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's used primarily to remove the newline from the end of an
634 input record, but is much more efficient than C<s/\n//> because it neither
635 scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
639 chop; # avoid \n on last field
644 If VARIABLE is a hash, it chops the hash's values, but not its keys.
646 You can actually chop anything that's an lvalue, including an assignment:
649 chop($answer = <STDIN>);
651 If you chop a list, each element is chopped. Only the value of the
652 last C<chop> is returned.
654 Note that C<chop> returns the last character. To return all but the last
655 character, use C<substr($string, 0, -1)>.
659 Changes the owner (and group) of a list of files. The first two
660 elements of the list must be the I<numeric> uid and gid, in that
661 order. A value of -1 in either position is interpreted by most
662 systems to leave that value unchanged. Returns the number of files
663 successfully changed.
665 $cnt = chown $uid, $gid, 'foo', 'bar';
666 chown $uid, $gid, @filenames;
668 Here's an example that looks up nonnumeric uids in the passwd file:
671 chomp($user = <STDIN>);
673 chomp($pattern = <STDIN>);
675 ($login,$pass,$uid,$gid) = getpwnam($user)
676 or die "$user not in passwd file";
678 @ary = glob($pattern); # expand filenames
679 chown $uid, $gid, @ary;
681 On most systems, you are not allowed to change the ownership of the
682 file unless you're the superuser, although you should be able to change
683 the group to any of your secondary groups. On insecure systems, these
684 restrictions may be relaxed, but this is not a portable assumption.
685 On POSIX systems, you can detect this condition this way:
687 use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
688 $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
694 Returns the character represented by that NUMBER in the character set.
695 For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
696 chr(0x263a) is a Unicode smiley face. Within the scope of C<use utf8>,
697 characters higher than 127 are encoded in Unicode; if you don't want
698 this, temporarily C<use bytes> or use C<pack("C*",...)>
700 For the reverse, use L</ord>.
701 See L<utf8> for more about Unicode.
703 If NUMBER is omitted, uses C<$_>.
705 =item chroot FILENAME
709 This function works like the system call by the same name: it makes the
710 named directory the new root directory for all further pathnames that
711 begin with a C</> by your process and all its children. (It doesn't
712 change your current working directory, which is unaffected.) For security
713 reasons, this call is restricted to the superuser. If FILENAME is
714 omitted, does a C<chroot> to C<$_>.
716 =item close FILEHANDLE
720 Closes the file or pipe associated with the file handle, returning true
721 only if stdio successfully flushes buffers and closes the system file
722 descriptor. Closes the currently selected filehandle if the argument
725 You don't have to close FILEHANDLE if you are immediately going to do
726 another C<open> on it, because C<open> will close it for you. (See
727 C<open>.) However, an explicit C<close> on an input file resets the line
728 counter (C<$.>), while the implicit close done by C<open> does not.
730 If the file handle came from a piped open C<close> will additionally
731 return false if one of the other system calls involved fails or if the
732 program exits with non-zero status. (If the only problem was that the
733 program exited non-zero C<$!> will be set to C<0>.) Closing a pipe
734 also waits for the process executing on the pipe to complete, in case you
735 want to look at the output of the pipe afterwards, and
736 implicitly puts the exit status value of that command into C<$?>.
738 Prematurely closing the read end of a pipe (i.e. before the process
739 writing to it at the other end has closed it) will result in a
740 SIGPIPE being delivered to the writer. If the other end can't
741 handle that, be sure to read all the data before closing the pipe.
745 open(OUTPUT, '|sort >foo') # pipe to sort
746 or die "Can't start sort: $!";
747 #... # print stuff to output
748 close OUTPUT # wait for sort to finish
749 or warn $! ? "Error closing sort pipe: $!"
750 : "Exit status $? from sort";
751 open(INPUT, 'foo') # get sort's results
752 or die "Can't open 'foo' for input: $!";
754 FILEHANDLE may be an expression whose value can be used as an indirect
755 filehandle, usually the real filehandle name.
757 =item closedir DIRHANDLE
759 Closes a directory opened by C<opendir> and returns the success of that
762 DIRHANDLE may be an expression whose value can be used as an indirect
763 dirhandle, usually the real dirhandle name.
765 =item connect SOCKET,NAME
767 Attempts to connect to a remote socket, just as the connect system call
768 does. Returns true if it succeeded, false otherwise. NAME should be a
769 packed address of the appropriate type for the socket. See the examples in
770 L<perlipc/"Sockets: Client/Server Communication">.
774 Actually a flow control statement rather than a function. If there is a
775 C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
776 C<foreach>), it is always executed just before the conditional is about to
777 be evaluated again, just like the third part of a C<for> loop in C. Thus
778 it can be used to increment a loop variable, even when the loop has been
779 continued via the C<next> statement (which is similar to the C C<continue>
782 C<last>, C<next>, or C<redo> may appear within a C<continue>
783 block. C<last> and C<redo> will behave as if they had been executed within
784 the main block. So will C<next>, but since it will execute a C<continue>
785 block, it may be more entertaining.
788 ### redo always comes here
791 ### next always comes here
793 # then back the top to re-check EXPR
795 ### last always comes here
797 Omitting the C<continue> section is semantically equivalent to using an
798 empty one, logically enough. In that case, C<next> goes directly back
799 to check the condition at the top of the loop.
805 Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
806 takes cosine of C<$_>.
808 For the inverse cosine operation, you may use the C<Math::Trig::acos()>
809 function, or use this relation:
811 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
813 =item crypt PLAINTEXT,SALT
815 Encrypts a string exactly like the crypt(3) function in the C library
816 (assuming that you actually have a version there that has not been
817 extirpated as a potential munition). This can prove useful for checking
818 the password file for lousy passwords, amongst other things. Only the
819 guys wearing white hats should do this.
821 Note that C<crypt> is intended to be a one-way function, much like breaking
822 eggs to make an omelette. There is no (known) corresponding decrypt
823 function. As a result, this function isn't all that useful for
824 cryptography. (For that, see your nearby CPAN mirror.)
826 When verifying an existing encrypted string you should use the encrypted
827 text as the salt (like C<crypt($plain, $crypted) eq $crypted>). This
828 allows your code to work with the standard C<crypt> and with more
829 exotic implementations. When choosing a new salt create a random two
830 character string whose characters come from the set C<[./0-9A-Za-z]>
831 (like C<join '', ('.', '/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>).
833 Here's an example that makes sure that whoever runs this program knows
836 $pwd = (getpwuid($<))[1];
840 chomp($word = <STDIN>);
844 if (crypt($word, $pwd) ne $pwd) {
850 Of course, typing in your own password to whoever asks you
853 The L<crypt> function is unsuitable for encrypting large quantities
854 of data, not least of all because you can't get the information
855 back. Look at the F<by-module/Crypt> and F<by-module/PGP> directories
856 on your favorite CPAN mirror for a slew of potentially useful
861 [This function has been largely superseded by the C<untie> function.]
863 Breaks the binding between a DBM file and a hash.
865 =item dbmopen HASH,DBNAME,MASK
867 [This function has been largely superseded by the C<tie> function.]
869 This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
870 hash. HASH is the name of the hash. (Unlike normal C<open>, the first
871 argument is I<not> a filehandle, even though it looks like one). DBNAME
872 is the name of the database (without the F<.dir> or F<.pag> extension if
873 any). If the database does not exist, it is created with protection
874 specified by MASK (as modified by the C<umask>). If your system supports
875 only the older DBM functions, you may perform only one C<dbmopen> in your
876 program. In older versions of Perl, if your system had neither DBM nor
877 ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
880 If you don't have write access to the DBM file, you can only read hash
881 variables, not set them. If you want to test whether you can write,
882 either use file tests or try setting a dummy hash entry inside an C<eval>,
883 which will trap the error.
885 Note that functions such as C<keys> and C<values> may return huge lists
886 when used on large DBM files. You may prefer to use the C<each>
887 function to iterate over large DBM files. Example:
889 # print out history file offsets
890 dbmopen(%HIST,'/usr/lib/news/history',0666);
891 while (($key,$val) = each %HIST) {
892 print $key, ' = ', unpack('L',$val), "\n";
896 See also L<AnyDBM_File> for a more general description of the pros and
897 cons of the various dbm approaches, as well as L<DB_File> for a particularly
900 You can control which DBM library you use by loading that library
901 before you call dbmopen():
904 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
905 or die "Can't open netscape history file: $!";
911 Returns a Boolean value telling whether EXPR has a value other than
912 the undefined value C<undef>. If EXPR is not present, C<$_> will be
915 Many operations return C<undef> to indicate failure, end of file,
916 system error, uninitialized variable, and other exceptional
917 conditions. This function allows you to distinguish C<undef> from
918 other values. (A simple Boolean test will not distinguish among
919 C<undef>, zero, the empty string, and C<"0">, which are all equally
920 false.) Note that since C<undef> is a valid scalar, its presence
921 doesn't I<necessarily> indicate an exceptional condition: C<pop>
922 returns C<undef> when its argument is an empty array, I<or> when the
923 element to return happens to be C<undef>.
925 You may also use C<defined(&func)> to check whether subroutine C<&func>
926 has ever been defined. The return value is unaffected by any forward
927 declarations of C<&foo>. Note that a subroutine which is not defined
928 may still be callable: its package may have an C<AUTOLOAD> method that
929 makes it spring into existence the first time that it is called -- see
932 Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
933 used to report whether memory for that aggregate has ever been
934 allocated. This behavior may disappear in future versions of Perl.
935 You should instead use a simple test for size:
937 if (@an_array) { print "has array elements\n" }
938 if (%a_hash) { print "has hash members\n" }
940 When used on a hash element, it tells you whether the value is defined,
941 not whether the key exists in the hash. Use L</exists> for the latter
946 print if defined $switch{'D'};
947 print "$val\n" while defined($val = pop(@ary));
948 die "Can't readlink $sym: $!"
949 unless defined($value = readlink $sym);
950 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
951 $debugging = 0 unless defined $debugging;
953 Note: Many folks tend to overuse C<defined>, and then are surprised to
954 discover that the number C<0> and C<""> (the zero-length string) are, in fact,
955 defined values. For example, if you say
959 The pattern match succeeds, and C<$1> is defined, despite the fact that it
960 matched "nothing". But it didn't really match nothing--rather, it
961 matched something that happened to be zero characters long. This is all
962 very above-board and honest. When a function returns an undefined value,
963 it's an admission that it couldn't give you an honest answer. So you
964 should use C<defined> only when you're questioning the integrity of what
965 you're trying to do. At other times, a simple comparison to C<0> or C<""> is
968 See also L</undef>, L</exists>, L</ref>.
972 Given an expression that specifies a hash element, array element, hash slice,
973 or array slice, deletes the specified element(s) from the hash or array.
974 In the case of an array, if the array elements happen to be at the end,
975 the size of the array will shrink to the highest element that tests
976 true for exists() (or 0 if no such element exists).
978 Returns each element so deleted or the undefined value if there was no such
979 element. Deleting from C<$ENV{}> modifies the environment. Deleting from
980 a hash tied to a DBM file deletes the entry from the DBM file. Deleting
981 from a C<tie>d hash or array may not necessarily return anything.
983 Deleting an array element effectively returns that position of the array
984 to its initial, uninitialized state. Subsequently testing for the same
985 element with exists() will return false. Note that deleting array
986 elements in the middle of an array will not shift the index of the ones
987 after them down--use splice() for that. See L</exists>.
989 The following (inefficiently) deletes all the values of %HASH and @ARRAY:
991 foreach $key (keys %HASH) {
995 foreach $index (0 .. $#ARRAY) {
996 delete $ARRAY[$index];
1001 delete @HASH{keys %HASH};
1003 delete @ARRAY[0 .. $#ARRAY];
1005 But both of these are slower than just assigning the empty list
1006 or undefining %HASH or @ARRAY:
1008 %HASH = (); # completely empty %HASH
1009 undef %HASH; # forget %HASH ever existed
1011 @ARRAY = (); # completely empty @ARRAY
1012 undef @ARRAY; # forget @ARRAY ever existed
1014 Note that the EXPR can be arbitrarily complicated as long as the final
1015 operation is a hash element, array element, hash slice, or array slice
1018 delete $ref->[$x][$y]{$key};
1019 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
1021 delete $ref->[$x][$y][$index];
1022 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
1026 Outside an C<eval>, prints the value of LIST to C<STDERR> and
1027 exits with the current value of C<$!> (errno). If C<$!> is C<0>,
1028 exits with the value of C<<< ($? >> 8) >>> (backtick `command`
1029 status). If C<<< ($? >> 8) >>> is C<0>, exits with C<255>. Inside
1030 an C<eval(),> the error message is stuffed into C<$@> and the
1031 C<eval> is terminated with the undefined value. This makes
1032 C<die> the way to raise an exception.
1034 Equivalent examples:
1036 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
1037 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
1039 If the value of EXPR does not end in a newline, the current script line
1040 number and input line number (if any) are also printed, and a newline
1041 is supplied. Note that the "input line number" (also known as "chunk")
1042 is subject to whatever notion of "line" happens to be currently in
1043 effect, and is also available as the special variable C<$.>.
1044 See L<perlvar/"$/"> and L<perlvar/"$.">.
1046 Hint: sometimes appending C<", stopped"> to your message
1047 will cause it to make better sense when the string C<"at foo line 123"> is
1048 appended. Suppose you are running script "canasta".
1050 die "/etc/games is no good";
1051 die "/etc/games is no good, stopped";
1053 produce, respectively
1055 /etc/games is no good at canasta line 123.
1056 /etc/games is no good, stopped at canasta line 123.
1058 See also exit(), warn(), and the Carp module.
1060 If LIST is empty and C<$@> already contains a value (typically from a
1061 previous eval) that value is reused after appending C<"\t...propagated">.
1062 This is useful for propagating exceptions:
1065 die unless $@ =~ /Expected exception/;
1067 If C<$@> is empty then the string C<"Died"> is used.
1069 die() can also be called with a reference argument. If this happens to be
1070 trapped within an eval(), $@ contains the reference. This behavior permits
1071 a more elaborate exception handling implementation using objects that
1072 maintain arbitrary state about the nature of the exception. Such a scheme
1073 is sometimes preferable to matching particular string values of $@ using
1074 regular expressions. Here's an example:
1076 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
1078 if (ref($@) && UNIVERSAL::isa($@,"Some::Module::Exception")) {
1079 # handle Some::Module::Exception
1082 # handle all other possible exceptions
1086 Because perl will stringify uncaught exception messages before displaying
1087 them, you may want to overload stringification operations on such custom
1088 exception objects. See L<overload> for details about that.
1090 You can arrange for a callback to be run just before the C<die>
1091 does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
1092 handler will be called with the error text and can change the error
1093 message, if it sees fit, by calling C<die> again. See
1094 L<perlvar/$SIG{expr}> for details on setting C<%SIG> entries, and
1095 L<"eval BLOCK"> for some examples. Although this feature was meant
1096 to be run only right before your program was to exit, this is not
1097 currently the case--the C<$SIG{__DIE__}> hook is currently called
1098 even inside eval()ed blocks/strings! If one wants the hook to do
1099 nothing in such situations, put
1103 as the first line of the handler (see L<perlvar/$^S>). Because
1104 this promotes strange action at a distance, this counterintuitive
1105 behavior may be fixed in a future release.
1109 Not really a function. Returns the value of the last command in the
1110 sequence of commands indicated by BLOCK. When modified by a loop
1111 modifier, executes the BLOCK once before testing the loop condition.
1112 (On other statements the loop modifiers test the conditional first.)
1114 C<do BLOCK> does I<not> count as a loop, so the loop control statements
1115 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1116 See L<perlsyn> for alternative strategies.
1118 =item do SUBROUTINE(LIST)
1120 A deprecated form of subroutine call. See L<perlsub>.
1124 Uses the value of EXPR as a filename and executes the contents of the
1125 file as a Perl script. Its primary use is to include subroutines
1126 from a Perl subroutine library.
1132 scalar eval `cat stat.pl`;
1134 except that it's more efficient and concise, keeps track of the current
1135 filename for error messages, searches the @INC libraries, and updates
1136 C<%INC> if the file is found. See L<perlvar/Predefined Names> for these
1137 variables. It also differs in that code evaluated with C<do FILENAME>
1138 cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1139 same, however, in that it does reparse the file every time you call it,
1140 so you probably don't want to do this inside a loop.
1142 If C<do> cannot read the file, it returns undef and sets C<$!> to the
1143 error. If C<do> can read the file but cannot compile it, it
1144 returns undef and sets an error message in C<$@>. If the file is
1145 successfully compiled, C<do> returns the value of the last expression
1148 Note that inclusion of library modules is better done with the
1149 C<use> and C<require> operators, which also do automatic error checking
1150 and raise an exception if there's a problem.
1152 You might like to use C<do> to read in a program configuration
1153 file. Manual error checking can be done this way:
1155 # read in config files: system first, then user
1156 for $file ("/share/prog/defaults.rc",
1157 "$ENV{HOME}/.someprogrc")
1159 unless ($return = do $file) {
1160 warn "couldn't parse $file: $@" if $@;
1161 warn "couldn't do $file: $!" unless defined $return;
1162 warn "couldn't run $file" unless $return;
1170 This function causes an immediate core dump. See also the B<-u>
1171 command-line switch in L<perlrun>, which does the same thing.
1172 Primarily this is so that you can use the B<undump> program (not
1173 supplied) to turn your core dump into an executable binary after
1174 having initialized all your variables at the beginning of the
1175 program. When the new binary is executed it will begin by executing
1176 a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1177 Think of it as a goto with an intervening core dump and reincarnation.
1178 If C<LABEL> is omitted, restarts the program from the top.
1180 B<WARNING>: Any files opened at the time of the dump will I<not>
1181 be open any more when the program is reincarnated, with possible
1182 resulting confusion on the part of Perl.
1184 This function is now largely obsolete, partly because it's very
1185 hard to convert a core file into an executable, and because the
1186 real compiler backends for generating portable bytecode and compilable
1187 C code have superseded it.
1189 If you're looking to use L<dump> to speed up your program, consider
1190 generating bytecode or native C code as described in L<perlcc>. If
1191 you're just trying to accelerate a CGI script, consider using the
1192 C<mod_perl> extension to B<Apache>, or the CPAN module, Fast::CGI.
1193 You might also consider autoloading or selfloading, which at least
1194 make your program I<appear> to run faster.
1198 When called in list context, returns a 2-element list consisting of the
1199 key and value for the next element of a hash, so that you can iterate over
1200 it. When called in scalar context, returns only the key for the next
1201 element in the hash.
1203 Entries are returned in an apparently random order. The actual random
1204 order is subject to change in future versions of perl, but it is guaranteed
1205 to be in the same order as either the C<keys> or C<values> function
1206 would produce on the same (unmodified) hash.
1208 When the hash is entirely read, a null array is returned in list context
1209 (which when assigned produces a false (C<0>) value), and C<undef> in
1210 scalar context. The next call to C<each> after that will start iterating
1211 again. There is a single iterator for each hash, shared by all C<each>,
1212 C<keys>, and C<values> function calls in the program; it can be reset by
1213 reading all the elements from the hash, or by evaluating C<keys HASH> or
1214 C<values HASH>. If you add or delete elements of a hash while you're
1215 iterating over it, you may get entries skipped or duplicated, so
1216 don't. Exception: It is always safe to delete the item most recently
1217 returned by C<each()>, which means that the following code will work:
1219 while (($key, $value) = each %hash) {
1221 delete $hash{$key}; # This is safe
1224 The following prints out your environment like the printenv(1) program,
1225 only in a different order:
1227 while (($key,$value) = each %ENV) {
1228 print "$key=$value\n";
1231 See also C<keys>, C<values> and C<sort>.
1233 =item eof FILEHANDLE
1239 Returns 1 if the next read on FILEHANDLE will return end of file, or if
1240 FILEHANDLE is not open. FILEHANDLE may be an expression whose value
1241 gives the real filehandle. (Note that this function actually
1242 reads a character and then C<ungetc>s it, so isn't very useful in an
1243 interactive context.) Do not read from a terminal file (or call
1244 C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
1245 as terminals may lose the end-of-file condition if you do.
1247 An C<eof> without an argument uses the last file read. Using C<eof()>
1248 with empty parentheses is very different. It refers to the pseudo file
1249 formed from the files listed on the command line and accessed via the
1250 C<< <> >> operator. Since C<< <> >> isn't explicitly opened,
1251 as a normal filehandle is, an C<eof()> before C<< <> >> has been
1252 used will cause C<@ARGV> to be examined to determine if input is
1255 In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to
1256 detect the end of each file, C<eof()> will only detect the end of the
1257 last file. Examples:
1259 # reset line numbering on each input file
1261 next if /^\s*#/; # skip comments
1264 close ARGV if eof; # Not eof()!
1267 # insert dashes just before last line of last file
1269 if (eof()) { # check for end of current file
1270 print "--------------\n";
1271 close(ARGV); # close or last; is needed if we
1272 # are reading from the terminal
1277 Practical hint: you almost never need to use C<eof> in Perl, because the
1278 input operators typically return C<undef> when they run out of data, or if
1285 In the first form, the return value of EXPR is parsed and executed as if it
1286 were a little Perl program. The value of the expression (which is itself
1287 determined within scalar context) is first parsed, and if there weren't any
1288 errors, executed in the lexical context of the current Perl program, so
1289 that any variable settings or subroutine and format definitions remain
1290 afterwards. Note that the value is parsed every time the eval executes.
1291 If EXPR is omitted, evaluates C<$_>. This form is typically used to
1292 delay parsing and subsequent execution of the text of EXPR until run time.
1294 In the second form, the code within the BLOCK is parsed only once--at the
1295 same time the code surrounding the eval itself was parsed--and executed
1296 within the context of the current Perl program. This form is typically
1297 used to trap exceptions more efficiently than the first (see below), while
1298 also providing the benefit of checking the code within BLOCK at compile
1301 The final semicolon, if any, may be omitted from the value of EXPR or within
1304 In both forms, the value returned is the value of the last expression
1305 evaluated inside the mini-program; a return statement may be also used, just
1306 as with subroutines. The expression providing the return value is evaluated
1307 in void, scalar, or list context, depending on the context of the eval itself.
1308 See L</wantarray> for more on how the evaluation context can be determined.
1310 If there is a syntax error or runtime error, or a C<die> statement is
1311 executed, an undefined value is returned by C<eval>, and C<$@> is set to the
1312 error message. If there was no error, C<$@> is guaranteed to be a null
1313 string. Beware that using C<eval> neither silences perl from printing
1314 warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
1315 To do either of those, you have to use the C<$SIG{__WARN__}> facility. See
1316 L</warn> and L<perlvar>.
1318 Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1319 determining whether a particular feature (such as C<socket> or C<symlink>)
1320 is implemented. It is also Perl's exception trapping mechanism, where
1321 the die operator is used to raise exceptions.
1323 If the code to be executed doesn't vary, you may use the eval-BLOCK
1324 form to trap run-time errors without incurring the penalty of
1325 recompiling each time. The error, if any, is still returned in C<$@>.
1328 # make divide-by-zero nonfatal
1329 eval { $answer = $a / $b; }; warn $@ if $@;
1331 # same thing, but less efficient
1332 eval '$answer = $a / $b'; warn $@ if $@;
1334 # a compile-time error
1335 eval { $answer = }; # WRONG
1338 eval '$answer ='; # sets $@
1340 Due to the current arguably broken state of C<__DIE__> hooks, when using
1341 the C<eval{}> form as an exception trap in libraries, you may wish not
1342 to trigger any C<__DIE__> hooks that user code may have installed.
1343 You can use the C<local $SIG{__DIE__}> construct for this purpose,
1344 as shown in this example:
1346 # a very private exception trap for divide-by-zero
1347 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1350 This is especially significant, given that C<__DIE__> hooks can call
1351 C<die> again, which has the effect of changing their error messages:
1353 # __DIE__ hooks may modify error messages
1355 local $SIG{'__DIE__'} =
1356 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
1357 eval { die "foo lives here" };
1358 print $@ if $@; # prints "bar lives here"
1361 Because this promotes action at a distance, this counterintuitive behavior
1362 may be fixed in a future release.
1364 With an C<eval>, you should be especially careful to remember what's
1365 being looked at when:
1371 eval { $x }; # CASE 4
1373 eval "\$$x++"; # CASE 5
1376 Cases 1 and 2 above behave identically: they run the code contained in
1377 the variable $x. (Although case 2 has misleading double quotes making
1378 the reader wonder what else might be happening (nothing is).) Cases 3
1379 and 4 likewise behave in the same way: they run the code C<'$x'>, which
1380 does nothing but return the value of $x. (Case 4 is preferred for
1381 purely visual reasons, but it also has the advantage of compiling at
1382 compile-time instead of at run-time.) Case 5 is a place where
1383 normally you I<would> like to use double quotes, except that in this
1384 particular situation, you can just use symbolic references instead, as
1387 C<eval BLOCK> does I<not> count as a loop, so the loop control statements
1388 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1392 =item exec PROGRAM LIST
1394 The C<exec> function executes a system command I<and never returns>--
1395 use C<system> instead of C<exec> if you want it to return. It fails and
1396 returns false only if the command does not exist I<and> it is executed
1397 directly instead of via your system's command shell (see below).
1399 Since it's a common mistake to use C<exec> instead of C<system>, Perl
1400 warns you if there is a following statement which isn't C<die>, C<warn>,
1401 or C<exit> (if C<-w> is set - but you always do that). If you
1402 I<really> want to follow an C<exec> with some other statement, you
1403 can use one of these styles to avoid the warning:
1405 exec ('foo') or print STDERR "couldn't exec foo: $!";
1406 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
1408 If there is more than one argument in LIST, or if LIST is an array
1409 with more than one value, calls execvp(3) with the arguments in LIST.
1410 If there is only one scalar argument or an array with one element in it,
1411 the argument is checked for shell metacharacters, and if there are any,
1412 the entire argument is passed to the system's command shell for parsing
1413 (this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1414 If there are no shell metacharacters in the argument, it is split into
1415 words and passed directly to C<execvp>, which is more efficient.
1418 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1419 exec "sort $outfile | uniq";
1421 If you don't really want to execute the first argument, but want to lie
1422 to the program you are executing about its own name, you can specify
1423 the program you actually want to run as an "indirect object" (without a
1424 comma) in front of the LIST. (This always forces interpretation of the
1425 LIST as a multivalued list, even if there is only a single scalar in
1428 $shell = '/bin/csh';
1429 exec $shell '-sh'; # pretend it's a login shell
1433 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1435 When the arguments get executed via the system shell, results will
1436 be subject to its quirks and capabilities. See L<perlop/"`STRING`">
1439 Using an indirect object with C<exec> or C<system> is also more
1440 secure. This usage (which also works fine with system()) forces
1441 interpretation of the arguments as a multivalued list, even if the
1442 list had just one argument. That way you're safe from the shell
1443 expanding wildcards or splitting up words with whitespace in them.
1445 @args = ( "echo surprise" );
1447 exec @args; # subject to shell escapes
1449 exec { $args[0] } @args; # safe even with one-arg list
1451 The first version, the one without the indirect object, ran the I<echo>
1452 program, passing it C<"surprise"> an argument. The second version
1453 didn't--it tried to run a program literally called I<"echo surprise">,
1454 didn't find it, and set C<$?> to a non-zero value indicating failure.
1456 Beginning with v5.6.0, Perl will attempt to flush all files opened for
1457 output before the exec, but this may not be supported on some platforms
1458 (see L<perlport>). To be safe, you may need to set C<$|> ($AUTOFLUSH
1459 in English) or call the C<autoflush()> method of C<IO::Handle> on any
1460 open handles in order to avoid lost output.
1462 Note that C<exec> will not call your C<END> blocks, nor will it call
1463 any C<DESTROY> methods in your objects.
1467 Given an expression that specifies a hash element or array element,
1468 returns true if the specified element in the hash or array has ever
1469 been initialized, even if the corresponding value is undefined. The
1470 element is not autovivified if it doesn't exist.
1472 print "Exists\n" if exists $hash{$key};
1473 print "Defined\n" if defined $hash{$key};
1474 print "True\n" if $hash{$key};
1476 print "Exists\n" if exists $array[$index];
1477 print "Defined\n" if defined $array[$index];
1478 print "True\n" if $array[$index];
1480 A hash or array element can be true only if it's defined, and defined if
1481 it exists, but the reverse doesn't necessarily hold true.
1483 Given an expression that specifies the name of a subroutine,
1484 returns true if the specified subroutine has ever been declared, even
1485 if it is undefined. Mentioning a subroutine name for exists or defined
1486 does not count as declaring it. Note that a subroutine which does not
1487 exist may still be callable: its package may have an C<AUTOLOAD>
1488 method that makes it spring into existence the first time that it is
1489 called -- see L<perlsub>.
1491 print "Exists\n" if exists &subroutine;
1492 print "Defined\n" if defined &subroutine;
1494 Note that the EXPR can be arbitrarily complicated as long as the final
1495 operation is a hash or array key lookup or subroutine name:
1497 if (exists $ref->{A}->{B}->{$key}) { }
1498 if (exists $hash{A}{B}{$key}) { }
1500 if (exists $ref->{A}->{B}->[$ix]) { }
1501 if (exists $hash{A}{B}[$ix]) { }
1503 if (exists &{$ref->{A}{B}{$key}}) { }
1505 Although the deepest nested array or hash will not spring into existence
1506 just because its existence was tested, any intervening ones will.
1507 Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
1508 into existence due to the existence test for the $key element above.
1509 This happens anywhere the arrow operator is used, including even:
1512 if (exists $ref->{"Some key"}) { }
1513 print $ref; # prints HASH(0x80d3d5c)
1515 This surprising autovivification in what does not at first--or even
1516 second--glance appear to be an lvalue context may be fixed in a future
1519 See L<perlref/"Pseudo-hashes: Using an array as a hash"> for specifics
1520 on how exists() acts when used on a pseudo-hash.
1522 Use of a subroutine call, rather than a subroutine name, as an argument
1523 to exists() is an error.
1526 exists &sub(); # Error
1530 Evaluates EXPR and exits immediately with that value. Example:
1533 exit 0 if $ans =~ /^[Xx]/;
1535 See also C<die>. If EXPR is omitted, exits with C<0> status. The only
1536 universally recognized values for EXPR are C<0> for success and C<1>
1537 for error; other values are subject to interpretation depending on the
1538 environment in which the Perl program is running. For example, exiting
1539 69 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
1540 the mailer to return the item undelivered, but that's not true everywhere.
1542 Don't use C<exit> to abort a subroutine if there's any chance that
1543 someone might want to trap whatever error happened. Use C<die> instead,
1544 which can be trapped by an C<eval>.
1546 The exit() function does not always exit immediately. It calls any
1547 defined C<END> routines first, but these C<END> routines may not
1548 themselves abort the exit. Likewise any object destructors that need to
1549 be called are called before the real exit. If this is a problem, you
1550 can call C<POSIX:_exit($status)> to avoid END and destructor processing.
1551 See L<perlmod> for details.
1557 Returns I<e> (the natural logarithm base) to the power of EXPR.
1558 If EXPR is omitted, gives C<exp($_)>.
1560 =item fcntl FILEHANDLE,FUNCTION,SCALAR
1562 Implements the fcntl(2) function. You'll probably have to say
1566 first to get the correct constant definitions. Argument processing and
1567 value return works just like C<ioctl> below.
1571 fcntl($filehandle, F_GETFL, $packed_return_buffer)
1572 or die "can't fcntl F_GETFL: $!";
1574 You don't have to check for C<defined> on the return from C<fnctl>.
1575 Like C<ioctl>, it maps a C<0> return from the system call into
1576 C<"0 but true"> in Perl. This string is true in boolean context and C<0>
1577 in numeric context. It is also exempt from the normal B<-w> warnings
1578 on improper numeric conversions.
1580 Note that C<fcntl> will produce a fatal error if used on a machine that
1581 doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
1582 manpage to learn what functions are available on your system.
1584 =item fileno FILEHANDLE
1586 Returns the file descriptor for a filehandle, or undefined if the
1587 filehandle is not open. This is mainly useful for constructing
1588 bitmaps for C<select> and low-level POSIX tty-handling operations.
1589 If FILEHANDLE is an expression, the value is taken as an indirect
1590 filehandle, generally its name.
1592 You can use this to find out whether two handles refer to the
1593 same underlying descriptor:
1595 if (fileno(THIS) == fileno(THAT)) {
1596 print "THIS and THAT are dups\n";
1599 =item flock FILEHANDLE,OPERATION
1601 Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
1602 for success, false on failure. Produces a fatal error if used on a
1603 machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
1604 C<flock> is Perl's portable file locking interface, although it locks
1605 only entire files, not records.
1607 Two potentially non-obvious but traditional C<flock> semantics are
1608 that it waits indefinitely until the lock is granted, and that its locks
1609 B<merely advisory>. Such discretionary locks are more flexible, but offer
1610 fewer guarantees. This means that files locked with C<flock> may be
1611 modified by programs that do not also use C<flock>. See L<perlport>,
1612 your port's specific documentation, or your system-specific local manpages
1613 for details. It's best to assume traditional behavior if you're writing
1614 portable programs. (But if you're not, you should as always feel perfectly
1615 free to write for your own system's idiosyncrasies (sometimes called
1616 "features"). Slavish adherence to portability concerns shouldn't get
1617 in the way of your getting your job done.)
1619 OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1620 LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
1621 you can use the symbolic names if you import them from the Fcntl module,
1622 either individually, or as a group using the ':flock' tag. LOCK_SH
1623 requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
1624 releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
1625 LOCK_SH or LOCK_EX then C<flock> will return immediately rather than blocking
1626 waiting for the lock (check the return status to see if you got it).
1628 To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
1629 before locking or unlocking it.
1631 Note that the emulation built with lockf(3) doesn't provide shared
1632 locks, and it requires that FILEHANDLE be open with write intent. These
1633 are the semantics that lockf(3) implements. Most if not all systems
1634 implement lockf(3) in terms of fcntl(2) locking, though, so the
1635 differing semantics shouldn't bite too many people.
1637 Note also that some versions of C<flock> cannot lock things over the
1638 network; you would need to use the more system-specific C<fcntl> for
1639 that. If you like you can force Perl to ignore your system's flock(2)
1640 function, and so provide its own fcntl(2)-based emulation, by passing
1641 the switch C<-Ud_flock> to the F<Configure> program when you configure
1644 Here's a mailbox appender for BSD systems.
1646 use Fcntl ':flock'; # import LOCK_* constants
1649 flock(MBOX,LOCK_EX);
1650 # and, in case someone appended
1651 # while we were waiting...
1656 flock(MBOX,LOCK_UN);
1659 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1660 or die "Can't open mailbox: $!";
1663 print MBOX $msg,"\n\n";
1666 On systems that support a real flock(), locks are inherited across fork()
1667 calls, whereas those that must resort to the more capricious fcntl()
1668 function lose the locks, making it harder to write servers.
1670 See also L<DB_File> for other flock() examples.
1674 Does a fork(2) system call to create a new process running the
1675 same program at the same point. It returns the child pid to the
1676 parent process, C<0> to the child process, or C<undef> if the fork is
1677 unsuccessful. File descriptors (and sometimes locks on those descriptors)
1678 are shared, while everything else is copied. On most systems supporting
1679 fork(), great care has gone into making it extremely efficient (for
1680 example, using copy-on-write technology on data pages), making it the
1681 dominant paradigm for multitasking over the last few decades.
1683 Beginning with v5.6.0, Perl will attempt to flush all files opened for
1684 output before forking the child process, but this may not be supported
1685 on some platforms (see L<perlport>). To be safe, you may need to set
1686 C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
1687 C<IO::Handle> on any open handles in order to avoid duplicate output.
1689 If you C<fork> without ever waiting on your children, you will
1690 accumulate zombies. On some systems, you can avoid this by setting
1691 C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
1692 forking and reaping moribund children.
1694 Note that if your forked child inherits system file descriptors like
1695 STDIN and STDOUT that are actually connected by a pipe or socket, even
1696 if you exit, then the remote server (such as, say, a CGI script or a
1697 backgrounded job launched from a remote shell) won't think you're done.
1698 You should reopen those to F</dev/null> if it's any issue.
1702 Declare a picture format for use by the C<write> function. For
1706 Test: @<<<<<<<< @||||| @>>>>>
1707 $str, $%, '$' . int($num)
1711 $num = $cost/$quantity;
1715 See L<perlform> for many details and examples.
1717 =item formline PICTURE,LIST
1719 This is an internal function used by C<format>s, though you may call it,
1720 too. It formats (see L<perlform>) a list of values according to the
1721 contents of PICTURE, placing the output into the format output
1722 accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
1723 Eventually, when a C<write> is done, the contents of
1724 C<$^A> are written to some filehandle, but you could also read C<$^A>
1725 yourself and then set C<$^A> back to C<"">. Note that a format typically
1726 does one C<formline> per line of form, but the C<formline> function itself
1727 doesn't care how many newlines are embedded in the PICTURE. This means
1728 that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
1729 You may therefore need to use multiple formlines to implement a single
1730 record format, just like the format compiler.
1732 Be careful if you put double quotes around the picture, because an C<@>
1733 character may be taken to mean the beginning of an array name.
1734 C<formline> always returns true. See L<perlform> for other examples.
1736 =item getc FILEHANDLE
1740 Returns the next character from the input file attached to FILEHANDLE,
1741 or the undefined value at end of file, or if there was an error.
1742 If FILEHANDLE is omitted, reads from STDIN. This is not particularly
1743 efficient. However, it cannot be used by itself to fetch single
1744 characters without waiting for the user to hit enter. For that, try
1745 something more like:
1748 system "stty cbreak </dev/tty >/dev/tty 2>&1";
1751 system "stty", '-icanon', 'eol', "\001";
1757 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
1760 system "stty", 'icanon', 'eol', '^@'; # ASCII null
1764 Determination of whether $BSD_STYLE should be set
1765 is left as an exercise to the reader.
1767 The C<POSIX::getattr> function can do this more portably on
1768 systems purporting POSIX compliance. See also the C<Term::ReadKey>
1769 module from your nearest CPAN site; details on CPAN can be found on
1774 Implements the C library function of the same name, which on most
1775 systems returns the current login from F</etc/utmp>, if any. If null,
1778 $login = getlogin || getpwuid($<) || "Kilroy";
1780 Do not consider C<getlogin> for authentication: it is not as
1781 secure as C<getpwuid>.
1783 =item getpeername SOCKET
1785 Returns the packed sockaddr address of other end of the SOCKET connection.
1788 $hersockaddr = getpeername(SOCK);
1789 ($port, $iaddr) = sockaddr_in($hersockaddr);
1790 $herhostname = gethostbyaddr($iaddr, AF_INET);
1791 $herstraddr = inet_ntoa($iaddr);
1795 Returns the current process group for the specified PID. Use
1796 a PID of C<0> to get the current process group for the
1797 current process. Will raise an exception if used on a machine that
1798 doesn't implement getpgrp(2). If PID is omitted, returns process
1799 group of current process. Note that the POSIX version of C<getpgrp>
1800 does not accept a PID argument, so only C<PID==0> is truly portable.
1804 Returns the process id of the parent process.
1806 =item getpriority WHICH,WHO
1808 Returns the current priority for a process, a process group, or a user.
1809 (See L<getpriority(2)>.) Will raise a fatal exception if used on a
1810 machine that doesn't implement getpriority(2).
1816 =item gethostbyname NAME
1818 =item getnetbyname NAME
1820 =item getprotobyname NAME
1826 =item getservbyname NAME,PROTO
1828 =item gethostbyaddr ADDR,ADDRTYPE
1830 =item getnetbyaddr ADDR,ADDRTYPE
1832 =item getprotobynumber NUMBER
1834 =item getservbyport PORT,PROTO
1852 =item sethostent STAYOPEN
1854 =item setnetent STAYOPEN
1856 =item setprotoent STAYOPEN
1858 =item setservent STAYOPEN
1872 These routines perform the same functions as their counterparts in the
1873 system library. In list context, the return values from the
1874 various get routines are as follows:
1876 ($name,$passwd,$uid,$gid,
1877 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
1878 ($name,$passwd,$gid,$members) = getgr*
1879 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
1880 ($name,$aliases,$addrtype,$net) = getnet*
1881 ($name,$aliases,$proto) = getproto*
1882 ($name,$aliases,$port,$proto) = getserv*
1884 (If the entry doesn't exist you get a null list.)
1886 The exact meaning of the $gcos field varies but it usually contains
1887 the real name of the user (as opposed to the login name) and other
1888 information pertaining to the user. Beware, however, that in many
1889 system users are able to change this information and therefore it
1890 cannot be trusted and therefore the $gcos is tainted (see
1891 L<perlsec>). The $passwd and $shell, user's encrypted password and
1892 login shell, are also tainted, because of the same reason.
1894 In scalar context, you get the name, unless the function was a
1895 lookup by name, in which case you get the other thing, whatever it is.
1896 (If the entry doesn't exist you get the undefined value.) For example:
1898 $uid = getpwnam($name);
1899 $name = getpwuid($num);
1901 $gid = getgrnam($name);
1902 $name = getgrgid($num;
1906 In I<getpw*()> the fields $quota, $comment, and $expire are special
1907 cases in the sense that in many systems they are unsupported. If the
1908 $quota is unsupported, it is an empty scalar. If it is supported, it
1909 usually encodes the disk quota. If the $comment field is unsupported,
1910 it is an empty scalar. If it is supported it usually encodes some
1911 administrative comment about the user. In some systems the $quota
1912 field may be $change or $age, fields that have to do with password
1913 aging. In some systems the $comment field may be $class. The $expire
1914 field, if present, encodes the expiration period of the account or the
1915 password. For the availability and the exact meaning of these fields
1916 in your system, please consult your getpwnam(3) documentation and your
1917 F<pwd.h> file. You can also find out from within Perl what your
1918 $quota and $comment fields mean and whether you have the $expire field
1919 by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
1920 C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password
1921 files are only supported if your vendor has implemented them in the
1922 intuitive fashion that calling the regular C library routines gets the
1923 shadow versions if you're running under privilege or if there exists
1924 the shadow(3) functions as found in System V ( this includes Solaris
1925 and Linux.) Those systems which implement a proprietary shadow password
1926 facility are unlikely to be supported.
1928 The $members value returned by I<getgr*()> is a space separated list of
1929 the login names of the members of the group.
1931 For the I<gethost*()> functions, if the C<h_errno> variable is supported in
1932 C, it will be returned to you via C<$?> if the function call fails. The
1933 C<@addrs> value returned by a successful call is a list of the raw
1934 addresses returned by the corresponding system library call. In the
1935 Internet domain, each address is four bytes long and you can unpack it
1936 by saying something like:
1938 ($a,$b,$c,$d) = unpack('C4',$addr[0]);
1940 The Socket library makes this slightly easier:
1943 $iaddr = inet_aton("127.1"); # or whatever address
1944 $name = gethostbyaddr($iaddr, AF_INET);
1946 # or going the other way
1947 $straddr = inet_ntoa($iaddr);
1949 If you get tired of remembering which element of the return list
1950 contains which return value, by-name interfaces are provided
1951 in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
1952 C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
1953 and C<User::grent>. These override the normal built-ins, supplying
1954 versions that return objects with the appropriate names
1955 for each field. For example:
1959 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
1961 Even though it looks like they're the same method calls (uid),
1962 they aren't, because a C<File::stat> object is different from
1963 a C<User::pwent> object.
1965 =item getsockname SOCKET
1967 Returns the packed sockaddr address of this end of the SOCKET connection,
1968 in case you don't know the address because you have several different
1969 IPs that the connection might have come in on.
1972 $mysockaddr = getsockname(SOCK);
1973 ($port, $myaddr) = sockaddr_in($mysockaddr);
1974 printf "Connect to %s [%s]\n",
1975 scalar gethostbyaddr($myaddr, AF_INET),
1978 =item getsockopt SOCKET,LEVEL,OPTNAME
1980 Returns the socket option requested, or undef if there is an error.
1986 Returns the value of EXPR with filename expansions such as the
1987 standard Unix shell F</bin/csh> would do. This is the internal function
1988 implementing the C<< <*.c> >> operator, but you can use it directly.
1989 If EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is
1990 discussed in more detail in L<perlop/"I/O Operators">.
1992 Beginning with v5.6.0, this operator is implemented using the standard
1993 C<File::Glob> extension. See L<File::Glob> for details.
1997 Converts a time as returned by the time function to a 8-element list
1998 with the time localized for the standard Greenwich time zone.
1999 Typically used as follows:
2002 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday) =
2005 All list elements are numeric, and come straight out of the C `struct
2006 tm'. $sec, $min, and $hour are the seconds, minutes, and hours of the
2007 specified time. $mday is the day of the month, and $mon is the month
2008 itself, in the range C<0..11> with 0 indicating January and 11
2009 indicating December. $year is the number of years since 1900. That
2010 is, $year is C<123> in year 2023. $wday is the day of the week, with
2011 0 indicating Sunday and 3 indicating Wednesday. $yday is the day of
2012 the year, in the range C<0..364> (or C<0..365> in leap years.)
2014 Note that the $year element is I<not> simply the last two digits of
2015 the year. If you assume it is, then you create non-Y2K-compliant
2016 programs--and you wouldn't want to do that, would you?
2018 The proper way to get a complete 4-digit year is simply:
2022 And to get the last two digits of the year (e.g., '01' in 2001) do:
2024 $year = sprintf("%02d", $year % 100);
2026 If EXPR is omitted, C<gmtime()> uses the current time (C<gmtime(time)>).
2028 In scalar context, C<gmtime()> returns the ctime(3) value:
2030 $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
2032 Also see the C<timegm> function provided by the C<Time::Local> module,
2033 and the strftime(3) function available via the POSIX module.
2035 This scalar value is B<not> locale dependent (see L<perllocale>), but
2036 is instead a Perl builtin. Also see the C<Time::Local> module, and the
2037 strftime(3) and mktime(3) functions available via the POSIX module. To
2038 get somewhat similar but locale dependent date strings, set up your
2039 locale environment variables appropriately (please see L<perllocale>)
2040 and try for example:
2042 use POSIX qw(strftime);
2043 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
2045 Note that the C<%a> and C<%b> escapes, which represent the short forms
2046 of the day of the week and the month of the year, may not necessarily
2047 be three characters wide in all locales.
2055 The C<goto-LABEL> form finds the statement labeled with LABEL and resumes
2056 execution there. It may not be used to go into any construct that
2057 requires initialization, such as a subroutine or a C<foreach> loop. It
2058 also can't be used to go into a construct that is optimized away,
2059 or to get out of a block or subroutine given to C<sort>.
2060 It can be used to go almost anywhere else within the dynamic scope,
2061 including out of subroutines, but it's usually better to use some other
2062 construct such as C<last> or C<die>. The author of Perl has never felt the
2063 need to use this form of C<goto> (in Perl, that is--C is another matter).
2065 The C<goto-EXPR> form expects a label name, whose scope will be resolved
2066 dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
2067 necessarily recommended if you're optimizing for maintainability:
2069 goto ("FOO", "BAR", "GLARCH")[$i];
2071 The C<goto-&NAME> form is quite different from the other forms of C<goto>.
2072 In fact, it isn't a goto in the normal sense at all, and doesn't have
2073 the stigma associated with other gotos. Instead, it
2074 substitutes a call to the named subroutine for the currently running
2075 subroutine. This is used by C<AUTOLOAD> subroutines that wish to load
2076 another subroutine and then pretend that the other subroutine had been
2077 called in the first place (except that any modifications to C<@_>
2078 in the current subroutine are propagated to the other subroutine.)
2079 After the C<goto>, not even C<caller> will be able to tell that this
2080 routine was called first.
2082 NAME needn't be the name of a subroutine; it can be a scalar variable
2083 containing a code reference, or a block which evaluates to a code
2086 =item grep BLOCK LIST
2088 =item grep EXPR,LIST
2090 This is similar in spirit to, but not the same as, grep(1) and its
2091 relatives. In particular, it is not limited to using regular expressions.
2093 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2094 C<$_> to each element) and returns the list value consisting of those
2095 elements for which the expression evaluated to true. In scalar
2096 context, returns the number of times the expression was true.
2098 @foo = grep(!/^#/, @bar); # weed out comments
2102 @foo = grep {!/^#/} @bar; # weed out comments
2104 Note that C<$_> is an alias to the list value, so it can be used to
2105 modify the elements of the LIST. While this is useful and supported,
2106 it can cause bizarre results if the elements of LIST are not variables.
2107 Similarly, grep returns aliases into the original list, much as a for
2108 loop's index variable aliases the list elements. That is, modifying an
2109 element of a list returned by grep (for example, in a C<foreach>, C<map>
2110 or another C<grep>) actually modifies the element in the original list.
2111 This is usually something to be avoided when writing clear code.
2113 See also L</map> for a list composed of the results of the BLOCK or EXPR.
2119 Interprets EXPR as a hex string and returns the corresponding value.
2120 (To convert strings that might start with either 0, 0x, or 0b, see
2121 L</oct>.) If EXPR is omitted, uses C<$_>.
2123 print hex '0xAf'; # prints '175'
2124 print hex 'aF'; # same
2126 Hex strings may only represent integers. Strings that would cause
2127 integer overflow trigger a warning.
2131 There is no builtin C<import> function. It is just an ordinary
2132 method (subroutine) defined (or inherited) by modules that wish to export
2133 names to another module. The C<use> function calls the C<import> method
2134 for the package used. See also L</use()>, L<perlmod>, and L<Exporter>.
2136 =item index STR,SUBSTR,POSITION
2138 =item index STR,SUBSTR
2140 The index function searches for one string within another, but without
2141 the wildcard-like behavior of a full regular-expression pattern match.
2142 It returns the position of the first occurrence of SUBSTR in STR at
2143 or after POSITION. If POSITION is omitted, starts searching from the
2144 beginning of the string. The return value is based at C<0> (or whatever
2145 you've set the C<$[> variable to--but don't do that). If the substring
2146 is not found, returns one less than the base, ordinarily C<-1>.
2152 Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
2153 You should not use this function for rounding: one because it truncates
2154 towards C<0>, and two because machine representations of floating point
2155 numbers can sometimes produce counterintuitive results. For example,
2156 C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
2157 because it's really more like -268.99999999999994315658 instead. Usually,
2158 the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
2159 functions will serve you better than will int().
2161 =item ioctl FILEHANDLE,FUNCTION,SCALAR
2163 Implements the ioctl(2) function. You'll probably first have to say
2165 require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph
2167 to get the correct function definitions. If F<ioctl.ph> doesn't
2168 exist or doesn't have the correct definitions you'll have to roll your
2169 own, based on your C header files such as F<< <sys/ioctl.h> >>.
2170 (There is a Perl script called B<h2ph> that comes with the Perl kit that
2171 may help you in this, but it's nontrivial.) SCALAR will be read and/or
2172 written depending on the FUNCTION--a pointer to the string value of SCALAR
2173 will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
2174 has no string value but does have a numeric value, that value will be
2175 passed rather than a pointer to the string value. To guarantee this to be
2176 true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
2177 functions may be needed to manipulate the values of structures used by
2180 The return value of C<ioctl> (and C<fcntl>) is as follows:
2182 if OS returns: then Perl returns:
2184 0 string "0 but true"
2185 anything else that number
2187 Thus Perl returns true on success and false on failure, yet you can
2188 still easily determine the actual value returned by the operating
2191 $retval = ioctl(...) || -1;
2192 printf "System returned %d\n", $retval;
2194 The special string "C<0> but true" is exempt from B<-w> complaints
2195 about improper numeric conversions.
2197 Here's an example of setting a filehandle named C<REMOTE> to be
2198 non-blocking at the system level. You'll have to negotiate C<$|>
2199 on your own, though.
2201 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
2203 $flags = fcntl(REMOTE, F_GETFL, 0)
2204 or die "Can't get flags for the socket: $!\n";
2206 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
2207 or die "Can't set flags for the socket: $!\n";
2209 =item join EXPR,LIST
2211 Joins the separate strings of LIST into a single string with fields
2212 separated by the value of EXPR, and returns that new string. Example:
2214 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
2216 Beware that unlike C<split>, C<join> doesn't take a pattern as its
2217 first argument. Compare L</split>.
2221 Returns a list consisting of all the keys of the named hash. (In
2222 scalar context, returns the number of keys.) The keys are returned in
2223 an apparently random order. The actual random order is subject to
2224 change in future versions of perl, but it is guaranteed to be the same
2225 order as either the C<values> or C<each> function produces (given
2226 that the hash has not been modified). As a side effect, it resets
2229 Here is yet another way to print your environment:
2232 @values = values %ENV;
2234 print pop(@keys), '=', pop(@values), "\n";
2237 or how about sorted by key:
2239 foreach $key (sort(keys %ENV)) {
2240 print $key, '=', $ENV{$key}, "\n";
2243 The returned values are copies of the original keys in the hash, so
2244 modifying them will not affect the original hash. Compare L</values>.
2246 To sort a hash by value, you'll need to use a C<sort> function.
2247 Here's a descending numeric sort of a hash by its values:
2249 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
2250 printf "%4d %s\n", $hash{$key}, $key;
2253 As an lvalue C<keys> allows you to increase the number of hash buckets
2254 allocated for the given hash. This can gain you a measure of efficiency if
2255 you know the hash is going to get big. (This is similar to pre-extending
2256 an array by assigning a larger number to $#array.) If you say
2260 then C<%hash> will have at least 200 buckets allocated for it--256 of them,
2261 in fact, since it rounds up to the next power of two. These
2262 buckets will be retained even if you do C<%hash = ()>, use C<undef
2263 %hash> if you want to free the storage while C<%hash> is still in scope.
2264 You can't shrink the number of buckets allocated for the hash using
2265 C<keys> in this way (but you needn't worry about doing this by accident,
2266 as trying has no effect).
2268 See also C<each>, C<values> and C<sort>.
2270 =item kill SIGNAL, LIST
2272 Sends a signal to a list of processes. Returns the number of
2273 processes successfully signaled (which is not necessarily the
2274 same as the number actually killed).
2276 $cnt = kill 1, $child1, $child2;
2279 If SIGNAL is zero, no signal is sent to the process. This is a
2280 useful way to check that the process is alive and hasn't changed
2281 its UID. See L<perlport> for notes on the portability of this
2284 Unlike in the shell, if SIGNAL is negative, it kills
2285 process groups instead of processes. (On System V, a negative I<PROCESS>
2286 number will also kill process groups, but that's not portable.) That
2287 means you usually want to use positive not negative signals. You may also
2288 use a signal name in quotes. See L<perlipc/"Signals"> for details.
2294 The C<last> command is like the C<break> statement in C (as used in
2295 loops); it immediately exits the loop in question. If the LABEL is
2296 omitted, the command refers to the innermost enclosing loop. The
2297 C<continue> block, if any, is not executed:
2299 LINE: while (<STDIN>) {
2300 last LINE if /^$/; # exit when done with header
2304 C<last> cannot be used to exit a block which returns a value such as
2305 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2306 a grep() or map() operation.
2308 Note that a block by itself is semantically identical to a loop
2309 that executes once. Thus C<last> can be used to effect an early
2310 exit out of such a block.
2312 See also L</continue> for an illustration of how C<last>, C<next>, and
2319 Returns an lowercased version of EXPR. This is the internal function
2320 implementing the C<\L> escape in double-quoted strings.
2321 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
2324 If EXPR is omitted, uses C<$_>.
2330 Returns the value of EXPR with the first character lowercased. This is
2331 the internal function implementing the C<\l> escape in double-quoted strings.
2332 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
2334 If EXPR is omitted, uses C<$_>.
2340 Returns the length in characters of the value of EXPR. If EXPR is
2341 omitted, returns length of C<$_>. Note that this cannot be used on
2342 an entire array or hash to find out how many elements these have.
2343 For that, use C<scalar @array> and C<scalar keys %hash> respectively.
2345 =item link OLDFILE,NEWFILE
2347 Creates a new filename linked to the old filename. Returns true for
2348 success, false otherwise.
2350 =item listen SOCKET,QUEUESIZE
2352 Does the same thing that the listen system call does. Returns true if
2353 it succeeded, false otherwise. See the example in L<perlipc/"Sockets: Client/Server Communication">.
2357 You really probably want to be using C<my> instead, because C<local> isn't
2358 what most people think of as "local". See
2359 L<perlsub/"Private Variables via my()"> for details.
2361 A local modifies the listed variables to be local to the enclosing
2362 block, file, or eval. If more than one value is listed, the list must
2363 be placed in parentheses. See L<perlsub/"Temporary Values via local()">
2364 for details, including issues with tied arrays and hashes.
2366 =item localtime EXPR
2368 Converts a time as returned by the time function to a 9-element list
2369 with the time analyzed for the local time zone. Typically used as
2373 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
2376 All list elements are numeric, and come straight out of the C `struct
2377 tm'. $sec, $min, and $hour are the seconds, minutes, and hours of the
2378 specified time. $mday is the day of the month, and $mon is the month
2379 itself, in the range C<0..11> with 0 indicating January and 11
2380 indicating December. $year is the number of years since 1900. That
2381 is, $year is C<123> in year 2023. $wday is the day of the week, with
2382 0 indicating Sunday and 3 indicating Wednesday. $yday is the day of
2383 the year, in the range C<0..364> (or C<0..365> in leap years.) $isdst
2384 is true if the specified time occurs during daylight savings time,
2387 Note that the $year element is I<not> simply the last two digits of
2388 the year. If you assume it is, then you create non-Y2K-compliant
2389 programs--and you wouldn't want to do that, would you?
2391 The proper way to get a complete 4-digit year is simply:
2395 And to get the last two digits of the year (e.g., '01' in 2001) do:
2397 $year = sprintf("%02d", $year % 100);
2399 If EXPR is omitted, C<localtime()> uses the current time (C<localtime(time)>).
2401 In scalar context, C<localtime()> returns the ctime(3) value:
2403 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
2405 This scalar value is B<not> locale dependent, see L<perllocale>, but
2406 instead a Perl builtin. Also see the C<Time::Local> module
2407 (to convert the second, minutes, hours, ... back to seconds since the
2408 stroke of midnight the 1st of January 1970, the value returned by
2409 time()), and the strftime(3) and mktime(3) functions available via the
2410 POSIX module. To get somewhat similar but locale dependent date
2411 strings, set up your locale environment variables appropriately
2412 (please see L<perllocale>) and try for example:
2414 use POSIX qw(strftime);
2415 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
2417 Note that the C<%a> and C<%b>, the short forms of the day of the week
2418 and the month of the year, may not necessarily be three characters wide.
2424 This function places an advisory lock on a variable, subroutine,
2425 or referenced object contained in I<THING> until the lock goes out
2426 of scope. This is a built-in function only if your version of Perl
2427 was built with threading enabled, and if you've said C<use Threads>.
2428 Otherwise a user-defined function by this name will be called. See
2435 Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
2436 returns log of C<$_>. To get the log of another base, use basic algebra:
2437 The base-N log of a number is equal to the natural log of that number
2438 divided by the natural log of N. For example:
2442 return log($n)/log(10);
2445 See also L</exp> for the inverse operation.
2451 Does the same thing as the C<stat> function (including setting the
2452 special C<_> filehandle) but stats a symbolic link instead of the file
2453 the symbolic link points to. If symbolic links are unimplemented on
2454 your system, a normal C<stat> is done.
2456 If EXPR is omitted, stats C<$_>.
2460 The match operator. See L<perlop>.
2462 =item map BLOCK LIST
2466 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2467 C<$_> to each element) and returns the list value composed of the
2468 results of each such evaluation. In scalar context, returns the
2469 total number of elements so generated. Evaluates BLOCK or EXPR in
2470 list context, so each element of LIST may produce zero, one, or
2471 more elements in the returned value.
2473 @chars = map(chr, @nums);
2475 translates a list of numbers to the corresponding characters. And
2477 %hash = map { getkey($_) => $_ } @array;
2479 is just a funny way to write
2482 foreach $_ (@array) {
2483 $hash{getkey($_)} = $_;
2486 Note that C<$_> is an alias to the list value, so it can be used to
2487 modify the elements of the LIST. While this is useful and supported,
2488 it can cause bizarre results if the elements of LIST are not variables.
2489 Using a regular C<foreach> loop for this purpose would be clearer in
2490 most cases. See also L</grep> for an array composed of those items of
2491 the original list for which the BLOCK or EXPR evaluates to true.
2493 C<{> starts both hash references and blocks, so C<map { ...> could be either
2494 the start of map BLOCK LIST or map EXPR, LIST. Because perl doesn't look
2495 ahead for the closing C<}> it has to take a guess at which its dealing with
2496 based what it finds just after the C<{>. Usually it gets it right, but if it
2497 doesn't it won't realize something is wrong until it gets to the C<}> and
2498 encounters the missing (or unexpected) comma. The syntax error will be
2499 reported close to the C<}> but you'll need to change something near the C<{>
2500 such as using a unary C<+> to give perl some help:
2502 %hash = map { "\L$_", 1 } @array # perl guesses EXPR. wrong
2503 %hash = map { +"\L$_", 1 } @array # perl guesses BLOCK. right
2504 %hash = map { ("\L$_", 1) } @array # this also works
2505 %hash = map { lc($_), 1 } @array # as does this.
2506 %hash = map +( lc($_), 1 ), @array # this is EXPR and works!
2508 %hash = map ( lc($_), 1 ), @array # evaluates to (1, @array)
2510 or to force an anon hash constructor use C<+{>
2512 @hashes = map +{ lc($_), 1 }, @array # EXPR, so needs , at end
2514 and you get list of anonymous hashes each with only 1 entry.
2516 =item mkdir FILENAME,MASK
2518 =item mkdir FILENAME
2520 Creates the directory specified by FILENAME, with permissions
2521 specified by MASK (as modified by C<umask>). If it succeeds it
2522 returns true, otherwise it returns false and sets C<$!> (errno).
2523 If omitted, MASK defaults to 0777.
2525 In general, it is better to create directories with permissive MASK,
2526 and let the user modify that with their C<umask>, than it is to supply
2527 a restrictive MASK and give the user no way to be more permissive.
2528 The exceptions to this rule are when the file or directory should be
2529 kept private (mail files, for instance). The perlfunc(1) entry on
2530 C<umask> discusses the choice of MASK in more detail.
2532 Note that according to the POSIX 1003.1-1996 the FILENAME may have any
2533 number of trailing slashes. Some operating and filesystems do not get
2534 this right, so Perl automatically removes all trailing slashes to keep
2537 =item msgctl ID,CMD,ARG
2539 Calls the System V IPC function msgctl(2). You'll probably have to say
2543 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
2544 then ARG must be a variable which will hold the returned C<msqid_ds>
2545 structure. Returns like C<ioctl>: the undefined value for error,
2546 C<"0 but true"> for zero, or the actual return value otherwise. See also
2547 L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::Semaphore> documentation.
2549 =item msgget KEY,FLAGS
2551 Calls the System V IPC function msgget(2). Returns the message queue
2552 id, or the undefined value if there is an error. See also
2553 L<perlipc/"SysV IPC"> and C<IPC::SysV> and C<IPC::Msg> documentation.
2555 =item msgrcv ID,VAR,SIZE,TYPE,FLAGS
2557 Calls the System V IPC function msgrcv to receive a message from
2558 message queue ID into variable VAR with a maximum message size of
2559 SIZE. Note that when a message is received, the message type as a
2560 native long integer will be the first thing in VAR, followed by the
2561 actual message. This packing may be opened with C<unpack("l! a*")>.
2562 Taints the variable. Returns true if successful, or false if there is
2563 an error. See also L<perlipc/"SysV IPC">, C<IPC::SysV>, and
2564 C<IPC::SysV::Msg> documentation.
2566 =item msgsnd ID,MSG,FLAGS
2568 Calls the System V IPC function msgsnd to send the message MSG to the
2569 message queue ID. MSG must begin with the native long integer message
2570 type, and be followed by the length of the actual message, and finally
2571 the message itself. This kind of packing can be achieved with
2572 C<pack("l! a*", $type, $message)>. Returns true if successful,
2573 or false if there is an error. See also C<IPC::SysV>
2574 and C<IPC::SysV::Msg> documentation.
2578 =item my EXPR : ATTRIBUTES
2580 A C<my> declares the listed variables to be local (lexically) to the
2581 enclosing block, file, or C<eval>. If
2582 more than one value is listed, the list must be placed in parentheses. See
2583 L<perlsub/"Private Variables via my()"> for details.
2589 The C<next> command is like the C<continue> statement in C; it starts
2590 the next iteration of the loop:
2592 LINE: while (<STDIN>) {
2593 next LINE if /^#/; # discard comments
2597 Note that if there were a C<continue> block on the above, it would get
2598 executed even on discarded lines. If the LABEL is omitted, the command
2599 refers to the innermost enclosing loop.
2601 C<next> cannot be used to exit a block which returns a value such as
2602 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2603 a grep() or map() operation.
2605 Note that a block by itself is semantically identical to a loop
2606 that executes once. Thus C<next> will exit such a block early.
2608 See also L</continue> for an illustration of how C<last>, C<next>, and
2611 =item no Module LIST
2613 See the L</use> function, which C<no> is the opposite of.
2619 Interprets EXPR as an octal string and returns the corresponding
2620 value. (If EXPR happens to start off with C<0x>, interprets it as a
2621 hex string. If EXPR starts off with C<0b>, it is interpreted as a
2622 binary string.) The following will handle decimal, binary, octal, and
2623 hex in the standard Perl or C notation:
2625 $val = oct($val) if $val =~ /^0/;
2627 If EXPR is omitted, uses C<$_>. To go the other way (produce a number
2628 in octal), use sprintf() or printf():
2630 $perms = (stat("filename"))[2] & 07777;
2631 $oct_perms = sprintf "%lo", $perms;
2633 The oct() function is commonly used when a string such as C<644> needs
2634 to be converted into a file mode, for example. (Although perl will
2635 automatically convert strings into numbers as needed, this automatic
2636 conversion assumes base 10.)
2638 =item open FILEHANDLE,MODE,LIST
2640 =item open FILEHANDLE,EXPR
2642 =item open FILEHANDLE
2644 Opens the file whose filename is given by EXPR, and associates it with
2645 FILEHANDLE. If FILEHANDLE is an expression, its value is used as the
2646 name of the real filehandle wanted. (This is considered a symbolic
2647 reference, so C<use strict 'refs'> should I<not> be in effect.)
2649 If EXPR is omitted, the scalar
2650 variable of the same name as the FILEHANDLE contains the filename.
2651 (Note that lexical variables--those declared with C<my>--will not work
2652 for this purpose; so if you're using C<my>, specify EXPR in your call
2653 to open.) See L<perlopentut> for a kinder, gentler explanation of opening
2656 If MODE is C<< '<' >> or nothing, the file is opened for input.
2657 If MODE is C<< '>' >>, the file is truncated and opened for
2658 output, being created if necessary. If MODE is C<<< '>>' >>>,
2659 the file is opened for appending, again being created if necessary.
2660 You can put a C<'+'> in front of the C<< '>' >> or C<< '<' >> to indicate that
2661 you want both read and write access to the file; thus C<< '+<' >> is almost
2662 always preferred for read/write updates--the C<< '+>' >> mode would clobber the
2663 file first. You can't usually use either read-write mode for updating
2664 textfiles, since they have variable length records. See the B<-i>
2665 switch in L<perlrun> for a better approach. The file is created with
2666 permissions of C<0666> modified by the process' C<umask> value.
2668 These various prefixes correspond to the fopen(3) modes of C<'r'>, C<'r+'>,
2669 C<'w'>, C<'w+'>, C<'a'>, and C<'a+'>.
2671 In the 2-arguments (and 1-argument) form of the call the mode and
2672 filename should be concatenated (in this order), possibly separated by
2673 spaces. It is possible to omit the mode if the mode is C<< '<' >>.
2675 If the filename begins with C<'|'>, the filename is interpreted as a
2676 command to which output is to be piped, and if the filename ends with a
2677 C<'|'>, the filename is interpreted as a command which pipes output to
2678 us. See L<perlipc/"Using open() for IPC">
2679 for more examples of this. (You are not allowed to C<open> to a command
2680 that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
2681 and L<perlipc/"Bidirectional Communication with Another Process">
2684 If MODE is C<'|-'>, the filename is interpreted as a
2685 command to which output is to be piped, and if MODE is
2686 C<'-|'>, the filename is interpreted as a command which pipes output to
2687 us. In the 2-arguments (and 1-argument) form one should replace dash
2688 (C<'-'>) with the command. See L<perlipc/"Using open() for IPC">
2689 for more examples of this. (You are not allowed to C<open> to a command
2690 that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
2691 and L<perlipc/"Bidirectional Communication"> for alternatives.)
2693 In the 2-arguments (and 1-argument) form opening C<'-'> opens STDIN
2694 and opening C<< '>-' >> opens STDOUT.
2697 nonzero upon success, the undefined value otherwise. If the C<open>
2698 involved a pipe, the return value happens to be the pid of the
2701 If you're unfortunate enough to be running Perl on a system that
2702 distinguishes between text files and binary files (modern operating
2703 systems don't care), then you should check out L</binmode> for tips for
2704 dealing with this. The key distinction between systems that need C<binmode>
2705 and those that don't is their text file formats. Systems like Unix, MacOS, and
2706 Plan9, which delimit lines with a single character, and which encode that
2707 character in C as C<"\n">, do not need C<binmode>. The rest need it.
2709 When opening a file, it's usually a bad idea to continue normal execution
2710 if the request failed, so C<open> is frequently used in connection with
2711 C<die>. Even if C<die> won't do what you want (say, in a CGI script,
2712 where you want to make a nicely formatted error message (but there are
2713 modules that can help with that problem)) you should always check
2714 the return value from opening a file. The infrequent exception is when
2715 working with an unopened filehandle is actually what you want to do.
2720 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
2721 while (<ARTICLE>) {...
2723 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
2724 # if the open fails, output is discarded
2726 open(DBASE, '+<', 'dbase.mine') # open for update
2727 or die "Can't open 'dbase.mine' for update: $!";
2729 open(DBASE, '+<dbase.mine') # ditto
2730 or die "Can't open 'dbase.mine' for update: $!";
2732 open(ARTICLE, '-|', "caesar <$article") # decrypt article
2733 or die "Can't start caesar: $!";
2735 open(ARTICLE, "caesar <$article |") # ditto
2736 or die "Can't start caesar: $!";
2738 open(EXTRACT, "|sort >/tmp/Tmp$$") # $$ is our process id
2739 or die "Can't start sort: $!";
2741 # process argument list of files along with any includes
2743 foreach $file (@ARGV) {
2744 process($file, 'fh00');
2748 my($filename, $input) = @_;
2749 $input++; # this is a string increment
2750 unless (open($input, $filename)) {
2751 print STDERR "Can't open $filename: $!\n";
2756 while (<$input>) { # note use of indirection
2757 if (/^#include "(.*)"/) {
2758 process($1, $input);
2765 You may also, in the Bourne shell tradition, specify an EXPR beginning
2766 with C<< '>&' >>, in which case the rest of the string is interpreted as the
2767 name of a filehandle (or file descriptor, if numeric) to be
2768 duped and opened. You may use C<&> after C<< > >>, C<<< >> >>>,
2769 C<< < >>, C<< +> >>, C<<< +>> >>>, and C<< +< >>. The
2770 mode you specify should match the mode of the original filehandle.
2771 (Duping a filehandle does not take into account any existing contents of
2772 stdio buffers.) Duping file handles is not yet supported for 3-argument
2775 Here is a script that saves, redirects, and restores STDOUT and
2779 open(OLDOUT, ">&STDOUT");
2780 open(OLDERR, ">&STDERR");
2782 open(STDOUT, '>', "foo.out") || die "Can't redirect stdout";
2783 open(STDERR, ">&STDOUT") || die "Can't dup stdout";
2785 select(STDERR); $| = 1; # make unbuffered
2786 select(STDOUT); $| = 1; # make unbuffered
2788 print STDOUT "stdout 1\n"; # this works for
2789 print STDERR "stderr 1\n"; # subprocesses too
2794 open(STDOUT, ">&OLDOUT");
2795 open(STDERR, ">&OLDERR");
2797 print STDOUT "stdout 2\n";
2798 print STDERR "stderr 2\n";
2800 If you specify C<< '<&=N' >>, where C<N> is a number, then Perl will do an
2801 equivalent of C's C<fdopen> of that file descriptor; this is more
2802 parsimonious of file descriptors. For example:
2804 open(FILEHANDLE, "<&=$fd")
2806 Note that this feature depends on the fdopen() C library function.
2807 On many UNIX systems, fdopen() is known to fail when file descriptors
2808 exceed a certain value, typically 255. If you need more file
2809 descriptors than that, consider rebuilding Perl to use the C<sfio>
2812 If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'>
2813 with 2-arguments (or 1-argument) form of open(), then
2814 there is an implicit fork done, and the return value of open is the pid
2815 of the child within the parent process, and C<0> within the child
2816 process. (Use C<defined($pid)> to determine whether the open was successful.)
2817 The filehandle behaves normally for the parent, but i/o to that
2818 filehandle is piped from/to the STDOUT/STDIN of the child process.
2819 In the child process the filehandle isn't opened--i/o happens from/to
2820 the new STDOUT or STDIN. Typically this is used like the normal
2821 piped open when you want to exercise more control over just how the
2822 pipe command gets executed, such as when you are running setuid, and
2823 don't want to have to scan shell commands for metacharacters.
2824 The following triples are more or less equivalent:
2826 open(FOO, "|tr '[a-z]' '[A-Z]'");
2827 open(FOO, '|-', "tr '[a-z]' '[A-Z]'");
2828 open(FOO, '|-') || exec 'tr', '[a-z]', '[A-Z]';
2830 open(FOO, "cat -n '$file'|");
2831 open(FOO, '-|', "cat -n '$file'");
2832 open(FOO, '-|') || exec 'cat', '-n', $file;
2834 See L<perlipc/"Safe Pipe Opens"> for more examples of this.
2836 Beginning with v5.6.0, Perl will attempt to flush all files opened for
2837 output before any operation that may do a fork, but this may not be
2838 supported on some platforms (see L<perlport>). To be safe, you may need
2839 to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
2840 of C<IO::Handle> on any open handles.
2842 On systems that support a
2843 close-on-exec flag on files, the flag will be set for the newly opened
2844 file descriptor as determined by the value of $^F. See L<perlvar/$^F>.
2846 Closing any piped filehandle causes the parent process to wait for the
2847 child to finish, and returns the status value in C<$?>.
2849 The filename passed to 2-argument (or 1-argument) form of open()
2850 will have leading and trailing
2851 whitespace deleted, and the normal redirection characters
2852 honored. This property, known as "magic open",
2853 can often be used to good effect. A user could specify a filename of
2854 F<"rsh cat file |">, or you could change certain filenames as needed:
2856 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
2857 open(FH, $filename) or die "Can't open $filename: $!";
2859 Use 3-argument form to open a file with arbitrary weird characters in it,
2861 open(FOO, '<', $file);
2863 otherwise it's necessary to protect any leading and trailing whitespace:
2865 $file =~ s#^(\s)#./$1#;
2866 open(FOO, "< $file\0");
2868 (this may not work on some bizarre filesystems). One should
2869 conscientiously choose between the I<magic> and 3-arguments form
2874 will allow the user to specify an argument of the form C<"rsh cat file |">,
2875 but will not work on a filename which happens to have a trailing space, while
2877 open IN, '<', $ARGV[0];
2879 will have exactly the opposite restrictions.
2881 If you want a "real" C C<open> (see L<open(2)> on your system), then you
2882 should use the C<sysopen> function, which involves no such magic (but
2883 may use subtly different filemodes than Perl open(), which is mapped
2884 to C fopen()). This is
2885 another way to protect your filenames from interpretation. For example:
2888 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
2889 or die "sysopen $path: $!";
2890 $oldfh = select(HANDLE); $| = 1; select($oldfh);
2891 print HANDLE "stuff $$\n";
2893 print "File contains: ", <HANDLE>;
2895 Using the constructor from the C<IO::Handle> package (or one of its
2896 subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous
2897 filehandles that have the scope of whatever variables hold references to
2898 them, and automatically close whenever and however you leave that scope:
2902 sub read_myfile_munged {
2904 my $handle = new IO::File;
2905 open($handle, "myfile") or die "myfile: $!";
2907 or return (); # Automatically closed here.
2908 mung $first or die "mung failed"; # Or here.
2909 return $first, <$handle> if $ALL; # Or here.
2913 See L</seek> for some details about mixing reading and writing.
2915 =item opendir DIRHANDLE,EXPR
2917 Opens a directory named EXPR for processing by C<readdir>, C<telldir>,
2918 C<seekdir>, C<rewinddir>, and C<closedir>. Returns true if successful.
2919 DIRHANDLEs have their own namespace separate from FILEHANDLEs.
2925 Returns the numeric (ASCII or Unicode) value of the first character of EXPR. If
2926 EXPR is omitted, uses C<$_>. For the reverse, see L</chr>.
2927 See L<utf8> for more about Unicode.
2931 An C<our> declares the listed variables to be valid globals within
2932 the enclosing block, file, or C<eval>. That is, it has the same
2933 scoping rules as a "my" declaration, but does not create a local
2934 variable. If more than one value is listed, the list must be placed
2935 in parentheses. The C<our> declaration has no semantic effect unless
2936 "use strict vars" is in effect, in which case it lets you use the
2937 declared global variable without qualifying it with a package name.
2938 (But only within the lexical scope of the C<our> declaration. In this
2939 it differs from "use vars", which is package scoped.)
2941 An C<our> declaration declares a global variable that will be visible
2942 across its entire lexical scope, even across package boundaries. The
2943 package in which the variable is entered is determined at the point
2944 of the declaration, not at the point of use. This means the following
2948 our $bar; # declares $Foo::bar for rest of lexical scope
2952 print $bar; # prints 20
2954 Multiple C<our> declarations in the same lexical scope are allowed
2955 if they are in different packages. If they happened to be in the same
2956 package, Perl will emit warnings if you have asked for them.
2960 our $bar; # declares $Foo::bar for rest of lexical scope
2964 our $bar = 30; # declares $Bar::bar for rest of lexical scope
2965 print $bar; # prints 30
2967 our $bar; # emits warning
2969 =item pack TEMPLATE,LIST
2971 Takes a LIST of values and converts it into a string using the rules
2972 given by the TEMPLATE. The resulting string is the concatenation of
2973 the converted values. Typically, each converted value looks
2974 like its machine-level representation. For example, on 32-bit machines
2975 a converted integer may be represented by a sequence of 4 bytes.
2978 sequence of characters that give the order and type of values, as
2981 a A string with arbitrary binary data, will be null padded.
2982 A An ASCII string, will be space padded.
2983 Z A null terminated (asciz) string, will be null padded.
2985 b A bit string (ascending bit order inside each byte, like vec()).
2986 B A bit string (descending bit order inside each byte).
2987 h A hex string (low nybble first).
2988 H A hex string (high nybble first).
2990 c A signed char value.
2991 C An unsigned char value. Only does bytes. See U for Unicode.
2993 s A signed short value.
2994 S An unsigned short value.
2995 (This 'short' is _exactly_ 16 bits, which may differ from
2996 what a local C compiler calls 'short'. If you want
2997 native-length shorts, use the '!' suffix.)
2999 i A signed integer value.
3000 I An unsigned integer value.
3001 (This 'integer' is _at_least_ 32 bits wide. Its exact
3002 size depends on what a local C compiler calls 'int',
3003 and may even be larger than the 'long' described in
3006 l A signed long value.
3007 L An unsigned long value.
3008 (This 'long' is _exactly_ 32 bits, which may differ from
3009 what a local C compiler calls 'long'. If you want
3010 native-length longs, use the '!' suffix.)
3012 n An unsigned short in "network" (big-endian) order.
3013 N An unsigned long in "network" (big-endian) order.
3014 v An unsigned short in "VAX" (little-endian) order.
3015 V An unsigned long in "VAX" (little-endian) order.
3016 (These 'shorts' and 'longs' are _exactly_ 16 bits and
3017 _exactly_ 32 bits, respectively.)
3019 q A signed quad (64-bit) value.
3020 Q An unsigned quad value.
3021 (Quads are available only if your system supports 64-bit
3022 integer values _and_ if Perl has been compiled to support those.
3023 Causes a fatal error otherwise.)
3025 f A single-precision float in the native format.
3026 d A double-precision float in the native format.
3028 p A pointer to a null-terminated string.
3029 P A pointer to a structure (fixed-length string).
3031 u A uuencoded string.
3032 U A Unicode character number. Encodes to UTF-8 internally.
3033 Works even if C<use utf8> is not in effect.
3035 w A BER compressed integer. Its bytes represent an unsigned
3036 integer in base 128, most significant digit first, with as
3037 few digits as possible. Bit eight (the high bit) is set
3038 on each byte except the last.
3042 @ Null fill to absolute position.
3044 The following rules apply:
3050 Each letter may optionally be followed by a number giving a repeat
3051 count. With all types except C<a>, C<A>, C<Z>, C<b>, C<B>, C<h>,
3052 C<H>, and C<P> the pack function will gobble up that many values from
3053 the LIST. A C<*> for the repeat count means to use however many items are
3054 left, except for C<@>, C<x>, C<X>, where it is equivalent
3055 to C<0>, and C<u>, where it is equivalent to 1 (or 45, what is the
3058 When used with C<Z>, C<*> results in the addition of a trailing null
3059 byte (so the packed result will be one longer than the byte C<length>
3062 The repeat count for C<u> is interpreted as the maximal number of bytes
3063 to encode per line of output, with 0 and 1 replaced by 45.
3067 The C<a>, C<A>, and C<Z> types gobble just one value, but pack it as a
3068 string of length count, padding with nulls or spaces as necessary. When
3069 unpacking, C<A> strips trailing spaces and nulls, C<Z> strips everything
3070 after the first null, and C<a> returns data verbatim. When packing,
3071 C<a>, and C<Z> are equivalent.
3073 If the value-to-pack is too long, it is truncated. If too long and an
3074 explicit count is provided, C<Z> packs only C<$count-1> bytes, followed
3075 by a null byte. Thus C<Z> always packs a trailing null byte under
3080 Likewise, the C<b> and C<B> fields pack a string that many bits long.
3081 Each byte of the input field of pack() generates 1 bit of the result.
3082 Each result bit is based on the least-significant bit of the corresponding
3083 input byte, i.e., on C<ord($byte)%2>. In particular, bytes C<"0"> and
3084 C<"1"> generate bits 0 and 1, as do bytes C<"\0"> and C<"\1">.
3086 Starting from the beginning of the input string of pack(), each 8-tuple
3087 of bytes is converted to 1 byte of output. With format C<b>
3088 the first byte of the 8-tuple determines the least-significant bit of a
3089 byte, and with format C<B> it determines the most-significant bit of
3092 If the length of the input string is not exactly divisible by 8, the
3093 remainder is packed as if the input string were padded by null bytes
3094 at the end. Similarly, during unpack()ing the "extra" bits are ignored.
3096 If the input string of pack() is longer than needed, extra bytes are ignored.
3097 A C<*> for the repeat count of pack() means to use all the bytes of
3098 the input field. On unpack()ing the bits are converted to a string
3099 of C<"0">s and C<"1">s.
3103 The C<h> and C<H> fields pack a string that many nybbles (4-bit groups,
3104 representable as hexadecimal digits, 0-9a-f) long.
3106 Each byte of the input field of pack() generates 4 bits of the result.
3107 For non-alphabetical bytes the result is based on the 4 least-significant
3108 bits of the input byte, i.e., on C<ord($byte)%16>. In particular,
3109 bytes C<"0"> and C<"1"> generate nybbles 0 and 1, as do bytes
3110 C<"\0"> and C<"\1">. For bytes C<"a".."f"> and C<"A".."F"> the result
3111 is compatible with the usual hexadecimal digits, so that C<"a"> and
3112 C<"A"> both generate the nybble C<0xa==10>. The result for bytes
3113 C<"g".."z"> and C<"G".."Z"> is not well-defined.
3115 Starting from the beginning of the input string of pack(), each pair
3116 of bytes is converted to 1 byte of output. With format C<h> the
3117 first byte of the pair determines the least-significant nybble of the
3118 output byte, and with format C<H> it determines the most-significant
3121 If the length of the input string is not even, it behaves as if padded
3122 by a null byte at the end. Similarly, during unpack()ing the "extra"
3123 nybbles are ignored.
3125 If the input string of pack() is longer than needed, extra bytes are ignored.
3126 A C<*> for the repeat count of pack() means to use all the bytes of
3127 the input field. On unpack()ing the bits are converted to a string
3128 of hexadecimal digits.
3132 The C<p> type packs a pointer to a null-terminated string. You are
3133 responsible for ensuring the string is not a temporary value (which can
3134 potentially get deallocated before you get around to using the packed result).
3135 The C<P> type packs a pointer to a structure of the size indicated by the
3136 length. A NULL pointer is created if the corresponding value for C<p> or
3137 C<P> is C<undef>, similarly for unpack().
3141 The C</> template character allows packing and unpacking of strings where
3142 the packed structure contains a byte count followed by the string itself.
3143 You write I<length-item>C</>I<string-item>.
3145 The I<length-item> can be any C<pack> template letter,
3146 and describes how the length value is packed.
3147 The ones likely to be of most use are integer-packing ones like
3148 C<n> (for Java strings), C<w> (for ASN.1 or SNMP)
3149 and C<N> (for Sun XDR).
3151 The I<string-item> must, at present, be C<"A*">, C<"a*"> or C<"Z*">.
3152 For C<unpack> the length of the string is obtained from the I<length-item>,
3153 but if you put in the '*' it will be ignored.
3155 unpack 'C/a', "\04Gurusamy"; gives 'Guru'
3156 unpack 'a3/A* A*', '007 Bond J '; gives (' Bond','J')
3157 pack 'n/a* w/a*','hello,','world'; gives "\000\006hello,\005world"
3159 The I<length-item> is not returned explicitly from C<unpack>.
3161 Adding a count to the I<length-item> letter is unlikely to do anything
3162 useful, unless that letter is C<A>, C<a> or C<Z>. Packing with a
3163 I<length-item> of C<a> or C<Z> may introduce C<"\000"> characters,
3164 which Perl does not regard as legal in numeric strings.
3168 The integer types C<s>, C<S>, C<l>, and C<L> may be
3169 immediately followed by a C<!> suffix to signify native shorts or
3170 longs--as you can see from above for example a bare C<l> does mean
3171 exactly 32 bits, the native C<long> (as seen by the local C compiler)
3172 may be larger. This is an issue mainly in 64-bit platforms. You can
3173 see whether using C<!> makes any difference by
3175 print length(pack("s")), " ", length(pack("s!")), "\n";
3176 print length(pack("l")), " ", length(pack("l!")), "\n";
3178 C<i!> and C<I!> also work but only because of completeness;
3179 they are identical to C<i> and C<I>.
3181 The actual sizes (in bytes) of native shorts, ints, longs, and long
3182 longs on the platform where Perl was built are also available via
3186 print $Config{shortsize}, "\n";
3187 print $Config{intsize}, "\n";
3188 print $Config{longsize}, "\n";
3189 print $Config{longlongsize}, "\n";
3191 (The C<$Config{longlongsize}> will be undefine if your system does
3192 not support long longs.)
3196 The integer formats C<s>, C<S>, C<i>, C<I>, C<l>, and C<L>
3197 are inherently non-portable between processors and operating systems
3198 because they obey the native byteorder and endianness. For example a
3199 4-byte integer 0x12345678 (305419896 decimal) be ordered natively
3200 (arranged in and handled by the CPU registers) into bytes as
3202 0x12 0x34 0x56 0x78 # big-endian
3203 0x78 0x56 0x34 0x12 # little-endian
3205 Basically, the Intel and VAX CPUs are little-endian, while everybody
3206 else, for example Motorola m68k/88k, PPC, Sparc, HP PA, Power, and
3207 Cray are big-endian. Alpha and MIPS can be either: Digital/Compaq
3208 used/uses them in little-endian mode; SGI/Cray uses them in big-endian mode.
3210 The names `big-endian' and `little-endian' are comic references to
3211 the classic "Gulliver's Travels" (via the paper "On Holy Wars and a
3212 Plea for Peace" by Danny Cohen, USC/ISI IEN 137, April 1, 1980) and
3213 the egg-eating habits of the Lilliputians.
3215 Some systems may have even weirder byte orders such as
3220 You can see your system's preference with
3222 print join(" ", map { sprintf "%#02x", $_ }
3223 unpack("C*",pack("L",0x12345678))), "\n";
3225 The byteorder on the platform where Perl was built is also available
3229 print $Config{byteorder}, "\n";
3231 Byteorders C<'1234'> and C<'12345678'> are little-endian, C<'4321'>
3232 and C<'87654321'> are big-endian.
3234 If you want portable packed integers use the formats C<n>, C<N>,
3235 C<v>, and C<V>, their byte endianness and size is known.
3236 See also L<perlport>.
3240 Real numbers (floats and doubles) are in the native machine format only;
3241 due to the multiplicity of floating formats around, and the lack of a
3242 standard "network" representation, no facility for interchange has been
3243 made. This means that packed floating point data written on one machine
3244 may not be readable on another - even if both use IEEE floating point
3245 arithmetic (as the endian-ness of the memory representation is not part
3246 of the IEEE spec). See also L<perlport>.
3248 Note that Perl uses doubles internally for all numeric calculation, and
3249 converting from double into float and thence back to double again will
3250 lose precision (i.e., C<unpack("f", pack("f", $foo)>) will not in general
3255 If the pattern begins with a C<U>, the resulting string will be treated
3256 as Unicode-encoded. You can force UTF8 encoding on in a string with an
3257 initial C<U0>, and the bytes that follow will be interpreted as Unicode
3258 characters. If you don't want this to happen, you can begin your pattern
3259 with C<C0> (or anything else) to force Perl not to UTF8 encode your
3260 string, and then follow this with a C<U*> somewhere in your pattern.
3264 You must yourself do any alignment or padding by inserting for example
3265 enough C<'x'>es while packing. There is no way to pack() and unpack()
3266 could know where the bytes are going to or coming from. Therefore
3267 C<pack> (and C<unpack>) handle their output and input as flat
3272 A comment in a TEMPLATE starts with C<#> and goes to the end of line.
3276 If TEMPLATE requires more arguments to pack() than actually given, pack()
3277 assumes additional C<""> arguments. If TEMPLATE requires less arguments
3278 to pack() than actually given, extra arguments are ignored.
3284 $foo = pack("CCCC",65,66,67,68);
3286 $foo = pack("C4",65,66,67,68);
3288 $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);
3289 # same thing with Unicode circled letters
3291 $foo = pack("ccxxcc",65,66,67,68);
3294 # note: the above examples featuring "C" and "c" are true
3295 # only on ASCII and ASCII-derived systems such as ISO Latin 1
3296 # and UTF-8. In EBCDIC the first example would be
3297 # $foo = pack("CCCC",193,194,195,196);
3299 $foo = pack("s2",1,2);
3300 # "\1\0\2\0" on little-endian
3301 # "\0\1\0\2" on big-endian
3303 $foo = pack("a4","abcd","x","y","z");
3306 $foo = pack("aaaa","abcd","x","y","z");
3309 $foo = pack("a14","abcdefg");
3310 # "abcdefg\0\0\0\0\0\0\0"
3312 $foo = pack("i9pl", gmtime);
3313 # a real struct tm (on my system anyway)
3315 $utmp_template = "Z8 Z8 Z16 L";
3316 $utmp = pack($utmp_template, @utmp1);
3317 # a struct utmp (BSDish)
3319 @utmp2 = unpack($utmp_template, $utmp);
3320 # "@utmp1" eq "@utmp2"
3323 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
3326 $foo = pack('sx2l', 12, 34);
3327 # short 12, two zero bytes padding, long 34
3328 $bar = pack('s@4l', 12, 34);
3329 # short 12, zero fill to position 4, long 34
3332 The same template may generally also be used in unpack().
3334 =item package NAMESPACE
3338 Declares the compilation unit as being in the given namespace. The scope
3339 of the package declaration is from the declaration itself through the end
3340 of the enclosing block, file, or eval (the same as the C<my> operator).
3341 All further unqualified dynamic identifiers will be in this namespace.
3342 A package statement affects only dynamic variables--including those
3343 you've used C<local> on--but I<not> lexical variables, which are created
3344 with C<my>. Typically it would be the first declaration in a file to
3345 be included by the C<require> or C<use> operator. You can switch into a
3346 package in more than one place; it merely influences which symbol table
3347 is used by the compiler for the rest of that block. You can refer to
3348 variables and filehandles in other packages by prefixing the identifier
3349 with the package name and a double colon: C<$Package::Variable>.
3350 If the package name is null, the C<main> package as assumed. That is,
3351 C<$::sail> is equivalent to C<$main::sail> (as well as to C<$main'sail>,
3352 still seen in older code).
3354 If NAMESPACE is omitted, then there is no current package, and all
3355 identifiers must be fully qualified or lexicals. This is stricter
3356 than C<use strict>, since it also extends to function names.
3358 See L<perlmod/"Packages"> for more information about packages, modules,
3359 and classes. See L<perlsub> for other scoping issues.
3361 =item pipe READHANDLE,WRITEHANDLE
3363 Opens a pair of connected pipes like the corresponding system call.
3364 Note that if you set up a loop of piped processes, deadlock can occur
3365 unless you are very careful. In addition, note that Perl's pipes use
3366 stdio buffering, so you may need to set C<$|> to flush your WRITEHANDLE
3367 after each command, depending on the application.
3369 See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
3370 for examples of such things.
3372 On systems that support a close-on-exec flag on files, the flag will be set
3373 for the newly opened file descriptors as determined by the value of $^F.
3380 Pops and returns the last value of the array, shortening the array by
3381 one element. Has an effect similar to
3385 If there are no elements in the array, returns the undefined value
3386 (although this may happen at other times as well). If ARRAY is
3387 omitted, pops the C<@ARGV> array in the main program, and the C<@_>
3388 array in subroutines, just like C<shift>.
3394 Returns the offset of where the last C<m//g> search left off for the variable
3395 in question (C<$_> is used when the variable is not specified). May be
3396 modified to change that offset. Such modification will also influence
3397 the C<\G> zero-width assertion in regular expressions. See L<perlre> and
3400 =item print FILEHANDLE LIST
3406 Prints a string or a list of strings. Returns true if successful.
3407 FILEHANDLE may be a scalar variable name, in which case the variable
3408 contains the name of or a reference to the filehandle, thus introducing
3409 one level of indirection. (NOTE: If FILEHANDLE is a variable and
3410 the next token is a term, it may be misinterpreted as an operator
3411 unless you interpose a C<+> or put parentheses around the arguments.)
3412 If FILEHANDLE is omitted, prints by default to standard output (or
3413 to the last selected output channel--see L</select>). If LIST is
3414 also omitted, prints C<$_> to the currently selected output channel.
3415 To set the default output channel to something other than STDOUT
3416 use the select operation. The current value of C<$,> (if any) is
3417 printed between each LIST item. The current value of C<$\> (if
3418 any) is printed after the entire LIST has been printed. Because
3419 print takes a LIST, anything in the LIST is evaluated in list
3420 context, and any subroutine that you call will have one or more of
3421 its expressions evaluated in list context. Also be careful not to
3422 follow the print keyword with a left parenthesis unless you want
3423 the corresponding right parenthesis to terminate the arguments to
3424 the print--interpose a C<+> or put parentheses around all the
3427 Note that if you're storing FILEHANDLES in an array or other expression,
3428 you will have to use a block returning its value instead:
3430 print { $files[$i] } "stuff\n";
3431 print { $OK ? STDOUT : STDERR } "stuff\n";
3433 =item printf FILEHANDLE FORMAT, LIST
3435 =item printf FORMAT, LIST
3437 Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that C<$\>
3438 (the output record separator) is not appended. The first argument
3439 of the list will be interpreted as the C<printf> format. If C<use locale> is
3440 in effect, the character used for the decimal point in formatted real numbers
3441 is affected by the LC_NUMERIC locale. See L<perllocale>.
3443 Don't fall into the trap of using a C<printf> when a simple
3444 C<print> would do. The C<print> is more efficient and less
3447 =item prototype FUNCTION
3449 Returns the prototype of a function as a string (or C<undef> if the
3450 function has no prototype). FUNCTION is a reference to, or the name of,
3451 the function whose prototype you want to retrieve.
3453 If FUNCTION is a string starting with C<CORE::>, the rest is taken as a
3454 name for Perl builtin. If the builtin is not I<overridable> (such as
3455 C<qw//>) or its arguments cannot be expressed by a prototype (such as
3456 C<system>) returns C<undef> because the builtin does not really behave
3457 like a Perl function. Otherwise, the string describing the equivalent
3458 prototype is returned.
3460 =item push ARRAY,LIST
3462 Treats ARRAY as a stack, and pushes the values of LIST
3463 onto the end of ARRAY. The length of ARRAY increases by the length of
3464 LIST. Has the same effect as
3467 $ARRAY[++$#ARRAY] = $value;
3470 but is more efficient. Returns the new number of elements in the array.
3484 Generalized quotes. See L<perlop/"Regexp Quote-Like Operators">.
3486 =item quotemeta EXPR
3490 Returns the value of EXPR with all non-"word"
3491 characters backslashed. (That is, all characters not matching
3492 C</[A-Za-z_0-9]/> will be preceded by a backslash in the
3493 returned string, regardless of any locale settings.)
3494 This is the internal function implementing
3495 the C<\Q> escape in double-quoted strings.
3497 If EXPR is omitted, uses C<$_>.
3503 Returns a random fractional number greater than or equal to C<0> and less
3504 than the value of EXPR. (EXPR should be positive.) If EXPR is
3505 omitted, the value C<1> is used. Automatically calls C<srand> unless
3506 C<srand> has already been called. See also C<srand>.
3508 (Note: If your rand function consistently returns numbers that are too
3509 large or too small, then your version of Perl was probably compiled
3510 with the wrong number of RANDBITS.)
3512 =item read FILEHANDLE,SCALAR,LENGTH,OFFSET
3514 =item read FILEHANDLE,SCALAR,LENGTH
3516 Attempts to read LENGTH bytes of data into variable SCALAR from the
3517 specified FILEHANDLE. Returns the number of bytes actually read, C<0>
3518 at end of file, or undef if there was an error. SCALAR will be grown
3519 or shrunk to the length actually read. If SCALAR needs growing, the
3520 new bytes will be zero bytes. An OFFSET may be specified to place
3521 the read data into some other place in SCALAR than the beginning.
3522 The call is actually implemented in terms of stdio's fread(3) call.
3523 To get a true read(2) system call, see C<sysread>.
3525 =item readdir DIRHANDLE
3527 Returns the next directory entry for a directory opened by C<opendir>.
3528 If used in list context, returns all the rest of the entries in the
3529 directory. If there are no more entries, returns an undefined value in
3530 scalar context or a null list in list context.
3532 If you're planning to filetest the return values out of a C<readdir>, you'd
3533 better prepend the directory in question. Otherwise, because we didn't
3534 C<chdir> there, it would have been testing the wrong file.
3536 opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!";
3537 @dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR);
3542 Reads from the filehandle whose typeglob is contained in EXPR. In scalar
3543 context, each call reads and returns the next line, until end-of-file is
3544 reached, whereupon the subsequent call returns undef. In list context,
3545 reads until end-of-file is reached and returns a list of lines. Note that
3546 the notion of "line" used here is however you may have defined it
3547 with C<$/> or C<$INPUT_RECORD_SEPARATOR>). See L<perlvar/"$/">.
3549 When C<$/> is set to C<undef>, when readline() is in scalar
3550 context (i.e. file slurp mode), and when an empty file is read, it
3551 returns C<''> the first time, followed by C<undef> subsequently.
3553 This is the internal function implementing the C<< <EXPR> >>
3554 operator, but you can use it directly. The C<< <EXPR> >>
3555 operator is discussed in more detail in L<perlop/"I/O Operators">.
3558 $line = readline(*STDIN); # same thing
3564 Returns the value of a symbolic link, if symbolic links are
3565 implemented. If not, gives a fatal error. If there is some system
3566 error, returns the undefined value and sets C<$!> (errno). If EXPR is
3567 omitted, uses C<$_>.
3571 EXPR is executed as a system command.
3572 The collected standard output of the command is returned.
3573 In scalar context, it comes back as a single (potentially
3574 multi-line) string. In list context, returns a list of lines
3575 (however you've defined lines with C<$/> or C<$INPUT_RECORD_SEPARATOR>).
3576 This is the internal function implementing the C<qx/EXPR/>
3577 operator, but you can use it directly. The C<qx/EXPR/>
3578 operator is discussed in more detail in L<perlop/"I/O Operators">.
3580 =item recv SOCKET,SCALAR,LENGTH,FLAGS
3582 Receives a message on a socket. Attempts to receive LENGTH bytes of
3583 data into variable SCALAR from the specified SOCKET filehandle. SCALAR
3584 will be grown or shrunk to the length actually read. Takes the same
3585 flags as the system call of the same name. Returns the address of the
3586 sender if SOCKET's protocol supports this; returns an empty string
3587 otherwise. If there's an error, returns the undefined value. This call
3588 is actually implemented in terms of recvfrom(2) system call. See
3589 L<perlipc/"UDP: Message Passing"> for examples.
3595 The C<redo> command restarts the loop block without evaluating the
3596 conditional again. The C<continue> block, if any, is not executed. If
3597 the LABEL is omitted, the command refers to the innermost enclosing
3598 loop. This command is normally used by programs that want to lie to
3599 themselves about what was just input:
3601 # a simpleminded Pascal comment stripper
3602 # (warning: assumes no { or } in strings)
3603 LINE: while (<STDIN>) {
3604 while (s|({.*}.*){.*}|$1 |) {}
3609 if (/}/) { # end of comment?
3618 C<redo> cannot be used to retry a block which returns a value such as
3619 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
3620 a grep() or map() operation.
3622 Note that a block by itself is semantically identical to a loop
3623 that executes once. Thus C<redo> inside such a block will effectively
3624 turn it into a looping construct.
3626 See also L</continue> for an illustration of how C<last>, C<next>, and
3633 Returns a true value if EXPR is a reference, false otherwise. If EXPR
3634 is not specified, C<$_> will be used. The value returned depends on the
3635 type of thing the reference is a reference to.
3636 Builtin types include:
3646 If the referenced object has been blessed into a package, then that package
3647 name is returned instead. You can think of C<ref> as a C<typeof> operator.
3649 if (ref($r) eq "HASH") {
3650 print "r is a reference to a hash.\n";
3653 print "r is not a reference at all.\n";
3655 if (UNIVERSAL::isa($r, "HASH")) { # for subclassing
3656 print "r is a reference to something that isa hash.\n";
3659 See also L<perlref>.
3661 =item rename OLDNAME,NEWNAME
3663 Changes the name of a file; an existing file NEWNAME will be
3664 clobbered. Returns true for success, false otherwise.
3666 Behavior of this function varies wildly depending on your system
3667 implementation. For example, it will usually not work across file system
3668 boundaries, even though the system I<mv> command sometimes compensates
3669 for this. Other restrictions include whether it works on directories,
3670 open files, or pre-existing files. Check L<perlport> and either the
3671 rename(2) manpage or equivalent system documentation for details.
3673 =item require VERSION
3679 Demands some semantics specified by EXPR, or by C<$_> if EXPR is not
3682 If a VERSION is specified as a literal of the form v5.6.1,
3683 demands that the current version of Perl (C<$^V> or $PERL_VERSION) be
3684 at least as recent as that version, at run time. (For compatibility
3685 with older versions of Perl, a numeric argument will also be interpreted
3686 as VERSION.) Compare with L</use>, which can do a similar check at
3689 require v5.6.1; # run time version check
3690 require 5.6.1; # ditto
3691 require 5.005_03; # float version allowed for compatibility
3693 Otherwise, demands that a library file be included if it hasn't already
3694 been included. The file is included via the do-FILE mechanism, which is
3695 essentially just a variety of C<eval>. Has semantics similar to the following
3700 return 1 if $INC{$filename};
3701 my($realfilename,$result);
3703 foreach $prefix (@INC) {
3704 $realfilename = "$prefix/$filename";
3705 if (-f $realfilename) {
3706 $INC{$filename} = $realfilename;
3707 $result = do $realfilename;
3711 die "Can't find $filename in \@INC";
3713 delete $INC{$filename} if $@ || !$result;
3715 die "$filename did not return true value" unless $result;
3719 Note that the file will not be included twice under the same specified
3720 name. The file must return true as the last statement to indicate
3721 successful execution of any initialization code, so it's customary to
3722 end such a file with C<1;> unless you're sure it'll return true
3723 otherwise. But it's better just to put the C<1;>, in case you add more
3726 If EXPR is a bareword, the require assumes a "F<.pm>" extension and
3727 replaces "F<::>" with "F</>" in the filename for you,
3728 to make it easy to load standard modules. This form of loading of
3729 modules does not risk altering your namespace.
3731 In other words, if you try this:
3733 require Foo::Bar; # a splendid bareword
3735 The require function will actually look for the "F<Foo/Bar.pm>" file in the
3736 directories specified in the C<@INC> array.
3738 But if you try this:
3740 $class = 'Foo::Bar';
3741 require $class; # $class is not a bareword
3743 require "Foo::Bar"; # not a bareword because of the ""
3745 The require function will look for the "F<Foo::Bar>" file in the @INC array and
3746 will complain about not finding "F<Foo::Bar>" there. In this case you can do:
3748 eval "require $class";
3750 For a yet-more-powerful import facility, see L</use> and L<perlmod>.
3756 Generally used in a C<continue> block at the end of a loop to clear
3757 variables and reset C<??> searches so that they work again. The
3758 expression is interpreted as a list of single characters (hyphens
3759 allowed for ranges). All variables and arrays beginning with one of
3760 those letters are reset to their pristine state. If the expression is
3761 omitted, one-match searches (C<?pattern?>) are reset to match again. Resets
3762 only variables or searches in the current package. Always returns
3765 reset 'X'; # reset all X variables
3766 reset 'a-z'; # reset lower case variables
3767 reset; # just reset ?one-time? searches
3769 Resetting C<"A-Z"> is not recommended because you'll wipe out your
3770 C<@ARGV> and C<@INC> arrays and your C<%ENV> hash. Resets only package
3771 variables--lexical variables are unaffected, but they clean themselves
3772 up on scope exit anyway, so you'll probably want to use them instead.
3779 Returns from a subroutine, C<eval>, or C<do FILE> with the value
3780 given in EXPR. Evaluation of EXPR may be in list, scalar, or void
3781 context, depending on how the return value will be used, and the context
3782 may vary from one execution to the next (see C<wantarray>). If no EXPR
3783 is given, returns an empty list in list context, the undefined value in
3784 scalar context, and (of course) nothing at all in a void context.
3786 (Note that in the absence of a explicit C<return>, a subroutine, eval,
3787 or do FILE will automatically return the value of the last expression
3792 In list context, returns a list value consisting of the elements
3793 of LIST in the opposite order. In scalar context, concatenates the
3794 elements of LIST and returns a string value with all characters
3795 in the opposite order.
3797 print reverse <>; # line tac, last line first
3799 undef $/; # for efficiency of <>
3800 print scalar reverse <>; # character tac, last line tsrif
3802 This operator is also handy for inverting a hash, although there are some
3803 caveats. If a value is duplicated in the original hash, only one of those
3804 can be represented as a key in the inverted hash. Also, this has to
3805 unwind one hash and build a whole new one, which may take some time
3806 on a large hash, such as from a DBM file.
3808 %by_name = reverse %by_address; # Invert the hash
3810 =item rewinddir DIRHANDLE
3812 Sets the current position to the beginning of the directory for the
3813 C<readdir> routine on DIRHANDLE.
3815 =item rindex STR,SUBSTR,POSITION
3817 =item rindex STR,SUBSTR
3819 Works just like index() except that it returns the position of the LAST
3820 occurrence of SUBSTR in STR. If POSITION is specified, returns the
3821 last occurrence at or before that position.
3823 =item rmdir FILENAME
3827 Deletes the directory specified by FILENAME if that directory is empty. If it
3828 succeeds it returns true, otherwise it returns false and sets C<$!> (errno). If
3829 FILENAME is omitted, uses C<$_>.
3833 The substitution operator. See L<perlop>.
3837 Forces EXPR to be interpreted in scalar context and returns the value
3840 @counts = ( scalar @a, scalar @b, scalar @c );
3842 There is no equivalent operator to force an expression to
3843 be interpolated in list context because in practice, this is never
3844 needed. If you really wanted to do so, however, you could use
3845 the construction C<@{[ (some expression) ]}>, but usually a simple
3846 C<(some expression)> suffices.
3848 Because C<scalar> is unary operator, if you accidentally use for EXPR a
3849 parenthesized list, this behaves as a scalar comma expression, evaluating
3850 all but the last element in void context and returning the final element
3851 evaluated in scalar context. This is seldom what you want.
3853 The following single statement:
3855 print uc(scalar(&foo,$bar)),$baz;
3857 is the moral equivalent of these two:
3860 print(uc($bar),$baz);
3862 See L<perlop> for more details on unary operators and the comma operator.
3864 =item seek FILEHANDLE,POSITION,WHENCE
3866 Sets FILEHANDLE's position, just like the C<fseek> call of C<stdio>.
3867 FILEHANDLE may be an expression whose value gives the name of the
3868 filehandle. The values for WHENCE are C<0> to set the new position to
3869 POSITION, C<1> to set it to the current position plus POSITION, and
3870 C<2> to set it to EOF plus POSITION (typically negative). For WHENCE
3871 you may use the constants C<SEEK_SET>, C<SEEK_CUR>, and C<SEEK_END>
3872 (start of the file, current position, end of the file) from the Fcntl
3873 module. Returns C<1> upon success, C<0> otherwise.
3875 If you want to position file for C<sysread> or C<syswrite>, don't use
3876 C<seek>--buffering makes its effect on the file's system position
3877 unpredictable and non-portable. Use C<sysseek> instead.
3879 Due to the rules and rigors of ANSI C, on some systems you have to do a
3880 seek whenever you switch between reading and writing. Amongst other
3881 things, this may have the effect of calling stdio's clearerr(3).
3882 A WHENCE of C<1> (C<SEEK_CUR>) is useful for not moving the file position:
3886 This is also useful for applications emulating C<tail -f>. Once you hit
3887 EOF on your read, and then sleep for a while, you might have to stick in a
3888 seek() to reset things. The C<seek> doesn't change the current position,
3889 but it I<does> clear the end-of-file condition on the handle, so that the
3890 next C<< <FILE> >> makes Perl try again to read something. We hope.
3892 If that doesn't work (some stdios are particularly cantankerous), then
3893 you may need something more like this:
3896 for ($curpos = tell(FILE); $_ = <FILE>;
3897 $curpos = tell(FILE)) {
3898 # search for some stuff and put it into files
3900 sleep($for_a_while);
3901 seek(FILE, $curpos, 0);
3904 =item seekdir DIRHANDLE,POS
3906 Sets the current position for the C<readdir> routine on DIRHANDLE. POS
3907 must be a value returned by C<telldir>. Has the same caveats about
3908 possible directory compaction as the corresponding system library
3911 =item select FILEHANDLE
3915 Returns the currently selected filehandle. Sets the current default
3916 filehandle for output, if FILEHANDLE is supplied. This has two
3917 effects: first, a C<write> or a C<print> without a filehandle will
3918 default to this FILEHANDLE. Second, references to variables related to
3919 output will refer to this output channel. For example, if you have to
3920 set the top of form format for more than one output channel, you might
3928 FILEHANDLE may be an expression whose value gives the name of the
3929 actual filehandle. Thus:
3931 $oldfh = select(STDERR); $| = 1; select($oldfh);
3933 Some programmers may prefer to think of filehandles as objects with
3934 methods, preferring to write the last example as:
3937 STDERR->autoflush(1);
3939 =item select RBITS,WBITS,EBITS,TIMEOUT
3941 This calls the select(2) system call with the bit masks specified, which
3942 can be constructed using C<fileno> and C<vec>, along these lines:
3944 $rin = $win = $ein = '';
3945 vec($rin,fileno(STDIN),1) = 1;
3946 vec($win,fileno(STDOUT),1) = 1;
3949 If you want to select on many filehandles you might wish to write a
3953 my(@fhlist) = split(' ',$_[0]);
3956 vec($bits,fileno($_),1) = 1;
3960 $rin = fhbits('STDIN TTY SOCK');
3964 ($nfound,$timeleft) =
3965 select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
3967 or to block until something becomes ready just do this
3969 $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
3971 Most systems do not bother to return anything useful in $timeleft, so
3972 calling select() in scalar context just returns $nfound.
3974 Any of the bit masks can also be undef. The timeout, if specified, is
3975 in seconds, which may be fractional. Note: not all implementations are
3976 capable of returning the$timeleft. If not, they always return
3977 $timeleft equal to the supplied $timeout.
3979 You can effect a sleep of 250 milliseconds this way:
3981 select(undef, undef, undef, 0.25);
3983 B<WARNING>: One should not attempt to mix buffered I/O (like C<read>
3984 or <FH>) with C<select>, except as permitted by POSIX, and even
3985 then only on POSIX systems. You have to use C<sysread> instead.
3987 =item semctl ID,SEMNUM,CMD,ARG
3989 Calls the System V IPC function C<semctl>. You'll probably have to say
3993 first to get the correct constant definitions. If CMD is IPC_STAT or
3994 GETALL, then ARG must be a variable which will hold the returned
3995 semid_ds structure or semaphore value array. Returns like C<ioctl>:
3996 the undefined value for error, "C<0 but true>" for zero, or the actual
3997 return value otherwise. The ARG must consist of a vector of native
3998 short integers, which may be created with C<pack("s!",(0)x$nsem)>.
3999 See also L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::Semaphore>
4002 =item semget KEY,NSEMS,FLAGS
4004 Calls the System V IPC function semget. Returns the semaphore id, or
4005 the undefined value if there is an error. See also
4006 L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::SysV::Semaphore>
4009 =item semop KEY,OPSTRING
4011 Calls the System V IPC function semop to perform semaphore operations
4012 such as signaling and waiting. OPSTRING must be a packed array of
4013 semop structures. Each semop structure can be generated with
4014 C<pack("sss", $semnum, $semop, $semflag)>. The number of semaphore
4015 operations is implied by the length of OPSTRING. Returns true if
4016 successful, or false if there is an error. As an example, the
4017 following code waits on semaphore $semnum of semaphore id $semid:
4019 $semop = pack("sss", $semnum, -1, 0);
4020 die "Semaphore trouble: $!\n" unless semop($semid, $semop);
4022 To signal the semaphore, replace C<-1> with C<1>. See also
4023 L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::SysV::Semaphore>
4026 =item send SOCKET,MSG,FLAGS,TO
4028 =item send SOCKET,MSG,FLAGS
4030 Sends a message on a socket. Takes the same flags as the system call
4031 of the same name. On unconnected sockets you must specify a
4032 destination to send TO, in which case it does a C C<sendto>. Returns
4033 the number of characters sent, or the undefined value if there is an
4034 error. The C system call sendmsg(2) is currently unimplemented.
4035 See L<perlipc/"UDP: Message Passing"> for examples.
4037 =item setpgrp PID,PGRP
4039 Sets the current process group for the specified PID, C<0> for the current
4040 process. Will produce a fatal error if used on a machine that doesn't
4041 implement POSIX setpgid(2) or BSD setpgrp(2). If the arguments are omitted,
4042 it defaults to C<0,0>. Note that the BSD 4.2 version of C<setpgrp> does not
4043 accept any arguments, so only C<setpgrp(0,0)> is portable. See also
4046 =item setpriority WHICH,WHO,PRIORITY
4048 Sets the current priority for a process, a process group, or a user.
4049 (See setpriority(2).) Will produce a fatal error if used on a machine
4050 that doesn't implement setpriority(2).
4052 =item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
4054 Sets the socket option requested. Returns undefined if there is an
4055 error. OPTVAL may be specified as C<undef> if you don't want to pass an
4062 Shifts the first value of the array off and returns it, shortening the
4063 array by 1 and moving everything down. If there are no elements in the
4064 array, returns the undefined value. If ARRAY is omitted, shifts the
4065 C<@_> array within the lexical scope of subroutines and formats, and the
4066 C<@ARGV> array at file scopes or within the lexical scopes established by
4067 the C<eval ''>, C<BEGIN {}>, C<INIT {}>, C<CHECK {}>, and C<END {}>
4070 See also C<unshift>, C<push>, and C<pop>. C<shift> and C<unshift> do the
4071 same thing to the left end of an array that C<pop> and C<push> do to the
4074 =item shmctl ID,CMD,ARG
4076 Calls the System V IPC function shmctl. You'll probably have to say
4080 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
4081 then ARG must be a variable which will hold the returned C<shmid_ds>
4082 structure. Returns like ioctl: the undefined value for error, "C<0> but
4083 true" for zero, or the actual return value otherwise.
4084 See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation.
4086 =item shmget KEY,SIZE,FLAGS
4088 Calls the System V IPC function shmget. Returns the shared memory
4089 segment id, or the undefined value if there is an error.
4090 See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation.
4092 =item shmread ID,VAR,POS,SIZE
4094 =item shmwrite ID,STRING,POS,SIZE
4096 Reads or writes the System V shared memory segment ID starting at
4097 position POS for size SIZE by attaching to it, copying in/out, and
4098 detaching from it. When reading, VAR must be a variable that will
4099 hold the data read. When writing, if STRING is too long, only SIZE
4100 bytes are used; if STRING is too short, nulls are written to fill out
4101 SIZE bytes. Return true if successful, or false if there is an error.
4102 shmread() taints the variable. See also L<perlipc/"SysV IPC">,
4103 C<IPC::SysV> documentation, and the C<IPC::Shareable> module from CPAN.
4105 =item shutdown SOCKET,HOW
4107 Shuts down a socket connection in the manner indicated by HOW, which
4108 has the same interpretation as in the system call of the same name.
4110 shutdown(SOCKET, 0); # I/we have stopped reading data
4111 shutdown(SOCKET, 1); # I/we have stopped writing data
4112 shutdown(SOCKET, 2); # I/we have stopped using this socket
4114 This is useful with sockets when you want to tell the other
4115 side you're done writing but not done reading, or vice versa.
4116 It's also a more insistent form of close because it also
4117 disables the file descriptor in any forked copies in other
4124 Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
4125 returns sine of C<$_>.
4127 For the inverse sine operation, you may use the C<Math::Trig::asin>
4128 function, or use this relation:
4130 sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
4136 Causes the script to sleep for EXPR seconds, or forever if no EXPR.
4137 May be interrupted if the process receives a signal such as C<SIGALRM>.
4138 Returns the number of seconds actually slept. You probably cannot
4139 mix C<alarm> and C<sleep> calls, because C<sleep> is often implemented
4142 On some older systems, it may sleep up to a full second less than what
4143 you requested, depending on how it counts seconds. Most modern systems
4144 always sleep the full amount. They may appear to sleep longer than that,
4145 however, because your process might not be scheduled right away in a
4146 busy multitasking system.
4148 For delays of finer granularity than one second, you may use Perl's
4149 C<syscall> interface to access setitimer(2) if your system supports
4150 it, or else see L</select> above. The Time::HiRes module from CPAN
4153 See also the POSIX module's C<pause> function.
4155 =item socket SOCKET,DOMAIN,TYPE,PROTOCOL
4157 Opens a socket of the specified kind and attaches it to filehandle
4158 SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for
4159 the system call of the same name. You should C<use Socket> first
4160 to get the proper definitions imported. See the examples in
4161 L<perlipc/"Sockets: Client/Server Communication">.
4163 On systems that support a close-on-exec flag on files, the flag will
4164 be set for the newly opened file descriptor, as determined by the
4165 value of $^F. See L<perlvar/$^F>.
4167 =item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
4169 Creates an unnamed pair of sockets in the specified domain, of the
4170 specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as
4171 for the system call of the same name. If unimplemented, yields a fatal
4172 error. Returns true if successful.
4174 On systems that support a close-on-exec flag on files, the flag will
4175 be set for the newly opened file descriptors, as determined by the value
4176 of $^F. See L<perlvar/$^F>.
4178 Some systems defined C<pipe> in terms of C<socketpair>, in which a call
4179 to C<pipe(Rdr, Wtr)> is essentially:
4182 socketpair(Rdr, Wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC);
4183 shutdown(Rdr, 1); # no more writing for reader
4184 shutdown(Wtr, 0); # no more reading for writer
4186 See L<perlipc> for an example of socketpair use.
4188 =item sort SUBNAME LIST
4190 =item sort BLOCK LIST
4194 Sorts the LIST and returns the sorted list value. If SUBNAME or BLOCK
4195 is omitted, C<sort>s in standard string comparison order. If SUBNAME is
4196 specified, it gives the name of a subroutine that returns an integer
4197 less than, equal to, or greater than C<0>, depending on how the elements
4198 of the list are to be ordered. (The C<< <=> >> and C<cmp>
4199 operators are extremely useful in such routines.) SUBNAME may be a
4200 scalar variable name (unsubscripted), in which case the value provides
4201 the name of (or a reference to) the actual subroutine to use. In place
4202 of a SUBNAME, you can provide a BLOCK as an anonymous, in-line sort
4205 If the subroutine's prototype is C<($$)>, the elements to be compared
4206 are passed by reference in C<@_>, as for a normal subroutine. This is
4207 slower than unprototyped subroutines, where the elements to be
4208 compared are passed into the subroutine
4209 as the package global variables $a and $b (see example below). Note that
4210 in the latter case, it is usually counter-productive to declare $a and
4213 In either case, the subroutine may not be recursive. The values to be
4214 compared are always passed by reference, so don't modify them.
4216 You also cannot exit out of the sort block or subroutine using any of the
4217 loop control operators described in L<perlsyn> or with C<goto>.
4219 When C<use locale> is in effect, C<sort LIST> sorts LIST according to the
4220 current collation locale. See L<perllocale>.
4225 @articles = sort @files;
4227 # same thing, but with explicit sort routine
4228 @articles = sort {$a cmp $b} @files;
4230 # now case-insensitively
4231 @articles = sort {uc($a) cmp uc($b)} @files;
4233 # same thing in reversed order
4234 @articles = sort {$b cmp $a} @files;
4236 # sort numerically ascending
4237 @articles = sort {$a <=> $b} @files;
4239 # sort numerically descending
4240 @articles = sort {$b <=> $a} @files;
4242 # this sorts the %age hash by value instead of key
4243 # using an in-line function
4244 @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
4246 # sort using explicit subroutine name
4248 $age{$a} <=> $age{$b}; # presuming numeric
4250 @sortedclass = sort byage @class;
4252 sub backwards { $b cmp $a }
4253 @harry = qw(dog cat x Cain Abel);
4254 @george = qw(gone chased yz Punished Axed);
4256 # prints AbelCaincatdogx
4257 print sort backwards @harry;
4258 # prints xdogcatCainAbel
4259 print sort @george, 'to', @harry;
4260 # prints AbelAxedCainPunishedcatchaseddoggonetoxyz
4262 # inefficiently sort by descending numeric compare using
4263 # the first integer after the first = sign, or the
4264 # whole record case-insensitively otherwise
4267 ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
4272 # same thing, but much more efficiently;
4273 # we'll build auxiliary indices instead
4277 push @nums, /=(\d+)/;
4282 $nums[$b] <=> $nums[$a]
4284 $caps[$a] cmp $caps[$b]
4288 # same thing, but without any temps
4289 @new = map { $_->[0] }
4290 sort { $b->[1] <=> $a->[1]
4293 } map { [$_, /=(\d+)/, uc($_)] } @old;
4295 # using a prototype allows you to use any comparison subroutine
4296 # as a sort subroutine (including other package's subroutines)
4298 sub backwards ($$) { $_[1] cmp $_[0]; } # $a and $b are not set here
4301 @new = sort other::backwards @old;
4303 If you're using strict, you I<must not> declare $a
4304 and $b as lexicals. They are package globals. That means
4305 if you're in the C<main> package and type
4307 @articles = sort {$b <=> $a} @files;
4309 then C<$a> and C<$b> are C<$main::a> and C<$main::b> (or C<$::a> and C<$::b>),
4310 but if you're in the C<FooPack> package, it's the same as typing
4312 @articles = sort {$FooPack::b <=> $FooPack::a} @files;
4314 The comparison function is required to behave. If it returns
4315 inconsistent results (sometimes saying C<$x[1]> is less than C<$x[2]> and
4316 sometimes saying the opposite, for example) the results are not
4319 =item splice ARRAY,OFFSET,LENGTH,LIST
4321 =item splice ARRAY,OFFSET,LENGTH
4323 =item splice ARRAY,OFFSET
4327 Removes the elements designated by OFFSET and LENGTH from an array, and
4328 replaces them with the elements of LIST, if any. In list context,
4329 returns the elements removed from the array. In scalar context,
4330 returns the last element removed, or C<undef> if no elements are
4331 removed. The array grows or shrinks as necessary.
4332 If OFFSET is negative then it starts that far from the end of the array.
4333 If LENGTH is omitted, removes everything from OFFSET onward.
4334 If LENGTH is negative, leaves that many elements off the end of the array.
4335 If both OFFSET and LENGTH are omitted, removes everything.
4337 The following equivalences hold (assuming C<$[ == 0>):
4339 push(@a,$x,$y) splice(@a,@a,0,$x,$y)
4340 pop(@a) splice(@a,-1)
4341 shift(@a) splice(@a,0,1)
4342 unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
4343 $a[$x] = $y splice(@a,$x,1,$y)
4345 Example, assuming array lengths are passed before arrays:
4347 sub aeq { # compare two list values
4348 my(@a) = splice(@_,0,shift);
4349 my(@b) = splice(@_,0,shift);
4350 return 0 unless @a == @b; # same len?
4352 return 0 if pop(@a) ne pop(@b);
4356 if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
4358 =item split /PATTERN/,EXPR,LIMIT
4360 =item split /PATTERN/,EXPR
4362 =item split /PATTERN/
4366 Splits a string into a list of strings and returns that list. By default,
4367 empty leading fields are preserved, and empty trailing ones are deleted.
4369 In scalar context, returns the number of fields found and splits into
4370 the C<@_> array. Use of split in scalar context is deprecated, however,
4371 because it clobbers your subroutine arguments.
4373 If EXPR is omitted, splits the C<$_> string. If PATTERN is also omitted,
4374 splits on whitespace (after skipping any leading whitespace). Anything
4375 matching PATTERN is taken to be a delimiter separating the fields. (Note
4376 that the delimiter may be longer than one character.)
4378 If LIMIT is specified and positive, splits into no more than that
4379 many fields (though it may split into fewer). If LIMIT is unspecified
4380 or zero, trailing null fields are stripped (which potential users
4381 of C<pop> would do well to remember). If LIMIT is negative, it is
4382 treated as if an arbitrarily large LIMIT had been specified.
4384 A pattern matching the null string (not to be confused with
4385 a null pattern C<//>, which is just one member of the set of patterns
4386 matching a null string) will split the value of EXPR into separate
4387 characters at each point it matches that way. For example:
4389 print join(':', split(/ */, 'hi there'));
4391 produces the output 'h:i:t:h:e:r:e'.
4393 Empty leading (or trailing) fields are produced when there positive width
4394 matches at the beginning (or end) of the string; a zero-width match at the
4395 beginning (or end) of the string does not produce an empty field. For
4398 print join(':', split(/(?=\w)/, 'hi there!'));
4400 produces the output 'h:i :t:h:e:r:e!'.
4402 The LIMIT parameter can be used to split a line partially
4404 ($login, $passwd, $remainder) = split(/:/, $_, 3);
4406 When assigning to a list, if LIMIT is omitted, Perl supplies a LIMIT
4407 one larger than the number of variables in the list, to avoid
4408 unnecessary work. For the list above LIMIT would have been 4 by
4409 default. In time critical applications it behooves you not to split
4410 into more fields than you really need.
4412 If the PATTERN contains parentheses, additional list elements are
4413 created from each matching substring in the delimiter.
4415 split(/([,-])/, "1-10,20", 3);
4417 produces the list value
4419 (1, '-', 10, ',', 20)
4421 If you had the entire header of a normal Unix email message in $header,
4422 you could split it up into fields and their values this way:
4424 $header =~ s/\n\s+/ /g; # fix continuation lines
4425 %hdrs = (UNIX_FROM => split /^(\S*?):\s*/m, $header);
4427 The pattern C</PATTERN/> may be replaced with an expression to specify
4428 patterns that vary at runtime. (To do runtime compilation only once,
4429 use C</$variable/o>.)
4431 As a special case, specifying a PATTERN of space (C<' '>) will split on
4432 white space just as C<split> with no arguments does. Thus, C<split(' ')> can
4433 be used to emulate B<awk>'s default behavior, whereas C<split(/ /)>
4434 will give you as many null initial fields as there are leading spaces.
4435 A C<split> on C</\s+/> is like a C<split(' ')> except that any leading
4436 whitespace produces a null first field. A C<split> with no arguments
4437 really does a C<split(' ', $_)> internally.
4439 A PATTERN of C</^/> is treated as if it were C</^/m>, since it isn't
4444 open(PASSWD, '/etc/passwd');
4446 ($login, $passwd, $uid, $gid,
4447 $gcos, $home, $shell) = split(/:/);
4451 (Note that $shell above will still have a newline on it. See L</chop>,
4452 L</chomp>, and L</join>.)
4454 =item sprintf FORMAT, LIST
4456 Returns a string formatted by the usual C<printf> conventions of the C
4457 library function C<sprintf>. See below for more details
4458 and see L<sprintf(3)> or L<printf(3)> on your system for an explanation of
4459 the general principles.
4463 # Format number with up to 8 leading zeroes
4464 $result = sprintf("%08d", $number);
4466 # Round number to 3 digits after decimal point
4467 $rounded = sprintf("%.3f", $number);
4469 Perl does its own C<sprintf> formatting--it emulates the C
4470 function C<sprintf>, but it doesn't use it (except for floating-point
4471 numbers, and even then only the standard modifiers are allowed). As a
4472 result, any non-standard extensions in your local C<sprintf> are not
4473 available from Perl.
4475 Unlike C<printf>, C<sprintf> does not do what you probably mean when you
4476 pass it an array as your first argument. The array is given scalar context,
4477 and instead of using the 0th element of the array as the format, Perl will
4478 use the count of elements in the array as the format, which is almost never
4481 Perl's C<sprintf> permits the following universally-known conversions:
4484 %c a character with the given number
4486 %d a signed integer, in decimal
4487 %u an unsigned integer, in decimal
4488 %o an unsigned integer, in octal
4489 %x an unsigned integer, in hexadecimal
4490 %e a floating-point number, in scientific notation
4491 %f a floating-point number, in fixed decimal notation
4492 %g a floating-point number, in %e or %f notation
4494 In addition, Perl permits the following widely-supported conversions:
4496 %X like %x, but using upper-case letters
4497 %E like %e, but using an upper-case "E"
4498 %G like %g, but with an upper-case "E" (if applicable)
4499 %b an unsigned integer, in binary
4500 %p a pointer (outputs the Perl value's address in hexadecimal)
4501 %n special: *stores* the number of characters output so far
4502 into the next variable in the parameter list
4504 Finally, for backward (and we do mean "backward") compatibility, Perl
4505 permits these unnecessary but widely-supported conversions:
4508 %D a synonym for %ld
4509 %U a synonym for %lu
4510 %O a synonym for %lo
4513 Note that the number of exponent digits in the scientific notation by
4514 C<%e>, C<%E>, C<%g> and C<%G> for numbers with the modulus of the
4515 exponent less than 100 is system-dependent: it may be three or less
4516 (zero-padded as necessary). In other words, 1.23 times ten to the
4517 99th may be either "1.23e99" or "1.23e099".
4519 Perl permits the following universally-known flags between the C<%>
4520 and the conversion letter:
4522 space prefix positive number with a space
4523 + prefix positive number with a plus sign
4524 - left-justify within the field
4525 0 use zeros, not spaces, to right-justify
4526 # prefix non-zero octal with "0", non-zero hex with "0x"
4527 number minimum field width
4528 .number "precision": digits after decimal point for
4529 floating-point, max length for string, minimum length
4531 l interpret integer as C type "long" or "unsigned long"
4532 h interpret integer as C type "short" or "unsigned short"
4533 If no flags, interpret integer as C type "int" or "unsigned"
4535 Perl supports parameter ordering, in other words, fetching the
4536 parameters in some explicitly specified "random" ordering as opposed
4537 to the default implicit sequential ordering. The syntax is, instead
4538 of the C<%> and C<*>, to use C<%>I<digits>C<$> and C<*>I<digits>C<$>,
4539 where the I<digits> is the wanted index, from one upwards. For example:
4541 printf "%2\$d %1\$d\n", 12, 34; # will print "34 12\n"
4542 printf "%*2\$d\n", 12, 3; # will print " 12\n"
4544 Note that using the reordering syntax does not interfere with the usual
4545 implicit sequential fetching of the parameters:
4547 printf "%2\$d %d\n", 12, 34; # will print "34 12\n"
4548 printf "%2\$d %d %d\n", 12, 34; # will print "34 12 34\n"
4549 printf "%3\$d %d %d\n", 12, 34, 56; # will print "56 12 34\n"
4550 printf "%2\$*3\$d %d\n", 12, 34, 3; # will print " 34 12\n"
4551 printf "%*3\$2\$d %d\n", 12, 34, 3; # will print " 34 12\n"
4553 There are also two Perl-specific flags:
4555 V interpret integer as Perl's standard integer type
4556 v interpret string as a vector of integers, output as
4557 numbers separated either by dots, or by an arbitrary
4558 string received from the argument list when the flag
4561 Where a number would appear in the flags, an asterisk (C<*>) may be
4562 used instead, in which case Perl uses the next item in the parameter
4563 list as the given number (that is, as the field width or precision).
4564 If a field width obtained through C<*> is negative, it has the same
4565 effect as the C<-> flag: left-justification.
4567 The C<v> flag is useful for displaying ordinal values of characters
4568 in arbitrary strings:
4570 printf "version is v%vd\n", $^V; # Perl's version
4571 printf "address is %*vX\n", ":", $addr; # IPv6 address
4572 printf "bits are %*vb\n", " ", $bits; # random bitstring
4574 If C<use locale> is in effect, the character used for the decimal
4575 point in formatted real numbers is affected by the LC_NUMERIC locale.
4578 If Perl understands "quads" (64-bit integers) (this requires
4579 either that the platform natively support quads or that Perl
4580 be specifically compiled to support quads), the characters
4584 print quads, and they may optionally be preceded by
4592 You can find out whether your Perl supports quads via L<Config>:
4595 ($Config{use64bitint} eq 'define' || $Config{longsize} == 8) &&
4598 If Perl understands "long doubles" (this requires that the platform
4599 support long doubles), the flags
4603 may optionally be preceded by
4611 You can find out whether your Perl supports long doubles via L<Config>:
4614 $Config{d_longdbl} eq 'define' && print "long doubles\n";
4620 Return the square root of EXPR. If EXPR is omitted, returns square
4621 root of C<$_>. Only works on non-negative operands, unless you've
4622 loaded the standard Math::Complex module.
4625 print sqrt(-2); # prints 1.4142135623731i
4631 Sets the random number seed for the C<rand> operator. If EXPR is
4632 omitted, uses a semi-random value supplied by the kernel (if it supports
4633 the F</dev/urandom> device) or based on the current time and process
4634 ID, among other things. In versions of Perl prior to 5.004 the default
4635 seed was just the current C<time>. This isn't a particularly good seed,
4636 so many old programs supply their own seed value (often C<time ^ $$> or
4637 C<time ^ ($$ + ($$ << 15))>), but that isn't necessary any more.
4639 In fact, it's usually not necessary to call C<srand> at all, because if
4640 it is not called explicitly, it is called implicitly at the first use of
4641 the C<rand> operator. However, this was not the case in version of Perl
4642 before 5.004, so if your script will run under older Perl versions, it
4643 should call C<srand>.
4645 Note that you need something much more random than the default seed for
4646 cryptographic purposes. Checksumming the compressed output of one or more
4647 rapidly changing operating system status programs is the usual method. For
4650 srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip`);
4652 If you're particularly concerned with this, see the C<Math::TrulyRandom>
4655 Do I<not> call C<srand> multiple times in your program unless you know
4656 exactly what you're doing and why you're doing it. The point of the
4657 function is to "seed" the C<rand> function so that C<rand> can produce
4658 a different sequence each time you run your program. Just do it once at the
4659 top of your program, or you I<won't> get random numbers out of C<rand>!
4661 Frequently called programs (like CGI scripts) that simply use
4665 for a seed can fall prey to the mathematical property that
4669 one-third of the time. So don't do that.
4671 =item stat FILEHANDLE
4677 Returns a 13-element list giving the status info for a file, either
4678 the file opened via FILEHANDLE, or named by EXPR. If EXPR is omitted,
4679 it stats C<$_>. Returns a null list if the stat fails. Typically used
4682 ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
4683 $atime,$mtime,$ctime,$blksize,$blocks)
4686 Not all fields are supported on all filesystem types. Here are the
4687 meaning of the fields:
4689 0 dev device number of filesystem
4691 2 mode file mode (type and permissions)
4692 3 nlink number of (hard) links to the file
4693 4 uid numeric user ID of file's owner
4694 5 gid numeric group ID of file's owner
4695 6 rdev the device identifier (special files only)
4696 7 size total size of file, in bytes
4697 8 atime last access time in seconds since the epoch
4698 9 mtime last modify time in seconds since the epoch
4699 10 ctime inode change time (NOT creation time!) in seconds since the epoch
4700 11 blksize preferred block size for file system I/O
4701 12 blocks actual number of blocks allocated
4703 (The epoch was at 00:00 January 1, 1970 GMT.)
4705 If stat is passed the special filehandle consisting of an underline, no
4706 stat is done, but the current contents of the stat structure from the
4707 last stat or filetest are returned. Example:
4709 if (-x $file && (($d) = stat(_)) && $d < 0) {
4710 print "$file is executable NFS file\n";
4713 (This works on machines only for which the device number is negative
4716 Because the mode contains both the file type and its permissions, you
4717 should mask off the file type portion and (s)printf using a C<"%o">
4718 if you want to see the real permissions.
4720 $mode = (stat($filename))[2];
4721 printf "Permissions are %04o\n", $mode & 07777;
4723 In scalar context, C<stat> returns a boolean value indicating success
4724 or failure, and, if successful, sets the information associated with
4725 the special filehandle C<_>.
4727 The File::stat module provides a convenient, by-name access mechanism:
4730 $sb = stat($filename);
4731 printf "File is %s, size is %s, perm %04o, mtime %s\n",
4732 $filename, $sb->size, $sb->mode & 07777,
4733 scalar localtime $sb->mtime;
4735 You can import symbolic mode constants (C<S_IF*>) and functions
4736 (C<S_IS*>) from the Fcntl module:
4740 $mode = (stat($filename))[2];
4742 $user_rwx = ($mode & S_IRWXU) >> 6;
4743 $group_read = ($mode & S_IRGRP) >> 3;
4744 $other_execute = $mode & S_IXOTH;
4746 printf "Permissions are %04o\n", S_ISMODE($mode), "\n";
4748 $is_setuid = $mode & S_ISUID;
4749 $is_setgid = S_ISDIR($mode);
4751 You could write the last two using the C<-u> and C<-d> operators.
4752 The commonly available S_IF* constants are
4754 # Permissions: read, write, execute, for user, group, others.
4756 S_IRWXU S_IRUSR S_IWUSR S_IXUSR
4757 S_IRWXG S_IRGRP S_IWGRP S_IXGRP
4758 S_IRWXO S_IROTH S_IWOTH S_IXOTH
4760 # Setuid/Setgid/Stickiness.
4762 S_ISUID S_ISGID S_ISVTX S_ISTXT
4764 # File types. Not necessarily all are available on your system.
4766 S_IFREG S_IFDIR S_IFLNK S_IFBLK S_ISCHR S_IFIFO S_IFSOCK S_IFWHT S_ENFMT
4768 # The following are compatibility aliases for S_IRUSR, S_IWUSR, S_IXUSR.
4770 S_IREAD S_IWRITE S_IEXEC
4772 and the S_IF* functions are
4774 S_IFMODE($mode) the part of $mode containing the permission bits
4775 and the setuid/setgid/sticky bits
4777 S_IFMT($mode) the part of $mode containing the file type
4778 which can be bit-anded with e.g. S_IFREG
4779 or with the following functions
4781 # The operators -f, -d, -l, -b, -c, -p, and -s.
4783 S_ISREG($mode) S_ISDIR($mode) S_ISLNK($mode)
4784 S_ISBLK($mode) S_ISCHR($mode) S_ISFIFO($mode) S_ISSOCK($mode)
4786 # No direct -X operator counterpart, but for the first one
4787 # the -g operator is often equivalent. The ENFMT stands for
4788 # record flocking enforcement, a platform-dependent feature.
4790 S_ISENFMT($mode) S_ISWHT($mode)
4792 See your native chmod(2) and stat(2) documentation for more details
4793 about the S_* constants.
4799 Takes extra time to study SCALAR (C<$_> if unspecified) in anticipation of
4800 doing many pattern matches on the string before it is next modified.
4801 This may or may not save time, depending on the nature and number of
4802 patterns you are searching on, and on the distribution of character
4803 frequencies in the string to be searched--you probably want to compare
4804 run times with and without it to see which runs faster. Those loops
4805 which scan for many short constant strings (including the constant
4806 parts of more complex patterns) will benefit most. You may have only
4807 one C<study> active at a time--if you study a different scalar the first
4808 is "unstudied". (The way C<study> works is this: a linked list of every
4809 character in the string to be searched is made, so we know, for
4810 example, where all the C<'k'> characters are. From each search string,
4811 the rarest character is selected, based on some static frequency tables
4812 constructed from some C programs and English text. Only those places
4813 that contain this "rarest" character are examined.)
4815 For example, here is a loop that inserts index producing entries
4816 before any line containing a certain pattern:
4820 print ".IX foo\n" if /\bfoo\b/;
4821 print ".IX bar\n" if /\bbar\b/;
4822 print ".IX blurfl\n" if /\bblurfl\b/;
4827 In searching for C</\bfoo\b/>, only those locations in C<$_> that contain C<f>
4828 will be looked at, because C<f> is rarer than C<o>. In general, this is
4829 a big win except in pathological cases. The only question is whether
4830 it saves you more time than it took to build the linked list in the
4833 Note that if you have to look for strings that you don't know till
4834 runtime, you can build an entire loop as a string and C<eval> that to
4835 avoid recompiling all your patterns all the time. Together with
4836 undefining C<$/> to input entire files as one record, this can be very
4837 fast, often faster than specialized programs like fgrep(1). The following
4838 scans a list of files (C<@files>) for a list of words (C<@words>), and prints
4839 out the names of those files that contain a match:
4841 $search = 'while (<>) { study;';
4842 foreach $word (@words) {
4843 $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
4848 eval $search; # this screams
4849 $/ = "\n"; # put back to normal input delimiter
4850 foreach $file (sort keys(%seen)) {
4858 =item sub NAME BLOCK
4860 This is subroutine definition, not a real function I<per se>. With just a
4861 NAME (and possibly prototypes or attributes), it's just a forward declaration.
4862 Without a NAME, it's an anonymous function declaration, and does actually
4863 return a value: the CODE ref of the closure you just created. See L<perlsub>
4864 and L<perlref> for details.
4866 =item substr EXPR,OFFSET,LENGTH,REPLACEMENT
4868 =item substr EXPR,OFFSET,LENGTH
4870 =item substr EXPR,OFFSET
4872 Extracts a substring out of EXPR and returns it. First character is at
4873 offset C<0>, or whatever you've set C<$[> to (but don't do that).
4874 If OFFSET is negative (or more precisely, less than C<$[>), starts
4875 that far from the end of the string. If LENGTH is omitted, returns
4876 everything to the end of the string. If LENGTH is negative, leaves that
4877 many characters off the end of the string.
4879 You can use the substr() function as an lvalue, in which case EXPR
4880 must itself be an lvalue. If you assign something shorter than LENGTH,
4881 the string will shrink, and if you assign something longer than LENGTH,
4882 the string will grow to accommodate it. To keep the string the same
4883 length you may need to pad or chop your value using C<sprintf>.
4885 If OFFSET and LENGTH specify a substring that is partly outside the
4886 string, only the part within the string is returned. If the substring
4887 is beyond either end of the string, substr() returns the undefined
4888 value and produces a warning. When used as an lvalue, specifying a
4889 substring that is entirely outside the string is a fatal error.
4890 Here's an example showing the behavior for boundary cases:
4893 substr($name, 4) = 'dy'; # $name is now 'freddy'
4894 my $null = substr $name, 6, 2; # returns '' (no warning)
4895 my $oops = substr $name, 7; # returns undef, with warning
4896 substr($name, 7) = 'gap'; # fatal error
4898 An alternative to using substr() as an lvalue is to specify the
4899 replacement string as the 4th argument. This allows you to replace
4900 parts of the EXPR and return what was there before in one operation,
4901 just as you can with splice().
4903 =item symlink OLDFILE,NEWFILE
4905 Creates a new filename symbolically linked to the old filename.
4906 Returns C<1> for success, C<0> otherwise. On systems that don't support
4907 symbolic links, produces a fatal error at run time. To check for that,
4910 $symlink_exists = eval { symlink("",""); 1 };
4914 Calls the system call specified as the first element of the list,
4915 passing the remaining elements as arguments to the system call. If
4916 unimplemented, produces a fatal error. The arguments are interpreted
4917 as follows: if a given argument is numeric, the argument is passed as
4918 an int. If not, the pointer to the string value is passed. You are
4919 responsible to make sure a string is pre-extended long enough to
4920 receive any result that might be written into a string. You can't use a
4921 string literal (or other read-only string) as an argument to C<syscall>
4922 because Perl has to assume that any string pointer might be written
4924 integer arguments are not literals and have never been interpreted in a
4925 numeric context, you may need to add C<0> to them to force them to look
4926 like numbers. This emulates the C<syswrite> function (or vice versa):
4928 require 'syscall.ph'; # may need to run h2ph
4930 syscall(&SYS_write, fileno(STDOUT), $s, length $s);
4932 Note that Perl supports passing of up to only 14 arguments to your system call,
4933 which in practice should usually suffice.
4935 Syscall returns whatever value returned by the system call it calls.
4936 If the system call fails, C<syscall> returns C<-1> and sets C<$!> (errno).
4937 Note that some system calls can legitimately return C<-1>. The proper
4938 way to handle such calls is to assign C<$!=0;> before the call and
4939 check the value of C<$!> if syscall returns C<-1>.
4941 There's a problem with C<syscall(&SYS_pipe)>: it returns the file
4942 number of the read end of the pipe it creates. There is no way
4943 to retrieve the file number of the other end. You can avoid this
4944 problem by using C<pipe> instead.
4946 =item sysopen FILEHANDLE,FILENAME,MODE
4948 =item sysopen FILEHANDLE,FILENAME,MODE,PERMS
4950 Opens the file whose filename is given by FILENAME, and associates it
4951 with FILEHANDLE. If FILEHANDLE is an expression, its value is used as
4952 the name of the real filehandle wanted. This function calls the
4953 underlying operating system's C<open> function with the parameters
4954 FILENAME, MODE, PERMS.
4956 The possible values and flag bits of the MODE parameter are
4957 system-dependent; they are available via the standard module C<Fcntl>.
4958 See the documentation of your operating system's C<open> to see which
4959 values and flag bits are available. You may combine several flags
4960 using the C<|>-operator.
4962 Some of the most common values are C<O_RDONLY> for opening the file in
4963 read-only mode, C<O_WRONLY> for opening the file in write-only mode,
4964 and C<O_RDWR> for opening the file in read-write mode, and.
4966 For historical reasons, some values work on almost every system
4967 supported by perl: zero means read-only, one means write-only, and two
4968 means read/write. We know that these values do I<not> work under
4969 OS/390 & VM/ESA Unix and on the Macintosh; you probably don't want to
4970 use them in new code.
4972 If the file named by FILENAME does not exist and the C<open> call creates
4973 it (typically because MODE includes the C<O_CREAT> flag), then the value of
4974 PERMS specifies the permissions of the newly created file. If you omit
4975 the PERMS argument to C<sysopen>, Perl uses the octal value C<0666>.
4976 These permission values need to be in octal, and are modified by your
4977 process's current C<umask>.
4979 In many systems the C<O_EXCL> flag is available for opening files in
4980 exclusive mode. This is B<not> locking: exclusiveness means here that
4981 if the file already exists, sysopen() fails. The C<O_EXCL> wins
4984 Sometimes you may want to truncate an already-existing file: C<O_TRUNC>.
4986 You should seldom if ever use C<0644> as argument to C<sysopen>, because
4987 that takes away the user's option to have a more permissive umask.
4988 Better to omit it. See the perlfunc(1) entry on C<umask> for more
4991 Note that C<sysopen> depends on the fdopen() C library function.
4992 On many UNIX systems, fdopen() is known to fail when file descriptors
4993 exceed a certain value, typically 255. If you need more file
4994 descriptors than that, consider rebuilding Perl to use the C<sfio>
4995 library, or perhaps using the POSIX::open() function.
4997 See L<perlopentut> for a kinder, gentler explanation of opening files.
4999 =item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
5001 =item sysread FILEHANDLE,SCALAR,LENGTH
5003 Attempts to read LENGTH bytes of data into variable SCALAR from the
5004 specified FILEHANDLE, using the system call read(2). It bypasses stdio,
5005 so mixing this with other kinds of reads, C<print>, C<write>,
5006 C<seek>, C<tell>, or C<eof> can cause confusion because stdio
5007 usually buffers data. Returns the number of bytes actually read, C<0>
5008 at end of file, or undef if there was an error. SCALAR will be grown or
5009 shrunk so that the last byte actually read is the last byte of the
5010 scalar after the read.
5012 An OFFSET may be specified to place the read data at some place in the
5013 string other than the beginning. A negative OFFSET specifies
5014 placement at that many bytes counting backwards from the end of the
5015 string. A positive OFFSET greater than the length of SCALAR results
5016 in the string being padded to the required size with C<"\0"> bytes before
5017 the result of the read is appended.
5019 There is no syseof() function, which is ok, since eof() doesn't work
5020 very well on device files (like ttys) anyway. Use sysread() and check
5021 for a return value for 0 to decide whether you're done.
5023 =item sysseek FILEHANDLE,POSITION,WHENCE
5025 Sets FILEHANDLE's system position using the system call lseek(2). It
5026 bypasses stdio, so mixing this with reads (other than C<sysread>),
5027 C<print>, C<write>, C<seek>, C<tell>, or C<eof> may cause confusion.
5028 FILEHANDLE may be an expression whose value gives the name of the
5029 filehandle. The values for WHENCE are C<0> to set the new position to
5030 POSITION, C<1> to set the it to the current position plus POSITION,
5031 and C<2> to set it to EOF plus POSITION (typically negative). For
5032 WHENCE, you may also use the constants C<SEEK_SET>, C<SEEK_CUR>, and
5033 C<SEEK_END> (start of the file, current position, end of the file)
5034 from the Fcntl module.
5036 Returns the new position, or the undefined value on failure. A position
5037 of zero is returned as the string C<"0 but true">; thus C<sysseek> returns
5038 true on success and false on failure, yet you can still easily determine
5043 =item system PROGRAM LIST
5045 Does exactly the same thing as C<exec LIST>, except that a fork is
5046 done first, and the parent process waits for the child process to
5047 complete. Note that argument processing varies depending on the
5048 number of arguments. If there is more than one argument in LIST,
5049 or if LIST is an array with more than one value, starts the program
5050 given by the first element of the list with arguments given by the
5051 rest of the list. If there is only one scalar argument, the argument
5052 is checked for shell metacharacters, and if there are any, the
5053 entire argument is passed to the system's command shell for parsing
5054 (this is C</bin/sh -c> on Unix platforms, but varies on other
5055 platforms). If there are no shell metacharacters in the argument,
5056 it is split into words and passed directly to C<execvp>, which is
5059 Beginning with v5.6.0, Perl will attempt to flush all files opened for
5060 output before any operation that may do a fork, but this may not be
5061 supported on some platforms (see L<perlport>). To be safe, you may need
5062 to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
5063 of C<IO::Handle> on any open handles.
5065 The return value is the exit status of the program as
5066 returned by the C<wait> call. To get the actual exit value divide by
5067 256. See also L</exec>. This is I<not> what you want to use to capture
5068 the output from a command, for that you should use merely backticks or
5069 C<qx//>, as described in L<perlop/"`STRING`">. Return value of -1
5070 indicates a failure to start the program (inspect $! for the reason).
5072 Like C<exec>, C<system> allows you to lie to a program about its name if
5073 you use the C<system PROGRAM LIST> syntax. Again, see L</exec>.
5075 Because C<system> and backticks block C<SIGINT> and C<SIGQUIT>, killing the
5076 program they're running doesn't actually interrupt your program.
5078 @args = ("command", "arg1", "arg2");
5080 or die "system @args failed: $?"
5082 You can check all the failure possibilities by inspecting
5085 $exit_value = $? >> 8;
5086 $signal_num = $? & 127;
5087 $dumped_core = $? & 128;
5089 When the arguments get executed via the system shell, results
5090 and return codes will be subject to its quirks and capabilities.
5091 See L<perlop/"`STRING`"> and L</exec> for details.
5093 =item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
5095 =item syswrite FILEHANDLE,SCALAR,LENGTH
5097 =item syswrite FILEHANDLE,SCALAR
5099 Attempts to write LENGTH bytes of data from variable SCALAR to the
5100 specified FILEHANDLE, using the system call write(2). If LENGTH
5101 is not specified, writes whole SCALAR. It bypasses stdio, so mixing
5102 this with reads (other than C<sysread())>, C<print>, C<write>,
5103 C<seek>, C<tell>, or C<eof> may cause confusion because stdio
5104 usually buffers data. Returns the number of bytes actually written,
5105 or C<undef> if there was an error. If the LENGTH is greater than
5106 the available data in the SCALAR after the OFFSET, only as much
5107 data as is available will be written.
5109 An OFFSET may be specified to write the data from some part of the
5110 string other than the beginning. A negative OFFSET specifies writing
5111 that many bytes counting backwards from the end of the string. In the
5112 case the SCALAR is empty you can use OFFSET but only zero offset.
5114 =item tell FILEHANDLE
5118 Returns the current position for FILEHANDLE, or -1 on error. FILEHANDLE
5119 may be an expression whose value gives the name of the actual filehandle.
5120 If FILEHANDLE is omitted, assumes the file last read.
5122 The return value of tell() for the standard streams like the STDIN
5123 depends on the operating system: it may return -1 or something else.
5124 tell() on pipes, fifos, and sockets usually returns -1.
5126 There is no C<systell> function. Use C<sysseek(FH, 0, 1)> for that.
5128 =item telldir DIRHANDLE
5130 Returns the current position of the C<readdir> routines on DIRHANDLE.
5131 Value may be given to C<seekdir> to access a particular location in a
5132 directory. Has the same caveats about possible directory compaction as
5133 the corresponding system library routine.
5135 =item tie VARIABLE,CLASSNAME,LIST
5137 This function binds a variable to a package class that will provide the
5138 implementation for the variable. VARIABLE is the name of the variable
5139 to be enchanted. CLASSNAME is the name of a class implementing objects
5140 of correct type. Any additional arguments are passed to the C<new>
5141 method of the class (meaning C<TIESCALAR>, C<TIEHANDLE>, C<TIEARRAY>,
5142 or C<TIEHASH>). Typically these are arguments such as might be passed
5143 to the C<dbm_open()> function of C. The object returned by the C<new>
5144 method is also returned by the C<tie> function, which would be useful
5145 if you want to access other methods in CLASSNAME.
5147 Note that functions such as C<keys> and C<values> may return huge lists
5148 when used on large objects, like DBM files. You may prefer to use the
5149 C<each> function to iterate over such. Example:
5151 # print out history file offsets
5153 tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
5154 while (($key,$val) = each %HIST) {
5155 print $key, ' = ', unpack('L',$val), "\n";
5159 A class implementing a hash should have the following methods:
5161 TIEHASH classname, LIST
5163 STORE this, key, value
5168 NEXTKEY this, lastkey
5172 A class implementing an ordinary array should have the following methods:
5174 TIEARRAY classname, LIST
5176 STORE this, key, value
5178 STORESIZE this, count
5184 SPLICE this, offset, length, LIST
5189 A class implementing a file handle should have the following methods:
5191 TIEHANDLE classname, LIST
5192 READ this, scalar, length, offset
5195 WRITE this, scalar, length, offset
5197 PRINTF this, format, LIST
5201 SEEK this, position, whence
5203 OPEN this, mode, LIST
5208 A class implementing a scalar should have the following methods:
5210 TIESCALAR classname, LIST
5216 Not all methods indicated above need be implemented. See L<perltie>,
5217 L<Tie::Hash>, L<Tie::Array>, L<Tie::Scalar>, and L<Tie::Handle>.
5219 Unlike C<dbmopen>, the C<tie> function will not use or require a module
5220 for you--you need to do that explicitly yourself. See L<DB_File>
5221 or the F<Config> module for interesting C<tie> implementations.
5223 For further details see L<perltie>, L<"tied VARIABLE">.
5227 Returns a reference to the object underlying VARIABLE (the same value
5228 that was originally returned by the C<tie> call that bound the variable
5229 to a package.) Returns the undefined value if VARIABLE isn't tied to a
5234 Returns the number of non-leap seconds since whatever time the system
5235 considers to be the epoch (that's 00:00:00, January 1, 1904 for MacOS,
5236 and 00:00:00 UTC, January 1, 1970 for most other systems).
5237 Suitable for feeding to C<gmtime> and C<localtime>.
5239 For measuring time in better granularity than one second,
5240 you may use either the Time::HiRes module from CPAN, or
5241 if you have gettimeofday(2), you may be able to use the
5242 C<syscall> interface of Perl, see L<perlfaq8> for details.
5246 Returns a four-element list giving the user and system times, in
5247 seconds, for this process and the children of this process.
5249 ($user,$system,$cuser,$csystem) = times;
5253 The transliteration operator. Same as C<y///>. See L<perlop>.
5255 =item truncate FILEHANDLE,LENGTH
5257 =item truncate EXPR,LENGTH
5259 Truncates the file opened on FILEHANDLE, or named by EXPR, to the
5260 specified length. Produces a fatal error if truncate isn't implemented
5261 on your system. Returns true if successful, the undefined value
5268 Returns an uppercased version of EXPR. This is the internal function
5269 implementing the C<\U> escape in double-quoted strings.
5270 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
5271 Under Unicode (C<use utf8>) it uses the standard Unicode uppercase mappings. (It
5272 does not attempt to do titlecase mapping on initial letters. See C<ucfirst> for that.)
5274 If EXPR is omitted, uses C<$_>.
5280 Returns the value of EXPR with the first character
5281 in uppercase (titlecase in Unicode). This is
5282 the internal function implementing the C<\u> escape in double-quoted strings.
5283 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
5286 If EXPR is omitted, uses C<$_>.
5292 Sets the umask for the process to EXPR and returns the previous value.
5293 If EXPR is omitted, merely returns the current umask.
5295 The Unix permission C<rwxr-x---> is represented as three sets of three
5296 bits, or three octal digits: C<0750> (the leading 0 indicates octal
5297 and isn't one of the digits). The C<umask> value is such a number
5298 representing disabled permissions bits. The permission (or "mode")
5299 values you pass C<mkdir> or C<sysopen> are modified by your umask, so
5300 even if you tell C<sysopen> to create a file with permissions C<0777>,
5301 if your umask is C<0022> then the file will actually be created with
5302 permissions C<0755>. If your C<umask> were C<0027> (group can't
5303 write; others can't read, write, or execute), then passing
5304 C<sysopen> C<0666> would create a file with mode C<0640> (C<0666 &~
5307 Here's some advice: supply a creation mode of C<0666> for regular
5308 files (in C<sysopen>) and one of C<0777> for directories (in
5309 C<mkdir>) and executable files. This gives users the freedom of
5310 choice: if they want protected files, they might choose process umasks
5311 of C<022>, C<027>, or even the particularly antisocial mask of C<077>.
5312 Programs should rarely if ever make policy decisions better left to
5313 the user. The exception to this is when writing files that should be
5314 kept private: mail files, web browser cookies, I<.rhosts> files, and
5317 If umask(2) is not implemented on your system and you are trying to
5318 restrict access for I<yourself> (i.e., (EXPR & 0700) > 0), produces a
5319 fatal error at run time. If umask(2) is not implemented and you are
5320 not trying to restrict access for yourself, returns C<undef>.
5322 Remember that a umask is a number, usually given in octal; it is I<not> a
5323 string of octal digits. See also L</oct>, if all you have is a string.
5329 Undefines the value of EXPR, which must be an lvalue. Use only on a
5330 scalar value, an array (using C<@>), a hash (using C<%>), a subroutine
5331 (using C<&>), or a typeglob (using <*>). (Saying C<undef $hash{$key}>
5332 will probably not do what you expect on most predefined variables or
5333 DBM list values, so don't do that; see L<delete>.) Always returns the
5334 undefined value. You can omit the EXPR, in which case nothing is
5335 undefined, but you still get an undefined value that you could, for
5336 instance, return from a subroutine, assign to a variable or pass as a
5337 parameter. Examples:
5340 undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'};
5344 undef *xyz; # destroys $xyz, @xyz, %xyz, &xyz, etc.
5345 return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it;
5346 select undef, undef, undef, 0.25;
5347 ($a, $b, undef, $c) = &foo; # Ignore third value returned
5349 Note that this is a unary operator, not a list operator.
5355 Deletes a list of files. Returns the number of files successfully
5358 $cnt = unlink 'a', 'b', 'c';
5362 Note: C<unlink> will not delete directories unless you are superuser and
5363 the B<-U> flag is supplied to Perl. Even if these conditions are
5364 met, be warned that unlinking a directory can inflict damage on your
5365 filesystem. Use C<rmdir> instead.
5367 If LIST is omitted, uses C<$_>.
5369 =item unpack TEMPLATE,EXPR
5371 C<unpack> does the reverse of C<pack>: it takes a string
5372 and expands it out into a list of values.
5373 (In scalar context, it returns merely the first value produced.)
5375 The string is broken into chunks described by the TEMPLATE. Each chunk
5376 is converted separately to a value. Typically, either the string is a result
5377 of C<pack>, or the bytes of the string represent a C structure of some
5380 The TEMPLATE has the same format as in the C<pack> function.
5381 Here's a subroutine that does substring:
5384 my($what,$where,$howmuch) = @_;
5385 unpack("x$where a$howmuch", $what);
5390 sub ordinal { unpack("c",$_[0]); } # same as ord()
5392 In addition to fields allowed in pack(), you may prefix a field with
5393 a %<number> to indicate that
5394 you want a <number>-bit checksum of the items instead of the items
5395 themselves. Default is a 16-bit checksum. Checksum is calculated by
5396 summing numeric values of expanded values (for string fields the sum of
5397 C<ord($char)> is taken, for bit fields the sum of zeroes and ones).
5399 For example, the following
5400 computes the same number as the System V sum program:
5404 unpack("%32C*",<>) % 65535;
5407 The following efficiently counts the number of set bits in a bit vector:
5409 $setbits = unpack("%32b*", $selectmask);
5411 The C<p> and C<P> formats should be used with care. Since Perl
5412 has no way of checking whether the value passed to C<unpack()>
5413 corresponds to a valid memory location, passing a pointer value that's
5414 not known to be valid is likely to have disastrous consequences.
5416 If the repeat count of a field is larger than what the remainder of
5417 the input string allows, repeat count is decreased. If the input string
5418 is longer than one described by the TEMPLATE, the rest is ignored.
5420 See L</pack> for more examples and notes.
5422 =item untie VARIABLE
5424 Breaks the binding between a variable and a package. (See C<tie>.)
5426 =item unshift ARRAY,LIST
5428 Does the opposite of a C<shift>. Or the opposite of a C<push>,
5429 depending on how you look at it. Prepends list to the front of the
5430 array, and returns the new number of elements in the array.
5432 unshift(ARGV, '-e') unless $ARGV[0] =~ /^-/;
5434 Note the LIST is prepended whole, not one element at a time, so the
5435 prepended elements stay in the same order. Use C<reverse> to do the
5438 =item use Module VERSION LIST
5440 =item use Module VERSION
5442 =item use Module LIST
5448 Imports some semantics into the current package from the named module,
5449 generally by aliasing certain subroutine or variable names into your
5450 package. It is exactly equivalent to
5452 BEGIN { require Module; import Module LIST; }
5454 except that Module I<must> be a bareword.
5456 VERSION, which can be specified as a literal of the form v5.6.1, demands
5457 that the current version of Perl (C<$^V> or $PERL_VERSION) be at least
5458 as recent as that version. (For compatibility with older versions of Perl,
5459 a numeric literal will also be interpreted as VERSION.) If the version
5460 of the running Perl interpreter is less than VERSION, then an error
5461 message is printed and Perl exits immediately without attempting to
5462 parse the rest of the file. Compare with L</require>, which can do a
5463 similar check at run time.
5465 use v5.6.1; # compile time version check
5467 use 5.005_03; # float version allowed for compatibility
5469 This is often useful if you need to check the current Perl version before
5470 C<use>ing library modules that have changed in incompatible ways from
5471 older versions of Perl. (We try not to do this more than we have to.)
5473 The C<BEGIN> forces the C<require> and C<import> to happen at compile time. The
5474 C<require> makes sure the module is loaded into memory if it hasn't been
5475 yet. The C<import> is not a builtin--it's just an ordinary static method
5476 call into the C<Module> package to tell the module to import the list of
5477 features back into the current package. The module can implement its
5478 C<import> method any way it likes, though most modules just choose to
5479 derive their C<import> method via inheritance from the C<Exporter> class that
5480 is defined in the C<Exporter> module. See L<Exporter>. If no C<import>
5481 method can be found then the call is skipped.
5483 If you do not want to call the package's C<import> method (for instance,
5484 to stop your namespace from being altered), explicitly supply the empty list:
5488 That is exactly equivalent to
5490 BEGIN { require Module }
5492 If the VERSION argument is present between Module and LIST, then the
5493 C<use> will call the VERSION method in class Module with the given
5494 version as an argument. The default VERSION method, inherited from
5495 the UNIVERSAL class, croaks if the given version is larger than the
5496 value of the variable C<$Module::VERSION>.
5498 Again, there is a distinction between omitting LIST (C<import> called
5499 with no arguments) and an explicit empty LIST C<()> (C<import> not
5500 called). Note that there is no comma after VERSION!
5502 Because this is a wide-open interface, pragmas (compiler directives)
5503 are also implemented this way. Currently implemented pragmas are:
5508 use sigtrap qw(SEGV BUS);
5509 use strict qw(subs vars refs);
5510 use subs qw(afunc blurfl);
5511 use warnings qw(all);
5513 Some of these pseudo-modules import semantics into the current
5514 block scope (like C<strict> or C<integer>, unlike ordinary modules,
5515 which import symbols into the current package (which are effective
5516 through the end of the file).
5518 There's a corresponding C<no> command that unimports meanings imported
5519 by C<use>, i.e., it calls C<unimport Module LIST> instead of C<import>.
5525 If no C<unimport> method can be found the call fails with a fatal error.
5527 See L<perlmodlib> for a list of standard modules and pragmas. See L<perlrun>
5528 for the C<-M> and C<-m> command-line options to perl that give C<use>
5529 functionality from the command-line.
5533 Changes the access and modification times on each file of a list of
5534 files. The first two elements of the list must be the NUMERICAL access
5535 and modification times, in that order. Returns the number of files
5536 successfully changed. The inode change time of each file is set
5537 to the current time. This code has the same effect as the C<touch>
5538 command if the files already exist:
5542 utime $now, $now, @ARGV;
5546 Returns a list consisting of all the values of the named hash. (In a
5547 scalar context, returns the number of values.) The values are
5548 returned in an apparently random order. The actual random order is
5549 subject to change in future versions of perl, but it is guaranteed to
5550 be the same order as either the C<keys> or C<each> function would
5551 produce on the same (unmodified) hash.
5553 Note that the values are not copied, which means modifying them will
5554 modify the contents of the hash:
5556 for (values %hash) { s/foo/bar/g } # modifies %hash values
5557 for (@hash{keys %hash}) { s/foo/bar/g } # same
5559 As a side effect, calling values() resets the HASH's internal iterator.
5560 See also C<keys>, C<each>, and C<sort>.
5562 =item vec EXPR,OFFSET,BITS
5564 Treats the string in EXPR as a bit vector made up of elements of
5565 width BITS, and returns the value of the element specified by OFFSET
5566 as an unsigned integer. BITS therefore specifies the number of bits
5567 that are reserved for each element in the bit vector. This must
5568 be a power of two from 1 to 32 (or 64, if your platform supports
5571 If BITS is 8, "elements" coincide with bytes of the input string.
5573 If BITS is 16 or more, bytes of the input string are grouped into chunks
5574 of size BITS/8, and each group is converted to a number as with
5575 pack()/unpack() with big-endian formats C<n>/C<N> (and analogously
5576 for BITS==64). See L<"pack"> for details.
5578 If bits is 4 or less, the string is broken into bytes, then the bits
5579 of each byte are broken into 8/BITS groups. Bits of a byte are
5580 numbered in a little-endian-ish way, as in C<0x01>, C<0x02>,
5581 C<0x04>, C<0x08>, C<0x10>, C<0x20>, C<0x40>, C<0x80>. For example,
5582 breaking the single input byte C<chr(0x36)> into two groups gives a list
5583 C<(0x6, 0x3)>; breaking it into 4 groups gives C<(0x2, 0x1, 0x3, 0x0)>.
5585 C<vec> may also be assigned to, in which case parentheses are needed
5586 to give the expression the correct precedence as in
5588 vec($image, $max_x * $x + $y, 8) = 3;
5590 If the selected element is outside the string, the value 0 is returned.
5591 If an element off the end of the string is written to, Perl will first
5592 extend the string with sufficiently many zero bytes. It is an error
5593 to try to write off the beginning of the string (i.e. negative OFFSET).
5595 The string should not contain any character with the value > 255 (which
5596 can only happen if you're using UTF8 encoding). If it does, it will be
5597 treated as something which is not UTF8 encoded. When the C<vec> was
5598 assigned to, other parts of your program will also no longer consider the
5599 string to be UTF8 encoded. In other words, if you do have such characters
5600 in your string, vec() will operate on the actual byte string, and not the
5601 conceptual character string.
5603 Strings created with C<vec> can also be manipulated with the logical
5604 operators C<|>, C<&>, C<^>, and C<~>. These operators will assume a bit
5605 vector operation is desired when both operands are strings.
5606 See L<perlop/"Bitwise String Operators">.
5608 The following code will build up an ASCII string saying C<'PerlPerlPerl'>.
5609 The comments show the string after each step. Note that this code works
5610 in the same way on big-endian or little-endian machines.
5613 vec($foo, 0, 32) = 0x5065726C; # 'Perl'
5615 # $foo eq "Perl" eq "\x50\x65\x72\x6C", 32 bits
5616 print vec($foo, 0, 8); # prints 80 == 0x50 == ord('P')
5618 vec($foo, 2, 16) = 0x5065; # 'PerlPe'
5619 vec($foo, 3, 16) = 0x726C; # 'PerlPerl'
5620 vec($foo, 8, 8) = 0x50; # 'PerlPerlP'
5621 vec($foo, 9, 8) = 0x65; # 'PerlPerlPe'
5622 vec($foo, 20, 4) = 2; # 'PerlPerlPe' . "\x02"
5623 vec($foo, 21, 4) = 7; # 'PerlPerlPer'
5625 vec($foo, 45, 2) = 3; # 'PerlPerlPer' . "\x0c"
5626 vec($foo, 93, 1) = 1; # 'PerlPerlPer' . "\x2c"
5627 vec($foo, 94, 1) = 1; # 'PerlPerlPerl'
5630 To transform a bit vector into a string or list of 0's and 1's, use these:
5632 $bits = unpack("b*", $vector);
5633 @bits = split(//, unpack("b*", $vector));
5635 If you know the exact length in bits, it can be used in place of the C<*>.
5637 Here is an example to illustrate how the bits actually fall in place:
5643 unpack("V",$_) 01234567890123456789012345678901
5644 ------------------------------------------------------------------
5649 for ($shift=0; $shift < $width; ++$shift) {
5650 for ($off=0; $off < 32/$width; ++$off) {
5651 $str = pack("B*", "0"x32);
5652 $bits = (1<<$shift);
5653 vec($str, $off, $width) = $bits;
5654 $res = unpack("b*",$str);
5655 $val = unpack("V", $str);
5662 vec($_,@#,@#) = @<< == @######### @>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
5663 $off, $width, $bits, $val, $res
5667 Regardless of the machine architecture on which it is run, the above
5668 example should print the following table:
5671 unpack("V",$_) 01234567890123456789012345678901
5672 ------------------------------------------------------------------
5673 vec($_, 0, 1) = 1 == 1 10000000000000000000000000000000
5674 vec($_, 1, 1) = 1 == 2 01000000000000000000000000000000
5675 vec($_, 2, 1) = 1 == 4 00100000000000000000000000000000
5676 vec($_, 3, 1) = 1 == 8 00010000000000000000000000000000
5677 vec($_, 4, 1) = 1 == 16 00001000000000000000000000000000
5678 vec($_, 5, 1) = 1 == 32 00000100000000000000000000000000
5679 vec($_, 6, 1) = 1 == 64 00000010000000000000000000000000
5680 vec($_, 7, 1) = 1 == 128 00000001000000000000000000000000
5681 vec($_, 8, 1) = 1 == 256 00000000100000000000000000000000
5682 vec($_, 9, 1) = 1 == 512 00000000010000000000000000000000
5683 vec($_,10, 1) = 1 == 1024 00000000001000000000000000000000
5684 vec($_,11, 1) = 1 == 2048 00000000000100000000000000000000
5685 vec($_,12, 1) = 1 == 4096 00000000000010000000000000000000
5686 vec($_,13, 1) = 1 == 8192 00000000000001000000000000000000
5687 vec($_,14, 1) = 1 == 16384 00000000000000100000000000000000
5688 vec($_,15, 1) = 1 == 32768 00000000000000010000000000000000
5689 vec($_,16, 1) = 1 == 65536 00000000000000001000000000000000
5690 vec($_,17, 1) = 1 == 131072 00000000000000000100000000000000
5691 vec($_,18, 1) = 1 == 262144 00000000000000000010000000000000
5692 vec($_,19, 1) = 1 == 524288 00000000000000000001000000000000
5693 vec($_,20, 1) = 1 == 1048576 00000000000000000000100000000000
5694 vec($_,21, 1) = 1 == 2097152 00000000000000000000010000000000
5695 vec($_,22, 1) = 1 == 4194304 00000000000000000000001000000000
5696 vec($_,23, 1) = 1 == 8388608 00000000000000000000000100000000
5697 vec($_,24, 1) = 1 == 16777216 00000000000000000000000010000000
5698 vec($_,25, 1) = 1 == 33554432 00000000000000000000000001000000
5699 vec($_,26, 1) = 1 == 67108864 00000000000000000000000000100000
5700 vec($_,27, 1) = 1 == 134217728 00000000000000000000000000010000
5701 vec($_,28, 1) = 1 == 268435456 00000000000000000000000000001000
5702 vec($_,29, 1) = 1 == 536870912 00000000000000000000000000000100
5703 vec($_,30, 1) = 1 == 1073741824 00000000000000000000000000000010
5704 vec($_,31, 1) = 1 == 2147483648 00000000000000000000000000000001
5705 vec($_, 0, 2) = 1 == 1 10000000000000000000000000000000
5706 vec($_, 1, 2) = 1 == 4 00100000000000000000000000000000
5707 vec($_, 2, 2) = 1 == 16 00001000000000000000000000000000
5708 vec($_, 3, 2) = 1 == 64 00000010000000000000000000000000
5709 vec($_, 4, 2) = 1 == 256 00000000100000000000000000000000
5710 vec($_, 5, 2) = 1 == 1024 00000000001000000000000000000000
5711 vec($_, 6, 2) = 1 == 4096 00000000000010000000000000000000
5712 vec($_, 7, 2) = 1 == 16384 00000000000000100000000000000000
5713 vec($_, 8, 2) = 1 == 65536 00000000000000001000000000000000
5714 vec($_, 9, 2) = 1 == 262144 00000000000000000010000000000000
5715 vec($_,10, 2) = 1 == 1048576 00000000000000000000100000000000
5716 vec($_,11, 2) = 1 == 4194304 00000000000000000000001000000000
5717 vec($_,12, 2) = 1 == 16777216 00000000000000000000000010000000
5718 vec($_,13, 2) = 1 == 67108864 00000000000000000000000000100000
5719 vec($_,14, 2) = 1 == 268435456 00000000000000000000000000001000
5720 vec($_,15, 2) = 1 == 1073741824 00000000000000000000000000000010
5721 vec($_, 0, 2) = 2 == 2 01000000000000000000000000000000
5722 vec($_, 1, 2) = 2 == 8 00010000000000000000000000000000
5723 vec($_, 2, 2) = 2 == 32 00000100000000000000000000000000
5724 vec($_, 3, 2) = 2 == 128 00000001000000000000000000000000
5725 vec($_, 4, 2) = 2 == 512 00000000010000000000000000000000
5726 vec($_, 5, 2) = 2 == 2048 00000000000100000000000000000000
5727 vec($_, 6, 2) = 2 == 8192 00000000000001000000000000000000
5728 vec($_, 7, 2) = 2 == 32768 00000000000000010000000000000000
5729 vec($_, 8, 2) = 2 == 131072 00000000000000000100000000000000
5730 vec($_, 9, 2) = 2 == 524288 00000000000000000001000000000000
5731 vec($_,10, 2) = 2 == 2097152 00000000000000000000010000000000
5732 vec($_,11, 2) = 2 == 8388608 00000000000000000000000100000000
5733 vec($_,12, 2) = 2 == 33554432 00000000000000000000000001000000
5734 vec($_,13, 2) = 2 == 134217728 00000000000000000000000000010000
5735 vec($_,14, 2) = 2 == 536870912 00000000000000000000000000000100
5736 vec($_,15, 2) = 2 == 2147483648 00000000000000000000000000000001
5737 vec($_, 0, 4) = 1 == 1 10000000000000000000000000000000
5738 vec($_, 1, 4) = 1 == 16 00001000000000000000000000000000
5739 vec($_, 2, 4) = 1 == 256 00000000100000000000000000000000
5740 vec($_, 3, 4) = 1 == 4096 00000000000010000000000000000000
5741 vec($_, 4, 4) = 1 == 65536 00000000000000001000000000000000
5742 vec($_, 5, 4) = 1 == 1048576 00000000000000000000100000000000
5743 vec($_, 6, 4) = 1 == 16777216 00000000000000000000000010000000
5744 vec($_, 7, 4) = 1 == 268435456 00000000000000000000000000001000
5745 vec($_, 0, 4) = 2 == 2 01000000000000000000000000000000
5746 vec($_, 1, 4) = 2 == 32 00000100000000000000000000000000
5747 vec($_, 2, 4) = 2 == 512 00000000010000000000000000000000
5748 vec($_, 3, 4) = 2 == 8192 00000000000001000000000000000000
5749 vec($_, 4, 4) = 2 == 131072 00000000000000000100000000000000
5750 vec($_, 5, 4) = 2 == 2097152 00000000000000000000010000000000
5751 vec($_, 6, 4) = 2 == 33554432 00000000000000000000000001000000
5752 vec($_, 7, 4) = 2 == 536870912 00000000000000000000000000000100
5753 vec($_, 0, 4) = 4 == 4 00100000000000000000000000000000
5754 vec($_, 1, 4) = 4 == 64 00000010000000000000000000000000
5755 vec($_, 2, 4) = 4 == 1024 00000000001000000000000000000000
5756 vec($_, 3, 4) = 4 == 16384 00000000000000100000000000000000
5757 vec($_, 4, 4) = 4 == 262144 00000000000000000010000000000000
5758 vec($_, 5, 4) = 4 == 4194304 00000000000000000000001000000000
5759 vec($_, 6, 4) = 4 == 67108864 00000000000000000000000000100000
5760 vec($_, 7, 4) = 4 == 1073741824 00000000000000000000000000000010
5761 vec($_, 0, 4) = 8 == 8 00010000000000000000000000000000
5762 vec($_, 1, 4) = 8 == 128 00000001000000000000000000000000
5763 vec($_, 2, 4) = 8 == 2048 00000000000100000000000000000000
5764 vec($_, 3, 4) = 8 == 32768 00000000000000010000000000000000
5765 vec($_, 4, 4) = 8 == 524288 00000000000000000001000000000000
5766 vec($_, 5, 4) = 8 == 8388608 00000000000000000000000100000000
5767 vec($_, 6, 4) = 8 == 134217728 00000000000000000000000000010000
5768 vec($_, 7, 4) = 8 == 2147483648 00000000000000000000000000000001
5769 vec($_, 0, 8) = 1 == 1 10000000000000000000000000000000
5770 vec($_, 1, 8) = 1 == 256 00000000100000000000000000000000
5771 vec($_, 2, 8) = 1 == 65536 00000000000000001000000000000000
5772 vec($_, 3, 8) = 1 == 16777216 00000000000000000000000010000000
5773 vec($_, 0, 8) = 2 == 2 01000000000000000000000000000000
5774 vec($_, 1, 8) = 2 == 512 00000000010000000000000000000000
5775 vec($_, 2, 8) = 2 == 131072 00000000000000000100000000000000
5776 vec($_, 3, 8) = 2 == 33554432 00000000000000000000000001000000
5777 vec($_, 0, 8) = 4 == 4 00100000000000000000000000000000
5778 vec($_, 1, 8) = 4 == 1024 00000000001000000000000000000000
5779 vec($_, 2, 8) = 4 == 262144 00000000000000000010000000000000
5780 vec($_, 3, 8) = 4 == 67108864 00000000000000000000000000100000
5781 vec($_, 0, 8) = 8 == 8 00010000000000000000000000000000
5782 vec($_, 1, 8) = 8 == 2048 00000000000100000000000000000000
5783 vec($_, 2, 8) = 8 == 524288 00000000000000000001000000000000
5784 vec($_, 3, 8) = 8 == 134217728 00000000000000000000000000010000
5785 vec($_, 0, 8) = 16 == 16 00001000000000000000000000000000
5786 vec($_, 1, 8) = 16 == 4096 00000000000010000000000000000000
5787 vec($_, 2, 8) = 16 == 1048576 00000000000000000000100000000000
5788 vec($_, 3, 8) = 16 == 268435456 00000000000000000000000000001000
5789 vec($_, 0, 8) = 32 == 32 00000100000000000000000000000000
5790 vec($_, 1, 8) = 32 == 8192 00000000000001000000000000000000
5791 vec($_, 2, 8) = 32 == 2097152 00000000000000000000010000000000
5792 vec($_, 3, 8) = 32 == 536870912 00000000000000000000000000000100
5793 vec($_, 0, 8) = 64 == 64 00000010000000000000000000000000
5794 vec($_, 1, 8) = 64 == 16384 00000000000000100000000000000000
5795 vec($_, 2, 8) = 64 == 4194304 00000000000000000000001000000000
5796 vec($_, 3, 8) = 64 == 1073741824 00000000000000000000000000000010
5797 vec($_, 0, 8) = 128 == 128 00000001000000000000000000000000
5798 vec($_, 1, 8) = 128 == 32768 00000000000000010000000000000000
5799 vec($_, 2, 8) = 128 == 8388608 00000000000000000000000100000000
5800 vec($_, 3, 8) = 128 == 2147483648 00000000000000000000000000000001
5804 Behaves like the wait(2) system call on your system: it waits for a child
5805 process to terminate and returns the pid of the deceased process, or
5806 C<-1> if there are no child processes. The status is returned in C<$?>.
5807 Note that a return value of C<-1> could mean that child processes are
5808 being automatically reaped, as described in L<perlipc>.
5810 =item waitpid PID,FLAGS
5812 Waits for a particular child process to terminate and returns the pid of
5813 the deceased process, or C<-1> if there is no such child process. On some
5814 systems, a value of 0 indicates that there are processes still running.
5815 The status is returned in C<$?>. If you say
5817 use POSIX ":sys_wait_h";
5820 $kid = waitpid(-1,&WNOHANG);
5823 then you can do a non-blocking wait for all pending zombie processes.
5824 Non-blocking wait is available on machines supporting either the
5825 waitpid(2) or wait4(2) system calls. However, waiting for a particular
5826 pid with FLAGS of C<0> is implemented everywhere. (Perl emulates the
5827 system call by remembering the status values of processes that have
5828 exited but have not been harvested by the Perl script yet.)
5830 Note that on some systems, a return value of C<-1> could mean that child
5831 processes are being automatically reaped. See L<perlipc> for details,
5832 and for other examples.
5836 Returns true if the context of the currently executing subroutine is
5837 looking for a list value. Returns false if the context is looking
5838 for a scalar. Returns the undefined value if the context is looking
5839 for no value (void context).
5841 return unless defined wantarray; # don't bother doing more
5842 my @a = complex_calculation();
5843 return wantarray ? @a : "@a";
5845 This function should have been named wantlist() instead.
5849 Produces a message on STDERR just like C<die>, but doesn't exit or throw
5852 If LIST is empty and C<$@> already contains a value (typically from a
5853 previous eval) that value is used after appending C<"\t...caught">
5854 to C<$@>. This is useful for staying almost, but not entirely similar to
5857 If C<$@> is empty then the string C<"Warning: Something's wrong"> is used.
5859 No message is printed if there is a C<$SIG{__WARN__}> handler
5860 installed. It is the handler's responsibility to deal with the message
5861 as it sees fit (like, for instance, converting it into a C<die>). Most
5862 handlers must therefore make arrangements to actually display the
5863 warnings that they are not prepared to deal with, by calling C<warn>
5864 again in the handler. Note that this is quite safe and will not
5865 produce an endless loop, since C<__WARN__> hooks are not called from
5868 You will find this behavior is slightly different from that of
5869 C<$SIG{__DIE__}> handlers (which don't suppress the error text, but can
5870 instead call C<die> again to change it).
5872 Using a C<__WARN__> handler provides a powerful way to silence all
5873 warnings (even the so-called mandatory ones). An example:
5875 # wipe out *all* compile-time warnings
5876 BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } }
5878 my $foo = 20; # no warning about duplicate my $foo,
5879 # but hey, you asked for it!
5880 # no compile-time or run-time warnings before here
5883 # run-time warnings enabled after here
5884 warn "\$foo is alive and $foo!"; # does show up
5886 See L<perlvar> for details on setting C<%SIG> entries, and for more
5887 examples. See the Carp module for other kinds of warnings using its
5888 carp() and cluck() functions.
5890 =item write FILEHANDLE
5896 Writes a formatted record (possibly multi-line) to the specified FILEHANDLE,
5897 using the format associated with that file. By default the format for
5898 a file is the one having the same name as the filehandle, but the
5899 format for the current output channel (see the C<select> function) may be set
5900 explicitly by assigning the name of the format to the C<$~> variable.
5902 Top of form processing is handled automatically: if there is
5903 insufficient room on the current page for the formatted record, the
5904 page is advanced by writing a form feed, a special top-of-page format
5905 is used to format the new page header, and then the record is written.
5906 By default the top-of-page format is the name of the filehandle with
5907 "_TOP" appended, but it may be dynamically set to the format of your
5908 choice by assigning the name to the C<$^> variable while the filehandle is
5909 selected. The number of lines remaining on the current page is in
5910 variable C<$->, which can be set to C<0> to force a new page.
5912 If FILEHANDLE is unspecified, output goes to the current default output
5913 channel, which starts out as STDOUT but may be changed by the
5914 C<select> operator. If the FILEHANDLE is an EXPR, then the expression
5915 is evaluated and the resulting string is used to look up the name of
5916 the FILEHANDLE at run time. For more on formats, see L<perlform>.
5918 Note that write is I<not> the opposite of C<read>. Unfortunately.
5922 The transliteration operator. Same as C<tr///>. See L<perlop>.