 on your system. Returns true if successful, the undefined value
5255 Returns an uppercased version of EXPR. This is the internal function
5256 implementing the C<\U> escape in double-quoted strings.
5257 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
5258 Under Unicode (C<use utf8>) it uses the standard Unicode uppercase mappings. (It
5259 does not attempt to do titlecase mapping on initial letters. See C<ucfirst> for that.)
5261 If EXPR is omitted, uses C<$_>.
5267 Returns the value of EXPR with the first character
5268 in uppercase (titlecase in Unicode). This is
5269 the internal function implementing the C<\u> escape in double-quoted strings.
5270 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
5273 If EXPR is omitted, uses C<$_>.
5279 Sets the umask for the process to EXPR and returns the previous value.
5280 If EXPR is omitted, merely returns the current umask.
5282 The Unix permission C<rwxr-x---> is represented as three sets of three
5283 bits, or three octal digits: C<0750> (the leading 0 indicates octal
5284 and isn't one of the digits). The C<umask> value is such a number
5285 representing disabled permissions bits. The permission (or "mode")
5286 values you pass C<mkdir> or C<sysopen> are modified by your umask, so
5287 even if you tell C<sysopen> to create a file with permissions C<0777>,
5288 if your umask is C<0022> then the file will actually be created with
5289 permissions C<0755>. If your C<umask> were C<0027> (group can't
5290 write; others can't read, write, or execute), then passing
5291 C<sysopen> C<0666> would create a file with mode C<0640> (C<0666 &~
5294 Here's some advice: supply a creation mode of C<0666> for regular
5295 files (in C<sysopen>) and one of C<0777> for directories (in
5296 C<mkdir>) and executable files. This gives users the freedom of
5297 choice: if they want protected files, they might choose process umasks
5298 of C<022>, C<027>, or even the particularly antisocial mask of C<077>.
5299 Programs should rarely if ever make policy decisions better left to
5300 the user. The exception to this is when writing files that should be
5301 kept private: mail files, web browser cookies, I<.rhosts> files, and
5304 If umask(2) is not implemented on your system and you are trying to
5305 restrict access for I<yourself> (i.e., (EXPR & 0700) > 0), produces a
5306 fatal error at run time. If umask(2) is not implemented and you are
5307 not trying to restrict access for yourself, returns C<undef>.
5309 Remember that a umask is a number, usually given in octal; it is I<not> a
5310 string of octal digits. See also L</oct>, if all you have is a string.
5316 Undefines the value of EXPR, which must be an lvalue. Use only on a
5317 scalar value, an array (using C<@>), a hash (using C<%>), a subroutine
5318 (using C<&>), or a typeglob (using <*>). (Saying C<undef $hash{$key}>
5319 will probably not do what you expect on most predefined variables or
5320 DBM list values, so don't do that; see L<delete>.) Always returns the
5321 undefined value. You can omit the EXPR, in which case nothing is
5322 undefined, but you still get an undefined value that you could, for
5323 instance, return from a subroutine, assign to a variable or pass as a
5324 parameter. Examples:
5327 undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'};
5331 undef *xyz; # destroys $xyz, @xyz, %xyz, &xyz, etc.
5332 return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it;
5333 select undef, undef, undef, 0.25;
5334 ($a, $b, undef, $c) = &foo; # Ignore third value returned
5336 Note that this is a unary operator, not a list operator.
5342 Deletes a list of files. Returns the number of files successfully
5345 $cnt = unlink 'a', 'b', 'c';
5349 Note: C<unlink> will not delete directories unless you are superuser and
5350 the B<-U> flag is supplied to Perl. Even if these conditions are
5351 met, be warned that unlinking a directory can inflict damage on your
5352 filesystem. Use C<rmdir> instead.
5354 If LIST is omitted, uses C<$_>.
5356 =item unpack TEMPLATE,EXPR
5358 C<unpack> does the reverse of C<pack>: it takes a string
5359 and expands it out into a list of values.
5360 (In scalar context, it returns merely the first value produced.)
5362 The string is broken into chunks described by the TEMPLATE. Each chunk
5363 is converted separately to a value. Typically, either the string is a result
5364 of C<pack>, or the bytes of the string represent a C structure of some
5367 The TEMPLATE has the same format as in the C<pack> function.
5368 Here's a subroutine that does substring:
5371 my($what,$where,$howmuch) = @_;
5372 unpack("x$where a$howmuch", $what);
5377 sub ordinal { unpack("c",$_[0]); } # same as ord()
5379 In addition to fields allowed in pack(), you may prefix a field with
5380 a %<number> to indicate that
5381 you want a <number>-bit checksum of the items instead of the items
5382 themselves. Default is a 16-bit checksum. Checksum is calculated by
5383 summing numeric values of expanded values (for string fields the sum of
5384 C<ord($char)> is taken, for bit fields the sum of zeroes and ones).
5386 For example, the following
5387 computes the same number as the System V sum program:
5391 unpack("%32C*",<>) % 65535;
5394 The following efficiently counts the number of set bits in a bit vector:
5396 $setbits = unpack("%32b*", $selectmask);
5398 The C<p> and C<P> formats should be used with care. Since Perl
5399 has no way of checking whether the value passed to C<unpack()>
5400 corresponds to a valid memory location, passing a pointer value that's
5401 not known to be valid is likely to have disastrous consequences.
5403 If the repeat count of a field is larger than what the remainder of
5404 the input string allows, repeat count is decreased. If the input string
5405 is longer than one described by the TEMPLATE, the rest is ignored.
5407 See L</pack> for more examples and notes.
5409 =item untie VARIABLE
5411 Breaks the binding between a variable and a package. (See C<tie>.)
5413 =item unshift ARRAY,LIST
5415 Does the opposite of a C<shift>. Or the opposite of a C<push>,
5416 depending on how you look at it. Prepends list to the front of the
5417 array, and returns the new number of elements in the array.
5419 unshift(ARGV, '-e') unless $ARGV[0] =~ /^-/;
5421 Note the LIST is prepended whole, not one element at a time, so the
5422 prepended elements stay in the same order. Use C<reverse> to do the
5425 =item use Module VERSION LIST
5427 =item use Module VERSION
5429 =item use Module LIST
5435 Imports some semantics into the current package from the named module,
5436 generally by aliasing certain subroutine or variable names into your
5437 package. It is exactly equivalent to
5439 BEGIN { require Module; import Module LIST; }
5441 except that Module I<must> be a bareword.
5443 VERSION, which can be specified as a literal of the form v5.6.1, demands
5444 that the current version of Perl (C<$^V> or $PERL_VERSION) be at least
5445 as recent as that version. (For compatibility with older versions of Perl,
5446 a numeric literal will also be interpreted as VERSION.) If the version
5447 of the running Perl interpreter is less than VERSION, then an error
5448 message is printed and Perl exits immediately without attempting to
5449 parse the rest of the file. Compare with L</require>, which can do a
5450 similar check at run time.
5452 use v5.6.1; # compile time version check
5454 use 5.005_03; # float version allowed for compatibility
5456 This is often useful if you need to check the current Perl version before
5457 C<use>ing library modules that have changed in incompatible ways from
5458 older versions of Perl. (We try not to do this more than we have to.)
5460 The C<BEGIN> forces the C<require> and C<import> to happen at compile time. The
5461 C<require> makes sure the module is loaded into memory if it hasn't been
5462 yet. The C<import> is not a builtin--it's just an ordinary static method
5463 call into the C<Module> package to tell the module to import the list of
5464 features back into the current package. The module can implement its
5465 C<import> method any way it likes, though most modules just choose to
5466 derive their C<import> method via inheritance from the C<Exporter> class that
5467 is defined in the C<Exporter> module. See L<Exporter>. If no C<import>
5468 method can be found then the call is skipped.
5470 If you do not want to call the package's C<import> method (for instance,
5471 to stop your namespace from being altered), explicitly supply the empty list:
5475 That is exactly equivalent to
5477 BEGIN { require Module }
5479 If the VERSION argument is present between Module and LIST, then the
5480 C<use> will call the VERSION method in class Module with the given
5481 version as an argument. The default VERSION method, inherited from
5482 the UNIVERSAL class, croaks if the given version is larger than the
5483 value of the variable C<$Module::VERSION>.
5485 Again, there is a distinction between omitting LIST (C<import> called
5486 with no arguments) and an explicit empty LIST C<()> (C<import> not
5487 called). Note that there is no comma after VERSION!
5489 Because this is a wide-open interface, pragmas (compiler directives)
5490 are also implemented this way. Currently implemented pragmas are:
5495 use sigtrap qw(SEGV BUS);
5496 use strict qw(subs vars refs);
5497 use subs qw(afunc blurfl);
5498 use warnings qw(all);
5500 Some of these pseudo-modules import semantics into the current
5501 block scope (like C<strict> or C<integer>, unlike ordinary modules,
5502 which import symbols into the current package (which are effective
5503 through the end of the file).
5505 There's a corresponding C<no> command that unimports meanings imported
5506 by C<use>, i.e., it calls C<unimport Module LIST> instead of C<import>.
5512 If no C<unimport> method can be found the call fails with a fatal error.
5514 See L<perlmodlib> for a list of standard modules and pragmas. See L<perlrun>
5515 for the C<-M> and C<-m> command-line options to perl that give C<use>
5516 functionality from the command-line.
5520 Changes the access and modification times on each file of a list of
5521 files. The first two elements of the list must be the NUMERICAL access
5522 and modification times, in that order. Returns the number of files
5523 successfully changed. The inode change time of each file is set
5524 to the current time. This code has the same effect as the C<touch>
5525 command if the files already exist:
5529 utime $now, $now, @ARGV;
5533 Returns a list consisting of all the values of the named hash. (In a
5534 scalar context, returns the number of values.) The values are
5535 returned in an apparently random order. The actual random order is
5536 subject to change in future versions of perl, but it is guaranteed to
5537 be the same order as either the C<keys> or C<each> function would
5538 produce on the same (unmodified) hash.
5540 Note that the values are not copied, which means modifying them will
5541 modify the contents of the hash:
5543 for (values %hash) { s/foo/bar/g } # modifies %hash values
5544 for (@hash{keys %hash}) { s/foo/bar/g } # same
5546 As a side effect, calling values() resets the HASH's internal iterator.
5547 See also C<keys>, C<each>, and C<sort>.
5549 =item vec EXPR,OFFSET,BITS
5551 Treats the string in EXPR as a bit vector made up of elements of
5552 width BITS, and returns the value of the element specified by OFFSET
5553 as an unsigned integer. BITS therefore specifies the number of bits
5554 that are reserved for each element in the bit vector. This must
5555 be a power of two from 1 to 32 (or 64, if your platform supports
5558 If BITS is 8, "elements" coincide with bytes of the input string.
5560 If BITS is 16 or more, bytes of the input string are grouped into chunks
5561 of size BITS/8, and each group is converted to a number as with
5562 pack()/unpack() with big-endian formats C<n>/C<N> (and analogously
5563 for BITS==64). See L<"pack"> for details.
5565 If bits is 4 or less, the string is broken into bytes, then the bits
5566 of each byte are broken into 8/BITS groups. Bits of a byte are
5567 numbered in a little-endian-ish way, as in C<0x01>, C<0x02>,
5568 C<0x04>, C<0x08>, C<0x10>, C<0x20>, C<0x40>, C<0x80>. For example,
5569 breaking the single input byte C<chr(0x36)> into two groups gives a list
5570 C<(0x6, 0x3)>; breaking it into 4 groups gives C<(0x2, 0x1, 0x3, 0x0)>.
5572 C<vec> may also be assigned to, in which case parentheses are needed
5573 to give the expression the correct precedence as in
5575 vec($image, $max_x * $x + $y, 8) = 3;
5577 If the selected element is outside the string, the value 0 is returned.
5578 If an element off the end of the string is written to, Perl will first
5579 extend the string with sufficiently many zero bytes. It is an error
5580 to try to write off the beginning of the string (i.e. negative OFFSET).
5582 The string should not contain any character with the value > 255 (which
5583 can only happen if you're using UTF8 encoding). If it does, it will be
5584 treated as something which is not UTF8 encoded. When the C<vec> was
5585 assigned to, other parts of your program will also no longer consider the
5586 string to be UTF8 encoded. In other words, if you do have such characters
5587 in your string, vec() will operate on the actual byte string, and not the
5588 conceptual character string.
5590 Strings created with C<vec> can also be manipulated with the logical
5591 operators C<|>, C<&>, C<^>, and C<~>. These operators will assume a bit
5592 vector operation is desired when both operands are strings.
5593 See L<perlop/"Bitwise String Operators">.
5595 The following code will build up an ASCII string saying C<'PerlPerlPerl'>.
5596 The comments show the string after each step. Note that this code works
5597 in the same way on big-endian or little-endian machines.
5600 vec($foo, 0, 32) = 0x5065726C; # 'Perl'
5602 # $foo eq "Perl" eq "\x50\x65\x72\x6C", 32 bits
5603 print vec($foo, 0, 8); # prints 80 == 0x50 == ord('P')
5605 vec($foo, 2, 16) = 0x5065; # 'PerlPe'
5606 vec($foo, 3, 16) = 0x726C; # 'PerlPerl'
5607 vec($foo, 8, 8) = 0x50; # 'PerlPerlP'
5608 vec($foo, 9, 8) = 0x65; # 'PerlPerlPe'
5609 vec($foo, 20, 4) = 2; # 'PerlPerlPe' . "\x02"
5610 vec($foo, 21, 4) = 7; # 'PerlPerlPer'
5612 vec($foo, 45, 2) = 3; # 'PerlPerlPer' . "\x0c"
5613 vec($foo, 93, 1) = 1; # 'PerlPerlPer' . "\x2c"
5614 vec($foo, 94, 1) = 1; # 'PerlPerlPerl'
5617 To transform a bit vector into a string or list of 0's and 1's, use these:
5619 $bits = unpack("b*", $vector);
5620 @bits = split(//, unpack("b*", $vector));
5622 If you know the exact length in bits, it can be used in place of the C<*>.
5624 Here is an example to illustrate how the bits actually fall in place:
5630 unpack("V",$_) 01234567890123456789012345678901
5631 ------------------------------------------------------------------
5636 for ($shift=0; $shift < $width; ++$shift) {
5637 for ($off=0; $off < 32/$width; ++$off) {
5638 $str = pack("B*", "0"x32);
5639 $bits = (1<<$shift);
5640 vec($str, $off, $width) = $bits;
5641 $res = unpack("b*",$str);
5642 $val = unpack("V", $str);
5649 vec($_,@#,@#) = @<< == @######### @>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
5650 $off, $width, $bits, $val, $res
5654 Regardless of the machine architecture on which it is run, the above
5655 example should print the following table:
5658 unpack("V",$_) 01234567890123456789012345678901
5659 ------------------------------------------------------------------
5660 vec($_, 0, 1) = 1 == 1 10000000000000000000000000000000
5661 vec($_, 1, 1) = 1 == 2 01000000000000000000000000000000
5662 vec($_, 2, 1) = 1 == 4 00100000000000000000000000000000
5663 vec($_, 3, 1) = 1 == 8 00010000000000000000000000000000
5664 vec($_, 4, 1) = 1 == 16 00001000000000000000000000000000
5665 vec($_, 5, 1) = 1 == 32 00000100000000000000000000000000
5666 vec($_, 6, 1) = 1 == 64 00000010000000000000000000000000
5667 vec($_, 7, 1) = 1 == 128 00000001000000000000000000000000
5668 vec($_, 8, 1) = 1 == 256 00000000100000000000000000000000
5669 vec($_, 9, 1) = 1 == 512 00000000010000000000000000000000
5670 vec($_,10, 1) = 1 == 1024 00000000001000000000000000000000
5671 vec($_,11, 1) = 1 == 2048 00000000000100000000000000000000
5672 vec($_,12, 1) = 1 == 4096 00000000000010000000000000000000
5673 vec($_,13, 1) = 1 == 8192 00000000000001000000000000000000
5674 vec($_,14, 1) = 1 == 16384 00000000000000100000000000000000
5675 vec($_,15, 1) = 1 == 32768 00000000000000010000000000000000
5676 vec($_,16, 1) = 1 == 65536 00000000000000001000000000000000
5677 vec($_,17, 1) = 1 == 131072 00000000000000000100000000000000
5678 vec($_,18, 1) = 1 == 262144 00000000000000000010000000000000
5679 vec($_,19, 1) = 1 == 524288 00000000000000000001000000000000
5680 vec($_,20, 1) = 1 == 1048576 00000000000000000000100000000000
5681 vec($_,21, 1) = 1 == 2097152 00000000000000000000010000000000
5682 vec($_,22, 1) = 1 == 4194304 00000000000000000000001000000000
5683 vec($_,23, 1) = 1 == 8388608 00000000000000000000000100000000
5684 vec($_,24, 1) = 1 == 16777216 00000000000000000000000010000000
5685 vec($_,25, 1) = 1 == 33554432 00000000000000000000000001000000
5686 vec($_,26, 1) = 1 == 67108864 00000000000000000000000000100000
5687 vec($_,27, 1) = 1 == 134217728 00000000000000000000000000010000
5688 vec($_,28, 1) = 1 == 268435456 00000000000000000000000000001000
5689 vec($_,29, 1) = 1 == 536870912 00000000000000000000000000000100
5690 vec($_,30, 1) = 1 == 1073741824 00000000000000000000000000000010
5691 vec($_,31, 1) = 1 == 2147483648 00000000000000000000000000000001
5692 vec($_, 0, 2) = 1 == 1 10000000000000000000000000000000
5693 vec($_, 1, 2) = 1 == 4 00100000000000000000000000000000
5694 vec($_, 2, 2) = 1 == 16 00001000000000000000000000000000
5695 vec($_, 3, 2) = 1 == 64 00000010000000000000000000000000
5696 vec($_, 4, 2) = 1 == 256 00000000100000000000000000000000
5697 vec($_, 5, 2) = 1 == 1024 00000000001000000000000000000000
5698 vec($_, 6, 2) = 1 == 4096 00000000000010000000000000000000
5699 vec($_, 7, 2) = 1 == 16384 00000000000000100000000000000000
5700 vec($_, 8, 2) = 1 == 65536 00000000000000001000000000000000
5701 vec($_, 9, 2) = 1 == 262144 00000000000000000010000000000000
5702 vec($_,10, 2) = 1 == 1048576 00000000000000000000100000000000
5703 vec($_,11, 2) = 1 == 4194304 00000000000000000000001000000000
5704 vec($_,12, 2) = 1 == 16777216 00000000000000000000000010000000
5705 vec($_,13, 2) = 1 == 67108864 00000000000000000000000000100000
5706 vec($_,14, 2) = 1 == 268435456 00000000000000000000000000001000
5707 vec($_,15, 2) = 1 == 1073741824 00000000000000000000000000000010
5708 vec($_, 0, 2) = 2 == 2 01000000000000000000000000000000
5709 vec($_, 1, 2) = 2 == 8 00010000000000000000000000000000
5710 vec($_, 2, 2) = 2 == 32 00000100000000000000000000000000
5711 vec($_, 3, 2) = 2 == 128 00000001000000000000000000000000
5712 vec($_, 4, 2) = 2 == 512 00000000010000000000000000000000
5713 vec($_, 5, 2) = 2 == 2048 00000000000100000000000000000000
5714 vec($_, 6, 2) = 2 == 8192 00000000000001000000000000000000
5715 vec($_, 7, 2) = 2 == 32768 00000000000000010000000000000000
5716 vec($_, 8, 2) = 2 == 131072 00000000000000000100000000000000
5717 vec($_, 9, 2) = 2 == 524288 00000000000000000001000000000000
5718 vec($_,10, 2) = 2 == 2097152 00000000000000000000010000000000
5719 vec($_,11, 2) = 2 == 8388608 00000000000000000000000100000000
5720 vec($_,12, 2) = 2 == 33554432 00000000000000000000000001000000
5721 vec($_,13, 2) = 2 == 134217728 00000000000000000000000000010000
5722 vec($_,14, 2) = 2 == 536870912 00000000000000000000000000000100
5723 vec($_,15, 2) = 2 == 2147483648 00000000000000000000000000000001
5724 vec($_, 0, 4) = 1 == 1 10000000000000000000000000000000
5725 vec($_, 1, 4) = 1 == 16 00001000000000000000000000000000
5726 vec($_, 2, 4) = 1 == 256 00000000100000000000000000000000
5727 vec($_, 3, 4) = 1 == 4096 00000000000010000000000000000000
5728 vec($_, 4, 4) = 1 == 65536 00000000000000001000000000000000
5729 vec($_, 5, 4) = 1 == 1048576 00000000000000000000100000000000
5730 vec($_, 6, 4) = 1 == 16777216 00000000000000000000000010000000
5731 vec($_, 7, 4) = 1 == 268435456 00000000000000000000000000001000
5732 vec($_, 0, 4) = 2 == 2 01000000000000000000000000000000
5733 vec($_, 1, 4) = 2 == 32 00000100000000000000000000000000
5734 vec($_, 2, 4) = 2 == 512 00000000010000000000000000000000
5735 vec($_, 3, 4) = 2 == 8192 00000000000001000000000000000000
5736 vec($_, 4, 4) = 2 == 131072 00000000000000000100000000000000
5737 vec($_, 5, 4) = 2 == 2097152 00000000000000000000010000000000
5738 vec($_, 6, 4) = 2 == 33554432 00000000000000000000000001000000
5739 vec($_, 7, 4) = 2 == 536870912 00000000000000000000000000000100
5740 vec($_, 0, 4) = 4 == 4 00100000000000000000000000000000
5741 vec($_, 1, 4) = 4 == 64 00000010000000000000000000000000
5742 vec($_, 2, 4) = 4 == 1024 00000000001000000000000000000000
5743 vec($_, 3, 4) = 4 == 16384 00000000000000100000000000000000
5744 vec($_, 4, 4) = 4 == 262144 00000000000000000010000000000000
5745 vec($_, 5, 4) = 4 == 4194304 00000000000000000000001000000000
5746 vec($_, 6, 4) = 4 == 67108864 00000000000000000000000000100000
5747 vec($_, 7, 4) = 4 == 1073741824 00000000000000000000000000000010
5748 vec($_, 0, 4) = 8 == 8 00010000000000000000000000000000
5749 vec($_, 1, 4) = 8 == 128 00000001000000000000000000000000
5750 vec($_, 2, 4) = 8 == 2048 00000000000100000000000000000000
5751 vec($_, 3, 4) = 8 == 32768 00000000000000010000000000000000
5752 vec($_, 4, 4) = 8 == 524288 00000000000000000001000000000000
5753 vec($_, 5, 4) = 8 == 8388608 00000000000000000000000100000000
5754 vec($_, 6, 4) = 8 == 134217728 00000000000000000000000000010000
5755 vec($_, 7, 4) = 8 == 2147483648 00000000000000000000000000000001
5756 vec($_, 0, 8) = 1 == 1 10000000000000000000000000000000
5757 vec($_, 1, 8) = 1 == 256 00000000100000000000000000000000
5758 vec($_, 2, 8) = 1 == 65536 00000000000000001000000000000000
5759 vec($_, 3, 8) = 1 == 16777216 00000000000000000000000010000000
5760 vec($_, 0, 8) = 2 == 2 01000000000000000000000000000000
5761 vec($_, 1, 8) = 2 == 512 00000000010000000000000000000000
5762 vec($_, 2, 8) = 2 == 131072 00000000000000000100000000000000
5763 vec($_, 3, 8) = 2 == 33554432 00000000000000000000000001000000
5764 vec($_, 0, 8) = 4 == 4 00100000000000000000000000000000
5765 vec($_, 1, 8) = 4 == 1024 00000000001000000000000000000000
5766 vec($_, 2, 8) = 4 == 262144 00000000000000000010000000000000
5767 vec($_, 3, 8) = 4 == 67108864 00000000000000000000000000100000
5768 vec($_, 0, 8) = 8 == 8 00010000000000000000000000000000
5769 vec($_, 1, 8) = 8 == 2048 00000000000100000000000000000000
5770 vec($_, 2, 8) = 8 == 524288 00000000000000000001000000000000
5771 vec($_, 3, 8) = 8 == 134217728 00000000000000000000000000010000
5772 vec($_, 0, 8) = 16 == 16 00001000000000000000000000000000
5773 vec($_, 1, 8) = 16 == 4096 00000000000010000000000000000000
5774 vec($_, 2, 8) = 16 == 1048576 00000000000000000000100000000000
5775 vec($_, 3, 8) = 16 == 268435456 00000000000000000000000000001000
5776 vec($_, 0, 8) = 32 == 32 00000100000000000000000000000000
5777 vec($_, 1, 8) = 32 == 8192 00000000000001000000000000000000
5778 vec($_, 2, 8) = 32 == 2097152 00000000000000000000010000000000
5779 vec($_, 3, 8) = 32 == 536870912 00000000000000000000000000000100
5780 vec($_, 0, 8) = 64 == 64 00000010000000000000000000000000
5781 vec($_, 1, 8) = 64 == 16384 00000000000000100000000000000000
5782 vec($_, 2, 8) = 64 == 4194304 00000000000000000000001000000000
5783 vec($_, 3, 8) = 64 == 1073741824 00000000000000000000000000000010
5784 vec($_, 0, 8) = 128 == 128 00000001000000000000000000000000
5785 vec($_, 1, 8) = 128 == 32768 00000000000000010000000000000000
5786 vec($_, 2, 8) = 128 == 8388608 00000000000000000000000100000000
5787 vec($_, 3, 8) = 128 == 2147483648 00000000000000000000000000000001
5791 Behaves like the wait(2) system call on your system: it waits for a child
5792 process to terminate and returns the pid of the deceased process, or
5793 C<-1> if there are no child processes. The status is returned in C<$?>.
5794 Note that a return value of C<-1> could mean that child processes are
5795 being automatically reaped, as described in L<perlipc>.
5797 =item waitpid PID,FLAGS
5799 Waits for a particular child process to terminate and returns the pid of
5800 the deceased process, or C<-1> if there is no such child process. On some
5801 systems, a value of 0 indicates that there are processes still running.
5802 The status is returned in C<$?>. If you say
5804 use POSIX ":sys_wait_h";
5807 $kid = waitpid(-1,&WNOHANG);
5810 then you can do a non-blocking wait for all pending zombie processes.
5811 Non-blocking wait is available on machines supporting either the
5812 waitpid(2) or wait4(2) system calls. However, waiting for a particular
5813 pid with FLAGS of C<0> is implemented everywhere. (Perl emulates the
5814 system call by remembering the status values of processes that have
5815 exited but have not been harvested by the Perl script yet.)
5817 Note that on some systems, a return value of C<-1> could mean that child
5818 processes are being automatically reaped. See L<perlipc> for details,
5819 and for other examples.
5823 Returns true if the context of the currently executing subroutine is
5824 looking for a list value. Returns false if the context is looking
5825 for a scalar. Returns the undefined value if the context is looking
5826 for no value (void context).
5828 return unless defined wantarray; # don't bother doing more
5829 my @a = complex_calculation();
5830 return wantarray ? @a : "@a";
5832 This function should have been named wantlist() instead.
5836 Produces a message on STDERR just like C<die>, but doesn't exit or throw
5839 If LIST is empty and C<$@> already contains a value (typically from a
5840 previous eval) that value is used after appending C<"\t...caught">
5841 to C<$@>. This is useful for staying almost, but not entirely similar to
5844 If C<$@> is empty then the string C<"Warning: Something's wrong"> is used.
5846 No message is printed if there is a C<$SIG{__WARN__}> handler
5847 installed. It is the handler's responsibility to deal with the message
5848 as it sees fit (like, for instance, converting it into a C<die>). Most
5849 handlers must therefore make arrangements to actually display the
5850 warnings that they are not prepared to deal with, by calling C<warn>
5851 again in the handler. Note that this is quite safe and will not
5852 produce an endless loop, since C<__WARN__> hooks are not called from
5855 You will find this behavior is slightly different from that of
5856 C<$SIG{__DIE__}> handlers (which don't suppress the error text, but can
5857 instead call C<die> again to change it).
5859 Using a C<__WARN__> handler provides a powerful way to silence all
5860 warnings (even the so-called mandatory ones). An example:
5862 # wipe out *all* compile-time warnings
5863 BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } }
5865 my $foo = 20; # no warning about duplicate my $foo,
5866 # but hey, you asked for it!
5867 # no compile-time or run-time warnings before here
5870 # run-time warnings enabled after here
5871 warn "\$foo is alive and $foo!"; # does show up
5873 See L<perlvar> for details on setting C<%SIG> entries, and for more
5874 examples. See the Carp module for other kinds of warnings using its
5875 carp() and cluck() functions.
5877 =item write FILEHANDLE
5883 Writes a formatted record (possibly multi-line) to the specified FILEHANDLE,
5884 using the format associated with that file. By default the format for
5885 a file is the one having the same name as the filehandle, but the
5886 format for the current output channel (see the C<select> function) may be set
5887 explicitly by assigning the name of the format to the C<$~> variable.
5889 Top of form processing is handled automatically: if there is
5890 insufficient room on the current page for the formatted record, the
5891 page is advanced by writing a form feed, a special top-of-page format
5892 is used to format the new page header, and then the record is written.
5893 By default the top-of-page format is the name of the filehandle with
5894 "_TOP" appended, but it may be dynamically set to the format of your
5895 choice by assigning the name to the C<$^> variable while the filehandle is
5896 selected. The number of lines remaining on the current page is in
5897 variable C<$->, which can be set to C<0> to force a new page.
5899 If FILEHANDLE is unspecified, output goes to the current default output
5900 channel, which starts out as STDOUT but may be changed by the
5901 C<select> operator. If the FILEHANDLE is an EXPR, then the expression
5902 is evaluated and the resulting string is used to look up the name of
5903 the FILEHANDLE at run time. For more on formats, see L<perlform>.
5905 Note that write is I<not> the opposite of C<read>. Unfortunately.
5909 The transliteration operator. Same as C<tr///>. See L<perlop>.