3 perlfunc - Perl builtin functions
7 The functions in this section can serve as terms in an expression.
8 They fall into two major categories: list operators and named unary
9 operators. These differ in their precedence relationship with a
10 following comma. (See the precedence table in L<perlop>.) List
11 operators take more than one argument, while unary operators can never
12 take more than one argument. Thus, a comma terminates the argument of
13 a unary operator, but merely separates the arguments of a list
14 operator. A unary operator generally provides a scalar context to its
15 argument, while a list operator may provide either scalar or list
16 contexts for its arguments. If it does both, the scalar arguments will
17 be first, and the list argument will follow. (Note that there can ever
18 be only one such list argument.) For instance, splice() has three scalar
19 arguments followed by a list, whereas gethostbyname() has four scalar
22 In the syntax descriptions that follow, list operators that expect a
23 list (and provide list context for the elements of the list) are shown
24 with LIST as an argument. Such a list may consist of any combination
25 of scalar arguments or list values; the list values will be included
26 in the list as if each individual element were interpolated at that
27 point in the list, forming a longer single-dimensional list value.
28 Elements of the LIST should be separated by commas.
30 Any function in the list below may be used either with or without
31 parentheses around its arguments. (The syntax descriptions omit the
32 parentheses.) If you use the parentheses, the simple (but occasionally
33 surprising) rule is this: It I<looks> like a function, therefore it I<is> a
34 function, and precedence doesn't matter. Otherwise it's a list
35 operator or unary operator, and precedence does matter. And whitespace
36 between the function and left parenthesis doesn't count--so you need to
39 print 1+2+4; # Prints 7.
40 print(1+2) + 4; # Prints 3.
41 print (1+2)+4; # Also prints 3!
42 print +(1+2)+4; # Prints 7.
43 print ((1+2)+4); # Prints 7.
45 If you run Perl with the B<-w> switch it can warn you about this. For
46 example, the third line above produces:
48 print (...) interpreted as function at - line 1.
49 Useless use of integer addition in void context at - line 1.
51 A few functions take no arguments at all, and therefore work as neither
52 unary nor list operators. These include such functions as C<time>
53 and C<endpwent>. For example, C<time+86_400> always means
56 For functions that can be used in either a scalar or list context,
57 nonabortive failure is generally indicated in a scalar context by
58 returning the undefined value, and in a list context by returning the
61 Remember the following important rule: There is B<no rule> that relates
62 the behavior of an expression in list context to its behavior in scalar
63 context, or vice versa. It might do two totally different things.
64 Each operator and function decides which sort of value it would be most
65 appropriate to return in scalar context. Some operators return the
66 length of the list that would have been returned in list context. Some
67 operators return the first value in the list. Some operators return the
68 last value in the list. Some operators return a count of successful
69 operations. In general, they do what you want, unless you want
72 An named array in scalar context is quite different from what would at
73 first glance appear to be a list in scalar context. You can't get a list
74 like C<(1,2,3)> into being in scalar context, because the compiler knows
75 the context at compile time. It would generate the scalar comma operator
76 there, not the list construction version of the comma. That means it
77 was never a list to start with.
79 In general, functions in Perl that serve as wrappers for system calls
80 of the same name (like chown(2), fork(2), closedir(2), etc.) all return
81 true when they succeed and C<undef> otherwise, as is usually mentioned
82 in the descriptions below. This is different from the C interfaces,
83 which return C<-1> on failure. Exceptions to this rule are C<wait>,
84 C<waitpid>, and C<syscall>. System calls also set the special C<$!>
85 variable on failure. Other functions do not, except accidentally.
87 =head2 Perl Functions by Category
89 Here are Perl's functions (including things that look like
90 functions, like some keywords and named operators)
91 arranged by category. Some functions appear in more
96 =item Functions for SCALARs or strings
98 C<chomp>, C<chop>, C<chr>, C<crypt>, C<hex>, C<index>, C<lc>, C<lcfirst>,
99 C<length>, C<oct>, C<ord>, C<pack>, C<q/STRING/>, C<qq/STRING/>, C<reverse>,
100 C<rindex>, C<sprintf>, C<substr>, C<tr///>, C<uc>, C<ucfirst>, C<y///>
102 =item Regular expressions and pattern matching
104 C<m//>, C<pos>, C<quotemeta>, C<s///>, C<split>, C<study>, C<qr//>
106 =item Numeric functions
108 C<abs>, C<atan2>, C<cos>, C<exp>, C<hex>, C<int>, C<log>, C<oct>, C<rand>,
109 C<sin>, C<sqrt>, C<srand>
111 =item Functions for real @ARRAYs
113 C<pop>, C<push>, C<shift>, C<splice>, C<unshift>
115 =item Functions for list data
117 C<grep>, C<join>, C<map>, C<qw/STRING/>, C<reverse>, C<sort>, C<unpack>
119 =item Functions for real %HASHes
121 C<delete>, C<each>, C<exists>, C<keys>, C<values>
123 =item Input and output functions
125 C<binmode>, C<close>, C<closedir>, C<dbmclose>, C<dbmopen>, C<die>, C<eof>,
126 C<fileno>, C<flock>, C<format>, C<getc>, C<print>, C<printf>, C<read>,
127 C<readdir>, C<rewinddir>, C<seek>, C<seekdir>, C<select>, C<syscall>,
128 C<sysread>, C<sysseek>, C<syswrite>, C<tell>, C<telldir>, C<truncate>,
131 =item Functions for fixed length data or records
133 C<pack>, C<read>, C<syscall>, C<sysread>, C<syswrite>, C<unpack>, C<vec>
135 =item Functions for filehandles, files, or directories
137 C<-I<X>>, C<chdir>, C<chmod>, C<chown>, C<chroot>, C<fcntl>, C<glob>,
138 C<ioctl>, C<link>, C<lstat>, C<mkdir>, C<open>, C<opendir>,
139 C<readlink>, C<rename>, C<rmdir>, C<stat>, C<symlink>, C<umask>,
142 =item Keywords related to the control flow of your perl program
144 C<caller>, C<continue>, C<die>, C<do>, C<dump>, C<eval>, C<exit>,
145 C<goto>, C<last>, C<next>, C<redo>, C<return>, C<sub>, C<wantarray>
147 =item Keywords related to scoping
149 C<caller>, C<import>, C<local>, C<my>, C<our>, C<package>, C<use>
151 =item Miscellaneous functions
153 C<defined>, C<dump>, C<eval>, C<formline>, C<local>, C<my>, C<our>, C<reset>,
154 C<scalar>, C<undef>, C<wantarray>
156 =item Functions for processes and process groups
158 C<alarm>, C<exec>, C<fork>, C<getpgrp>, C<getppid>, C<getpriority>, C<kill>,
159 C<pipe>, C<qx/STRING/>, C<setpgrp>, C<setpriority>, C<sleep>, C<system>,
160 C<times>, C<wait>, C<waitpid>
162 =item Keywords related to perl modules
164 C<do>, C<import>, C<no>, C<package>, C<require>, C<use>
166 =item Keywords related to classes and object-orientedness
168 C<bless>, C<dbmclose>, C<dbmopen>, C<package>, C<ref>, C<tie>, C<tied>,
171 =item Low-level socket functions
173 C<accept>, C<bind>, C<connect>, C<getpeername>, C<getsockname>,
174 C<getsockopt>, C<listen>, C<recv>, C<send>, C<setsockopt>, C<shutdown>,
175 C<sockatmark>, C<socket>, C<socketpair>
177 =item System V interprocess communication functions
179 C<msgctl>, C<msgget>, C<msgrcv>, C<msgsnd>, C<semctl>, C<semget>, C<semop>,
180 C<shmctl>, C<shmget>, C<shmread>, C<shmwrite>
182 =item Fetching user and group info
184 C<endgrent>, C<endhostent>, C<endnetent>, C<endpwent>, C<getgrent>,
185 C<getgrgid>, C<getgrnam>, C<getlogin>, C<getpwent>, C<getpwnam>,
186 C<getpwuid>, C<setgrent>, C<setpwent>
188 =item Fetching network info
190 C<endprotoent>, C<endservent>, C<gethostbyaddr>, C<gethostbyname>,
191 C<gethostent>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
192 C<getprotobyname>, C<getprotobynumber>, C<getprotoent>,
193 C<getservbyname>, C<getservbyport>, C<getservent>, C<sethostent>,
194 C<setnetent>, C<setprotoent>, C<setservent>
196 =item Time-related functions
198 C<gmtime>, C<localtime>, C<time>, C<times>
200 =item Functions new in perl5
202 C<abs>, C<bless>, C<chomp>, C<chr>, C<exists>, C<formline>, C<glob>,
203 C<import>, C<lc>, C<lcfirst>, C<map>, C<my>, C<no>, C<our>, C<prototype>,
204 C<qx>, C<qw>, C<readline>, C<readpipe>, C<ref>, C<sub*>, C<sysopen>, C<tie>,
205 C<tied>, C<uc>, C<ucfirst>, C<untie>, C<use>
207 * - C<sub> was a keyword in perl4, but in perl5 it is an
208 operator, which can be used in expressions.
210 =item Functions obsoleted in perl5
212 C<dbmclose>, C<dbmopen>
218 Perl was born in Unix and can therefore access all common Unix
219 system calls. In non-Unix environments, the functionality of some
220 Unix system calls may not be available, or details of the available
221 functionality may differ slightly. The Perl functions affected
224 C<-X>, C<binmode>, C<chmod>, C<chown>, C<chroot>, C<crypt>,
225 C<dbmclose>, C<dbmopen>, C<dump>, C<endgrent>, C<endhostent>,
226 C<endnetent>, C<endprotoent>, C<endpwent>, C<endservent>, C<exec>,
227 C<fcntl>, C<flock>, C<fork>, C<getgrent>, C<getgrgid>, C<gethostent>,
228 C<getlogin>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
229 C<getppid>, C<getprgp>, C<getpriority>, C<getprotobynumber>,
230 C<getprotoent>, C<getpwent>, C<getpwnam>, C<getpwuid>,
231 C<getservbyport>, C<getservent>, C<getsockopt>, C<glob>, C<ioctl>,
232 C<kill>, C<link>, C<lstat>, C<msgctl>, C<msgget>, C<msgrcv>,
233 C<msgsnd>, C<open>, C<pipe>, C<readlink>, C<rename>, C<select>, C<semctl>,
234 C<semget>, C<semop>, C<setgrent>, C<sethostent>, C<setnetent>,
235 C<setpgrp>, C<setpriority>, C<setprotoent>, C<setpwent>,
236 C<setservent>, C<setsockopt>, C<shmctl>, C<shmget>, C<shmread>,
237 C<shmwrite>, C<sockatmark>, C<socket>, C<socketpair>,
238 C<stat>, C<symlink>, C<syscall>, C<sysopen>, C<system>,
239 C<times>, C<truncate>, C<umask>, C<unlink>,
240 C<utime>, C<wait>, C<waitpid>
242 For more information about the portability of these functions, see
243 L<perlport> and other available platform-specific documentation.
245 =head2 Alphabetical Listing of Perl Functions
249 =item I<-X> FILEHANDLE
255 A file test, where X is one of the letters listed below. This unary
256 operator takes one argument, either a filename or a filehandle, and
257 tests the associated file to see if something is true about it. If the
258 argument is omitted, tests C<$_>, except for C<-t>, which tests STDIN.
259 Unless otherwise documented, it returns C<1> for true and C<''> for false, or
260 the undefined value if the file doesn't exist. Despite the funny
261 names, precedence is the same as any other named unary operator, and
262 the argument may be parenthesized like any other unary operator. The
263 operator may be any of:
264 X<-r>X<-w>X<-x>X<-o>X<-R>X<-W>X<-X>X<-O>X<-e>X<-z>X<-s>X<-f>X<-d>X<-l>X<-p>
265 X<-S>X<-b>X<-c>X<-t>X<-u>X<-g>X<-k>X<-T>X<-B>X<-M>X<-A>X<-C>
267 -r File is readable by effective uid/gid.
268 -w File is writable by effective uid/gid.
269 -x File is executable by effective uid/gid.
270 -o File is owned by effective uid.
272 -R File is readable by real uid/gid.
273 -W File is writable by real uid/gid.
274 -X File is executable by real uid/gid.
275 -O File is owned by real uid.
278 -z File has zero size (is empty).
279 -s File has nonzero size (returns size in bytes).
281 -f File is a plain file.
282 -d File is a directory.
283 -l File is a symbolic link.
284 -p File is a named pipe (FIFO), or Filehandle is a pipe.
286 -b File is a block special file.
287 -c File is a character special file.
288 -t Filehandle is opened to a tty.
290 -u File has setuid bit set.
291 -g File has setgid bit set.
292 -k File has sticky bit set.
294 -T File is an ASCII text file.
295 -B File is a "binary" file (opposite of -T).
297 -M Age of file in days when script started.
298 -A Same for access time.
299 -C Same for inode change time.
305 next unless -f $_; # ignore specials
309 The interpretation of the file permission operators C<-r>, C<-R>,
310 C<-w>, C<-W>, C<-x>, and C<-X> is by default based solely on the mode
311 of the file and the uids and gids of the user. There may be other
312 reasons you can't actually read, write, or execute the file. Such
313 reasons may be for example network filesystem access controls, ACLs
314 (access control lists), read-only filesystems, and unrecognized
317 Also note that, for the superuser on the local filesystems, the C<-r>,
318 C<-R>, C<-w>, and C<-W> tests always return 1, and C<-x> and C<-X> return 1
319 if any execute bit is set in the mode. Scripts run by the superuser
320 may thus need to do a stat() to determine the actual mode of the file,
321 or temporarily set their effective uid to something else.
323 If you are using ACLs, there is a pragma called C<filetest> that may
324 produce more accurate results than the bare stat() mode bits.
325 When under the C<use filetest 'access'> the above-mentioned filetests
326 will test whether the permission can (not) be granted using the
327 access() family of system calls. Also note that the C<-x> and C<-X> may
328 under this pragma return true even if there are no execute permission
329 bits set (nor any extra execute permission ACLs). This strangeness is
330 due to the underlying system calls' definitions. Read the
331 documentation for the C<filetest> pragma for more information.
333 Note that C<-s/a/b/> does not do a negated substitution. Saying
334 C<-exp($foo)> still works as expected, however--only single letters
335 following a minus are interpreted as file tests.
337 The C<-T> and C<-B> switches work as follows. The first block or so of the
338 file is examined for odd characters such as strange control codes or
339 characters with the high bit set. If too many strange characters (>30%)
340 are found, it's a C<-B> file, otherwise it's a C<-T> file. Also, any file
341 containing null in the first block is considered a binary file. If C<-T>
342 or C<-B> is used on a filehandle, the current stdio buffer is examined
343 rather than the first block. Both C<-T> and C<-B> return true on a null
344 file, or a file at EOF when testing a filehandle. Because you have to
345 read a file to do the C<-T> test, on most occasions you want to use a C<-f>
346 against the file first, as in C<next unless -f $file && -T $file>.
348 If any of the file tests (or either the C<stat> or C<lstat> operators) are given
349 the special filehandle consisting of a solitary underline, then the stat
350 structure of the previous file test (or stat operator) is used, saving
351 a system call. (This doesn't work with C<-t>, and you need to remember
352 that lstat() and C<-l> will leave values in the stat structure for the
353 symbolic link, not the real file.) Example:
355 print "Can do.\n" if -r $a || -w _ || -x _;
358 print "Readable\n" if -r _;
359 print "Writable\n" if -w _;
360 print "Executable\n" if -x _;
361 print "Setuid\n" if -u _;
362 print "Setgid\n" if -g _;
363 print "Sticky\n" if -k _;
364 print "Text\n" if -T _;
365 print "Binary\n" if -B _;
371 Returns the absolute value of its argument.
372 If VALUE is omitted, uses C<$_>.
374 =item accept NEWSOCKET,GENERICSOCKET
376 Accepts an incoming socket connect, just as the accept(2) system call
377 does. Returns the packed address if it succeeded, false otherwise.
378 See the example in L<perlipc/"Sockets: Client/Server Communication">.
380 On systems that support a close-on-exec flag on files, the flag will
381 be set for the newly opened file descriptor, as determined by the
382 value of $^F. See L<perlvar/$^F>.
388 Arranges to have a SIGALRM delivered to this process after the
389 specified number of seconds have elapsed. If SECONDS is not specified,
390 the value stored in C<$_> is used. (On some machines,
391 unfortunately, the elapsed time may be up to one second less than you
392 specified because of how seconds are counted.) Only one timer may be
393 counting at once. Each call disables the previous timer, and an
394 argument of C<0> may be supplied to cancel the previous timer without
395 starting a new one. The returned value is the amount of time remaining
396 on the previous timer.
398 For delays of finer granularity than one second, you may use Perl's
399 four-argument version of select() leaving the first three arguments
400 undefined, or you might be able to use the C<syscall> interface to
401 access setitimer(2) if your system supports it. The Time::HiRes module
402 from CPAN may also prove useful.
404 It is usually a mistake to intermix C<alarm> and C<sleep> calls.
405 (C<sleep> may be internally implemented in your system with C<alarm>)
407 If you want to use C<alarm> to time out a system call you need to use an
408 C<eval>/C<die> pair. You can't rely on the alarm causing the system call to
409 fail with C<$!> set to C<EINTR> because Perl sets up signal handlers to
410 restart system calls on some systems. Using C<eval>/C<die> always works,
411 modulo the caveats given in L<perlipc/"Signals">.
414 local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
416 $nread = sysread SOCKET, $buffer, $size;
420 die unless $@ eq "alarm\n"; # propagate unexpected errors
429 Returns the arctangent of Y/X in the range -PI to PI.
431 For the tangent operation, you may use the C<Math::Trig::tan>
432 function, or use the familiar relation:
434 sub tan { sin($_[0]) / cos($_[0]) }
436 =item bind SOCKET,NAME
438 Binds a network address to a socket, just as the bind system call
439 does. Returns true if it succeeded, false otherwise. NAME should be a
440 packed address of the appropriate type for the socket. See the examples in
441 L<perlipc/"Sockets: Client/Server Communication">.
443 =item binmode FILEHANDLE, DISCIPLINE
445 =item binmode FILEHANDLE
447 Arranges for FILEHANDLE to be read or written in "binary" or "text" mode
448 on systems where the run-time libraries distinguish between binary and
449 text files. If FILEHANDLE is an expression, the value is taken as the
450 name of the filehandle. DISCIPLINE can be either of C<":raw"> for
451 binary mode or C<":crlf"> for "text" mode. If the DISCIPLINE is
452 omitted, it defaults to C<":raw">.
454 binmode() should be called after open() but before any I/O is done on
457 On many systems binmode() currently has no effect, but in future, it
458 will be extended to support user-defined input and output disciplines.
459 On some systems binmode() is necessary when you're not working with a
460 text file. For the sake of portability it is a good idea to always use
461 it when appropriate, and to never use it when it isn't appropriate.
463 In other words: Regardless of platform, use binmode() on binary
464 files, and do not use binmode() on text files.
466 The C<open> pragma can be used to establish default disciplines.
469 The operating system, device drivers, C libraries, and Perl run-time
470 system all work together to let the programmer treat a single
471 character (C<\n>) as the line terminator, irrespective of the external
472 representation. On many operating systems, the native text file
473 representation matches the internal representation, but on some
474 platforms the external representation of C<\n> is made up of more than
477 Mac OS and all variants of Unix use a single character to end each line
478 in the external representation of text (even though that single
479 character is not necessarily the same across these platforms).
480 Consequently binmode() has no effect on these operating systems. In
481 other systems like VMS, MS-DOS and the various flavors of MS-Windows
482 your program sees a C<\n> as a simple C<\cJ>, but what's stored in text
483 files are the two characters C<\cM\cJ>. That means that, if you don't
484 use binmode() on these systems, C<\cM\cJ> sequences on disk will be
485 converted to C<\n> on input, and any C<\n> in your program will be
486 converted back to C<\cM\cJ> on output. This is what you want for text
487 files, but it can be disastrous for binary files.
489 Another consequence of using binmode() (on some systems) is that
490 special end-of-file markers will be seen as part of the data stream.
491 For systems from the Microsoft family this means that if your binary
492 data contains C<\cZ>, the I/O subsystem will regard it as the end of
493 the file, unless you use binmode().
495 binmode() is not only important for readline() and print() operations,
496 but also when using read(), seek(), sysread(), syswrite() and tell()
497 (see L<perlport> for more details). See the C<$/> and C<$\> variables
498 in L<perlvar> for how to manually set your input and output
499 line-termination sequences.
501 =item bless REF,CLASSNAME
505 This function tells the thingy referenced by REF that it is now an object
506 in the CLASSNAME package. If CLASSNAME is omitted, the current package
507 is used. Because a C<bless> is often the last thing in a constructor,
508 it returns the reference for convenience. Always use the two-argument
509 version if the function doing the blessing might be inherited by a
510 derived class. See L<perltoot> and L<perlobj> for more about the blessing
511 (and blessings) of objects.
513 Consider always blessing objects in CLASSNAMEs that are mixed case.
514 Namespaces with all lowercase names are considered reserved for
515 Perl pragmata. Builtin types have all uppercase names, so to prevent
516 confusion, you may wish to avoid such package names as well. Make sure
517 that CLASSNAME is a true value.
519 See L<perlmod/"Perl Modules">.
525 Returns the context of the current subroutine call. In scalar context,
526 returns the caller's package name if there is a caller, that is, if
527 we're in a subroutine or C<eval> or C<require>, and the undefined value
528 otherwise. In list context, returns
530 ($package, $filename, $line) = caller;
532 With EXPR, it returns some extra information that the debugger uses to
533 print a stack trace. The value of EXPR indicates how many call frames
534 to go back before the current one.
536 ($package, $filename, $line, $subroutine, $hasargs,
537 $wantarray, $evaltext, $is_require, $hints, $bitmask) = caller($i);
539 Here $subroutine may be C<(eval)> if the frame is not a subroutine
540 call, but an C<eval>. In such a case additional elements $evaltext and
541 C<$is_require> are set: C<$is_require> is true if the frame is created by a
542 C<require> or C<use> statement, $evaltext contains the text of the
543 C<eval EXPR> statement. In particular, for an C<eval BLOCK> statement,
544 $filename is C<(eval)>, but $evaltext is undefined. (Note also that
545 each C<use> statement creates a C<require> frame inside an C<eval EXPR>)
546 frame. C<$hasargs> is true if a new instance of C<@_> was set up for the
547 frame. C<$hints> and C<$bitmask> contain pragmatic hints that the caller
548 was compiled with. The C<$hints> and C<$bitmask> values are subject to
549 change between versions of Perl, and are not meant for external use.
551 Furthermore, when called from within the DB package, caller returns more
552 detailed information: it sets the list variable C<@DB::args> to be the
553 arguments with which the subroutine was invoked.
555 Be aware that the optimizer might have optimized call frames away before
556 C<caller> had a chance to get the information. That means that C<caller(N)>
557 might not return information about the call frame you expect it do, for
558 C<< N > 1 >>. In particular, C<@DB::args> might have information from the
559 previous time C<caller> was called.
563 Changes the working directory to EXPR, if possible. If EXPR is omitted,
564 changes to the directory specified by C<$ENV{HOME}>, if set; if not,
565 changes to the directory specified by C<$ENV{LOGDIR}>. If neither is
566 set, C<chdir> does nothing. It returns true upon success, false
567 otherwise. See the example under C<die>.
571 Changes the permissions of a list of files. The first element of the
572 list must be the numerical mode, which should probably be an octal
573 number, and which definitely should I<not> a string of octal digits:
574 C<0644> is okay, C<'0644'> is not. Returns the number of files
575 successfully changed. See also L</oct>, if all you have is a string.
577 $cnt = chmod 0755, 'foo', 'bar';
578 chmod 0755, @executables;
579 $mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to
581 $mode = '0644'; chmod oct($mode), 'foo'; # this is better
582 $mode = 0644; chmod $mode, 'foo'; # this is best
584 You can also import the symbolic C<S_I*> constants from the Fcntl
589 chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
590 # This is identical to the chmod 0755 of the above example.
598 This safer version of L</chop> removes any trailing string
599 that corresponds to the current value of C<$/> (also known as
600 $INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
601 number of characters removed from all its arguments. It's often used to
602 remove the newline from the end of an input record when you're worried
603 that the final record may be missing its newline. When in paragraph
604 mode (C<$/ = "">), it removes all trailing newlines from the string.
605 When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is
606 a reference to an integer or the like, see L<perlvar>) chomp() won't
608 If VARIABLE is omitted, it chomps C<$_>. Example:
611 chomp; # avoid \n on last field
616 If VARIABLE is a hash, it chomps the hash's values, but not its keys.
618 You can actually chomp anything that's an lvalue, including an assignment:
621 chomp($answer = <STDIN>);
623 If you chomp a list, each element is chomped, and the total number of
624 characters removed is returned.
632 Chops off the last character of a string and returns the character
633 chopped. It is much more efficient than C<s/.$//s> because it neither
634 scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
635 If VARIABLE is a hash, it chops the hash's values, but not its keys.
637 You can actually chop anything that's an lvalue, including an assignment.
639 If you chop a list, each element is chopped. Only the value of the
640 last C<chop> is returned.
642 Note that C<chop> returns the last character. To return all but the last
643 character, use C<substr($string, 0, -1)>.
647 Changes the owner (and group) of a list of files. The first two
648 elements of the list must be the I<numeric> uid and gid, in that
649 order. A value of -1 in either position is interpreted by most
650 systems to leave that value unchanged. Returns the number of files
651 successfully changed.
653 $cnt = chown $uid, $gid, 'foo', 'bar';
654 chown $uid, $gid, @filenames;
656 Here's an example that looks up nonnumeric uids in the passwd file:
659 chomp($user = <STDIN>);
661 chomp($pattern = <STDIN>);
663 ($login,$pass,$uid,$gid) = getpwnam($user)
664 or die "$user not in passwd file";
666 @ary = glob($pattern); # expand filenames
667 chown $uid, $gid, @ary;
669 On most systems, you are not allowed to change the ownership of the
670 file unless you're the superuser, although you should be able to change
671 the group to any of your secondary groups. On insecure systems, these
672 restrictions may be relaxed, but this is not a portable assumption.
673 On POSIX systems, you can detect this condition this way:
675 use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
676 $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
682 Returns the character represented by that NUMBER in the character set.
683 For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
684 chr(0x263a) is a Unicode smiley face. Within the scope of C<use utf8>,
685 characters higher than 127 are encoded in Unicode; if you don't want
686 this, temporarily C<use bytes> or use C<pack("C*",...)>
688 For the reverse, use L</ord>.
689 See L<utf8> for more about Unicode.
691 If NUMBER is omitted, uses C<$_>.
693 =item chroot FILENAME
697 This function works like the system call by the same name: it makes the
698 named directory the new root directory for all further pathnames that
699 begin with a C</> by your process and all its children. (It doesn't
700 change your current working directory, which is unaffected.) For security
701 reasons, this call is restricted to the superuser. If FILENAME is
702 omitted, does a C<chroot> to C<$_>.
704 =item close FILEHANDLE
708 Closes the file or pipe associated with the file handle, returning true
709 only if stdio successfully flushes buffers and closes the system file
710 descriptor. Closes the currently selected filehandle if the argument
713 You don't have to close FILEHANDLE if you are immediately going to do
714 another C<open> on it, because C<open> will close it for you. (See
715 C<open>.) However, an explicit C<close> on an input file resets the line
716 counter (C<$.>), while the implicit close done by C<open> does not.
718 If the file handle came from a piped open C<close> will additionally
719 return false if one of the other system calls involved fails or if the
720 program exits with non-zero status. (If the only problem was that the
721 program exited non-zero C<$!> will be set to C<0>.) Closing a pipe
722 also waits for the process executing on the pipe to complete, in case you
723 want to look at the output of the pipe afterwards, and
724 implicitly puts the exit status value of that command into C<$?>.
726 Prematurely closing the read end of a pipe (i.e. before the process
727 writing to it at the other end has closed it) will result in a
728 SIGPIPE being delivered to the writer. If the other end can't
729 handle that, be sure to read all the data before closing the pipe.
733 open(OUTPUT, '|sort >foo') # pipe to sort
734 or die "Can't start sort: $!";
735 #... # print stuff to output
736 close OUTPUT # wait for sort to finish
737 or warn $! ? "Error closing sort pipe: $!"
738 : "Exit status $? from sort";
739 open(INPUT, 'foo') # get sort's results
740 or die "Can't open 'foo' for input: $!";
742 FILEHANDLE may be an expression whose value can be used as an indirect
743 filehandle, usually the real filehandle name.
745 =item closedir DIRHANDLE
747 Closes a directory opened by C<opendir> and returns the success of that
750 DIRHANDLE may be an expression whose value can be used as an indirect
751 dirhandle, usually the real dirhandle name.
753 =item connect SOCKET,NAME
755 Attempts to connect to a remote socket, just as the connect system call
756 does. Returns true if it succeeded, false otherwise. NAME should be a
757 packed address of the appropriate type for the socket. See the examples in
758 L<perlipc/"Sockets: Client/Server Communication">.
762 Actually a flow control statement rather than a function. If there is a
763 C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
764 C<foreach>), it is always executed just before the conditional is about to
765 be evaluated again, just like the third part of a C<for> loop in C. Thus
766 it can be used to increment a loop variable, even when the loop has been
767 continued via the C<next> statement (which is similar to the C C<continue>
770 C<last>, C<next>, or C<redo> may appear within a C<continue>
771 block. C<last> and C<redo> will behave as if they had been executed within
772 the main block. So will C<next>, but since it will execute a C<continue>
773 block, it may be more entertaining.
776 ### redo always comes here
779 ### next always comes here
781 # then back the top to re-check EXPR
783 ### last always comes here
785 Omitting the C<continue> section is semantically equivalent to using an
786 empty one, logically enough. In that case, C<next> goes directly back
787 to check the condition at the top of the loop.
793 Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
794 takes cosine of C<$_>.
796 For the inverse cosine operation, you may use the C<Math::Trig::acos()>
797 function, or use this relation:
799 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
801 =item crypt PLAINTEXT,SALT
803 Encrypts a string exactly like the crypt(3) function in the C library
804 (assuming that you actually have a version there that has not been
805 extirpated as a potential munition). This can prove useful for checking
806 the password file for lousy passwords, amongst other things. Only the
807 guys wearing white hats should do this.
809 Note that C<crypt> is intended to be a one-way function, much like breaking
810 eggs to make an omelette. There is no (known) corresponding decrypt
811 function. As a result, this function isn't all that useful for
812 cryptography. (For that, see your nearby CPAN mirror.)
814 When verifying an existing encrypted string you should use the encrypted
815 text as the salt (like C<crypt($plain, $crypted) eq $crypted>). This
816 allows your code to work with the standard C<crypt> and with more
817 exotic implementations. When choosing a new salt create a random two
818 character string whose characters come from the set C<[./0-9A-Za-z]>
819 (like C<join '', ('.', '/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>).
821 Here's an example that makes sure that whoever runs this program knows
824 $pwd = (getpwuid($<))[1];
828 chomp($word = <STDIN>);
832 if (crypt($word, $pwd) ne $pwd) {
838 Of course, typing in your own password to whoever asks you
841 The L<crypt> function is unsuitable for encrypting large quantities
842 of data, not least of all because you can't get the information
843 back. Look at the F<by-module/Crypt> and F<by-module/PGP> directories
844 on your favorite CPAN mirror for a slew of potentially useful
849 [This function has been largely superseded by the C<untie> function.]
851 Breaks the binding between a DBM file and a hash.
853 =item dbmopen HASH,DBNAME,MASK
855 [This function has been largely superseded by the C<tie> function.]
857 This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
858 hash. HASH is the name of the hash. (Unlike normal C<open>, the first
859 argument is I<not> a filehandle, even though it looks like one). DBNAME
860 is the name of the database (without the F<.dir> or F<.pag> extension if
861 any). If the database does not exist, it is created with protection
862 specified by MASK (as modified by the C<umask>). If your system supports
863 only the older DBM functions, you may perform only one C<dbmopen> in your
864 program. In older versions of Perl, if your system had neither DBM nor
865 ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
868 If you don't have write access to the DBM file, you can only read hash
869 variables, not set them. If you want to test whether you can write,
870 either use file tests or try setting a dummy hash entry inside an C<eval>,
871 which will trap the error.
873 Note that functions such as C<keys> and C<values> may return huge lists
874 when used on large DBM files. You may prefer to use the C<each>
875 function to iterate over large DBM files. Example:
877 # print out history file offsets
878 dbmopen(%HIST,'/usr/lib/news/history',0666);
879 while (($key,$val) = each %HIST) {
880 print $key, ' = ', unpack('L',$val), "\n";
884 See also L<AnyDBM_File> for a more general description of the pros and
885 cons of the various dbm approaches, as well as L<DB_File> for a particularly
888 You can control which DBM library you use by loading that library
889 before you call dbmopen():
892 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
893 or die "Can't open netscape history file: $!";
899 Returns a Boolean value telling whether EXPR has a value other than
900 the undefined value C<undef>. If EXPR is not present, C<$_> will be
903 Many operations return C<undef> to indicate failure, end of file,
904 system error, uninitialized variable, and other exceptional
905 conditions. This function allows you to distinguish C<undef> from
906 other values. (A simple Boolean test will not distinguish among
907 C<undef>, zero, the empty string, and C<"0">, which are all equally
908 false.) Note that since C<undef> is a valid scalar, its presence
909 doesn't I<necessarily> indicate an exceptional condition: C<pop>
910 returns C<undef> when its argument is an empty array, I<or> when the
911 element to return happens to be C<undef>.
913 You may also use C<defined(&func)> to check whether subroutine C<&func>
914 has ever been defined. The return value is unaffected by any forward
915 declarations of C<&foo>. Note that a subroutine which is not defined
916 may still be callable: its package may have an C<AUTOLOAD> method that
917 makes it spring into existence the first time that it is called -- see
920 Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
921 used to report whether memory for that aggregate has ever been
922 allocated. This behavior may disappear in future versions of Perl.
923 You should instead use a simple test for size:
925 if (@an_array) { print "has array elements\n" }
926 if (%a_hash) { print "has hash members\n" }
928 When used on a hash element, it tells you whether the value is defined,
929 not whether the key exists in the hash. Use L</exists> for the latter
934 print if defined $switch{'D'};
935 print "$val\n" while defined($val = pop(@ary));
936 die "Can't readlink $sym: $!"
937 unless defined($value = readlink $sym);
938 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
939 $debugging = 0 unless defined $debugging;
941 Note: Many folks tend to overuse C<defined>, and then are surprised to
942 discover that the number C<0> and C<""> (the zero-length string) are, in fact,
943 defined values. For example, if you say
947 The pattern match succeeds, and C<$1> is defined, despite the fact that it
948 matched "nothing". But it didn't really match nothing--rather, it
949 matched something that happened to be zero characters long. This is all
950 very above-board and honest. When a function returns an undefined value,
951 it's an admission that it couldn't give you an honest answer. So you
952 should use C<defined> only when you're questioning the integrity of what
953 you're trying to do. At other times, a simple comparison to C<0> or C<""> is
956 See also L</undef>, L</exists>, L</ref>.
960 Given an expression that specifies a hash element, array element, hash slice,
961 or array slice, deletes the specified element(s) from the hash or array.
962 In the case of an array, if the array elements happen to be at the end,
963 the size of the array will shrink to the highest element that tests
964 true for exists() (or 0 if no such element exists).
966 Returns each element so deleted or the undefined value if there was no such
967 element. Deleting from C<$ENV{}> modifies the environment. Deleting from
968 a hash tied to a DBM file deletes the entry from the DBM file. Deleting
969 from a C<tie>d hash or array may not necessarily return anything.
971 Deleting an array element effectively returns that position of the array
972 to its initial, uninitialized state. Subsequently testing for the same
973 element with exists() will return false. Note that deleting array
974 elements in the middle of an array will not shift the index of the ones
975 after them down--use splice() for that. See L</exists>.
977 The following (inefficiently) deletes all the values of %HASH and @ARRAY:
979 foreach $key (keys %HASH) {
983 foreach $index (0 .. $#ARRAY) {
984 delete $ARRAY[$index];
989 delete @HASH{keys %HASH};
991 delete @ARRAY[0 .. $#ARRAY];
993 But both of these are slower than just assigning the empty list
994 or undefining %HASH or @ARRAY:
996 %HASH = (); # completely empty %HASH
997 undef %HASH; # forget %HASH ever existed
999 @ARRAY = (); # completely empty @ARRAY
1000 undef @ARRAY; # forget @ARRAY ever existed
1002 Note that the EXPR can be arbitrarily complicated as long as the final
1003 operation is a hash element, array element, hash slice, or array slice
1006 delete $ref->[$x][$y]{$key};
1007 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
1009 delete $ref->[$x][$y][$index];
1010 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
1014 Outside an C<eval>, prints the value of LIST to C<STDERR> and
1015 exits with the current value of C<$!> (errno). If C<$!> is C<0>,
1016 exits with the value of C<<< ($? >> 8) >>> (backtick `command`
1017 status). If C<<< ($? >> 8) >>> is C<0>, exits with C<255>. Inside
1018 an C<eval(),> the error message is stuffed into C<$@> and the
1019 C<eval> is terminated with the undefined value. This makes
1020 C<die> the way to raise an exception.
1022 Equivalent examples:
1024 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
1025 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
1027 If the value of EXPR does not end in a newline, the current script line
1028 number and input line number (if any) are also printed, and a newline
1029 is supplied. Note that the "input line number" (also known as "chunk")
1030 is subject to whatever notion of "line" happens to be currently in
1031 effect, and is also available as the special variable C<$.>.
1032 See L<perlvar/"$/"> and L<perlvar/"$.">.
1034 Hint: sometimes appending C<", stopped"> to your message
1035 will cause it to make better sense when the string C<"at foo line 123"> is
1036 appended. Suppose you are running script "canasta".
1038 die "/etc/games is no good";
1039 die "/etc/games is no good, stopped";
1041 produce, respectively
1043 /etc/games is no good at canasta line 123.
1044 /etc/games is no good, stopped at canasta line 123.
1046 See also exit(), warn(), and the Carp module.
1048 If LIST is empty and C<$@> already contains a value (typically from a
1049 previous eval) that value is reused after appending C<"\t...propagated">.
1050 This is useful for propagating exceptions:
1053 die unless $@ =~ /Expected exception/;
1055 If C<$@> is empty then the string C<"Died"> is used.
1057 die() can also be called with a reference argument. If this happens to be
1058 trapped within an eval(), $@ contains the reference. This behavior permits
1059 a more elaborate exception handling implementation using objects that
1060 maintain arbitrary state about the nature of the exception. Such a scheme
1061 is sometimes preferable to matching particular string values of $@ using
1062 regular expressions. Here's an example:
1064 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
1066 if (ref($@) && UNIVERSAL::isa($@,"Some::Module::Exception")) {
1067 # handle Some::Module::Exception
1070 # handle all other possible exceptions
1074 Because perl will stringify uncaught exception messages before displaying
1075 them, you may want to overload stringification operations on such custom
1076 exception objects. See L<overload> for details about that.
1078 You can arrange for a callback to be run just before the C<die>
1079 does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
1080 handler will be called with the error text and can change the error
1081 message, if it sees fit, by calling C<die> again. See
1082 L<perlvar/$SIG{expr}> for details on setting C<%SIG> entries, and
1083 L<"eval BLOCK"> for some examples. Although this feature was meant
1084 to be run only right before your program was to exit, this is not
1085 currently the case--the C<$SIG{__DIE__}> hook is currently called
1086 even inside eval()ed blocks/strings! If one wants the hook to do
1087 nothing in such situations, put
1091 as the first line of the handler (see L<perlvar/$^S>). Because
1092 this promotes strange action at a distance, this counterintuitive
1093 behavior may be fixed in a future release.
1097 Not really a function. Returns the value of the last command in the
1098 sequence of commands indicated by BLOCK. When modified by a loop
1099 modifier, executes the BLOCK once before testing the loop condition.
1100 (On other statements the loop modifiers test the conditional first.)
1102 C<do BLOCK> does I<not> count as a loop, so the loop control statements
1103 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1104 See L<perlsyn> for alternative strategies.
1106 =item do SUBROUTINE(LIST)
1108 A deprecated form of subroutine call. See L<perlsub>.
1112 Uses the value of EXPR as a filename and executes the contents of the
1113 file as a Perl script. Its primary use is to include subroutines
1114 from a Perl subroutine library.
1122 except that it's more efficient and concise, keeps track of the current
1123 filename for error messages, searches the @INC libraries, and updates
1124 C<%INC> if the file is found. See L<perlvar/Predefined Names> for these
1125 variables. It also differs in that code evaluated with C<do FILENAME>
1126 cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1127 same, however, in that it does reparse the file every time you call it,
1128 so you probably don't want to do this inside a loop.
1130 If C<do> cannot read the file, it returns undef and sets C<$!> to the
1131 error. If C<do> can read the file but cannot compile it, it
1132 returns undef and sets an error message in C<$@>. If the file is
1133 successfully compiled, C<do> returns the value of the last expression
1136 Note that inclusion of library modules is better done with the
1137 C<use> and C<require> operators, which also do automatic error checking
1138 and raise an exception if there's a problem.
1140 You might like to use C<do> to read in a program configuration
1141 file. Manual error checking can be done this way:
1143 # read in config files: system first, then user
1144 for $file ("/share/prog/defaults.rc",
1145 "$ENV{HOME}/.someprogrc")
1147 unless ($return = do $file) {
1148 warn "couldn't parse $file: $@" if $@;
1149 warn "couldn't do $file: $!" unless defined $return;
1150 warn "couldn't run $file" unless $return;
1158 This function causes an immediate core dump. See also the B<-u>
1159 command-line switch in L<perlrun>, which does the same thing.
1160 Primarily this is so that you can use the B<undump> program (not
1161 supplied) to turn your core dump into an executable binary after
1162 having initialized all your variables at the beginning of the
1163 program. When the new binary is executed it will begin by executing
1164 a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1165 Think of it as a goto with an intervening core dump and reincarnation.
1166 If C<LABEL> is omitted, restarts the program from the top.
1168 B<WARNING>: Any files opened at the time of the dump will I<not>
1169 be open any more when the program is reincarnated, with possible
1170 resulting confusion on the part of Perl.
1172 This function is now largely obsolete, partly because it's very
1173 hard to convert a core file into an executable, and because the
1174 real compiler backends for generating portable bytecode and compilable
1175 C code have superseded it.
1177 If you're looking to use L<dump> to speed up your program, consider
1178 generating bytecode or native C code as described in L<perlcc>. If
1179 you're just trying to accelerate a CGI script, consider using the
1180 C<mod_perl> extension to B<Apache>, or the CPAN module, Fast::CGI.
1181 You might also consider autoloading or selfloading, which at least
1182 make your program I<appear> to run faster.
1186 When called in list context, returns a 2-element list consisting of the
1187 key and value for the next element of a hash, so that you can iterate over
1188 it. When called in scalar context, returns only the key for the next
1189 element in the hash.
1191 Entries are returned in an apparently random order. The actual random
1192 order is subject to change in future versions of perl, but it is guaranteed
1193 to be in the same order as either the C<keys> or C<values> function
1194 would produce on the same (unmodified) hash.
1196 When the hash is entirely read, a null array is returned in list context
1197 (which when assigned produces a false (C<0>) value), and C<undef> in
1198 scalar context. The next call to C<each> after that will start iterating
1199 again. There is a single iterator for each hash, shared by all C<each>,
1200 C<keys>, and C<values> function calls in the program; it can be reset by
1201 reading all the elements from the hash, or by evaluating C<keys HASH> or
1202 C<values HASH>. If you add or delete elements of a hash while you're
1203 iterating over it, you may get entries skipped or duplicated, so
1204 don't. Exception: It is always safe to delete the item most recently
1205 returned by C<each()>, which means that the following code will work:
1207 while (($key, $value) = each %hash) {
1209 delete $hash{$key}; # This is safe
1212 The following prints out your environment like the printenv(1) program,
1213 only in a different order:
1215 while (($key,$value) = each %ENV) {
1216 print "$key=$value\n";
1219 See also C<keys>, C<values> and C<sort>.
1221 =item eof FILEHANDLE
1227 Returns 1 if the next read on FILEHANDLE will return end of file, or if
1228 FILEHANDLE is not open. FILEHANDLE may be an expression whose value
1229 gives the real filehandle. (Note that this function actually
1230 reads a character and then C<ungetc>s it, so isn't very useful in an
1231 interactive context.) Do not read from a terminal file (or call
1232 C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
1233 as terminals may lose the end-of-file condition if you do.
1235 An C<eof> without an argument uses the last file read. Using C<eof()>
1236 with empty parentheses is very different. It refers to the pseudo file
1237 formed from the files listed on the command line and accessed via the
1238 C<< <> >> operator. Since C<< <> >> isn't explicitly opened,
1239 as a normal filehandle is, an C<eof()> before C<< <> >> has been
1240 used will cause C<@ARGV> to be examined to determine if input is
1243 In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to
1244 detect the end of each file, C<eof()> will only detect the end of the
1245 last file. Examples:
1247 # reset line numbering on each input file
1249 next if /^\s*#/; # skip comments
1252 close ARGV if eof; # Not eof()!
1255 # insert dashes just before last line of last file
1257 if (eof()) { # check for end of current file
1258 print "--------------\n";
1259 close(ARGV); # close or last; is needed if we
1260 # are reading from the terminal
1265 Practical hint: you almost never need to use C<eof> in Perl, because the
1266 input operators typically return C<undef> when they run out of data, or if
1273 In the first form, the return value of EXPR is parsed and executed as if it
1274 were a little Perl program. The value of the expression (which is itself
1275 determined within scalar context) is first parsed, and if there weren't any
1276 errors, executed in the lexical context of the current Perl program, so
1277 that any variable settings or subroutine and format definitions remain
1278 afterwards. Note that the value is parsed every time the eval executes.
1279 If EXPR is omitted, evaluates C<$_>. This form is typically used to
1280 delay parsing and subsequent execution of the text of EXPR until run time.
1282 In the second form, the code within the BLOCK is parsed only once--at the
1283 same time the code surrounding the eval itself was parsed--and executed
1284 within the context of the current Perl program. This form is typically
1285 used to trap exceptions more efficiently than the first (see below), while
1286 also providing the benefit of checking the code within BLOCK at compile
1289 The final semicolon, if any, may be omitted from the value of EXPR or within
1292 In both forms, the value returned is the value of the last expression
1293 evaluated inside the mini-program; a return statement may be also used, just
1294 as with subroutines. The expression providing the return value is evaluated
1295 in void, scalar, or list context, depending on the context of the eval itself.
1296 See L</wantarray> for more on how the evaluation context can be determined.
1298 If there is a syntax error or runtime error, or a C<die> statement is
1299 executed, an undefined value is returned by C<eval>, and C<$@> is set to the
1300 error message. If there was no error, C<$@> is guaranteed to be a null
1301 string. Beware that using C<eval> neither silences perl from printing
1302 warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
1303 To do either of those, you have to use the C<$SIG{__WARN__}> facility. See
1304 L</warn> and L<perlvar>.
1306 Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1307 determining whether a particular feature (such as C<socket> or C<symlink>)
1308 is implemented. It is also Perl's exception trapping mechanism, where
1309 the die operator is used to raise exceptions.
1311 If the code to be executed doesn't vary, you may use the eval-BLOCK
1312 form to trap run-time errors without incurring the penalty of
1313 recompiling each time. The error, if any, is still returned in C<$@>.
1316 # make divide-by-zero nonfatal
1317 eval { $answer = $a / $b; }; warn $@ if $@;
1319 # same thing, but less efficient
1320 eval '$answer = $a / $b'; warn $@ if $@;
1322 # a compile-time error
1323 eval { $answer = }; # WRONG
1326 eval '$answer ='; # sets $@
1328 Due to the current arguably broken state of C<__DIE__> hooks, when using
1329 the C<eval{}> form as an exception trap in libraries, you may wish not
1330 to trigger any C<__DIE__> hooks that user code may have installed.
1331 You can use the C<local $SIG{__DIE__}> construct for this purpose,
1332 as shown in this example:
1334 # a very private exception trap for divide-by-zero
1335 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1338 This is especially significant, given that C<__DIE__> hooks can call
1339 C<die> again, which has the effect of changing their error messages:
1341 # __DIE__ hooks may modify error messages
1343 local $SIG{'__DIE__'} =
1344 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
1345 eval { die "foo lives here" };
1346 print $@ if $@; # prints "bar lives here"
1349 Because this promotes action at a distance, this counterintuitive behavior
1350 may be fixed in a future release.
1352 With an C<eval>, you should be especially careful to remember what's
1353 being looked at when:
1359 eval { $x }; # CASE 4
1361 eval "\$$x++"; # CASE 5
1364 Cases 1 and 2 above behave identically: they run the code contained in
1365 the variable $x. (Although case 2 has misleading double quotes making
1366 the reader wonder what else might be happening (nothing is).) Cases 3
1367 and 4 likewise behave in the same way: they run the code C<'$x'>, which
1368 does nothing but return the value of $x. (Case 4 is preferred for
1369 purely visual reasons, but it also has the advantage of compiling at
1370 compile-time instead of at run-time.) Case 5 is a place where
1371 normally you I<would> like to use double quotes, except that in this
1372 particular situation, you can just use symbolic references instead, as
1375 C<eval BLOCK> does I<not> count as a loop, so the loop control statements
1376 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1380 =item exec PROGRAM LIST
1382 The C<exec> function executes a system command I<and never returns>--
1383 use C<system> instead of C<exec> if you want it to return. It fails and
1384 returns false only if the command does not exist I<and> it is executed
1385 directly instead of via your system's command shell (see below).
1387 Since it's a common mistake to use C<exec> instead of C<system>, Perl
1388 warns you if there is a following statement which isn't C<die>, C<warn>,
1389 or C<exit> (if C<-w> is set - but you always do that). If you
1390 I<really> want to follow an C<exec> with some other statement, you
1391 can use one of these styles to avoid the warning:
1393 exec ('foo') or print STDERR "couldn't exec foo: $!";
1394 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
1396 If there is more than one argument in LIST, or if LIST is an array
1397 with more than one value, calls execvp(3) with the arguments in LIST.
1398 If there is only one scalar argument or an array with one element in it,
1399 the argument is checked for shell metacharacters, and if there are any,
1400 the entire argument is passed to the system's command shell for parsing
1401 (this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1402 If there are no shell metacharacters in the argument, it is split into
1403 words and passed directly to C<execvp>, which is more efficient.
1406 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1407 exec "sort $outfile | uniq";
1409 If you don't really want to execute the first argument, but want to lie
1410 to the program you are executing about its own name, you can specify
1411 the program you actually want to run as an "indirect object" (without a
1412 comma) in front of the LIST. (This always forces interpretation of the
1413 LIST as a multivalued list, even if there is only a single scalar in
1416 $shell = '/bin/csh';
1417 exec $shell '-sh'; # pretend it's a login shell
1421 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1423 When the arguments get executed via the system shell, results will
1424 be subject to its quirks and capabilities. See L<perlop/"`STRING`">
1427 Using an indirect object with C<exec> or C<system> is also more
1428 secure. This usage (which also works fine with system()) forces
1429 interpretation of the arguments as a multivalued list, even if the
1430 list had just one argument. That way you're safe from the shell
1431 expanding wildcards or splitting up words with whitespace in them.
1433 @args = ( "echo surprise" );
1435 exec @args; # subject to shell escapes
1437 exec { $args[0] } @args; # safe even with one-arg list
1439 The first version, the one without the indirect object, ran the I<echo>
1440 program, passing it C<"surprise"> an argument. The second version
1441 didn't--it tried to run a program literally called I<"echo surprise">,
1442 didn't find it, and set C<$?> to a non-zero value indicating failure.
1444 Beginning with v5.6.0, Perl will attempt to flush all files opened for
1445 output before the exec, but this may not be supported on some platforms
1446 (see L<perlport>). To be safe, you may need to set C<$|> ($AUTOFLUSH
1447 in English) or call the C<autoflush()> method of C<IO::Handle> on any
1448 open handles in order to avoid lost output.
1450 Note that C<exec> will not call your C<END> blocks, nor will it call
1451 any C<DESTROY> methods in your objects.
1455 Given an expression that specifies a hash element or array element,
1456 returns true if the specified element in the hash or array has ever
1457 been initialized, even if the corresponding value is undefined. The
1458 element is not autovivified if it doesn't exist.
1460 print "Exists\n" if exists $hash{$key};
1461 print "Defined\n" if defined $hash{$key};
1462 print "True\n" if $hash{$key};
1464 print "Exists\n" if exists $array[$index];
1465 print "Defined\n" if defined $array[$index];
1466 print "True\n" if $array[$index];
1468 A hash or array element can be true only if it's defined, and defined if
1469 it exists, but the reverse doesn't necessarily hold true.
1471 Given an expression that specifies the name of a subroutine,
1472 returns true if the specified subroutine has ever been declared, even
1473 if it is undefined. Mentioning a subroutine name for exists or defined
1474 does not count as declaring it. Note that a subroutine which does not
1475 exist may still be callable: its package may have an C<AUTOLOAD>
1476 method that makes it spring into existence the first time that it is
1477 called -- see L<perlsub>.
1479 print "Exists\n" if exists &subroutine;
1480 print "Defined\n" if defined &subroutine;
1482 Note that the EXPR can be arbitrarily complicated as long as the final
1483 operation is a hash or array key lookup or subroutine name:
1485 if (exists $ref->{A}->{B}->{$key}) { }
1486 if (exists $hash{A}{B}{$key}) { }
1488 if (exists $ref->{A}->{B}->[$ix]) { }
1489 if (exists $hash{A}{B}[$ix]) { }
1491 if (exists &{$ref->{A}{B}{$key}}) { }
1493 Although the deepest nested array or hash will not spring into existence
1494 just because its existence was tested, any intervening ones will.
1495 Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
1496 into existence due to the existence test for the $key element above.
1497 This happens anywhere the arrow operator is used, including even:
1500 if (exists $ref->{"Some key"}) { }
1501 print $ref; # prints HASH(0x80d3d5c)
1503 This surprising autovivification in what does not at first--or even
1504 second--glance appear to be an lvalue context may be fixed in a future
1507 See L<perlref/"Pseudo-hashes: Using an array as a hash"> for specifics
1508 on how exists() acts when used on a pseudo-hash.
1510 Use of a subroutine call, rather than a subroutine name, as an argument
1511 to exists() is an error.
1514 exists &sub(); # Error
1518 Evaluates EXPR and exits immediately with that value. Example:
1521 exit 0 if $ans =~ /^[Xx]/;
1523 See also C<die>. If EXPR is omitted, exits with C<0> status. The only
1524 universally recognized values for EXPR are C<0> for success and C<1>
1525 for error; other values are subject to interpretation depending on the
1526 environment in which the Perl program is running. For example, exiting
1527 69 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
1528 the mailer to return the item undelivered, but that's not true everywhere.
1530 Don't use C<exit> to abort a subroutine if there's any chance that
1531 someone might want to trap whatever error happened. Use C<die> instead,
1532 which can be trapped by an C<eval>.
1534 The exit() function does not always exit immediately. It calls any
1535 defined C<END> routines first, but these C<END> routines may not
1536 themselves abort the exit. Likewise any object destructors that need to
1537 be called are called before the real exit. If this is a problem, you
1538 can call C<POSIX:_exit($status)> to avoid END and destructor processing.
1539 See L<perlmod> for details.
1545 Returns I<e> (the natural logarithm base) to the power of EXPR.
1546 If EXPR is omitted, gives C<exp($_)>.
1548 =item fcntl FILEHANDLE,FUNCTION,SCALAR
1550 Implements the fcntl(2) function. You'll probably have to say
1554 first to get the correct constant definitions. Argument processing and
1555 value return works just like C<ioctl> below.
1559 fcntl($filehandle, F_GETFL, $packed_return_buffer)
1560 or die "can't fcntl F_GETFL: $!";
1562 You don't have to check for C<defined> on the return from C<fnctl>.
1563 Like C<ioctl>, it maps a C<0> return from the system call into
1564 C<"0 but true"> in Perl. This string is true in boolean context and C<0>
1565 in numeric context. It is also exempt from the normal B<-w> warnings
1566 on improper numeric conversions.
1568 Note that C<fcntl> will produce a fatal error if used on a machine that
1569 doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
1570 manpage to learn what functions are available on your system.
1572 =item fileno FILEHANDLE
1574 Returns the file descriptor for a filehandle, or undefined if the
1575 filehandle is not open. This is mainly useful for constructing
1576 bitmaps for C<select> and low-level POSIX tty-handling operations.
1577 If FILEHANDLE is an expression, the value is taken as an indirect
1578 filehandle, generally its name.
1580 You can use this to find out whether two handles refer to the
1581 same underlying descriptor:
1583 if (fileno(THIS) == fileno(THAT)) {
1584 print "THIS and THAT are dups\n";
1587 (Filehandles connected to memory objects via new features of C<open> may
1588 return undefined even though they are open.)
1591 =item flock FILEHANDLE,OPERATION
1593 Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
1594 for success, false on failure. Produces a fatal error if used on a
1595 machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
1596 C<flock> is Perl's portable file locking interface, although it locks
1597 only entire files, not records.
1599 Two potentially non-obvious but traditional C<flock> semantics are
1600 that it waits indefinitely until the lock is granted, and that its locks
1601 B<merely advisory>. Such discretionary locks are more flexible, but offer
1602 fewer guarantees. This means that files locked with C<flock> may be
1603 modified by programs that do not also use C<flock>. See L<perlport>,
1604 your port's specific documentation, or your system-specific local manpages
1605 for details. It's best to assume traditional behavior if you're writing
1606 portable programs. (But if you're not, you should as always feel perfectly
1607 free to write for your own system's idiosyncrasies (sometimes called
1608 "features"). Slavish adherence to portability concerns shouldn't get
1609 in the way of your getting your job done.)
1611 OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1612 LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
1613 you can use the symbolic names if you import them from the Fcntl module,
1614 either individually, or as a group using the ':flock' tag. LOCK_SH
1615 requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
1616 releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
1617 LOCK_SH or LOCK_EX then C<flock> will return immediately rather than blocking
1618 waiting for the lock (check the return status to see if you got it).
1620 To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
1621 before locking or unlocking it.
1623 Note that the emulation built with lockf(3) doesn't provide shared
1624 locks, and it requires that FILEHANDLE be open with write intent. These
1625 are the semantics that lockf(3) implements. Most if not all systems
1626 implement lockf(3) in terms of fcntl(2) locking, though, so the
1627 differing semantics shouldn't bite too many people.
1629 Note also that some versions of C<flock> cannot lock things over the
1630 network; you would need to use the more system-specific C<fcntl> for
1631 that. If you like you can force Perl to ignore your system's flock(2)
1632 function, and so provide its own fcntl(2)-based emulation, by passing
1633 the switch C<-Ud_flock> to the F<Configure> program when you configure
1636 Here's a mailbox appender for BSD systems.
1638 use Fcntl ':flock'; # import LOCK_* constants
1641 flock(MBOX,LOCK_EX);
1642 # and, in case someone appended
1643 # while we were waiting...
1648 flock(MBOX,LOCK_UN);
1651 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1652 or die "Can't open mailbox: $!";
1655 print MBOX $msg,"\n\n";
1658 On systems that support a real flock(), locks are inherited across fork()
1659 calls, whereas those that must resort to the more capricious fcntl()
1660 function lose the locks, making it harder to write servers.
1662 See also L<DB_File> for other flock() examples.
1666 Does a fork(2) system call to create a new process running the
1667 same program at the same point. It returns the child pid to the
1668 parent process, C<0> to the child process, or C<undef> if the fork is
1669 unsuccessful. File descriptors (and sometimes locks on those descriptors)
1670 are shared, while everything else is copied. On most systems supporting
1671 fork(), great care has gone into making it extremely efficient (for
1672 example, using copy-on-write technology on data pages), making it the
1673 dominant paradigm for multitasking over the last few decades.
1675 Beginning with v5.6.0, Perl will attempt to flush all files opened for
1676 output before forking the child process, but this may not be supported
1677 on some platforms (see L<perlport>). To be safe, you may need to set
1678 C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
1679 C<IO::Handle> on any open handles in order to avoid duplicate output.
1681 If you C<fork> without ever waiting on your children, you will
1682 accumulate zombies. On some systems, you can avoid this by setting
1683 C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
1684 forking and reaping moribund children.
1686 Note that if your forked child inherits system file descriptors like
1687 STDIN and STDOUT that are actually connected by a pipe or socket, even
1688 if you exit, then the remote server (such as, say, a CGI script or a
1689 backgrounded job launched from a remote shell) won't think you're done.
1690 You should reopen those to F</dev/null> if it's any issue.
1694 Declare a picture format for use by the C<write> function. For
1698 Test: @<<<<<<<< @||||| @>>>>>
1699 $str, $%, '$' . int($num)
1703 $num = $cost/$quantity;
1707 See L<perlform> for many details and examples.
1709 =item formline PICTURE,LIST
1711 This is an internal function used by C<format>s, though you may call it,
1712 too. It formats (see L<perlform>) a list of values according to the
1713 contents of PICTURE, placing the output into the format output
1714 accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
1715 Eventually, when a C<write> is done, the contents of
1716 C<$^A> are written to some filehandle, but you could also read C<$^A>
1717 yourself and then set C<$^A> back to C<"">. Note that a format typically
1718 does one C<formline> per line of form, but the C<formline> function itself
1719 doesn't care how many newlines are embedded in the PICTURE. This means
1720 that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
1721 You may therefore need to use multiple formlines to implement a single
1722 record format, just like the format compiler.
1724 Be careful if you put double quotes around the picture, because an C<@>
1725 character may be taken to mean the beginning of an array name.
1726 C<formline> always returns true. See L<perlform> for other examples.
1728 =item getc FILEHANDLE
1732 Returns the next character from the input file attached to FILEHANDLE,
1733 or the undefined value at end of file, or if there was an error.
1734 If FILEHANDLE is omitted, reads from STDIN. This is not particularly
1735 efficient. However, it cannot be used by itself to fetch single
1736 characters without waiting for the user to hit enter. For that, try
1737 something more like:
1740 system "stty cbreak </dev/tty >/dev/tty 2>&1";
1743 system "stty", '-icanon', 'eol', "\001";
1749 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
1752 system "stty", 'icanon', 'eol', '^@'; # ASCII null
1756 Determination of whether $BSD_STYLE should be set
1757 is left as an exercise to the reader.
1759 The C<POSIX::getattr> function can do this more portably on
1760 systems purporting POSIX compliance. See also the C<Term::ReadKey>
1761 module from your nearest CPAN site; details on CPAN can be found on
1766 Implements the C library function of the same name, which on most
1767 systems returns the current login from F</etc/utmp>, if any. If null,
1770 $login = getlogin || getpwuid($<) || "Kilroy";
1772 Do not consider C<getlogin> for authentication: it is not as
1773 secure as C<getpwuid>.
1775 =item getpeername SOCKET
1777 Returns the packed sockaddr address of other end of the SOCKET connection.
1780 $hersockaddr = getpeername(SOCK);
1781 ($port, $iaddr) = sockaddr_in($hersockaddr);
1782 $herhostname = gethostbyaddr($iaddr, AF_INET);
1783 $herstraddr = inet_ntoa($iaddr);
1787 Returns the current process group for the specified PID. Use
1788 a PID of C<0> to get the current process group for the
1789 current process. Will raise an exception if used on a machine that
1790 doesn't implement getpgrp(2). If PID is omitted, returns process
1791 group of current process. Note that the POSIX version of C<getpgrp>
1792 does not accept a PID argument, so only C<PID==0> is truly portable.
1796 Returns the process id of the parent process.
1798 =item getpriority WHICH,WHO
1800 Returns the current priority for a process, a process group, or a user.
1801 (See L<getpriority(2)>.) Will raise a fatal exception if used on a
1802 machine that doesn't implement getpriority(2).
1808 =item gethostbyname NAME
1810 =item getnetbyname NAME
1812 =item getprotobyname NAME
1818 =item getservbyname NAME,PROTO
1820 =item gethostbyaddr ADDR,ADDRTYPE
1822 =item getnetbyaddr ADDR,ADDRTYPE
1824 =item getprotobynumber NUMBER
1826 =item getservbyport PORT,PROTO
1844 =item sethostent STAYOPEN
1846 =item setnetent STAYOPEN
1848 =item setprotoent STAYOPEN
1850 =item setservent STAYOPEN
1864 These routines perform the same functions as their counterparts in the
1865 system library. In list context, the return values from the
1866 various get routines are as follows:
1868 ($name,$passwd,$uid,$gid,
1869 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
1870 ($name,$passwd,$gid,$members) = getgr*
1871 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
1872 ($name,$aliases,$addrtype,$net) = getnet*
1873 ($name,$aliases,$proto) = getproto*
1874 ($name,$aliases,$port,$proto) = getserv*
1876 (If the entry doesn't exist you get a null list.)
1878 The exact meaning of the $gcos field varies but it usually contains
1879 the real name of the user (as opposed to the login name) and other
1880 information pertaining to the user. Beware, however, that in many
1881 system users are able to change this information and therefore it
1882 cannot be trusted and therefore the $gcos is tainted (see
1883 L<perlsec>). The $passwd and $shell, user's encrypted password and
1884 login shell, are also tainted, because of the same reason.
1886 In scalar context, you get the name, unless the function was a
1887 lookup by name, in which case you get the other thing, whatever it is.
1888 (If the entry doesn't exist you get the undefined value.) For example:
1890 $uid = getpwnam($name);
1891 $name = getpwuid($num);
1893 $gid = getgrnam($name);
1894 $name = getgrgid($num;
1898 In I<getpw*()> the fields $quota, $comment, and $expire are special
1899 cases in the sense that in many systems they are unsupported. If the
1900 $quota is unsupported, it is an empty scalar. If it is supported, it
1901 usually encodes the disk quota. If the $comment field is unsupported,
1902 it is an empty scalar. If it is supported it usually encodes some
1903 administrative comment about the user. In some systems the $quota
1904 field may be $change or $age, fields that have to do with password
1905 aging. In some systems the $comment field may be $class. The $expire
1906 field, if present, encodes the expiration period of the account or the
1907 password. For the availability and the exact meaning of these fields
1908 in your system, please consult your getpwnam(3) documentation and your
1909 F<pwd.h> file. You can also find out from within Perl what your
1910 $quota and $comment fields mean and whether you have the $expire field
1911 by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
1912 C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password
1913 files are only supported if your vendor has implemented them in the
1914 intuitive fashion that calling the regular C library routines gets the
1915 shadow versions if you're running under privilege or if there exists
1916 the shadow(3) functions as found in System V ( this includes Solaris
1917 and Linux.) Those systems which implement a proprietary shadow password
1918 facility are unlikely to be supported.
1920 The $members value returned by I<getgr*()> is a space separated list of
1921 the login names of the members of the group.
1923 For the I<gethost*()> functions, if the C<h_errno> variable is supported in
1924 C, it will be returned to you via C<$?> if the function call fails. The
1925 C<@addrs> value returned by a successful call is a list of the raw
1926 addresses returned by the corresponding system library call. In the
1927 Internet domain, each address is four bytes long and you can unpack it
1928 by saying something like:
1930 ($a,$b,$c,$d) = unpack('C4',$addr[0]);
1932 The Socket library makes this slightly easier:
1935 $iaddr = inet_aton("127.1"); # or whatever address
1936 $name = gethostbyaddr($iaddr, AF_INET);
1938 # or going the other way
1939 $straddr = inet_ntoa($iaddr);
1941 If you get tired of remembering which element of the return list
1942 contains which return value, by-name interfaces are provided
1943 in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
1944 C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
1945 and C<User::grent>. These override the normal built-ins, supplying
1946 versions that return objects with the appropriate names
1947 for each field. For example:
1951 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
1953 Even though it looks like they're the same method calls (uid),
1954 they aren't, because a C<File::stat> object is different from
1955 a C<User::pwent> object.
1957 =item getsockname SOCKET
1959 Returns the packed sockaddr address of this end of the SOCKET connection,
1960 in case you don't know the address because you have several different
1961 IPs that the connection might have come in on.
1964 $mysockaddr = getsockname(SOCK);
1965 ($port, $myaddr) = sockaddr_in($mysockaddr);
1966 printf "Connect to %s [%s]\n",
1967 scalar gethostbyaddr($myaddr, AF_INET),
1970 =item getsockopt SOCKET,LEVEL,OPTNAME
1972 Returns the socket option requested, or undef if there is an error.
1978 Returns the value of EXPR with filename expansions such as the
1979 standard Unix shell F</bin/csh> would do. This is the internal function
1980 implementing the C<< <*.c> >> operator, but you can use it directly.
1981 If EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is
1982 discussed in more detail in L<perlop/"I/O Operators">.
1984 Beginning with v5.6.0, this operator is implemented using the standard
1985 C<File::Glob> extension. See L<File::Glob> for details.
1989 Converts a time as returned by the time function to a 8-element list
1990 with the time localized for the standard Greenwich time zone.
1991 Typically used as follows:
1994 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday) =
1997 All list elements are numeric, and come straight out of the C `struct
1998 tm'. $sec, $min, and $hour are the seconds, minutes, and hours of the
1999 specified time. $mday is the day of the month, and $mon is the month
2000 itself, in the range C<0..11> with 0 indicating January and 11
2001 indicating December. $year is the number of years since 1900. That
2002 is, $year is C<123> in year 2023. $wday is the day of the week, with
2003 0 indicating Sunday and 3 indicating Wednesday. $yday is the day of
2004 the year, in the range C<0..364> (or C<0..365> in leap years.)
2006 Note that the $year element is I<not> simply the last two digits of
2007 the year. If you assume it is, then you create non-Y2K-compliant
2008 programs--and you wouldn't want to do that, would you?
2010 The proper way to get a complete 4-digit year is simply:
2014 And to get the last two digits of the year (e.g., '01' in 2001) do:
2016 $year = sprintf("%02d", $year % 100);
2018 If EXPR is omitted, C<gmtime()> uses the current time (C<gmtime(time)>).
2020 In scalar context, C<gmtime()> returns the ctime(3) value:
2022 $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
2024 Also see the C<timegm> function provided by the C<Time::Local> module,
2025 and the strftime(3) function available via the POSIX module.
2027 This scalar value is B<not> locale dependent (see L<perllocale>), but
2028 is instead a Perl builtin. Also see the C<Time::Local> module, and the
2029 strftime(3) and mktime(3) functions available via the POSIX module. To
2030 get somewhat similar but locale dependent date strings, set up your
2031 locale environment variables appropriately (please see L<perllocale>)
2032 and try for example:
2034 use POSIX qw(strftime);
2035 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
2037 Note that the C<%a> and C<%b> escapes, which represent the short forms
2038 of the day of the week and the month of the year, may not necessarily
2039 be three characters wide in all locales.
2047 The C<goto-LABEL> form finds the statement labeled with LABEL and resumes
2048 execution there. It may not be used to go into any construct that
2049 requires initialization, such as a subroutine or a C<foreach> loop. It
2050 also can't be used to go into a construct that is optimized away,
2051 or to get out of a block or subroutine given to C<sort>.
2052 It can be used to go almost anywhere else within the dynamic scope,
2053 including out of subroutines, but it's usually better to use some other
2054 construct such as C<last> or C<die>. The author of Perl has never felt the
2055 need to use this form of C<goto> (in Perl, that is--C is another matter).
2057 The C<goto-EXPR> form expects a label name, whose scope will be resolved
2058 dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
2059 necessarily recommended if you're optimizing for maintainability:
2061 goto ("FOO", "BAR", "GLARCH")[$i];
2063 The C<goto-&NAME> form is quite different from the other forms of C<goto>.
2064 In fact, it isn't a goto in the normal sense at all, and doesn't have
2065 the stigma associated with other gotos. Instead, it
2066 substitutes a call to the named subroutine for the currently running
2067 subroutine. This is used by C<AUTOLOAD> subroutines that wish to load
2068 another subroutine and then pretend that the other subroutine had been
2069 called in the first place (except that any modifications to C<@_>
2070 in the current subroutine are propagated to the other subroutine.)
2071 After the C<goto>, not even C<caller> will be able to tell that this
2072 routine was called first.
2074 NAME needn't be the name of a subroutine; it can be a scalar variable
2075 containing a code reference, or a block which evaluates to a code
2078 =item grep BLOCK LIST
2080 =item grep EXPR,LIST
2082 This is similar in spirit to, but not the same as, grep(1) and its
2083 relatives. In particular, it is not limited to using regular expressions.
2085 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2086 C<$_> to each element) and returns the list value consisting of those
2087 elements for which the expression evaluated to true. In scalar
2088 context, returns the number of times the expression was true.
2090 @foo = grep(!/^#/, @bar); # weed out comments
2094 @foo = grep {!/^#/} @bar; # weed out comments
2096 Note that C<$_> is an alias to the list value, so it can be used to
2097 modify the elements of the LIST. While this is useful and supported,
2098 it can cause bizarre results if the elements of LIST are not variables.
2099 Similarly, grep returns aliases into the original list, much as a for
2100 loop's index variable aliases the list elements. That is, modifying an
2101 element of a list returned by grep (for example, in a C<foreach>, C<map>
2102 or another C<grep>) actually modifies the element in the original list.
2103 This is usually something to be avoided when writing clear code.
2105 See also L</map> for a list composed of the results of the BLOCK or EXPR.
2111 Interprets EXPR as a hex string and returns the corresponding value.
2112 (To convert strings that might start with either 0, 0x, or 0b, see
2113 L</oct>.) If EXPR is omitted, uses C<$_>.
2115 print hex '0xAf'; # prints '175'
2116 print hex 'aF'; # same
2118 Hex strings may only represent integers. Strings that would cause
2119 integer overflow trigger a warning.
2123 There is no builtin C<import> function. It is just an ordinary
2124 method (subroutine) defined (or inherited) by modules that wish to export
2125 names to another module. The C<use> function calls the C<import> method
2126 for the package used. See also L</use>, L<perlmod>, and L<Exporter>.
2128 =item index STR,SUBSTR,POSITION
2130 =item index STR,SUBSTR
2132 The index function searches for one string within another, but without
2133 the wildcard-like behavior of a full regular-expression pattern match.
2134 It returns the position of the first occurrence of SUBSTR in STR at
2135 or after POSITION. If POSITION is omitted, starts searching from the
2136 beginning of the string. The return value is based at C<0> (or whatever
2137 you've set the C<$[> variable to--but don't do that). If the substring
2138 is not found, returns one less than the base, ordinarily C<-1>.
2144 Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
2145 You should not use this function for rounding: one because it truncates
2146 towards C<0>, and two because machine representations of floating point
2147 numbers can sometimes produce counterintuitive results. For example,
2148 C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
2149 because it's really more like -268.99999999999994315658 instead. Usually,
2150 the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
2151 functions will serve you better than will int().
2153 =item ioctl FILEHANDLE,FUNCTION,SCALAR
2155 Implements the ioctl(2) function. You'll probably first have to say
2157 require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph
2159 to get the correct function definitions. If F<ioctl.ph> doesn't
2160 exist or doesn't have the correct definitions you'll have to roll your
2161 own, based on your C header files such as F<< <sys/ioctl.h> >>.
2162 (There is a Perl script called B<h2ph> that comes with the Perl kit that
2163 may help you in this, but it's nontrivial.) SCALAR will be read and/or
2164 written depending on the FUNCTION--a pointer to the string value of SCALAR
2165 will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
2166 has no string value but does have a numeric value, that value will be
2167 passed rather than a pointer to the string value. To guarantee this to be
2168 true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
2169 functions may be needed to manipulate the values of structures used by
2172 The return value of C<ioctl> (and C<fcntl>) is as follows:
2174 if OS returns: then Perl returns:
2176 0 string "0 but true"
2177 anything else that number
2179 Thus Perl returns true on success and false on failure, yet you can
2180 still easily determine the actual value returned by the operating
2183 $retval = ioctl(...) || -1;
2184 printf "System returned %d\n", $retval;
2186 The special string "C<0> but true" is exempt from B<-w> complaints
2187 about improper numeric conversions.
2189 Here's an example of setting a filehandle named C<REMOTE> to be
2190 non-blocking at the system level. You'll have to negotiate C<$|>
2191 on your own, though.
2193 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
2195 $flags = fcntl(REMOTE, F_GETFL, 0)
2196 or die "Can't get flags for the socket: $!\n";
2198 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
2199 or die "Can't set flags for the socket: $!\n";
2201 =item join EXPR,LIST
2203 Joins the separate strings of LIST into a single string with fields
2204 separated by the value of EXPR, and returns that new string. Example:
2206 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
2208 Beware that unlike C<split>, C<join> doesn't take a pattern as its
2209 first argument. Compare L</split>.
2213 Returns a list consisting of all the keys of the named hash. (In
2214 scalar context, returns the number of keys.) The keys are returned in
2215 an apparently random order. The actual random order is subject to
2216 change in future versions of perl, but it is guaranteed to be the same
2217 order as either the C<values> or C<each> function produces (given
2218 that the hash has not been modified). As a side effect, it resets
2221 Here is yet another way to print your environment:
2224 @values = values %ENV;
2226 print pop(@keys), '=', pop(@values), "\n";
2229 or how about sorted by key:
2231 foreach $key (sort(keys %ENV)) {
2232 print $key, '=', $ENV{$key}, "\n";
2235 The returned values are copies of the original keys in the hash, so
2236 modifying them will not affect the original hash. Compare L</values>.
2238 To sort a hash by value, you'll need to use a C<sort> function.
2239 Here's a descending numeric sort of a hash by its values:
2241 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
2242 printf "%4d %s\n", $hash{$key}, $key;
2245 As an lvalue C<keys> allows you to increase the number of hash buckets
2246 allocated for the given hash. This can gain you a measure of efficiency if
2247 you know the hash is going to get big. (This is similar to pre-extending
2248 an array by assigning a larger number to $#array.) If you say
2252 then C<%hash> will have at least 200 buckets allocated for it--256 of them,
2253 in fact, since it rounds up to the next power of two. These
2254 buckets will be retained even if you do C<%hash = ()>, use C<undef
2255 %hash> if you want to free the storage while C<%hash> is still in scope.
2256 You can't shrink the number of buckets allocated for the hash using
2257 C<keys> in this way (but you needn't worry about doing this by accident,
2258 as trying has no effect).
2260 See also C<each>, C<values> and C<sort>.
2262 =item kill SIGNAL, LIST
2264 Sends a signal to a list of processes. Returns the number of
2265 processes successfully signaled (which is not necessarily the
2266 same as the number actually killed).
2268 $cnt = kill 1, $child1, $child2;
2271 If SIGNAL is zero, no signal is sent to the process. This is a
2272 useful way to check that the process is alive and hasn't changed
2273 its UID. See L<perlport> for notes on the portability of this
2276 Unlike in the shell, if SIGNAL is negative, it kills
2277 process groups instead of processes. (On System V, a negative I<PROCESS>
2278 number will also kill process groups, but that's not portable.) That
2279 means you usually want to use positive not negative signals. You may also
2280 use a signal name in quotes. See L<perlipc/"Signals"> for details.
2286 The C<last> command is like the C<break> statement in C (as used in
2287 loops); it immediately exits the loop in question. If the LABEL is
2288 omitted, the command refers to the innermost enclosing loop. The
2289 C<continue> block, if any, is not executed:
2291 LINE: while (<STDIN>) {
2292 last LINE if /^$/; # exit when done with header
2296 C<last> cannot be used to exit a block which returns a value such as
2297 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2298 a grep() or map() operation.
2300 Note that a block by itself is semantically identical to a loop
2301 that executes once. Thus C<last> can be used to effect an early
2302 exit out of such a block.
2304 See also L</continue> for an illustration of how C<last>, C<next>, and
2311 Returns an lowercased version of EXPR. This is the internal function
2312 implementing the C<\L> escape in double-quoted strings.
2313 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
2316 If EXPR is omitted, uses C<$_>.
2322 Returns the value of EXPR with the first character lowercased. This is
2323 the internal function implementing the C<\l> escape in double-quoted strings.
2324 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
2326 If EXPR is omitted, uses C<$_>.
2332 Returns the length in characters of the value of EXPR. If EXPR is
2333 omitted, returns length of C<$_>. Note that this cannot be used on
2334 an entire array or hash to find out how many elements these have.
2335 For that, use C<scalar @array> and C<scalar keys %hash> respectively.
2337 =item link OLDFILE,NEWFILE
2339 Creates a new filename linked to the old filename. Returns true for
2340 success, false otherwise.
2342 =item listen SOCKET,QUEUESIZE
2344 Does the same thing that the listen system call does. Returns true if
2345 it succeeded, false otherwise. See the example in
2346 L<perlipc/"Sockets: Client/Server Communication">.
2350 You really probably want to be using C<my> instead, because C<local> isn't
2351 what most people think of as "local". See
2352 L<perlsub/"Private Variables via my()"> for details.
2354 A local modifies the listed variables to be local to the enclosing
2355 block, file, or eval. If more than one value is listed, the list must
2356 be placed in parentheses. See L<perlsub/"Temporary Values via local()">
2357 for details, including issues with tied arrays and hashes.
2359 =item localtime EXPR
2361 Converts a time as returned by the time function to a 9-element list
2362 with the time analyzed for the local time zone. Typically used as
2366 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
2369 All list elements are numeric, and come straight out of the C `struct
2370 tm'. $sec, $min, and $hour are the seconds, minutes, and hours of the
2371 specified time. $mday is the day of the month, and $mon is the month
2372 itself, in the range C<0..11> with 0 indicating January and 11
2373 indicating December. $year is the number of years since 1900. That
2374 is, $year is C<123> in year 2023. $wday is the day of the week, with
2375 0 indicating Sunday and 3 indicating Wednesday. $yday is the day of
2376 the year, in the range C<0..364> (or C<0..365> in leap years.) $isdst
2377 is true if the specified time occurs during daylight savings time,
2380 Note that the $year element is I<not> simply the last two digits of
2381 the year. If you assume it is, then you create non-Y2K-compliant
2382 programs--and you wouldn't want to do that, would you?
2384 The proper way to get a complete 4-digit year is simply:
2388 And to get the last two digits of the year (e.g., '01' in 2001) do:
2390 $year = sprintf("%02d", $year % 100);
2392 If EXPR is omitted, C<localtime()> uses the current time (C<localtime(time)>).
2394 In scalar context, C<localtime()> returns the ctime(3) value:
2396 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
2398 This scalar value is B<not> locale dependent, see L<perllocale>, but
2399 instead a Perl builtin. Also see the C<Time::Local> module
2400 (to convert the second, minutes, hours, ... back to seconds since the
2401 stroke of midnight the 1st of January 1970, the value returned by
2402 time()), and the strftime(3) and mktime(3) functions available via the
2403 POSIX module. To get somewhat similar but locale dependent date
2404 strings, set up your locale environment variables appropriately
2405 (please see L<perllocale>) and try for example:
2407 use POSIX qw(strftime);
2408 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
2410 Note that the C<%a> and C<%b>, the short forms of the day of the week
2411 and the month of the year, may not necessarily be three characters wide.
2417 This function places an advisory lock on a variable, subroutine,
2418 or referenced object contained in I<THING> until the lock goes out
2419 of scope. This is a built-in function only if your version of Perl
2420 was built with threading enabled, and if you've said C<use Threads>.
2421 Otherwise a user-defined function by this name will be called. See
2428 Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
2429 returns log of C<$_>. To get the log of another base, use basic algebra:
2430 The base-N log of a number is equal to the natural log of that number
2431 divided by the natural log of N. For example:
2435 return log($n)/log(10);
2438 See also L</exp> for the inverse operation.
2444 Does the same thing as the C<stat> function (including setting the
2445 special C<_> filehandle) but stats a symbolic link instead of the file
2446 the symbolic link points to. If symbolic links are unimplemented on
2447 your system, a normal C<stat> is done.
2449 If EXPR is omitted, stats C<$_>.
2453 The match operator. See L<perlop>.
2455 =item map BLOCK LIST
2459 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2460 C<$_> to each element) and returns the list value composed of the
2461 results of each such evaluation. In scalar context, returns the
2462 total number of elements so generated. Evaluates BLOCK or EXPR in
2463 list context, so each element of LIST may produce zero, one, or
2464 more elements in the returned value.
2466 @chars = map(chr, @nums);
2468 translates a list of numbers to the corresponding characters. And
2470 %hash = map { getkey($_) => $_ } @array;
2472 is just a funny way to write
2475 foreach $_ (@array) {
2476 $hash{getkey($_)} = $_;
2479 Note that C<$_> is an alias to the list value, so it can be used to
2480 modify the elements of the LIST. While this is useful and supported,
2481 it can cause bizarre results if the elements of LIST are not variables.
2482 Using a regular C<foreach> loop for this purpose would be clearer in
2483 most cases. See also L</grep> for an array composed of those items of
2484 the original list for which the BLOCK or EXPR evaluates to true.
2486 C<{> starts both hash references and blocks, so C<map { ...> could be either
2487 the start of map BLOCK LIST or map EXPR, LIST. Because perl doesn't look
2488 ahead for the closing C<}> it has to take a guess at which its dealing with
2489 based what it finds just after the C<{>. Usually it gets it right, but if it
2490 doesn't it won't realize something is wrong until it gets to the C<}> and
2491 encounters the missing (or unexpected) comma. The syntax error will be
2492 reported close to the C<}> but you'll need to change something near the C<{>
2493 such as using a unary C<+> to give perl some help:
2495 %hash = map { "\L$_", 1 } @array # perl guesses EXPR. wrong
2496 %hash = map { +"\L$_", 1 } @array # perl guesses BLOCK. right
2497 %hash = map { ("\L$_", 1) } @array # this also works
2498 %hash = map { lc($_), 1 } @array # as does this.
2499 %hash = map +( lc($_), 1 ), @array # this is EXPR and works!
2501 %hash = map ( lc($_), 1 ), @array # evaluates to (1, @array)
2503 or to force an anon hash constructor use C<+{>
2505 @hashes = map +{ lc($_), 1 }, @array # EXPR, so needs , at end
2507 and you get list of anonymous hashes each with only 1 entry.
2509 =item mkdir FILENAME,MASK
2511 =item mkdir FILENAME
2513 Creates the directory specified by FILENAME, with permissions
2514 specified by MASK (as modified by C<umask>). If it succeeds it
2515 returns true, otherwise it returns false and sets C<$!> (errno).
2516 If omitted, MASK defaults to 0777.
2518 In general, it is better to create directories with permissive MASK,
2519 and let the user modify that with their C<umask>, than it is to supply
2520 a restrictive MASK and give the user no way to be more permissive.
2521 The exceptions to this rule are when the file or directory should be
2522 kept private (mail files, for instance). The perlfunc(1) entry on
2523 C<umask> discusses the choice of MASK in more detail.
2525 Note that according to the POSIX 1003.1-1996 the FILENAME may have any
2526 number of trailing slashes. Some operating and filesystems do not get
2527 this right, so Perl automatically removes all trailing slashes to keep
2530 =item msgctl ID,CMD,ARG
2532 Calls the System V IPC function msgctl(2). You'll probably have to say
2536 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
2537 then ARG must be a variable which will hold the returned C<msqid_ds>
2538 structure. Returns like C<ioctl>: the undefined value for error,
2539 C<"0 but true"> for zero, or the actual return value otherwise. See also
2540 L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::Semaphore> documentation.
2542 =item msgget KEY,FLAGS
2544 Calls the System V IPC function msgget(2). Returns the message queue
2545 id, or the undefined value if there is an error. See also
2546 L<perlipc/"SysV IPC"> and C<IPC::SysV> and C<IPC::Msg> documentation.
2548 =item msgrcv ID,VAR,SIZE,TYPE,FLAGS
2550 Calls the System V IPC function msgrcv to receive a message from
2551 message queue ID into variable VAR with a maximum message size of
2552 SIZE. Note that when a message is received, the message type as a
2553 native long integer will be the first thing in VAR, followed by the
2554 actual message. This packing may be opened with C<unpack("l! a*")>.
2555 Taints the variable. Returns true if successful, or false if there is
2556 an error. See also L<perlipc/"SysV IPC">, C<IPC::SysV>, and
2557 C<IPC::SysV::Msg> documentation.
2559 =item msgsnd ID,MSG,FLAGS
2561 Calls the System V IPC function msgsnd to send the message MSG to the
2562 message queue ID. MSG must begin with the native long integer message
2563 type, and be followed by the length of the actual message, and finally
2564 the message itself. This kind of packing can be achieved with
2565 C<pack("l! a*", $type, $message)>. Returns true if successful,
2566 or false if there is an error. See also C<IPC::SysV>
2567 and C<IPC::SysV::Msg> documentation.
2571 =item my EXPR : ATTRIBUTES
2573 A C<my> declares the listed variables to be local (lexically) to the
2574 enclosing block, file, or C<eval>. If
2575 more than one value is listed, the list must be placed in parentheses. See
2576 L<perlsub/"Private Variables via my()"> for details.
2582 The C<next> command is like the C<continue> statement in C; it starts
2583 the next iteration of the loop:
2585 LINE: while (<STDIN>) {
2586 next LINE if /^#/; # discard comments
2590 Note that if there were a C<continue> block on the above, it would get
2591 executed even on discarded lines. If the LABEL is omitted, the command
2592 refers to the innermost enclosing loop.
2594 C<next> cannot be used to exit a block which returns a value such as
2595 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2596 a grep() or map() operation.
2598 Note that a block by itself is semantically identical to a loop
2599 that executes once. Thus C<next> will exit such a block early.
2601 See also L</continue> for an illustration of how C<last>, C<next>, and
2604 =item no Module LIST
2606 See the L</use> function, which C<no> is the opposite of.
2612 Interprets EXPR as an octal string and returns the corresponding
2613 value. (If EXPR happens to start off with C<0x>, interprets it as a
2614 hex string. If EXPR starts off with C<0b>, it is interpreted as a
2615 binary string.) The following will handle decimal, binary, octal, and
2616 hex in the standard Perl or C notation:
2618 $val = oct($val) if $val =~ /^0/;
2620 If EXPR is omitted, uses C<$_>. To go the other way (produce a number
2621 in octal), use sprintf() or printf():
2623 $perms = (stat("filename"))[2] & 07777;
2624 $oct_perms = sprintf "%lo", $perms;
2626 The oct() function is commonly used when a string such as C<644> needs
2627 to be converted into a file mode, for example. (Although perl will
2628 automatically convert strings into numbers as needed, this automatic
2629 conversion assumes base 10.)
2631 =item open FILEHANDLE,MODE,LIST
2633 =item open FILEHANDLE,EXPR
2635 =item open FILEHANDLE
2637 Opens the file whose filename is given by EXPR, and associates it with
2638 FILEHANDLE. If FILEHANDLE is an expression, its value is used as the
2639 name of the real filehandle wanted. (This is considered a symbolic
2640 reference, so C<use strict 'refs'> should I<not> be in effect.)
2642 If EXPR is omitted, the scalar
2643 variable of the same name as the FILEHANDLE contains the filename.
2644 (Note that lexical variables--those declared with C<my>--will not work
2645 for this purpose; so if you're using C<my>, specify EXPR in your call
2646 to open.) See L<perlopentut> for a kinder, gentler explanation of opening
2649 If MODE is C<< '<' >> or nothing, the file is opened for input.
2650 If MODE is C<< '>' >>, the file is truncated and opened for
2651 output, being created if necessary. If MODE is C<<< '>>' >>>,
2652 the file is opened for appending, again being created if necessary.
2653 You can put a C<'+'> in front of the C<< '>' >> or C<< '<' >> to indicate that
2654 you want both read and write access to the file; thus C<< '+<' >> is almost
2655 always preferred for read/write updates--the C<< '+>' >> mode would clobber the
2656 file first. You can't usually use either read-write mode for updating
2657 textfiles, since they have variable length records. See the B<-i>
2658 switch in L<perlrun> for a better approach. The file is created with
2659 permissions of C<0666> modified by the process' C<umask> value.
2661 These various prefixes correspond to the fopen(3) modes of C<'r'>, C<'r+'>,
2662 C<'w'>, C<'w+'>, C<'a'>, and C<'a+'>.
2664 In the 2-arguments (and 1-argument) form of the call the mode and
2665 filename should be concatenated (in this order), possibly separated by
2666 spaces. It is possible to omit the mode if the mode is C<< '<' >>.
2668 If the filename begins with C<'|'>, the filename is interpreted as a
2669 command to which output is to be piped, and if the filename ends with a
2670 C<'|'>, the filename is interpreted as a command which pipes output to
2671 us. See L<perlipc/"Using open() for IPC">
2672 for more examples of this. (You are not allowed to C<open> to a command
2673 that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
2674 and L<perlipc/"Bidirectional Communication with Another Process">
2677 If MODE is C<'|-'>, the filename is interpreted as a
2678 command to which output is to be piped, and if MODE is
2679 C<'-|'>, the filename is interpreted as a command which pipes output to
2680 us. In the 2-arguments (and 1-argument) form one should replace dash
2681 (C<'-'>) with the command. See L<perlipc/"Using open() for IPC">
2682 for more examples of this. (You are not allowed to C<open> to a command
2683 that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
2684 and L<perlipc/"Bidirectional Communication"> for alternatives.)
2686 In the 2-arguments (and 1-argument) form opening C<'-'> opens STDIN
2687 and opening C<< '>-' >> opens STDOUT.
2690 nonzero upon success, the undefined value otherwise. If the C<open>
2691 involved a pipe, the return value happens to be the pid of the
2694 If you're unfortunate enough to be running Perl on a system that
2695 distinguishes between text files and binary files (modern operating
2696 systems don't care), then you should check out L</binmode> for tips for
2697 dealing with this. The key distinction between systems that need C<binmode>
2698 and those that don't is their text file formats. Systems like Unix, MacOS, and
2699 Plan9, which delimit lines with a single character, and which encode that
2700 character in C as C<"\n">, do not need C<binmode>. The rest need it.
2702 When opening a file, it's usually a bad idea to continue normal execution
2703 if the request failed, so C<open> is frequently used in connection with
2704 C<die>. Even if C<die> won't do what you want (say, in a CGI script,
2705 where you want to make a nicely formatted error message (but there are
2706 modules that can help with that problem)) you should always check
2707 the return value from opening a file. The infrequent exception is when
2708 working with an unopened filehandle is actually what you want to do.
2710 As a special case the 3 arg form with a read/write mode and the third argument
2713 open(TMP, "+>", undef) or die ...
2715 opens a filehandle to an anonymous temporary file.
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.) If you use the 3 arg form then you can pass either a number,
2773 the name of a filehandle or the normal "reference to a glob".
2775 Here is a script that saves, redirects, and restores STDOUT and
2779 open(my $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 open(FILEHANDLE, "<&=", $fd)
2808 Note that if perl is using the standard C libaries fdopen() then on many UNIX systems,
2809 fdopen() is known to fail when file descriptors
2810 exceed a certain value, typically 255. If you need more file
2811 descriptors than that, consider rebuilding Perl to use the C<PerlIO>.
2813 If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'>
2814 with 2-arguments (or 1-argument) form of open(), then
2815 there is an implicit fork done, and the return value of open is the pid
2816 of the child within the parent process, and C<0> within the child
2817 process. (Use C<defined($pid)> to determine whether the open was successful.)
2818 The filehandle behaves normally for the parent, but i/o to that
2819 filehandle is piped from/to the STDOUT/STDIN of the child process.
2820 In the child process the filehandle isn't opened--i/o happens from/to
2821 the new STDOUT or STDIN. Typically this is used like the normal
2822 piped open when you want to exercise more control over just how the
2823 pipe command gets executed, such as when you are running setuid, and
2824 don't want to have to scan shell commands for metacharacters.
2825 The following triples are more or less equivalent:
2827 open(FOO, "|tr '[a-z]' '[A-Z]'");
2828 open(FOO, '|-', "tr '[a-z]' '[A-Z]'");
2829 open(FOO, '|-') || exec 'tr', '[a-z]', '[A-Z]';
2830 open(FOO, '|-', "tr", '[a-z]', '[A-Z]');
2832 open(FOO, "cat -n '$file'|");
2833 open(FOO, '-|', "cat -n '$file'");
2834 open(FOO, '-|') || exec 'cat', '-n', $file;
2835 open(FOO, '-|', "cat", '-n', $file);
2837 The last example in each block shows the pipe as "list form", which is
2838 not yet supported on all platforms.
2840 See L<perlipc/"Safe Pipe Opens"> for more examples of this.
2842 Beginning with v5.6.0, Perl will attempt to flush all files opened for
2843 output before any operation that may do a fork, but this may not be
2844 supported on some platforms (see L<perlport>). To be safe, you may need
2845 to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
2846 of C<IO::Handle> on any open handles.
2848 On systems that support a
2849 close-on-exec flag on files, the flag will be set for the newly opened
2850 file descriptor as determined by the value of $^F. See L<perlvar/$^F>.
2852 Closing any piped filehandle causes the parent process to wait for the
2853 child to finish, and returns the status value in C<$?>.
2855 The filename passed to 2-argument (or 1-argument) form of open()
2856 will have leading and trailing
2857 whitespace deleted, and the normal redirection characters
2858 honored. This property, known as "magic open",
2859 can often be used to good effect. A user could specify a filename of
2860 F<"rsh cat file |">, or you could change certain filenames as needed:
2862 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
2863 open(FH, $filename) or die "Can't open $filename: $!";
2865 Use 3-argument form to open a file with arbitrary weird characters in it,
2867 open(FOO, '<', $file);
2869 otherwise it's necessary to protect any leading and trailing whitespace:
2871 $file =~ s#^(\s)#./$1#;
2872 open(FOO, "< $file\0");
2874 (this may not work on some bizarre filesystems). One should
2875 conscientiously choose between the I<magic> and 3-arguments form
2880 will allow the user to specify an argument of the form C<"rsh cat file |">,
2881 but will not work on a filename which happens to have a trailing space, while
2883 open IN, '<', $ARGV[0];
2885 will have exactly the opposite restrictions.
2887 If you want a "real" C C<open> (see L<open(2)> on your system), then you
2888 should use the C<sysopen> function, which involves no such magic (but
2889 may use subtly different filemodes than Perl open(), which is mapped
2890 to C fopen()). This is
2891 another way to protect your filenames from interpretation. For example:
2894 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
2895 or die "sysopen $path: $!";
2896 $oldfh = select(HANDLE); $| = 1; select($oldfh);
2897 print HANDLE "stuff $$\n";
2899 print "File contains: ", <HANDLE>;
2901 Using the constructor from the C<IO::Handle> package (or one of its
2902 subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous
2903 filehandles that have the scope of whatever variables hold references to
2904 them, and automatically close whenever and however you leave that scope:
2908 sub read_myfile_munged {
2910 my $handle = new IO::File;
2911 open($handle, "myfile") or die "myfile: $!";
2913 or return (); # Automatically closed here.
2914 mung $first or die "mung failed"; # Or here.
2915 return $first, <$handle> if $ALL; # Or here.
2919 See L</seek> for some details about mixing reading and writing.
2921 =item opendir DIRHANDLE,EXPR
2923 Opens a directory named EXPR for processing by C<readdir>, C<telldir>,
2924 C<seekdir>, C<rewinddir>, and C<closedir>. Returns true if successful.
2925 DIRHANDLEs have their own namespace separate from FILEHANDLEs.
2931 Returns the numeric (ASCII or Unicode) value of the first character of EXPR. If
2932 EXPR is omitted, uses C<$_>. For the reverse, see L</chr>.
2933 See L<utf8> for more about Unicode.
2937 An C<our> declares the listed variables to be valid globals within
2938 the enclosing block, file, or C<eval>. That is, it has the same
2939 scoping rules as a "my" declaration, but does not create a local
2940 variable. If more than one value is listed, the list must be placed
2941 in parentheses. The C<our> declaration has no semantic effect unless
2942 "use strict vars" is in effect, in which case it lets you use the
2943 declared global variable without qualifying it with a package name.
2944 (But only within the lexical scope of the C<our> declaration. In this
2945 it differs from "use vars", which is package scoped.)
2947 An C<our> declaration declares a global variable that will be visible
2948 across its entire lexical scope, even across package boundaries. The
2949 package in which the variable is entered is determined at the point
2950 of the declaration, not at the point of use. This means the following
2954 our $bar; # declares $Foo::bar for rest of lexical scope
2958 print $bar; # prints 20
2960 Multiple C<our> declarations in the same lexical scope are allowed
2961 if they are in different packages. If they happened to be in the same
2962 package, Perl will emit warnings if you have asked for them.
2966 our $bar; # declares $Foo::bar for rest of lexical scope
2970 our $bar = 30; # declares $Bar::bar for rest of lexical scope
2971 print $bar; # prints 30
2973 our $bar; # emits warning
2975 =item pack TEMPLATE,LIST
2977 Takes a LIST of values and converts it into a string using the rules
2978 given by the TEMPLATE. The resulting string is the concatenation of
2979 the converted values. Typically, each converted value looks
2980 like its machine-level representation. For example, on 32-bit machines
2981 a converted integer may be represented by a sequence of 4 bytes.
2984 sequence of characters that give the order and type of values, as
2987 a A string with arbitrary binary data, will be null padded.
2988 A An ASCII string, will be space padded.
2989 Z A null terminated (asciz) string, will be null padded.
2991 b A bit string (ascending bit order inside each byte, like vec()).
2992 B A bit string (descending bit order inside each byte).
2993 h A hex string (low nybble first).
2994 H A hex string (high nybble first).
2996 c A signed char value.
2997 C An unsigned char value. Only does bytes. See U for Unicode.
2999 s A signed short value.
3000 S An unsigned short value.
3001 (This 'short' is _exactly_ 16 bits, which may differ from
3002 what a local C compiler calls 'short'. If you want
3003 native-length shorts, use the '!' suffix.)
3005 i A signed integer value.
3006 I An unsigned integer value.
3007 (This 'integer' is _at_least_ 32 bits wide. Its exact
3008 size depends on what a local C compiler calls 'int',
3009 and may even be larger than the 'long' described in
3012 l A signed long value.
3013 L An unsigned long value.
3014 (This 'long' is _exactly_ 32 bits, which may differ from
3015 what a local C compiler calls 'long'. If you want
3016 native-length longs, use the '!' suffix.)
3018 n An unsigned short in "network" (big-endian) order.
3019 N An unsigned long in "network" (big-endian) order.
3020 v An unsigned short in "VAX" (little-endian) order.
3021 V An unsigned long in "VAX" (little-endian) order.
3022 (These 'shorts' and 'longs' are _exactly_ 16 bits and
3023 _exactly_ 32 bits, respectively.)
3025 q A signed quad (64-bit) value.
3026 Q An unsigned quad value.
3027 (Quads are available only if your system supports 64-bit
3028 integer values _and_ if Perl has been compiled to support those.
3029 Causes a fatal error otherwise.)
3031 f A single-precision float in the native format.
3032 d A double-precision float in the native format.
3034 p A pointer to a null-terminated string.
3035 P A pointer to a structure (fixed-length string).
3037 u A uuencoded string.
3038 U A Unicode character number. Encodes to UTF-8 internally.
3039 Works even if C<use utf8> is not in effect.
3041 w A BER compressed integer. Its bytes represent an unsigned
3042 integer in base 128, most significant digit first, with as
3043 few digits as possible. Bit eight (the high bit) is set
3044 on each byte except the last.
3048 @ Null fill to absolute position.
3050 The following rules apply:
3056 Each letter may optionally be followed by a number giving a repeat
3057 count. With all types except C<a>, C<A>, C<Z>, C<b>, C<B>, C<h>,
3058 C<H>, and C<P> the pack function will gobble up that many values from
3059 the LIST. A C<*> for the repeat count means to use however many items are
3060 left, except for C<@>, C<x>, C<X>, where it is equivalent
3061 to C<0>, and C<u>, where it is equivalent to 1 (or 45, what is the
3064 When used with C<Z>, C<*> results in the addition of a trailing null
3065 byte (so the packed result will be one longer than the byte C<length>
3068 The repeat count for C<u> is interpreted as the maximal number of bytes
3069 to encode per line of output, with 0 and 1 replaced by 45.
3073 The C<a>, C<A>, and C<Z> types gobble just one value, but pack it as a
3074 string of length count, padding with nulls or spaces as necessary. When
3075 unpacking, C<A> strips trailing spaces and nulls, C<Z> strips everything
3076 after the first null, and C<a> returns data verbatim. When packing,
3077 C<a>, and C<Z> are equivalent.
3079 If the value-to-pack is too long, it is truncated. If too long and an
3080 explicit count is provided, C<Z> packs only C<$count-1> bytes, followed
3081 by a null byte. Thus C<Z> always packs a trailing null byte under
3086 Likewise, the C<b> and C<B> fields pack a string that many bits long.
3087 Each byte of the input field of pack() generates 1 bit of the result.
3088 Each result bit is based on the least-significant bit of the corresponding
3089 input byte, i.e., on C<ord($byte)%2>. In particular, bytes C<"0"> and
3090 C<"1"> generate bits 0 and 1, as do bytes C<"\0"> and C<"\1">.
3092 Starting from the beginning of the input string of pack(), each 8-tuple
3093 of bytes is converted to 1 byte of output. With format C<b>
3094 the first byte of the 8-tuple determines the least-significant bit of a
3095 byte, and with format C<B> it determines the most-significant bit of
3098 If the length of the input string is not exactly divisible by 8, the
3099 remainder is packed as if the input string were padded by null bytes
3100 at the end. Similarly, during unpack()ing the "extra" bits are ignored.
3102 If the input string of pack() is longer than needed, extra bytes are ignored.
3103 A C<*> for the repeat count of pack() means to use all the bytes of
3104 the input field. On unpack()ing the bits are converted to a string
3105 of C<"0">s and C<"1">s.
3109 The C<h> and C<H> fields pack a string that many nybbles (4-bit groups,
3110 representable as hexadecimal digits, 0-9a-f) long.
3112 Each byte of the input field of pack() generates 4 bits of the result.
3113 For non-alphabetical bytes the result is based on the 4 least-significant
3114 bits of the input byte, i.e., on C<ord($byte)%16>. In particular,
3115 bytes C<"0"> and C<"1"> generate nybbles 0 and 1, as do bytes
3116 C<"\0"> and C<"\1">. For bytes C<"a".."f"> and C<"A".."F"> the result
3117 is compatible with the usual hexadecimal digits, so that C<"a"> and
3118 C<"A"> both generate the nybble C<0xa==10>. The result for bytes
3119 C<"g".."z"> and C<"G".."Z"> is not well-defined.
3121 Starting from the beginning of the input string of pack(), each pair
3122 of bytes is converted to 1 byte of output. With format C<h> the
3123 first byte of the pair determines the least-significant nybble of the
3124 output byte, and with format C<H> it determines the most-significant
3127 If the length of the input string is not even, it behaves as if padded
3128 by a null byte at the end. Similarly, during unpack()ing the "extra"
3129 nybbles are ignored.
3131 If the input string of pack() is longer than needed, extra bytes are ignored.
3132 A C<*> for the repeat count of pack() means to use all the bytes of
3133 the input field. On unpack()ing the bits are converted to a string
3134 of hexadecimal digits.
3138 The C<p> type packs a pointer to a null-terminated string. You are
3139 responsible for ensuring the string is not a temporary value (which can
3140 potentially get deallocated before you get around to using the packed result).
3141 The C<P> type packs a pointer to a structure of the size indicated by the
3142 length. A NULL pointer is created if the corresponding value for C<p> or
3143 C<P> is C<undef>, similarly for unpack().
3147 The C</> template character allows packing and unpacking of strings where
3148 the packed structure contains a byte count followed by the string itself.
3149 You write I<length-item>C</>I<string-item>.
3151 The I<length-item> can be any C<pack> template letter,
3152 and describes how the length value is packed.
3153 The ones likely to be of most use are integer-packing ones like
3154 C<n> (for Java strings), C<w> (for ASN.1 or SNMP)
3155 and C<N> (for Sun XDR).
3157 The I<string-item> must, at present, be C<"A*">, C<"a*"> or C<"Z*">.
3158 For C<unpack> the length of the string is obtained from the I<length-item>,
3159 but if you put in the '*' it will be ignored.
3161 unpack 'C/a', "\04Gurusamy"; gives 'Guru'
3162 unpack 'a3/A* A*', '007 Bond J '; gives (' Bond','J')
3163 pack 'n/a* w/a*','hello,','world'; gives "\000\006hello,\005world"
3165 The I<length-item> is not returned explicitly from C<unpack>.
3167 Adding a count to the I<length-item> letter is unlikely to do anything
3168 useful, unless that letter is C<A>, C<a> or C<Z>. Packing with a
3169 I<length-item> of C<a> or C<Z> may introduce C<"\000"> characters,
3170 which Perl does not regard as legal in numeric strings.
3174 The integer types C<s>, C<S>, C<l>, and C<L> may be
3175 immediately followed by a C<!> suffix to signify native shorts or
3176 longs--as you can see from above for example a bare C<l> does mean
3177 exactly 32 bits, the native C<long> (as seen by the local C compiler)
3178 may be larger. This is an issue mainly in 64-bit platforms. You can
3179 see whether using C<!> makes any difference by
3181 print length(pack("s")), " ", length(pack("s!")), "\n";
3182 print length(pack("l")), " ", length(pack("l!")), "\n";
3184 C<i!> and C<I!> also work but only because of completeness;
3185 they are identical to C<i> and C<I>.
3187 The actual sizes (in bytes) of native shorts, ints, longs, and long
3188 longs on the platform where Perl was built are also available via
3192 print $Config{shortsize}, "\n";
3193 print $Config{intsize}, "\n";
3194 print $Config{longsize}, "\n";
3195 print $Config{longlongsize}, "\n";
3197 (The C<$Config{longlongsize}> will be undefine if your system does
3198 not support long longs.)
3202 The integer formats C<s>, C<S>, C<i>, C<I>, C<l>, and C<L>
3203 are inherently non-portable between processors and operating systems
3204 because they obey the native byteorder and endianness. For example a
3205 4-byte integer 0x12345678 (305419896 decimal) be ordered natively
3206 (arranged in and handled by the CPU registers) into bytes as
3208 0x12 0x34 0x56 0x78 # big-endian
3209 0x78 0x56 0x34 0x12 # little-endian
3211 Basically, the Intel and VAX CPUs are little-endian, while everybody
3212 else, for example Motorola m68k/88k, PPC, Sparc, HP PA, Power, and
3213 Cray are big-endian. Alpha and MIPS can be either: Digital/Compaq
3214 used/uses them in little-endian mode; SGI/Cray uses them in big-endian mode.
3216 The names `big-endian' and `little-endian' are comic references to
3217 the classic "Gulliver's Travels" (via the paper "On Holy Wars and a
3218 Plea for Peace" by Danny Cohen, USC/ISI IEN 137, April 1, 1980) and
3219 the egg-eating habits of the Lilliputians.
3221 Some systems may have even weirder byte orders such as
3226 You can see your system's preference with
3228 print join(" ", map { sprintf "%#02x", $_ }
3229 unpack("C*",pack("L",0x12345678))), "\n";
3231 The byteorder on the platform where Perl was built is also available
3235 print $Config{byteorder}, "\n";
3237 Byteorders C<'1234'> and C<'12345678'> are little-endian, C<'4321'>
3238 and C<'87654321'> are big-endian.
3240 If you want portable packed integers use the formats C<n>, C<N>,
3241 C<v>, and C<V>, their byte endianness and size is known.
3242 See also L<perlport>.
3246 Real numbers (floats and doubles) are in the native machine format only;
3247 due to the multiplicity of floating formats around, and the lack of a
3248 standard "network" representation, no facility for interchange has been
3249 made. This means that packed floating point data written on one machine
3250 may not be readable on another - even if both use IEEE floating point
3251 arithmetic (as the endian-ness of the memory representation is not part
3252 of the IEEE spec). See also L<perlport>.
3254 Note that Perl uses doubles internally for all numeric calculation, and
3255 converting from double into float and thence back to double again will
3256 lose precision (i.e., C<unpack("f", pack("f", $foo)>) will not in general
3261 If the pattern begins with a C<U>, the resulting string will be treated
3262 as Unicode-encoded. You can force UTF8 encoding on in a string with an
3263 initial C<U0>, and the bytes that follow will be interpreted as Unicode
3264 characters. If you don't want this to happen, you can begin your pattern
3265 with C<C0> (or anything else) to force Perl not to UTF8 encode your
3266 string, and then follow this with a C<U*> somewhere in your pattern.
3270 You must yourself do any alignment or padding by inserting for example
3271 enough C<'x'>es while packing. There is no way to pack() and unpack()
3272 could know where the bytes are going to or coming from. Therefore
3273 C<pack> (and C<unpack>) handle their output and input as flat
3278 A comment in a TEMPLATE starts with C<#> and goes to the end of line.
3282 If TEMPLATE requires more arguments to pack() than actually given, pack()
3283 assumes additional C<""> arguments. If TEMPLATE requires less arguments
3284 to pack() than actually given, extra arguments are ignored.
3290 $foo = pack("CCCC",65,66,67,68);
3292 $foo = pack("C4",65,66,67,68);
3294 $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);
3295 # same thing with Unicode circled letters
3297 $foo = pack("ccxxcc",65,66,67,68);
3300 # note: the above examples featuring "C" and "c" are true
3301 # only on ASCII and ASCII-derived systems such as ISO Latin 1
3302 # and UTF-8. In EBCDIC the first example would be
3303 # $foo = pack("CCCC",193,194,195,196);
3305 $foo = pack("s2",1,2);
3306 # "\1\0\2\0" on little-endian
3307 # "\0\1\0\2" on big-endian
3309 $foo = pack("a4","abcd","x","y","z");
3312 $foo = pack("aaaa","abcd","x","y","z");
3315 $foo = pack("a14","abcdefg");
3316 # "abcdefg\0\0\0\0\0\0\0"
3318 $foo = pack("i9pl", gmtime);
3319 # a real struct tm (on my system anyway)
3321 $utmp_template = "Z8 Z8 Z16 L";
3322 $utmp = pack($utmp_template, @utmp1);
3323 # a struct utmp (BSDish)
3325 @utmp2 = unpack($utmp_template, $utmp);
3326 # "@utmp1" eq "@utmp2"
3329 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
3332 $foo = pack('sx2l', 12, 34);
3333 # short 12, two zero bytes padding, long 34
3334 $bar = pack('s@4l', 12, 34);
3335 # short 12, zero fill to position 4, long 34
3338 The same template may generally also be used in unpack().
3340 =item package NAMESPACE
3344 Declares the compilation unit as being in the given namespace. The scope
3345 of the package declaration is from the declaration itself through the end
3346 of the enclosing block, file, or eval (the same as the C<my> operator).
3347 All further unqualified dynamic identifiers will be in this namespace.
3348 A package statement affects only dynamic variables--including those
3349 you've used C<local> on--but I<not> lexical variables, which are created
3350 with C<my>. Typically it would be the first declaration in a file to
3351 be included by the C<require> or C<use> operator. You can switch into a
3352 package in more than one place; it merely influences which symbol table
3353 is used by the compiler for the rest of that block. You can refer to
3354 variables and filehandles in other packages by prefixing the identifier
3355 with the package name and a double colon: C<$Package::Variable>.
3356 If the package name is null, the C<main> package as assumed. That is,
3357 C<$::sail> is equivalent to C<$main::sail> (as well as to C<$main'sail>,
3358 still seen in older code).
3360 If NAMESPACE is omitted, then there is no current package, and all
3361 identifiers must be fully qualified or lexicals. This is stricter
3362 than C<use strict>, since it also extends to function names.
3364 See L<perlmod/"Packages"> for more information about packages, modules,
3365 and classes. See L<perlsub> for other scoping issues.
3367 =item pipe READHANDLE,WRITEHANDLE
3369 Opens a pair of connected pipes like the corresponding system call.
3370 Note that if you set up a loop of piped processes, deadlock can occur
3371 unless you are very careful. In addition, note that Perl's pipes use
3372 stdio buffering, so you may need to set C<$|> to flush your WRITEHANDLE
3373 after each command, depending on the application.
3375 See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
3376 for examples of such things.
3378 On systems that support a close-on-exec flag on files, the flag will be set
3379 for the newly opened file descriptors as determined by the value of $^F.
3386 Pops and returns the last value of the array, shortening the array by
3387 one element. Has an effect similar to
3391 If there are no elements in the array, returns the undefined value
3392 (although this may happen at other times as well). If ARRAY is
3393 omitted, pops the C<@ARGV> array in the main program, and the C<@_>
3394 array in subroutines, just like C<shift>.
3400 Returns the offset of where the last C<m//g> search left off for the variable
3401 in question (C<$_> is used when the variable is not specified). May be
3402 modified to change that offset. Such modification will also influence
3403 the C<\G> zero-width assertion in regular expressions. See L<perlre> and
3406 =item print FILEHANDLE LIST
3412 Prints a string or a list of strings. Returns true if successful.
3413 FILEHANDLE may be a scalar variable name, in which case the variable
3414 contains the name of or a reference to the filehandle, thus introducing
3415 one level of indirection. (NOTE: If FILEHANDLE is a variable and
3416 the next token is a term, it may be misinterpreted as an operator
3417 unless you interpose a C<+> or put parentheses around the arguments.)
3418 If FILEHANDLE is omitted, prints by default to standard output (or
3419 to the last selected output channel--see L</select>). If LIST is
3420 also omitted, prints C<$_> to the currently selected output channel.
3421 To set the default output channel to something other than STDOUT
3422 use the select operation. The current value of C<$,> (if any) is
3423 printed between each LIST item. The current value of C<$\> (if
3424 any) is printed after the entire LIST has been printed. Because
3425 print takes a LIST, anything in the LIST is evaluated in list
3426 context, and any subroutine that you call will have one or more of
3427 its expressions evaluated in list context. Also be careful not to
3428 follow the print keyword with a left parenthesis unless you want
3429 the corresponding right parenthesis to terminate the arguments to
3430 the print--interpose a C<+> or put parentheses around all the
3433 Note that if you're storing FILEHANDLES in an array or other expression,
3434 you will have to use a block returning its value instead:
3436 print { $files[$i] } "stuff\n";
3437 print { $OK ? STDOUT : STDERR } "stuff\n";
3439 =item printf FILEHANDLE FORMAT, LIST
3441 =item printf FORMAT, LIST
3443 Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that C<$\>
3444 (the output record separator) is not appended. The first argument
3445 of the list will be interpreted as the C<printf> format. If C<use locale> is
3446 in effect, the character used for the decimal point in formatted real numbers
3447 is affected by the LC_NUMERIC locale. See L<perllocale>.
3449 Don't fall into the trap of using a C<printf> when a simple
3450 C<print> would do. The C<print> is more efficient and less
3453 =item prototype FUNCTION
3455 Returns the prototype of a function as a string (or C<undef> if the
3456 function has no prototype). FUNCTION is a reference to, or the name of,
3457 the function whose prototype you want to retrieve.
3459 If FUNCTION is a string starting with C<CORE::>, the rest is taken as a
3460 name for Perl builtin. If the builtin is not I<overridable> (such as
3461 C<qw//>) or its arguments cannot be expressed by a prototype (such as
3462 C<system>) returns C<undef> because the builtin does not really behave
3463 like a Perl function. Otherwise, the string describing the equivalent
3464 prototype is returned.
3466 =item push ARRAY,LIST
3468 Treats ARRAY as a stack, and pushes the values of LIST
3469 onto the end of ARRAY. The length of ARRAY increases by the length of
3470 LIST. Has the same effect as
3473 $ARRAY[++$#ARRAY] = $value;
3476 but is more efficient. Returns the new number of elements in the array.
3488 Generalized quotes. See L<perlop/"Regexp Quote-Like Operators">.
3490 =item quotemeta EXPR
3494 Returns the value of EXPR with all non-"word"
3495 characters backslashed. (That is, all characters not matching
3496 C</[A-Za-z_0-9]/> will be preceded by a backslash in the
3497 returned string, regardless of any locale settings.)
3498 This is the internal function implementing
3499 the C<\Q> escape in double-quoted strings.
3501 If EXPR is omitted, uses C<$_>.
3507 Returns a random fractional number greater than or equal to C<0> and less
3508 than the value of EXPR. (EXPR should be positive.) If EXPR is
3509 omitted, the value C<1> is used. Automatically calls C<srand> unless
3510 C<srand> has already been called. See also C<srand>.
3512 Apply C<int()> to the value returned by C<rand()> if you want random
3513 integers instead of random fractional numbers. For example,
3517 returns a random integer between C<0> and C<9>, inclusive.
3519 (Note: If your rand function consistently returns numbers that are too
3520 large or too small, then your version of Perl was probably compiled
3521 with the wrong number of RANDBITS.)
3523 =item read FILEHANDLE,SCALAR,LENGTH,OFFSET
3525 =item read FILEHANDLE,SCALAR,LENGTH
3527 Attempts to read LENGTH bytes of data into variable SCALAR from the
3528 specified FILEHANDLE. Returns the number of bytes actually read, C<0>
3529 at end of file, or undef if there was an error. SCALAR will be grown
3530 or shrunk to the length actually read. If SCALAR needs growing, the
3531 new bytes will be zero bytes. An OFFSET may be specified to place
3532 the read data into some other place in SCALAR than the beginning.
3533 The call is actually implemented in terms of stdio's fread(3) call.
3534 To get a true read(2) system call, see C<sysread>.
3536 =item readdir DIRHANDLE
3538 Returns the next directory entry for a directory opened by C<opendir>.
3539 If used in list context, returns all the rest of the entries in the
3540 directory. If there are no more entries, returns an undefined value in
3541 scalar context or a null list in list context.
3543 If you're planning to filetest the return values out of a C<readdir>, you'd
3544 better prepend the directory in question. Otherwise, because we didn't
3545 C<chdir> there, it would have been testing the wrong file.
3547 opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!";
3548 @dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR);
3553 Reads from the filehandle whose typeglob is contained in EXPR. In scalar
3554 context, each call reads and returns the next line, until end-of-file is
3555 reached, whereupon the subsequent call returns undef. In list context,
3556 reads until end-of-file is reached and returns a list of lines. Note that
3557 the notion of "line" used here is however you may have defined it
3558 with C<$/> or C<$INPUT_RECORD_SEPARATOR>). See L<perlvar/"$/">.
3560 When C<$/> is set to C<undef>, when readline() is in scalar
3561 context (i.e. file slurp mode), and when an empty file is read, it
3562 returns C<''> the first time, followed by C<undef> subsequently.
3564 This is the internal function implementing the C<< <EXPR> >>
3565 operator, but you can use it directly. The C<< <EXPR> >>
3566 operator is discussed in more detail in L<perlop/"I/O Operators">.
3569 $line = readline(*STDIN); # same thing
3575 Returns the value of a symbolic link, if symbolic links are
3576 implemented. If not, gives a fatal error. If there is some system
3577 error, returns the undefined value and sets C<$!> (errno). If EXPR is
3578 omitted, uses C<$_>.
3582 EXPR is executed as a system command.
3583 The collected standard output of the command is returned.
3584 In scalar context, it comes back as a single (potentially
3585 multi-line) string. In list context, returns a list of lines
3586 (however you've defined lines with C<$/> or C<$INPUT_RECORD_SEPARATOR>).
3587 This is the internal function implementing the C<qx/EXPR/>
3588 operator, but you can use it directly. The C<qx/EXPR/>
3589 operator is discussed in more detail in L<perlop/"I/O Operators">.
3591 =item recv SOCKET,SCALAR,LENGTH,FLAGS
3593 Receives a message on a socket. Attempts to receive LENGTH bytes of
3594 data into variable SCALAR from the specified SOCKET filehandle. SCALAR
3595 will be grown or shrunk to the length actually read. Takes the same
3596 flags as the system call of the same name. Returns the address of the
3597 sender if SOCKET's protocol supports this; returns an empty string
3598 otherwise. If there's an error, returns the undefined value. This call
3599 is actually implemented in terms of recvfrom(2) system call. See
3600 L<perlipc/"UDP: Message Passing"> for examples.
3606 The C<redo> command restarts the loop block without evaluating the
3607 conditional again. The C<continue> block, if any, is not executed. If
3608 the LABEL is omitted, the command refers to the innermost enclosing
3609 loop. This command is normally used by programs that want to lie to
3610 themselves about what was just input:
3612 # a simpleminded Pascal comment stripper
3613 # (warning: assumes no { or } in strings)
3614 LINE: while (<STDIN>) {
3615 while (s|({.*}.*){.*}|$1 |) {}
3620 if (/}/) { # end of comment?
3629 C<redo> cannot be used to retry a block which returns a value such as
3630 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
3631 a grep() or map() operation.
3633 Note that a block by itself is semantically identical to a loop
3634 that executes once. Thus C<redo> inside such a block will effectively
3635 turn it into a looping construct.
3637 See also L</continue> for an illustration of how C<last>, C<next>, and
3644 Returns a true value if EXPR is a reference, false otherwise. If EXPR
3645 is not specified, C<$_> will be used. The value returned depends on the
3646 type of thing the reference is a reference to.
3647 Builtin types include:
3657 If the referenced object has been blessed into a package, then that package
3658 name is returned instead. You can think of C<ref> as a C<typeof> operator.
3660 if (ref($r) eq "HASH") {
3661 print "r is a reference to a hash.\n";
3664 print "r is not a reference at all.\n";
3666 if (UNIVERSAL::isa($r, "HASH")) { # for subclassing
3667 print "r is a reference to something that isa hash.\n";
3670 See also L<perlref>.
3672 =item rename OLDNAME,NEWNAME
3674 Changes the name of a file; an existing file NEWNAME will be
3675 clobbered. Returns true for success, false otherwise.
3677 Behavior of this function varies wildly depending on your system
3678 implementation. For example, it will usually not work across file system
3679 boundaries, even though the system I<mv> command sometimes compensates
3680 for this. Other restrictions include whether it works on directories,
3681 open files, or pre-existing files. Check L<perlport> and either the
3682 rename(2) manpage or equivalent system documentation for details.
3684 =item require VERSION
3690 Demands some semantics specified by EXPR, or by C<$_> if EXPR is not
3693 If a VERSION is specified as a literal of the form v5.6.1,
3694 demands that the current version of Perl (C<$^V> or $PERL_VERSION) be
3695 at least as recent as that version, at run time. (For compatibility
3696 with older versions of Perl, a numeric argument will also be interpreted
3697 as VERSION.) Compare with L</use>, which can do a similar check at
3700 require v5.6.1; # run time version check
3701 require 5.6.1; # ditto
3702 require 5.005_03; # float version allowed for compatibility
3704 Otherwise, demands that a library file be included if it hasn't already
3705 been included. The file is included via the do-FILE mechanism, which is
3706 essentially just a variety of C<eval>. Has semantics similar to the following
3711 return 1 if $INC{$filename};
3712 my($realfilename,$result);
3714 foreach $prefix (@INC) {
3715 $realfilename = "$prefix/$filename";
3716 if (-f $realfilename) {
3717 $INC{$filename} = $realfilename;
3718 $result = do $realfilename;
3722 die "Can't find $filename in \@INC";
3724 delete $INC{$filename} if $@ || !$result;
3726 die "$filename did not return true value" unless $result;
3730 Note that the file will not be included twice under the same specified
3731 name. The file must return true as the last statement to indicate
3732 successful execution of any initialization code, so it's customary to
3733 end such a file with C<1;> unless you're sure it'll return true
3734 otherwise. But it's better just to put the C<1;>, in case you add more
3737 If EXPR is a bareword, the require assumes a "F<.pm>" extension and
3738 replaces "F<::>" with "F</>" in the filename for you,
3739 to make it easy to load standard modules. This form of loading of
3740 modules does not risk altering your namespace.
3742 In other words, if you try this:
3744 require Foo::Bar; # a splendid bareword
3746 The require function will actually look for the "F<Foo/Bar.pm>" file in the
3747 directories specified in the C<@INC> array.
3749 But if you try this:
3751 $class = 'Foo::Bar';
3752 require $class; # $class is not a bareword
3754 require "Foo::Bar"; # not a bareword because of the ""
3756 The require function will look for the "F<Foo::Bar>" file in the @INC array and
3757 will complain about not finding "F<Foo::Bar>" there. In this case you can do:
3759 eval "require $class";
3761 For a yet-more-powerful import facility, see L</use> and L<perlmod>.
3767 Generally used in a C<continue> block at the end of a loop to clear
3768 variables and reset C<??> searches so that they work again. The
3769 expression is interpreted as a list of single characters (hyphens
3770 allowed for ranges). All variables and arrays beginning with one of
3771 those letters are reset to their pristine state. If the expression is
3772 omitted, one-match searches (C<?pattern?>) are reset to match again. Resets
3773 only variables or searches in the current package. Always returns
3776 reset 'X'; # reset all X variables
3777 reset 'a-z'; # reset lower case variables
3778 reset; # just reset ?one-time? searches
3780 Resetting C<"A-Z"> is not recommended because you'll wipe out your
3781 C<@ARGV> and C<@INC> arrays and your C<%ENV> hash. Resets only package
3782 variables--lexical variables are unaffected, but they clean themselves
3783 up on scope exit anyway, so you'll probably want to use them instead.
3790 Returns from a subroutine, C<eval>, or C<do FILE> with the value
3791 given in EXPR. Evaluation of EXPR may be in list, scalar, or void
3792 context, depending on how the return value will be used, and the context
3793 may vary from one execution to the next (see C<wantarray>). If no EXPR
3794 is given, returns an empty list in list context, the undefined value in
3795 scalar context, and (of course) nothing at all in a void context.
3797 (Note that in the absence of a explicit C<return>, a subroutine, eval,
3798 or do FILE will automatically return the value of the last expression
3803 In list context, returns a list value consisting of the elements
3804 of LIST in the opposite order. In scalar context, concatenates the
3805 elements of LIST and returns a string value with all characters
3806 in the opposite order.
3808 print reverse <>; # line tac, last line first
3810 undef $/; # for efficiency of <>
3811 print scalar reverse <>; # character tac, last line tsrif
3813 This operator is also handy for inverting a hash, although there are some
3814 caveats. If a value is duplicated in the original hash, only one of those
3815 can be represented as a key in the inverted hash. Also, this has to
3816 unwind one hash and build a whole new one, which may take some time
3817 on a large hash, such as from a DBM file.
3819 %by_name = reverse %by_address; # Invert the hash
3821 =item rewinddir DIRHANDLE
3823 Sets the current position to the beginning of the directory for the
3824 C<readdir> routine on DIRHANDLE.
3826 =item rindex STR,SUBSTR,POSITION
3828 =item rindex STR,SUBSTR
3830 Works just like index() except that it returns the position of the LAST
3831 occurrence of SUBSTR in STR. If POSITION is specified, returns the
3832 last occurrence at or before that position.
3834 =item rmdir FILENAME
3838 Deletes the directory specified by FILENAME if that directory is empty. If it
3839 succeeds it returns true, otherwise it returns false and sets C<$!> (errno). If
3840 FILENAME is omitted, uses C<$_>.
3844 The substitution operator. See L<perlop>.
3848 Forces EXPR to be interpreted in scalar context and returns the value
3851 @counts = ( scalar @a, scalar @b, scalar @c );
3853 There is no equivalent operator to force an expression to
3854 be interpolated in list context because in practice, this is never
3855 needed. If you really wanted to do so, however, you could use
3856 the construction C<@{[ (some expression) ]}>, but usually a simple
3857 C<(some expression)> suffices.
3859 Because C<scalar> is unary operator, if you accidentally use for EXPR a
3860 parenthesized list, this behaves as a scalar comma expression, evaluating
3861 all but the last element in void context and returning the final element
3862 evaluated in scalar context. This is seldom what you want.
3864 The following single statement:
3866 print uc(scalar(&foo,$bar)),$baz;
3868 is the moral equivalent of these two:
3871 print(uc($bar),$baz);
3873 See L<perlop> for more details on unary operators and the comma operator.
3875 =item seek FILEHANDLE,POSITION,WHENCE
3877 Sets FILEHANDLE's position, just like the C<fseek> call of C<stdio>.
3878 FILEHANDLE may be an expression whose value gives the name of the
3879 filehandle. The values for WHENCE are C<0> to set the new position to
3880 POSITION, C<1> to set it to the current position plus POSITION, and
3881 C<2> to set it to EOF plus POSITION (typically negative). For WHENCE
3882 you may use the constants C<SEEK_SET>, C<SEEK_CUR>, and C<SEEK_END>
3883 (start of the file, current position, end of the file) from the Fcntl
3884 module. Returns C<1> upon success, C<0> otherwise.
3886 If you want to position file for C<sysread> or C<syswrite>, don't use
3887 C<seek>--buffering makes its effect on the file's system position
3888 unpredictable and non-portable. Use C<sysseek> instead.
3890 Due to the rules and rigors of ANSI C, on some systems you have to do a
3891 seek whenever you switch between reading and writing. Amongst other
3892 things, this may have the effect of calling stdio's clearerr(3).
3893 A WHENCE of C<1> (C<SEEK_CUR>) is useful for not moving the file position:
3897 This is also useful for applications emulating C<tail -f>. Once you hit
3898 EOF on your read, and then sleep for a while, you might have to stick in a
3899 seek() to reset things. The C<seek> doesn't change the current position,
3900 but it I<does> clear the end-of-file condition on the handle, so that the
3901 next C<< <FILE> >> makes Perl try again to read something. We hope.
3903 If that doesn't work (some stdios are particularly cantankerous), then
3904 you may need something more like this:
3907 for ($curpos = tell(FILE); $_ = <FILE>;
3908 $curpos = tell(FILE)) {
3909 # search for some stuff and put it into files
3911 sleep($for_a_while);
3912 seek(FILE, $curpos, 0);
3915 =item seekdir DIRHANDLE,POS
3917 Sets the current position for the C<readdir> routine on DIRHANDLE. POS
3918 must be a value returned by C<telldir>. Has the same caveats about
3919 possible directory compaction as the corresponding system library
3922 =item select FILEHANDLE
3926 Returns the currently selected filehandle. Sets the current default
3927 filehandle for output, if FILEHANDLE is supplied. This has two
3928 effects: first, a C<write> or a C<print> without a filehandle will
3929 default to this FILEHANDLE. Second, references to variables related to
3930 output will refer to this output channel. For example, if you have to
3931 set the top of form format for more than one output channel, you might
3939 FILEHANDLE may be an expression whose value gives the name of the
3940 actual filehandle. Thus:
3942 $oldfh = select(STDERR); $| = 1; select($oldfh);
3944 Some programmers may prefer to think of filehandles as objects with
3945 methods, preferring to write the last example as:
3948 STDERR->autoflush(1);
3950 =item select RBITS,WBITS,EBITS,TIMEOUT
3952 This calls the select(2) system call with the bit masks specified, which
3953 can be constructed using C<fileno> and C<vec>, along these lines:
3955 $rin = $win = $ein = '';
3956 vec($rin,fileno(STDIN),1) = 1;
3957 vec($win,fileno(STDOUT),1) = 1;
3960 If you want to select on many filehandles you might wish to write a
3964 my(@fhlist) = split(' ',$_[0]);
3967 vec($bits,fileno($_),1) = 1;
3971 $rin = fhbits('STDIN TTY SOCK');
3975 ($nfound,$timeleft) =
3976 select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
3978 or to block until something becomes ready just do this
3980 $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
3982 Most systems do not bother to return anything useful in $timeleft, so
3983 calling select() in scalar context just returns $nfound.
3985 Any of the bit masks can also be undef. The timeout, if specified, is
3986 in seconds, which may be fractional. Note: not all implementations are
3987 capable of returning the$timeleft. If not, they always return
3988 $timeleft equal to the supplied $timeout.
3990 You can effect a sleep of 250 milliseconds this way:
3992 select(undef, undef, undef, 0.25);
3994 B<WARNING>: One should not attempt to mix buffered I/O (like C<read>
3995 or <FH>) with C<select>, except as permitted by POSIX, and even
3996 then only on POSIX systems. You have to use C<sysread> instead.
3998 =item semctl ID,SEMNUM,CMD,ARG
4000 Calls the System V IPC function C<semctl>. You'll probably have to say
4004 first to get the correct constant definitions. If CMD is IPC_STAT or
4005 GETALL, then ARG must be a variable which will hold the returned
4006 semid_ds structure or semaphore value array. Returns like C<ioctl>:
4007 the undefined value for error, "C<0 but true>" for zero, or the actual
4008 return value otherwise. The ARG must consist of a vector of native
4009 short integers, which may be created with C<pack("s!",(0)x$nsem)>.
4010 See also L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::Semaphore>
4013 =item semget KEY,NSEMS,FLAGS
4015 Calls the System V IPC function semget. Returns the semaphore id, or
4016 the undefined value if there is an error. See also
4017 L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::SysV::Semaphore>
4020 =item semop KEY,OPSTRING
4022 Calls the System V IPC function semop to perform semaphore operations
4023 such as signaling and waiting. OPSTRING must be a packed array of
4024 semop structures. Each semop structure can be generated with
4025 C<pack("sss", $semnum, $semop, $semflag)>. The number of semaphore
4026 operations is implied by the length of OPSTRING. Returns true if
4027 successful, or false if there is an error. As an example, the
4028 following code waits on semaphore $semnum of semaphore id $semid:
4030 $semop = pack("sss", $semnum, -1, 0);
4031 die "Semaphore trouble: $!\n" unless semop($semid, $semop);
4033 To signal the semaphore, replace C<-1> with C<1>. See also
4034 L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::SysV::Semaphore>
4037 =item send SOCKET,MSG,FLAGS,TO
4039 =item send SOCKET,MSG,FLAGS
4041 Sends a message on a socket. Takes the same flags as the system call
4042 of the same name. On unconnected sockets you must specify a
4043 destination to send TO, in which case it does a C C<sendto>. Returns
4044 the number of characters sent, or the undefined value if there is an
4045 error. The C system call sendmsg(2) is currently unimplemented.
4046 See L<perlipc/"UDP: Message Passing"> for examples.
4048 =item setpgrp PID,PGRP
4050 Sets the current process group for the specified PID, C<0> for the current
4051 process. Will produce a fatal error if used on a machine that doesn't
4052 implement POSIX setpgid(2) or BSD setpgrp(2). If the arguments are omitted,
4053 it defaults to C<0,0>. Note that the BSD 4.2 version of C<setpgrp> does not
4054 accept any arguments, so only C<setpgrp(0,0)> is portable. See also
4057 =item setpriority WHICH,WHO,PRIORITY
4059 Sets the current priority for a process, a process group, or a user.
4060 (See setpriority(2).) Will produce a fatal error if used on a machine
4061 that doesn't implement setpriority(2).
4063 =item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
4065 Sets the socket option requested. Returns undefined if there is an
4066 error. OPTVAL may be specified as C<undef> if you don't want to pass an
4073 Shifts the first value of the array off and returns it, shortening the
4074 array by 1 and moving everything down. If there are no elements in the
4075 array, returns the undefined value. If ARRAY is omitted, shifts the
4076 C<@_> array within the lexical scope of subroutines and formats, and the
4077 C<@ARGV> array at file scopes or within the lexical scopes established by
4078 the C<eval ''>, C<BEGIN {}>, C<INIT {}>, C<CHECK {}>, and C<END {}>
4081 See also C<unshift>, C<push>, and C<pop>. C<shift> and C<unshift> do the
4082 same thing to the left end of an array that C<pop> and C<push> do to the
4085 =item shmctl ID,CMD,ARG
4087 Calls the System V IPC function shmctl. You'll probably have to say
4091 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
4092 then ARG must be a variable which will hold the returned C<shmid_ds>
4093 structure. Returns like ioctl: the undefined value for error, "C<0> but
4094 true" for zero, or the actual return value otherwise.
4095 See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation.
4097 =item shmget KEY,SIZE,FLAGS
4099 Calls the System V IPC function shmget. Returns the shared memory
4100 segment id, or the undefined value if there is an error.
4101 See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation.
4103 =item shmread ID,VAR,POS,SIZE
4105 =item shmwrite ID,STRING,POS,SIZE
4107 Reads or writes the System V shared memory segment ID starting at
4108 position POS for size SIZE by attaching to it, copying in/out, and
4109 detaching from it. When reading, VAR must be a variable that will
4110 hold the data read. When writing, if STRING is too long, only SIZE
4111 bytes are used; if STRING is too short, nulls are written to fill out
4112 SIZE bytes. Return true if successful, or false if there is an error.
4113 shmread() taints the variable. See also L<perlipc/"SysV IPC">,
4114 C<IPC::SysV> documentation, and the C<IPC::Shareable> module from CPAN.
4116 =item shutdown SOCKET,HOW
4118 Shuts down a socket connection in the manner indicated by HOW, which
4119 has the same interpretation as in the system call of the same name.
4121 shutdown(SOCKET, 0); # I/we have stopped reading data
4122 shutdown(SOCKET, 1); # I/we have stopped writing data
4123 shutdown(SOCKET, 2); # I/we have stopped using this socket
4125 This is useful with sockets when you want to tell the other
4126 side you're done writing but not done reading, or vice versa.
4127 It's also a more insistent form of close because it also
4128 disables the file descriptor in any forked copies in other
4135 Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
4136 returns sine of C<$_>.
4138 For the inverse sine operation, you may use the C<Math::Trig::asin>
4139 function, or use this relation:
4141 sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
4147 Causes the script to sleep for EXPR seconds, or forever if no EXPR.
4148 May be interrupted if the process receives a signal such as C<SIGALRM>.
4149 Returns the number of seconds actually slept. You probably cannot
4150 mix C<alarm> and C<sleep> calls, because C<sleep> is often implemented
4153 On some older systems, it may sleep up to a full second less than what
4154 you requested, depending on how it counts seconds. Most modern systems
4155 always sleep the full amount. They may appear to sleep longer than that,
4156 however, because your process might not be scheduled right away in a
4157 busy multitasking system.
4159 For delays of finer granularity than one second, you may use Perl's
4160 C<syscall> interface to access setitimer(2) if your system supports
4161 it, or else see L</select> above. The Time::HiRes module from CPAN
4164 See also the POSIX module's C<pause> function.
4166 =item sockatmark SOCKET
4168 Returns true if the socket is positioned at the out-of-band mark
4169 (also known as the urgent data mark), false otherwise. Use right
4170 after reading from the socket.
4172 Not available directly, one has to import the function from
4173 the IO::Socket extension
4175 use IO::Socket 'sockatmark';
4177 Even this doesn't guarantee that sockatmark() really is available,
4178 though, because sockatmark() is a relatively recent addition to
4179 the family of socket functions. If it is unavailable, attempt to
4182 IO::Socket::atmark not implemented on this architecture ...
4184 See also L<IO::Socket>.
4186 =item socket SOCKET,DOMAIN,TYPE,PROTOCOL
4188 Opens a socket of the specified kind and attaches it to filehandle
4189 SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for
4190 the system call of the same name. You should C<use Socket> first
4191 to get the proper definitions imported. See the examples in
4192 L<perlipc/"Sockets: Client/Server Communication">.
4194 On systems that support a close-on-exec flag on files, the flag will
4195 be set for the newly opened file descriptor, as determined by the
4196 value of $^F. See L<perlvar/$^F>.
4198 =item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
4200 Creates an unnamed pair of sockets in the specified domain, of the
4201 specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as
4202 for the system call of the same name. If unimplemented, yields a fatal
4203 error. Returns true if successful.
4205 On systems that support a close-on-exec flag on files, the flag will
4206 be set for the newly opened file descriptors, as determined by the value
4207 of $^F. See L<perlvar/$^F>.
4209 Some systems defined C<pipe> in terms of C<socketpair>, in which a call
4210 to C<pipe(Rdr, Wtr)> is essentially:
4213 socketpair(Rdr, Wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC);
4214 shutdown(Rdr, 1); # no more writing for reader
4215 shutdown(Wtr, 0); # no more reading for writer
4217 See L<perlipc> for an example of socketpair use.
4219 =item sort SUBNAME LIST
4221 =item sort BLOCK LIST
4225 Sorts the LIST and returns the sorted list value. If SUBNAME or BLOCK
4226 is omitted, C<sort>s in standard string comparison order. If SUBNAME is
4227 specified, it gives the name of a subroutine that returns an integer
4228 less than, equal to, or greater than C<0>, depending on how the elements
4229 of the list are to be ordered. (The C<< <=> >> and C<cmp>
4230 operators are extremely useful in such routines.) SUBNAME may be a
4231 scalar variable name (unsubscripted), in which case the value provides
4232 the name of (or a reference to) the actual subroutine to use. In place
4233 of a SUBNAME, you can provide a BLOCK as an anonymous, in-line sort
4236 If the subroutine's prototype is C<($$)>, the elements to be compared
4237 are passed by reference in C<@_>, as for a normal subroutine. This is
4238 slower than unprototyped subroutines, where the elements to be
4239 compared are passed into the subroutine
4240 as the package global variables $a and $b (see example below). Note that
4241 in the latter case, it is usually counter-productive to declare $a and
4244 In either case, the subroutine may not be recursive. The values to be
4245 compared are always passed by reference, so don't modify them.
4247 You also cannot exit out of the sort block or subroutine using any of the
4248 loop control operators described in L<perlsyn> or with C<goto>.
4250 When C<use locale> is in effect, C<sort LIST> sorts LIST according to the
4251 current collation locale. See L<perllocale>.
4256 @articles = sort @files;
4258 # same thing, but with explicit sort routine
4259 @articles = sort {$a cmp $b} @files;
4261 # now case-insensitively
4262 @articles = sort {uc($a) cmp uc($b)} @files;
4264 # same thing in reversed order
4265 @articles = sort {$b cmp $a} @files;
4267 # sort numerically ascending
4268 @articles = sort {$a <=> $b} @files;
4270 # sort numerically descending
4271 @articles = sort {$b <=> $a} @files;
4273 # this sorts the %age hash by value instead of key
4274 # using an in-line function
4275 @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
4277 # sort using explicit subroutine name
4279 $age{$a} <=> $age{$b}; # presuming numeric
4281 @sortedclass = sort byage @class;
4283 sub backwards { $b cmp $a }
4284 @harry = qw(dog cat x Cain Abel);
4285 @george = qw(gone chased yz Punished Axed);
4287 # prints AbelCaincatdogx
4288 print sort backwards @harry;
4289 # prints xdogcatCainAbel
4290 print sort @george, 'to', @harry;
4291 # prints AbelAxedCainPunishedcatchaseddoggonetoxyz
4293 # inefficiently sort by descending numeric compare using
4294 # the first integer after the first = sign, or the
4295 # whole record case-insensitively otherwise
4298 ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
4303 # same thing, but much more efficiently;
4304 # we'll build auxiliary indices instead
4308 push @nums, /=(\d+)/;
4313 $nums[$b] <=> $nums[$a]
4315 $caps[$a] cmp $caps[$b]
4319 # same thing, but without any temps
4320 @new = map { $_->[0] }
4321 sort { $b->[1] <=> $a->[1]
4324 } map { [$_, /=(\d+)/, uc($_)] } @old;
4326 # using a prototype allows you to use any comparison subroutine
4327 # as a sort subroutine (including other package's subroutines)
4329 sub backwards ($$) { $_[1] cmp $_[0]; } # $a and $b are not set here
4332 @new = sort other::backwards @old;
4334 If you're using strict, you I<must not> declare $a
4335 and $b as lexicals. They are package globals. That means
4336 if you're in the C<main> package and type
4338 @articles = sort {$b <=> $a} @files;
4340 then C<$a> and C<$b> are C<$main::a> and C<$main::b> (or C<$::a> and C<$::b>),
4341 but if you're in the C<FooPack> package, it's the same as typing
4343 @articles = sort {$FooPack::b <=> $FooPack::a} @files;
4345 The comparison function is required to behave. If it returns
4346 inconsistent results (sometimes saying C<$x[1]> is less than C<$x[2]> and
4347 sometimes saying the opposite, for example) the results are not
4350 =item splice ARRAY,OFFSET,LENGTH,LIST
4352 =item splice ARRAY,OFFSET,LENGTH
4354 =item splice ARRAY,OFFSET
4358 Removes the elements designated by OFFSET and LENGTH from an array, and
4359 replaces them with the elements of LIST, if any. In list context,
4360 returns the elements removed from the array. In scalar context,
4361 returns the last element removed, or C<undef> if no elements are
4362 removed. The array grows or shrinks as necessary.
4363 If OFFSET is negative then it starts that far from the end of the array.
4364 If LENGTH is omitted, removes everything from OFFSET onward.
4365 If LENGTH is negative, leaves that many elements off the end of the array.
4366 If both OFFSET and LENGTH are omitted, removes everything.
4368 The following equivalences hold (assuming C<$[ == 0>):
4370 push(@a,$x,$y) splice(@a,@a,0,$x,$y)
4371 pop(@a) splice(@a,-1)
4372 shift(@a) splice(@a,0,1)
4373 unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
4374 $a[$x] = $y splice(@a,$x,1,$y)
4376 Example, assuming array lengths are passed before arrays:
4378 sub aeq { # compare two list values
4379 my(@a) = splice(@_,0,shift);
4380 my(@b) = splice(@_,0,shift);
4381 return 0 unless @a == @b; # same len?
4383 return 0 if pop(@a) ne pop(@b);
4387 if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
4389 =item split /PATTERN/,EXPR,LIMIT
4391 =item split /PATTERN/,EXPR
4393 =item split /PATTERN/
4397 Splits a string into a list of strings and returns that list. By default,
4398 empty leading fields are preserved, and empty trailing ones are deleted.
4400 In scalar context, returns the number of fields found and splits into
4401 the C<@_> array. Use of split in scalar context is deprecated, however,
4402 because it clobbers your subroutine arguments.
4404 If EXPR is omitted, splits the C<$_> string. If PATTERN is also omitted,
4405 splits on whitespace (after skipping any leading whitespace). Anything
4406 matching PATTERN is taken to be a delimiter separating the fields. (Note
4407 that the delimiter may be longer than one character.)
4409 If LIMIT is specified and positive, splits into no more than that
4410 many fields (though it may split into fewer). If LIMIT is unspecified
4411 or zero, trailing null fields are stripped (which potential users
4412 of C<pop> would do well to remember). If LIMIT is negative, it is
4413 treated as if an arbitrarily large LIMIT had been specified.
4415 A pattern matching the null string (not to be confused with
4416 a null pattern C<//>, which is just one member of the set of patterns
4417 matching a null string) will split the value of EXPR into separate
4418 characters at each point it matches that way. For example:
4420 print join(':', split(/ */, 'hi there'));
4422 produces the output 'h:i:t:h:e:r:e'.
4424 Empty leading (or trailing) fields are produced when there positive width
4425 matches at the beginning (or end) of the string; a zero-width match at the
4426 beginning (or end) of the string does not produce an empty field. For
4429 print join(':', split(/(?=\w)/, 'hi there!'));
4431 produces the output 'h:i :t:h:e:r:e!'.
4433 The LIMIT parameter can be used to split a line partially
4435 ($login, $passwd, $remainder) = split(/:/, $_, 3);
4437 When assigning to a list, if LIMIT is omitted, Perl supplies a LIMIT
4438 one larger than the number of variables in the list, to avoid
4439 unnecessary work. For the list above LIMIT would have been 4 by
4440 default. In time critical applications it behooves you not to split
4441 into more fields than you really need.
4443 If the PATTERN contains parentheses, additional list elements are
4444 created from each matching substring in the delimiter.
4446 split(/([,-])/, "1-10,20", 3);
4448 produces the list value
4450 (1, '-', 10, ',', 20)
4452 If you had the entire header of a normal Unix email message in $header,
4453 you could split it up into fields and their values this way:
4455 $header =~ s/\n\s+/ /g; # fix continuation lines
4456 %hdrs = (UNIX_FROM => split /^(\S*?):\s*/m, $header);
4458 The pattern C</PATTERN/> may be replaced with an expression to specify
4459 patterns that vary at runtime. (To do runtime compilation only once,
4460 use C</$variable/o>.)
4462 As a special case, specifying a PATTERN of space (C<' '>) will split on
4463 white space just as C<split> with no arguments does. Thus, C<split(' ')> can
4464 be used to emulate B<awk>'s default behavior, whereas C<split(/ /)>
4465 will give you as many null initial fields as there are leading spaces.
4466 A C<split> on C</\s+/> is like a C<split(' ')> except that any leading
4467 whitespace produces a null first field. A C<split> with no arguments
4468 really does a C<split(' ', $_)> internally.
4470 A PATTERN of C</^/> is treated as if it were C</^/m>, since it isn't
4475 open(PASSWD, '/etc/passwd');
4478 ($login, $passwd, $uid, $gid,
4479 $gcos, $home, $shell) = split(/:/);
4484 =item sprintf FORMAT, LIST
4486 Returns a string formatted by the usual C<printf> conventions of the C
4487 library function C<sprintf>. See below for more details
4488 and see L<sprintf(3)> or L<printf(3)> on your system for an explanation of
4489 the general principles.
4493 # Format number with up to 8 leading zeroes
4494 $result = sprintf("%08d", $number);
4496 # Round number to 3 digits after decimal point
4497 $rounded = sprintf("%.3f", $number);
4499 Perl does its own C<sprintf> formatting--it emulates the C
4500 function C<sprintf>, but it doesn't use it (except for floating-point
4501 numbers, and even then only the standard modifiers are allowed). As a
4502 result, any non-standard extensions in your local C<sprintf> are not
4503 available from Perl.
4505 Unlike C<printf>, C<sprintf> does not do what you probably mean when you
4506 pass it an array as your first argument. The array is given scalar context,
4507 and instead of using the 0th element of the array as the format, Perl will
4508 use the count of elements in the array as the format, which is almost never
4511 Perl's C<sprintf> permits the following universally-known conversions:
4514 %c a character with the given number
4516 %d a signed integer, in decimal
4517 %u an unsigned integer, in decimal
4518 %o an unsigned integer, in octal
4519 %x an unsigned integer, in hexadecimal
4520 %e a floating-point number, in scientific notation
4521 %f a floating-point number, in fixed decimal notation
4522 %g a floating-point number, in %e or %f notation
4524 In addition, Perl permits the following widely-supported conversions:
4526 %X like %x, but using upper-case letters
4527 %E like %e, but using an upper-case "E"
4528 %G like %g, but with an upper-case "E" (if applicable)
4529 %b an unsigned integer, in binary
4530 %p a pointer (outputs the Perl value's address in hexadecimal)
4531 %n special: *stores* the number of characters output so far
4532 into the next variable in the parameter list
4534 Finally, for backward (and we do mean "backward") compatibility, Perl
4535 permits these unnecessary but widely-supported conversions:
4538 %D a synonym for %ld
4539 %U a synonym for %lu
4540 %O a synonym for %lo
4543 Note that the number of exponent digits in the scientific notation by
4544 C<%e>, C<%E>, C<%g> and C<%G> for numbers with the modulus of the
4545 exponent less than 100 is system-dependent: it may be three or less
4546 (zero-padded as necessary). In other words, 1.23 times ten to the
4547 99th may be either "1.23e99" or "1.23e099".
4549 Perl permits the following universally-known flags between the C<%>
4550 and the conversion letter:
4552 space prefix positive number with a space
4553 + prefix positive number with a plus sign
4554 - left-justify within the field
4555 0 use zeros, not spaces, to right-justify
4556 # prefix non-zero octal with "0", non-zero hex with "0x"
4557 number minimum field width
4558 .number "precision": digits after decimal point for
4559 floating-point, max length for string, minimum length
4561 l interpret integer as C type "long" or "unsigned long"
4562 h interpret integer as C type "short" or "unsigned short"
4563 If no flags, interpret integer as C type "int" or "unsigned"
4565 Perl supports parameter ordering, in other words, fetching the
4566 parameters in some explicitly specified "random" ordering as opposed
4567 to the default implicit sequential ordering. The syntax is, instead
4568 of the C<%> and C<*>, to use C<%>I<digits>C<$> and C<*>I<digits>C<$>,
4569 where the I<digits> is the wanted index, from one upwards. For example:
4571 printf "%2\$d %1\$d\n", 12, 34; # will print "34 12\n"
4572 printf "%*2\$d\n", 12, 3; # will print " 12\n"
4574 Note that using the reordering syntax does not interfere with the usual
4575 implicit sequential fetching of the parameters:
4577 printf "%2\$d %d\n", 12, 34; # will print "34 12\n"
4578 printf "%2\$d %d %d\n", 12, 34; # will print "34 12 34\n"
4579 printf "%3\$d %d %d\n", 12, 34, 56; # will print "56 12 34\n"
4580 printf "%2\$*3\$d %d\n", 12, 34, 3; # will print " 34 12\n"
4581 printf "%*3\$2\$d %d\n", 12, 34, 3; # will print " 34 12\n"
4583 There are also two Perl-specific flags:
4585 V interpret integer as Perl's standard integer type
4586 v interpret string as a vector of integers, output as
4587 numbers separated either by dots, or by an arbitrary
4588 string received from the argument list when the flag
4591 Where a number would appear in the flags, an asterisk (C<*>) may be
4592 used instead, in which case Perl uses the next item in the parameter
4593 list as the given number (that is, as the field width or precision).
4594 If a field width obtained through C<*> is negative, it has the same
4595 effect as the C<-> flag: left-justification.
4597 The C<v> flag is useful for displaying ordinal values of characters
4598 in arbitrary strings:
4600 printf "version is v%vd\n", $^V; # Perl's version
4601 printf "address is %*vX\n", ":", $addr; # IPv6 address
4602 printf "bits are %*vb\n", " ", $bits; # random bitstring
4604 If C<use locale> is in effect, the character used for the decimal
4605 point in formatted real numbers is affected by the LC_NUMERIC locale.
4608 If Perl understands "quads" (64-bit integers) (this requires
4609 either that the platform natively support quads or that Perl
4610 be specifically compiled to support quads), the characters
4614 print quads, and they may optionally be preceded by
4622 You can find out whether your Perl supports quads via L<Config>:
4625 ($Config{use64bitint} eq 'define' || $Config{longsize} == 8) &&
4628 If Perl understands "long doubles" (this requires that the platform
4629 support long doubles), the flags
4633 may optionally be preceded by
4641 You can find out whether your Perl supports long doubles via L<Config>:
4644 $Config{d_longdbl} eq 'define' && print "long doubles\n";
4650 Return the square root of EXPR. If EXPR is omitted, returns square
4651 root of C<$_>. Only works on non-negative operands, unless you've
4652 loaded the standard Math::Complex module.
4655 print sqrt(-2); # prints 1.4142135623731i
4661 Sets the random number seed for the C<rand> operator. If EXPR is
4662 omitted, uses a semi-random value supplied by the kernel (if it supports
4663 the F</dev/urandom> device) or based on the current time and process
4664 ID, among other things. In versions of Perl prior to 5.004 the default
4665 seed was just the current C<time>. This isn't a particularly good seed,
4666 so many old programs supply their own seed value (often C<time ^ $$> or
4667 C<time ^ ($$ + ($$ << 15))>), but that isn't necessary any more.
4669 In fact, it's usually not necessary to call C<srand> at all, because if
4670 it is not called explicitly, it is called implicitly at the first use of
4671 the C<rand> operator. However, this was not the case in version of Perl
4672 before 5.004, so if your script will run under older Perl versions, it
4673 should call C<srand>.
4675 Note that you need something much more random than the default seed for
4676 cryptographic purposes. Checksumming the compressed output of one or more
4677 rapidly changing operating system status programs is the usual method. For
4680 srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip`);
4682 If you're particularly concerned with this, see the C<Math::TrulyRandom>
4685 Do I<not> call C<srand> multiple times in your program unless you know
4686 exactly what you're doing and why you're doing it. The point of the
4687 function is to "seed" the C<rand> function so that C<rand> can produce
4688 a different sequence each time you run your program. Just do it once at the
4689 top of your program, or you I<won't> get random numbers out of C<rand>!
4691 Frequently called programs (like CGI scripts) that simply use
4695 for a seed can fall prey to the mathematical property that
4699 one-third of the time. So don't do that.
4701 =item stat FILEHANDLE
4707 Returns a 13-element list giving the status info for a file, either
4708 the file opened via FILEHANDLE, or named by EXPR. If EXPR is omitted,
4709 it stats C<$_>. Returns a null list if the stat fails. Typically used
4712 ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
4713 $atime,$mtime,$ctime,$blksize,$blocks)
4716 Not all fields are supported on all filesystem types. Here are the
4717 meaning of the fields:
4719 0 dev device number of filesystem
4721 2 mode file mode (type and permissions)
4722 3 nlink number of (hard) links to the file
4723 4 uid numeric user ID of file's owner
4724 5 gid numeric group ID of file's owner
4725 6 rdev the device identifier (special files only)
4726 7 size total size of file, in bytes
4727 8 atime last access time in seconds since the epoch
4728 9 mtime last modify time in seconds since the epoch
4729 10 ctime inode change time (NOT creation time!) in seconds since the epoch
4730 11 blksize preferred block size for file system I/O
4731 12 blocks actual number of blocks allocated
4733 (The epoch was at 00:00 January 1, 1970 GMT.)
4735 If stat is passed the special filehandle consisting of an underline, no
4736 stat is done, but the current contents of the stat structure from the
4737 last stat or filetest are returned. Example:
4739 if (-x $file && (($d) = stat(_)) && $d < 0) {
4740 print "$file is executable NFS file\n";
4743 (This works on machines only for which the device number is negative
4746 Because the mode contains both the file type and its permissions, you
4747 should mask off the file type portion and (s)printf using a C<"%o">
4748 if you want to see the real permissions.
4750 $mode = (stat($filename))[2];
4751 printf "Permissions are %04o\n", $mode & 07777;
4753 In scalar context, C<stat> returns a boolean value indicating success
4754 or failure, and, if successful, sets the information associated with
4755 the special filehandle C<_>.
4757 The File::stat module provides a convenient, by-name access mechanism:
4760 $sb = stat($filename);
4761 printf "File is %s, size is %s, perm %04o, mtime %s\n",
4762 $filename, $sb->size, $sb->mode & 07777,
4763 scalar localtime $sb->mtime;
4765 You can import symbolic mode constants (C<S_IF*>) and functions
4766 (C<S_IS*>) from the Fcntl module:
4770 $mode = (stat($filename))[2];
4772 $user_rwx = ($mode & S_IRWXU) >> 6;
4773 $group_read = ($mode & S_IRGRP) >> 3;
4774 $other_execute = $mode & S_IXOTH;
4776 printf "Permissions are %04o\n", S_ISMODE($mode), "\n";
4778 $is_setuid = $mode & S_ISUID;
4779 $is_setgid = S_ISDIR($mode);
4781 You could write the last two using the C<-u> and C<-d> operators.
4782 The commonly available S_IF* constants are
4784 # Permissions: read, write, execute, for user, group, others.
4786 S_IRWXU S_IRUSR S_IWUSR S_IXUSR
4787 S_IRWXG S_IRGRP S_IWGRP S_IXGRP
4788 S_IRWXO S_IROTH S_IWOTH S_IXOTH
4790 # Setuid/Setgid/Stickiness.
4792 S_ISUID S_ISGID S_ISVTX S_ISTXT
4794 # File types. Not necessarily all are available on your system.
4796 S_IFREG S_IFDIR S_IFLNK S_IFBLK S_ISCHR S_IFIFO S_IFSOCK S_IFWHT S_ENFMT
4798 # The following are compatibility aliases for S_IRUSR, S_IWUSR, S_IXUSR.
4800 S_IREAD S_IWRITE S_IEXEC
4802 and the S_IF* functions are
4804 S_IFMODE($mode) the part of $mode containing the permission bits
4805 and the setuid/setgid/sticky bits
4807 S_IFMT($mode) the part of $mode containing the file type
4808 which can be bit-anded with e.g. S_IFREG
4809 or with the following functions
4811 # The operators -f, -d, -l, -b, -c, -p, and -s.
4813 S_ISREG($mode) S_ISDIR($mode) S_ISLNK($mode)
4814 S_ISBLK($mode) S_ISCHR($mode) S_ISFIFO($mode) S_ISSOCK($mode)
4816 # No direct -X operator counterpart, but for the first one
4817 # the -g operator is often equivalent. The ENFMT stands for
4818 # record flocking enforcement, a platform-dependent feature.
4820 S_ISENFMT($mode) S_ISWHT($mode)
4822 See your native chmod(2) and stat(2) documentation for more details
4823 about the S_* constants.
4829 Takes extra time to study SCALAR (C<$_> if unspecified) in anticipation of
4830 doing many pattern matches on the string before it is next modified.
4831 This may or may not save time, depending on the nature and number of
4832 patterns you are searching on, and on the distribution of character
4833 frequencies in the string to be searched--you probably want to compare
4834 run times with and without it to see which runs faster. Those loops
4835 which scan for many short constant strings (including the constant
4836 parts of more complex patterns) will benefit most. You may have only
4837 one C<study> active at a time--if you study a different scalar the first
4838 is "unstudied". (The way C<study> works is this: a linked list of every
4839 character in the string to be searched is made, so we know, for
4840 example, where all the C<'k'> characters are. From each search string,
4841 the rarest character is selected, based on some static frequency tables
4842 constructed from some C programs and English text. Only those places
4843 that contain this "rarest" character are examined.)
4845 For example, here is a loop that inserts index producing entries
4846 before any line containing a certain pattern:
4850 print ".IX foo\n" if /\bfoo\b/;
4851 print ".IX bar\n" if /\bbar\b/;
4852 print ".IX blurfl\n" if /\bblurfl\b/;
4857 In searching for C</\bfoo\b/>, only those locations in C<$_> that contain C<f>
4858 will be looked at, because C<f> is rarer than C<o>. In general, this is
4859 a big win except in pathological cases. The only question is whether
4860 it saves you more time than it took to build the linked list in the
4863 Note that if you have to look for strings that you don't know till
4864 runtime, you can build an entire loop as a string and C<eval> that to
4865 avoid recompiling all your patterns all the time. Together with
4866 undefining C<$/> to input entire files as one record, this can be very
4867 fast, often faster than specialized programs like fgrep(1). The following
4868 scans a list of files (C<@files>) for a list of words (C<@words>), and prints
4869 out the names of those files that contain a match:
4871 $search = 'while (<>) { study;';
4872 foreach $word (@words) {
4873 $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
4878 eval $search; # this screams
4879 $/ = "\n"; # put back to normal input delimiter
4880 foreach $file (sort keys(%seen)) {
4888 =item sub NAME BLOCK
4890 This is subroutine definition, not a real function I<per se>. With just a
4891 NAME (and possibly prototypes or attributes), it's just a forward declaration.
4892 Without a NAME, it's an anonymous function declaration, and does actually
4893 return a value: the CODE ref of the closure you just created. See L<perlsub>
4894 and L<perlref> for details.
4896 =item substr EXPR,OFFSET,LENGTH,REPLACEMENT
4898 =item substr EXPR,OFFSET,LENGTH
4900 =item substr EXPR,OFFSET
4902 Extracts a substring out of EXPR and returns it. First character is at
4903 offset C<0>, or whatever you've set C<$[> to (but don't do that).
4904 If OFFSET is negative (or more precisely, less than C<$[>), starts
4905 that far from the end of the string. If LENGTH is omitted, returns
4906 everything to the end of the string. If LENGTH is negative, leaves that
4907 many characters off the end of the string.
4909 You can use the substr() function as an lvalue, in which case EXPR
4910 must itself be an lvalue. If you assign something shorter than LENGTH,
4911 the string will shrink, and if you assign something longer than LENGTH,
4912 the string will grow to accommodate it. To keep the string the same
4913 length you may need to pad or chop your value using C<sprintf>.
4915 If OFFSET and LENGTH specify a substring that is partly outside the
4916 string, only the part within the string is returned. If the substring
4917 is beyond either end of the string, substr() returns the undefined
4918 value and produces a warning. When used as an lvalue, specifying a
4919 substring that is entirely outside the string is a fatal error.
4920 Here's an example showing the behavior for boundary cases:
4923 substr($name, 4) = 'dy'; # $name is now 'freddy'
4924 my $null = substr $name, 6, 2; # returns '' (no warning)
4925 my $oops = substr $name, 7; # returns undef, with warning
4926 substr($name, 7) = 'gap'; # fatal error
4928 An alternative to using substr() as an lvalue is to specify the
4929 replacement string as the 4th argument. This allows you to replace
4930 parts of the EXPR and return what was there before in one operation,
4931 just as you can with splice().
4933 =item symlink OLDFILE,NEWFILE
4935 Creates a new filename symbolically linked to the old filename.
4936 Returns C<1> for success, C<0> otherwise. On systems that don't support
4937 symbolic links, produces a fatal error at run time. To check for that,
4940 $symlink_exists = eval { symlink("",""); 1 };
4944 Calls the system call specified as the first element of the list,
4945 passing the remaining elements as arguments to the system call. If
4946 unimplemented, produces a fatal error. The arguments are interpreted
4947 as follows: if a given argument is numeric, the argument is passed as
4948 an int. If not, the pointer to the string value is passed. You are
4949 responsible to make sure a string is pre-extended long enough to
4950 receive any result that might be written into a string. You can't use a
4951 string literal (or other read-only string) as an argument to C<syscall>
4952 because Perl has to assume that any string pointer might be written
4954 integer arguments are not literals and have never been interpreted in a
4955 numeric context, you may need to add C<0> to them to force them to look
4956 like numbers. This emulates the C<syswrite> function (or vice versa):
4958 require 'syscall.ph'; # may need to run h2ph
4960 syscall(&SYS_write, fileno(STDOUT), $s, length $s);
4962 Note that Perl supports passing of up to only 14 arguments to your system call,
4963 which in practice should usually suffice.
4965 Syscall returns whatever value returned by the system call it calls.
4966 If the system call fails, C<syscall> returns C<-1> and sets C<$!> (errno).
4967 Note that some system calls can legitimately return C<-1>. The proper
4968 way to handle such calls is to assign C<$!=0;> before the call and
4969 check the value of C<$!> if syscall returns C<-1>.
4971 There's a problem with C<syscall(&SYS_pipe)>: it returns the file
4972 number of the read end of the pipe it creates. There is no way
4973 to retrieve the file number of the other end. You can avoid this
4974 problem by using C<pipe> instead.
4976 =item sysopen FILEHANDLE,FILENAME,MODE
4978 =item sysopen FILEHANDLE,FILENAME,MODE,PERMS
4980 Opens the file whose filename is given by FILENAME, and associates it
4981 with FILEHANDLE. If FILEHANDLE is an expression, its value is used as
4982 the name of the real filehandle wanted. This function calls the
4983 underlying operating system's C<open> function with the parameters
4984 FILENAME, MODE, PERMS.
4986 The possible values and flag bits of the MODE parameter are
4987 system-dependent; they are available via the standard module C<Fcntl>.
4988 See the documentation of your operating system's C<open> to see which
4989 values and flag bits are available. You may combine several flags
4990 using the C<|>-operator.
4992 Some of the most common values are C<O_RDONLY> for opening the file in
4993 read-only mode, C<O_WRONLY> for opening the file in write-only mode,
4994 and C<O_RDWR> for opening the file in read-write mode, and.
4996 For historical reasons, some values work on almost every system
4997 supported by perl: zero means read-only, one means write-only, and two
4998 means read/write. We know that these values do I<not> work under
4999 OS/390 & VM/ESA Unix and on the Macintosh; you probably don't want to
5000 use them in new code.
5002 If the file named by FILENAME does not exist and the C<open> call creates
5003 it (typically because MODE includes the C<O_CREAT> flag), then the value of
5004 PERMS specifies the permissions of the newly created file. If you omit
5005 the PERMS argument to C<sysopen>, Perl uses the octal value C<0666>.
5006 These permission values need to be in octal, and are modified by your
5007 process's current C<umask>.
5009 In many systems the C<O_EXCL> flag is available for opening files in
5010 exclusive mode. This is B<not> locking: exclusiveness means here that
5011 if the file already exists, sysopen() fails. The C<O_EXCL> wins
5014 Sometimes you may want to truncate an already-existing file: C<O_TRUNC>.
5016 You should seldom if ever use C<0644> as argument to C<sysopen>, because
5017 that takes away the user's option to have a more permissive umask.
5018 Better to omit it. See the perlfunc(1) entry on C<umask> for more
5021 Note that C<sysopen> depends on the fdopen() C library function.
5022 On many UNIX systems, fdopen() is known to fail when file descriptors
5023 exceed a certain value, typically 255. If you need more file
5024 descriptors than that, consider rebuilding Perl to use the C<sfio>
5025 library, or perhaps using the POSIX::open() function.
5027 See L<perlopentut> for a kinder, gentler explanation of opening files.
5029 =item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
5031 =item sysread FILEHANDLE,SCALAR,LENGTH
5033 Attempts to read LENGTH bytes of data into variable SCALAR from the
5034 specified FILEHANDLE, using the system call read(2). It bypasses stdio,
5035 so mixing this with other kinds of reads, C<print>, C<write>,
5036 C<seek>, C<tell>, or C<eof> can cause confusion because stdio
5037 usually buffers data. Returns the number of bytes actually read, C<0>
5038 at end of file, or undef if there was an error. SCALAR will be grown or
5039 shrunk so that the last byte actually read is the last byte of the
5040 scalar after the read.
5042 An OFFSET may be specified to place the read data at some place in the
5043 string other than the beginning. A negative OFFSET specifies
5044 placement at that many bytes counting backwards from the end of the
5045 string. A positive OFFSET greater than the length of SCALAR results
5046 in the string being padded to the required size with C<"\0"> bytes before
5047 the result of the read is appended.
5049 There is no syseof() function, which is ok, since eof() doesn't work
5050 very well on device files (like ttys) anyway. Use sysread() and check
5051 for a return value for 0 to decide whether you're done.
5053 =item sysseek FILEHANDLE,POSITION,WHENCE
5055 Sets FILEHANDLE's system position using the system call lseek(2). It
5056 bypasses stdio, so mixing this with reads (other than C<sysread>),
5057 C<print>, C<write>, C<seek>, C<tell>, or C<eof> may cause confusion.
5058 FILEHANDLE may be an expression whose value gives the name of the
5059 filehandle. The values for WHENCE are C<0> to set the new position to
5060 POSITION, C<1> to set the it to the current position plus POSITION,
5061 and C<2> to set it to EOF plus POSITION (typically negative). For
5062 WHENCE, you may also use the constants C<SEEK_SET>, C<SEEK_CUR>, and
5063 C<SEEK_END> (start of the file, current position, end of the file)
5064 from the Fcntl module.
5066 Returns the new position, or the undefined value on failure. A position
5067 of zero is returned as the string C<"0 but true">; thus C<sysseek> returns
5068 true on success and false on failure, yet you can still easily determine
5073 =item system PROGRAM LIST
5075 Does exactly the same thing as C<exec LIST>, except that a fork is
5076 done first, and the parent process waits for the child process to
5077 complete. Note that argument processing varies depending on the
5078 number of arguments. If there is more than one argument in LIST,
5079 or if LIST is an array with more than one value, starts the program
5080 given by the first element of the list with arguments given by the
5081 rest of the list. If there is only one scalar argument, the argument
5082 is checked for shell metacharacters, and if there are any, the
5083 entire argument is passed to the system's command shell for parsing
5084 (this is C</bin/sh -c> on Unix platforms, but varies on other
5085 platforms). If there are no shell metacharacters in the argument,
5086 it is split into words and passed directly to C<execvp>, which is
5089 Beginning with v5.6.0, Perl will attempt to flush all files opened for
5090 output before any operation that may do a fork, but this may not be
5091 supported on some platforms (see L<perlport>). To be safe, you may need
5092 to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
5093 of C<IO::Handle> on any open handles.
5095 The return value is the exit status of the program as
5096 returned by the C<wait> call. To get the actual exit value divide by
5097 256. See also L</exec>. This is I<not> what you want to use to capture
5098 the output from a command, for that you should use merely backticks or
5099 C<qx//>, as described in L<perlop/"`STRING`">. Return value of -1
5100 indicates a failure to start the program (inspect $! for the reason).
5102 Like C<exec>, C<system> allows you to lie to a program about its name if
5103 you use the C<system PROGRAM LIST> syntax. Again, see L</exec>.
5105 Because C<system> and backticks block C<SIGINT> and C<SIGQUIT>, killing the
5106 program they're running doesn't actually interrupt your program.
5108 @args = ("command", "arg1", "arg2");
5110 or die "system @args failed: $?"
5112 You can check all the failure possibilities by inspecting
5115 $exit_value = $? >> 8;
5116 $signal_num = $? & 127;
5117 $dumped_core = $? & 128;
5119 When the arguments get executed via the system shell, results
5120 and return codes will be subject to its quirks and capabilities.
5121 See L<perlop/"`STRING`"> and L</exec> for details.
5123 =item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
5125 =item syswrite FILEHANDLE,SCALAR,LENGTH
5127 =item syswrite FILEHANDLE,SCALAR
5129 Attempts to write LENGTH bytes of data from variable SCALAR to the
5130 specified FILEHANDLE, using the system call write(2). If LENGTH
5131 is not specified, writes whole SCALAR. It bypasses stdio, so mixing
5132 this with reads (other than C<sysread())>, C<print>, C<write>,
5133 C<seek>, C<tell>, or C<eof> may cause confusion because stdio
5134 usually buffers data. Returns the number of bytes actually written,
5135 or C<undef> if there was an error. If the LENGTH is greater than
5136 the available data in the SCALAR after the OFFSET, only as much
5137 data as is available will be written.
5139 An OFFSET may be specified to write the data from some part of the
5140 string other than the beginning. A negative OFFSET specifies writing
5141 that many bytes counting backwards from the end of the string. In the
5142 case the SCALAR is empty you can use OFFSET but only zero offset.
5144 =item tell FILEHANDLE
5148 Returns the current position for FILEHANDLE, or -1 on error. FILEHANDLE
5149 may be an expression whose value gives the name of the actual filehandle.
5150 If FILEHANDLE is omitted, assumes the file last read.
5152 The return value of tell() for the standard streams like the STDIN
5153 depends on the operating system: it may return -1 or something else.
5154 tell() on pipes, fifos, and sockets usually returns -1.
5156 There is no C<systell> function. Use C<sysseek(FH, 0, 1)> for that.
5158 =item telldir DIRHANDLE
5160 Returns the current position of the C<readdir> routines on DIRHANDLE.
5161 Value may be given to C<seekdir> to access a particular location in a
5162 directory. Has the same caveats about possible directory compaction as
5163 the corresponding system library routine.
5165 =item tie VARIABLE,CLASSNAME,LIST
5167 This function binds a variable to a package class that will provide the
5168 implementation for the variable. VARIABLE is the name of the variable
5169 to be enchanted. CLASSNAME is the name of a class implementing objects
5170 of correct type. Any additional arguments are passed to the C<new>
5171 method of the class (meaning C<TIESCALAR>, C<TIEHANDLE>, C<TIEARRAY>,
5172 or C<TIEHASH>). Typically these are arguments such as might be passed
5173 to the C<dbm_open()> function of C. The object returned by the C<new>
5174 method is also returned by the C<tie> function, which would be useful
5175 if you want to access other methods in CLASSNAME.
5177 Note that functions such as C<keys> and C<values> may return huge lists
5178 when used on large objects, like DBM files. You may prefer to use the
5179 C<each> function to iterate over such. Example:
5181 # print out history file offsets
5183 tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
5184 while (($key,$val) = each %HIST) {
5185 print $key, ' = ', unpack('L',$val), "\n";
5189 A class implementing a hash should have the following methods:
5191 TIEHASH classname, LIST
5193 STORE this, key, value
5198 NEXTKEY this, lastkey
5202 A class implementing an ordinary array should have the following methods:
5204 TIEARRAY classname, LIST
5206 STORE this, key, value
5208 STORESIZE this, count
5214 SPLICE this, offset, length, LIST
5219 A class implementing a file handle should have the following methods:
5221 TIEHANDLE classname, LIST
5222 READ this, scalar, length, offset
5225 WRITE this, scalar, length, offset
5227 PRINTF this, format, LIST
5231 SEEK this, position, whence
5233 OPEN this, mode, LIST
5238 A class implementing a scalar should have the following methods:
5240 TIESCALAR classname, LIST
5246 Not all methods indicated above need be implemented. See L<perltie>,
5247 L<Tie::Hash>, L<Tie::Array>, L<Tie::Scalar>, and L<Tie::Handle>.
5249 Unlike C<dbmopen>, the C<tie> function will not use or require a module
5250 for you--you need to do that explicitly yourself. See L<DB_File>
5251 or the F<Config> module for interesting C<tie> implementations.
5253 For further details see L<perltie>, L<"tied VARIABLE">.
5257 Returns a reference to the object underlying VARIABLE (the same value
5258 that was originally returned by the C<tie> call that bound the variable
5259 to a package.) Returns the undefined value if VARIABLE isn't tied to a
5264 Returns the number of non-leap seconds since whatever time the system
5265 considers to be the epoch (that's 00:00:00, January 1, 1904 for MacOS,
5266 and 00:00:00 UTC, January 1, 1970 for most other systems).
5267 Suitable for feeding to C<gmtime> and C<localtime>.
5269 For measuring time in better granularity than one second,
5270 you may use either the Time::HiRes module from CPAN, or
5271 if you have gettimeofday(2), you may be able to use the
5272 C<syscall> interface of Perl, see L<perlfaq8> for details.
5276 Returns a four-element list giving the user and system times, in
5277 seconds, for this process and the children of this process.
5279 ($user,$system,$cuser,$csystem) = times;
5283 The transliteration operator. Same as C<y///>. See L<perlop>.
5285 =item truncate FILEHANDLE,LENGTH
5287 =item truncate EXPR,LENGTH
5289 Truncates the file opened on FILEHANDLE, or named by EXPR, to the
5290 specified length. Produces a fatal error if truncate isn't implemented
5291 on your system. Returns true if successful, the undefined value
5298 Returns an uppercased version of EXPR. This is the internal function
5299 implementing the C<\U> escape in double-quoted strings.
5300 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
5301 Under Unicode (C<use utf8>) it uses the standard Unicode uppercase mappings. (It
5302 does not attempt to do titlecase mapping on initial letters. See C<ucfirst> for that.)
5304 If EXPR is omitted, uses C<$_>.
5310 Returns the value of EXPR with the first character
5311 in uppercase (titlecase in Unicode). This is
5312 the internal function implementing the C<\u> escape in double-quoted strings.
5313 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
5316 If EXPR is omitted, uses C<$_>.
5322 Sets the umask for the process to EXPR and returns the previous value.
5323 If EXPR is omitted, merely returns the current umask.
5325 The Unix permission C<rwxr-x---> is represented as three sets of three
5326 bits, or three octal digits: C<0750> (the leading 0 indicates octal
5327 and isn't one of the digits). The C<umask> value is such a number
5328 representing disabled permissions bits. The permission (or "mode")
5329 values you pass C<mkdir> or C<sysopen> are modified by your umask, so
5330 even if you tell C<sysopen> to create a file with permissions C<0777>,
5331 if your umask is C<0022> then the file will actually be created with
5332 permissions C<0755>. If your C<umask> were C<0027> (group can't
5333 write; others can't read, write, or execute), then passing
5334 C<sysopen> C<0666> would create a file with mode C<0640> (C<0666 &~
5337 Here's some advice: supply a creation mode of C<0666> for regular
5338 files (in C<sysopen>) and one of C<0777> for directories (in
5339 C<mkdir>) and executable files. This gives users the freedom of
5340 choice: if they want protected files, they might choose process umasks
5341 of C<022>, C<027>, or even the particularly antisocial mask of C<077>.
5342 Programs should rarely if ever make policy decisions better left to
5343 the user. The exception to this is when writing files that should be
5344 kept private: mail files, web browser cookies, I<.rhosts> files, and
5347 If umask(2) is not implemented on your system and you are trying to
5348 restrict access for I<yourself> (i.e., (EXPR & 0700) > 0), produces a
5349 fatal error at run time. If umask(2) is not implemented and you are
5350 not trying to restrict access for yourself, returns C<undef>.
5352 Remember that a umask is a number, usually given in octal; it is I<not> a
5353 string of octal digits. See also L</oct>, if all you have is a string.
5359 Undefines the value of EXPR, which must be an lvalue. Use only on a
5360 scalar value, an array (using C<@>), a hash (using C<%>), a subroutine
5361 (using C<&>), or a typeglob (using <*>). (Saying C<undef $hash{$key}>
5362 will probably not do what you expect on most predefined variables or
5363 DBM list values, so don't do that; see L<delete>.) Always returns the
5364 undefined value. You can omit the EXPR, in which case nothing is
5365 undefined, but you still get an undefined value that you could, for
5366 instance, return from a subroutine, assign to a variable or pass as a
5367 parameter. Examples:
5370 undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'};
5374 undef *xyz; # destroys $xyz, @xyz, %xyz, &xyz, etc.
5375 return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it;
5376 select undef, undef, undef, 0.25;
5377 ($a, $b, undef, $c) = &foo; # Ignore third value returned
5379 Note that this is a unary operator, not a list operator.
5385 Deletes a list of files. Returns the number of files successfully
5388 $cnt = unlink 'a', 'b', 'c';
5392 Note: C<unlink> will not delete directories unless you are superuser and
5393 the B<-U> flag is supplied to Perl. Even if these conditions are
5394 met, be warned that unlinking a directory can inflict damage on your
5395 filesystem. Use C<rmdir> instead.
5397 If LIST is omitted, uses C<$_>.
5399 =item unpack TEMPLATE,EXPR
5401 C<unpack> does the reverse of C<pack>: it takes a string
5402 and expands it out into a list of values.
5403 (In scalar context, it returns merely the first value produced.)
5405 The string is broken into chunks described by the TEMPLATE. Each chunk
5406 is converted separately to a value. Typically, either the string is a result
5407 of C<pack>, or the bytes of the string represent a C structure of some
5410 The TEMPLATE has the same format as in the C<pack> function.
5411 Here's a subroutine that does substring:
5414 my($what,$where,$howmuch) = @_;
5415 unpack("x$where a$howmuch", $what);
5420 sub ordinal { unpack("c",$_[0]); } # same as ord()
5422 In addition to fields allowed in pack(), you may prefix a field with
5423 a %<number> to indicate that
5424 you want a <number>-bit checksum of the items instead of the items
5425 themselves. Default is a 16-bit checksum. Checksum is calculated by
5426 summing numeric values of expanded values (for string fields the sum of
5427 C<ord($char)> is taken, for bit fields the sum of zeroes and ones).
5429 For example, the following
5430 computes the same number as the System V sum program:
5434 unpack("%32C*",<>) % 65535;
5437 The following efficiently counts the number of set bits in a bit vector:
5439 $setbits = unpack("%32b*", $selectmask);
5441 The C<p> and C<P> formats should be used with care. Since Perl
5442 has no way of checking whether the value passed to C<unpack()>
5443 corresponds to a valid memory location, passing a pointer value that's
5444 not known to be valid is likely to have disastrous consequences.
5446 If the repeat count of a field is larger than what the remainder of
5447 the input string allows, repeat count is decreased. If the input string
5448 is longer than one described by the TEMPLATE, the rest is ignored.
5450 See L</pack> for more examples and notes.
5452 =item untie VARIABLE
5454 Breaks the binding between a variable and a package. (See C<tie>.)
5456 =item unshift ARRAY,LIST
5458 Does the opposite of a C<shift>. Or the opposite of a C<push>,
5459 depending on how you look at it. Prepends list to the front of the
5460 array, and returns the new number of elements in the array.
5462 unshift(ARGV, '-e') unless $ARGV[0] =~ /^-/;
5464 Note the LIST is prepended whole, not one element at a time, so the
5465 prepended elements stay in the same order. Use C<reverse> to do the
5468 =item use Module VERSION LIST
5470 =item use Module VERSION
5472 =item use Module LIST
5478 Imports some semantics into the current package from the named module,
5479 generally by aliasing certain subroutine or variable names into your
5480 package. It is exactly equivalent to
5482 BEGIN { require Module; import Module LIST; }
5484 except that Module I<must> be a bareword.
5486 VERSION, which can be specified as a literal of the form v5.6.1, demands
5487 that the current version of Perl (C<$^V> or $PERL_VERSION) be at least
5488 as recent as that version. (For compatibility with older versions of Perl,
5489 a numeric literal will also be interpreted as VERSION.) If the version
5490 of the running Perl interpreter is less than VERSION, then an error
5491 message is printed and Perl exits immediately without attempting to
5492 parse the rest of the file. Compare with L</require>, which can do a
5493 similar check at run time.
5495 use v5.6.1; # compile time version check
5497 use 5.005_03; # float version allowed for compatibility
5499 This is often useful if you need to check the current Perl version before
5500 C<use>ing library modules that have changed in incompatible ways from
5501 older versions of Perl. (We try not to do this more than we have to.)
5503 The C<BEGIN> forces the C<require> and C<import> to happen at compile time. The
5504 C<require> makes sure the module is loaded into memory if it hasn't been
5505 yet. The C<import> is not a builtin--it's just an ordinary static method
5506 call into the C<Module> package to tell the module to import the list of
5507 features back into the current package. The module can implement its
5508 C<import> method any way it likes, though most modules just choose to
5509 derive their C<import> method via inheritance from the C<Exporter> class that
5510 is defined in the C<Exporter> module. See L<Exporter>. If no C<import>
5511 method can be found then the call is skipped.
5513 If you do not want to call the package's C<import> method (for instance,
5514 to stop your namespace from being altered), explicitly supply the empty list:
5518 That is exactly equivalent to
5520 BEGIN { require Module }
5522 If the VERSION argument is present between Module and LIST, then the
5523 C<use> will call the VERSION method in class Module with the given
5524 version as an argument. The default VERSION method, inherited from
5525 the UNIVERSAL class, croaks if the given version is larger than the
5526 value of the variable C<$Module::VERSION>.
5528 Again, there is a distinction between omitting LIST (C<import> called
5529 with no arguments) and an explicit empty LIST C<()> (C<import> not
5530 called). Note that there is no comma after VERSION!
5532 Because this is a wide-open interface, pragmas (compiler directives)
5533 are also implemented this way. Currently implemented pragmas are:
5538 use sigtrap qw(SEGV BUS);
5539 use strict qw(subs vars refs);
5540 use subs qw(afunc blurfl);
5541 use warnings qw(all);
5543 Some of these pseudo-modules import semantics into the current
5544 block scope (like C<strict> or C<integer>, unlike ordinary modules,
5545 which import symbols into the current package (which are effective
5546 through the end of the file).
5548 There's a corresponding C<no> command that unimports meanings imported
5549 by C<use>, i.e., it calls C<unimport Module LIST> instead of C<import>.
5555 If no C<unimport> method can be found the call fails with a fatal error.
5557 See L<perlmodlib> for a list of standard modules and pragmas. See L<perlrun>
5558 for the C<-M> and C<-m> command-line options to perl that give C<use>
5559 functionality from the command-line.
5563 Changes the access and modification times on each file of a list of
5564 files. The first two elements of the list must be the NUMERICAL access
5565 and modification times, in that order. Returns the number of files
5566 successfully changed. The inode change time of each file is set
5567 to the current time. This code has the same effect as the C<touch>
5568 command if the files already exist:
5572 utime $now, $now, @ARGV;
5576 Returns a list consisting of all the values of the named hash. (In a
5577 scalar context, returns the number of values.) The values are
5578 returned in an apparently random order. The actual random order is
5579 subject to change in future versions of perl, but it is guaranteed to
5580 be the same order as either the C<keys> or C<each> function would
5581 produce on the same (unmodified) hash.
5583 Note that the values are not copied, which means modifying them will
5584 modify the contents of the hash:
5586 for (values %hash) { s/foo/bar/g } # modifies %hash values
5587 for (@hash{keys %hash}) { s/foo/bar/g } # same
5589 As a side effect, calling values() resets the HASH's internal iterator.
5590 See also C<keys>, C<each>, and C<sort>.
5592 =item vec EXPR,OFFSET,BITS
5594 Treats the string in EXPR as a bit vector made up of elements of
5595 width BITS, and returns the value of the element specified by OFFSET
5596 as an unsigned integer. BITS therefore specifies the number of bits
5597 that are reserved for each element in the bit vector. This must
5598 be a power of two from 1 to 32 (or 64, if your platform supports
5601 If BITS is 8, "elements" coincide with bytes of the input string.
5603 If BITS is 16 or more, bytes of the input string are grouped into chunks
5604 of size BITS/8, and each group is converted to a number as with
5605 pack()/unpack() with big-endian formats C<n>/C<N> (and analogously
5606 for BITS==64). See L<"pack"> for details.
5608 If bits is 4 or less, the string is broken into bytes, then the bits
5609 of each byte are broken into 8/BITS groups. Bits of a byte are
5610 numbered in a little-endian-ish way, as in C<0x01>, C<0x02>,
5611 C<0x04>, C<0x08>, C<0x10>, C<0x20>, C<0x40>, C<0x80>. For example,
5612 breaking the single input byte C<chr(0x36)> into two groups gives a list
5613 C<(0x6, 0x3)>; breaking it into 4 groups gives C<(0x2, 0x1, 0x3, 0x0)>.
5615 C<vec> may also be assigned to, in which case parentheses are needed
5616 to give the expression the correct precedence as in
5618 vec($image, $max_x * $x + $y, 8) = 3;
5620 If the selected element is outside the string, the value 0 is returned.
5621 If an element off the end of the string is written to, Perl will first
5622 extend the string with sufficiently many zero bytes. It is an error
5623 to try to write off the beginning of the string (i.e. negative OFFSET).
5625 The string should not contain any character with the value > 255 (which
5626 can only happen if you're using UTF8 encoding). If it does, it will be
5627 treated as something which is not UTF8 encoded. When the C<vec> was
5628 assigned to, other parts of your program will also no longer consider the
5629 string to be UTF8 encoded. In other words, if you do have such characters
5630 in your string, vec() will operate on the actual byte string, and not the
5631 conceptual character string.
5633 Strings created with C<vec> can also be manipulated with the logical
5634 operators C<|>, C<&>, C<^>, and C<~>. These operators will assume a bit
5635 vector operation is desired when both operands are strings.
5636 See L<perlop/"Bitwise String Operators">.
5638 The following code will build up an ASCII string saying C<'PerlPerlPerl'>.
5639 The comments show the string after each step. Note that this code works
5640 in the same way on big-endian or little-endian machines.
5643 vec($foo, 0, 32) = 0x5065726C; # 'Perl'
5645 # $foo eq "Perl" eq "\x50\x65\x72\x6C", 32 bits
5646 print vec($foo, 0, 8); # prints 80 == 0x50 == ord('P')
5648 vec($foo, 2, 16) = 0x5065; # 'PerlPe'
5649 vec($foo, 3, 16) = 0x726C; # 'PerlPerl'
5650 vec($foo, 8, 8) = 0x50; # 'PerlPerlP'
5651 vec($foo, 9, 8) = 0x65; # 'PerlPerlPe'
5652 vec($foo, 20, 4) = 2; # 'PerlPerlPe' . "\x02"
5653 vec($foo, 21, 4) = 7; # 'PerlPerlPer'
5655 vec($foo, 45, 2) = 3; # 'PerlPerlPer' . "\x0c"
5656 vec($foo, 93, 1) = 1; # 'PerlPerlPer' . "\x2c"
5657 vec($foo, 94, 1) = 1; # 'PerlPerlPerl'
5660 To transform a bit vector into a string or list of 0's and 1's, use these:
5662 $bits = unpack("b*", $vector);
5663 @bits = split(//, unpack("b*", $vector));
5665 If you know the exact length in bits, it can be used in place of the C<*>.
5667 Here is an example to illustrate how the bits actually fall in place:
5673 unpack("V",$_) 01234567890123456789012345678901
5674 ------------------------------------------------------------------
5679 for ($shift=0; $shift < $width; ++$shift) {
5680 for ($off=0; $off < 32/$width; ++$off) {
5681 $str = pack("B*", "0"x32);
5682 $bits = (1<<$shift);
5683 vec($str, $off, $width) = $bits;
5684 $res = unpack("b*",$str);
5685 $val = unpack("V", $str);
5692 vec($_,@#,@#) = @<< == @######### @>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
5693 $off, $width, $bits, $val, $res
5697 Regardless of the machine architecture on which it is run, the above
5698 example should print the following table:
5701 unpack("V",$_) 01234567890123456789012345678901
5702 ------------------------------------------------------------------
5703 vec($_, 0, 1) = 1 == 1 10000000000000000000000000000000
5704 vec($_, 1, 1) = 1 == 2 01000000000000000000000000000000
5705 vec($_, 2, 1) = 1 == 4 00100000000000000000000000000000
5706 vec($_, 3, 1) = 1 == 8 00010000000000000000000000000000
5707 vec($_, 4, 1) = 1 == 16 00001000000000000000000000000000
5708 vec($_, 5, 1) = 1 == 32 00000100000000000000000000000000
5709 vec($_, 6, 1) = 1 == 64 00000010000000000000000000000000
5710 vec($_, 7, 1) = 1 == 128 00000001000000000000000000000000
5711 vec($_, 8, 1) = 1 == 256 00000000100000000000000000000000
5712 vec($_, 9, 1) = 1 == 512 00000000010000000000000000000000
5713 vec($_,10, 1) = 1 == 1024 00000000001000000000000000000000
5714 vec($_,11, 1) = 1 == 2048 00000000000100000000000000000000
5715 vec($_,12, 1) = 1 == 4096 00000000000010000000000000000000
5716 vec($_,13, 1) = 1 == 8192 00000000000001000000000000000000
5717 vec($_,14, 1) = 1 == 16384 00000000000000100000000000000000
5718 vec($_,15, 1) = 1 == 32768 00000000000000010000000000000000
5719 vec($_,16, 1) = 1 == 65536 00000000000000001000000000000000
5720 vec($_,17, 1) = 1 == 131072 00000000000000000100000000000000
5721 vec($_,18, 1) = 1 == 262144 00000000000000000010000000000000
5722 vec($_,19, 1) = 1 == 524288 00000000000000000001000000000000
5723 vec($_,20, 1) = 1 == 1048576 00000000000000000000100000000000
5724 vec($_,21, 1) = 1 == 2097152 00000000000000000000010000000000
5725 vec($_,22, 1) = 1 == 4194304 00000000000000000000001000000000
5726 vec($_,23, 1) = 1 == 8388608 00000000000000000000000100000000
5727 vec($_,24, 1) = 1 == 16777216 00000000000000000000000010000000
5728 vec($_,25, 1) = 1 == 33554432 00000000000000000000000001000000
5729 vec($_,26, 1) = 1 == 67108864 00000000000000000000000000100000
5730 vec($_,27, 1) = 1 == 134217728 00000000000000000000000000010000
5731 vec($_,28, 1) = 1 == 268435456 00000000000000000000000000001000
5732 vec($_,29, 1) = 1 == 536870912 00000000000000000000000000000100
5733 vec($_,30, 1) = 1 == 1073741824 00000000000000000000000000000010
5734 vec($_,31, 1) = 1 == 2147483648 00000000000000000000000000000001
5735 vec($_, 0, 2) = 1 == 1 10000000000000000000000000000000
5736 vec($_, 1, 2) = 1 == 4 00100000000000000000000000000000
5737 vec($_, 2, 2) = 1 == 16 00001000000000000000000000000000
5738 vec($_, 3, 2) = 1 == 64 00000010000000000000000000000000
5739 vec($_, 4, 2) = 1 == 256 00000000100000000000000000000000
5740 vec($_, 5, 2) = 1 == 1024 00000000001000000000000000000000
5741 vec($_, 6, 2) = 1 == 4096 00000000000010000000000000000000
5742 vec($_, 7, 2) = 1 == 16384 00000000000000100000000000000000
5743 vec($_, 8, 2) = 1 == 65536 00000000000000001000000000000000
5744 vec($_, 9, 2) = 1 == 262144 00000000000000000010000000000000
5745 vec($_,10, 2) = 1 == 1048576 00000000000000000000100000000000
5746 vec($_,11, 2) = 1 == 4194304 00000000000000000000001000000000
5747 vec($_,12, 2) = 1 == 16777216 00000000000000000000000010000000
5748 vec($_,13, 2) = 1 == 67108864 00000000000000000000000000100000
5749 vec($_,14, 2) = 1 == 268435456 00000000000000000000000000001000
5750 vec($_,15, 2) = 1 == 1073741824 00000000000000000000000000000010
5751 vec($_, 0, 2) = 2 == 2 01000000000000000000000000000000
5752 vec($_, 1, 2) = 2 == 8 00010000000000000000000000000000
5753 vec($_, 2, 2) = 2 == 32 00000100000000000000000000000000
5754 vec($_, 3, 2) = 2 == 128 00000001000000000000000000000000
5755 vec($_, 4, 2) = 2 == 512 00000000010000000000000000000000
5756 vec($_, 5, 2) = 2 == 2048 00000000000100000000000000000000
5757 vec($_, 6, 2) = 2 == 8192 00000000000001000000000000000000
5758 vec($_, 7, 2) = 2 == 32768 00000000000000010000000000000000
5759 vec($_, 8, 2) = 2 == 131072 00000000000000000100000000000000
5760 vec($_, 9, 2) = 2 == 524288 00000000000000000001000000000000
5761 vec($_,10, 2) = 2 == 2097152 00000000000000000000010000000000
5762 vec($_,11, 2) = 2 == 8388608 00000000000000000000000100000000
5763 vec($_,12, 2) = 2 == 33554432 00000000000000000000000001000000
5764 vec($_,13, 2) = 2 == 134217728 00000000000000000000000000010000
5765 vec($_,14, 2) = 2 == 536870912 00000000000000000000000000000100
5766 vec($_,15, 2) = 2 == 2147483648 00000000000000000000000000000001
5767 vec($_, 0, 4) = 1 == 1 10000000000000000000000000000000
5768 vec($_, 1, 4) = 1 == 16 00001000000000000000000000000000
5769 vec($_, 2, 4) = 1 == 256 00000000100000000000000000000000
5770 vec($_, 3, 4) = 1 == 4096 00000000000010000000000000000000
5771 vec($_, 4, 4) = 1 == 65536 00000000000000001000000000000000
5772 vec($_, 5, 4) = 1 == 1048576 00000000000000000000100000000000
5773 vec($_, 6, 4) = 1 == 16777216 00000000000000000000000010000000
5774 vec($_, 7, 4) = 1 == 268435456 00000000000000000000000000001000
5775 vec($_, 0, 4) = 2 == 2 01000000000000000000000000000000
5776 vec($_, 1, 4) = 2 == 32 00000100000000000000000000000000
5777 vec($_, 2, 4) = 2 == 512 00000000010000000000000000000000
5778 vec($_, 3, 4) = 2 == 8192 00000000000001000000000000000000
5779 vec($_, 4, 4) = 2 == 131072 00000000000000000100000000000000
5780 vec($_, 5, 4) = 2 == 2097152 00000000000000000000010000000000
5781 vec($_, 6, 4) = 2 == 33554432 00000000000000000000000001000000
5782 vec($_, 7, 4) = 2 == 536870912 00000000000000000000000000000100
5783 vec($_, 0, 4) = 4 == 4 00100000000000000000000000000000
5784 vec($_, 1, 4) = 4 == 64 00000010000000000000000000000000
5785 vec($_, 2, 4) = 4 == 1024 00000000001000000000000000000000
5786 vec($_, 3, 4) = 4 == 16384 00000000000000100000000000000000
5787 vec($_, 4, 4) = 4 == 262144 00000000000000000010000000000000
5788 vec($_, 5, 4) = 4 == 4194304 00000000000000000000001000000000
5789 vec($_, 6, 4) = 4 == 67108864 00000000000000000000000000100000
5790 vec($_, 7, 4) = 4 == 1073741824 00000000000000000000000000000010
5791 vec($_, 0, 4) = 8 == 8 00010000000000000000000000000000
5792 vec($_, 1, 4) = 8 == 128 00000001000000000000000000000000
5793 vec($_, 2, 4) = 8 == 2048 00000000000100000000000000000000
5794 vec($_, 3, 4) = 8 == 32768 00000000000000010000000000000000
5795 vec($_, 4, 4) = 8 == 524288 00000000000000000001000000000000
5796 vec($_, 5, 4) = 8 == 8388608 00000000000000000000000100000000
5797 vec($_, 6, 4) = 8 == 134217728 00000000000000000000000000010000
5798 vec($_, 7, 4) = 8 == 2147483648 00000000000000000000000000000001
5799 vec($_, 0, 8) = 1 == 1 10000000000000000000000000000000
5800 vec($_, 1, 8) = 1 == 256 00000000100000000000000000000000
5801 vec($_, 2, 8) = 1 == 65536 00000000000000001000000000000000
5802 vec($_, 3, 8) = 1 == 16777216 00000000000000000000000010000000
5803 vec($_, 0, 8) = 2 == 2 01000000000000000000000000000000
5804 vec($_, 1, 8) = 2 == 512 00000000010000000000000000000000
5805 vec($_, 2, 8) = 2 == 131072 00000000000000000100000000000000
5806 vec($_, 3, 8) = 2 == 33554432 00000000000000000000000001000000
5807 vec($_, 0, 8) = 4 == 4 00100000000000000000000000000000
5808 vec($_, 1, 8) = 4 == 1024 00000000001000000000000000000000
5809 vec($_, 2, 8) = 4 == 262144 00000000000000000010000000000000
5810 vec($_, 3, 8) = 4 == 67108864 00000000000000000000000000100000
5811 vec($_, 0, 8) = 8 == 8 00010000000000000000000000000000
5812 vec($_, 1, 8) = 8 == 2048 00000000000100000000000000000000
5813 vec($_, 2, 8) = 8 == 524288 00000000000000000001000000000000
5814 vec($_, 3, 8) = 8 == 134217728 00000000000000000000000000010000
5815 vec($_, 0, 8) = 16 == 16 00001000000000000000000000000000
5816 vec($_, 1, 8) = 16 == 4096 00000000000010000000000000000000
5817 vec($_, 2, 8) = 16 == 1048576 00000000000000000000100000000000
5818 vec($_, 3, 8) = 16 == 268435456 00000000000000000000000000001000
5819 vec($_, 0, 8) = 32 == 32 00000100000000000000000000000000
5820 vec($_, 1, 8) = 32 == 8192 00000000000001000000000000000000
5821 vec($_, 2, 8) = 32 == 2097152 00000000000000000000010000000000
5822 vec($_, 3, 8) = 32 == 536870912 00000000000000000000000000000100
5823 vec($_, 0, 8) = 64 == 64 00000010000000000000000000000000
5824 vec($_, 1, 8) = 64 == 16384 00000000000000100000000000000000
5825 vec($_, 2, 8) = 64 == 4194304 00000000000000000000001000000000
5826 vec($_, 3, 8) = 64 == 1073741824 00000000000000000000000000000010
5827 vec($_, 0, 8) = 128 == 128 00000001000000000000000000000000
5828 vec($_, 1, 8) = 128 == 32768 00000000000000010000000000000000
5829 vec($_, 2, 8) = 128 == 8388608 00000000000000000000000100000000
5830 vec($_, 3, 8) = 128 == 2147483648 00000000000000000000000000000001
5834 Behaves like the wait(2) system call on your system: it waits for a child
5835 process to terminate and returns the pid of the deceased process, or
5836 C<-1> if there are no child processes. The status is returned in C<$?>.
5837 Note that a return value of C<-1> could mean that child processes are
5838 being automatically reaped, as described in L<perlipc>.
5840 =item waitpid PID,FLAGS
5842 Waits for a particular child process to terminate and returns the pid of
5843 the deceased process, or C<-1> if there is no such child process. On some
5844 systems, a value of 0 indicates that there are processes still running.
5845 The status is returned in C<$?>. If you say
5847 use POSIX ":sys_wait_h";
5850 $kid = waitpid(-1,&WNOHANG);
5853 then you can do a non-blocking wait for all pending zombie processes.
5854 Non-blocking wait is available on machines supporting either the
5855 waitpid(2) or wait4(2) system calls. However, waiting for a particular
5856 pid with FLAGS of C<0> is implemented everywhere. (Perl emulates the
5857 system call by remembering the status values of processes that have
5858 exited but have not been harvested by the Perl script yet.)
5860 Note that on some systems, a return value of C<-1> could mean that child
5861 processes are being automatically reaped. See L<perlipc> for details,
5862 and for other examples.
5866 Returns true if the context of the currently executing subroutine is
5867 looking for a list value. Returns false if the context is looking
5868 for a scalar. Returns the undefined value if the context is looking
5869 for no value (void context).
5871 return unless defined wantarray; # don't bother doing more
5872 my @a = complex_calculation();
5873 return wantarray ? @a : "@a";
5875 This function should have been named wantlist() instead.
5879 Produces a message on STDERR just like C<die>, but doesn't exit or throw
5882 If LIST is empty and C<$@> already contains a value (typically from a
5883 previous eval) that value is used after appending C<"\t...caught">
5884 to C<$@>. This is useful for staying almost, but not entirely similar to
5887 If C<$@> is empty then the string C<"Warning: Something's wrong"> is used.
5889 No message is printed if there is a C<$SIG{__WARN__}> handler
5890 installed. It is the handler's responsibility to deal with the message
5891 as it sees fit (like, for instance, converting it into a C<die>). Most
5892 handlers must therefore make arrangements to actually display the
5893 warnings that they are not prepared to deal with, by calling C<warn>
5894 again in the handler. Note that this is quite safe and will not
5895 produce an endless loop, since C<__WARN__> hooks are not called from
5898 You will find this behavior is slightly different from that of
5899 C<$SIG{__DIE__}> handlers (which don't suppress the error text, but can
5900 instead call C<die> again to change it).
5902 Using a C<__WARN__> handler provides a powerful way to silence all
5903 warnings (even the so-called mandatory ones). An example:
5905 # wipe out *all* compile-time warnings
5906 BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } }
5908 my $foo = 20; # no warning about duplicate my $foo,
5909 # but hey, you asked for it!
5910 # no compile-time or run-time warnings before here
5913 # run-time warnings enabled after here
5914 warn "\$foo is alive and $foo!"; # does show up
5916 See L<perlvar> for details on setting C<%SIG> entries, and for more
5917 examples. See the Carp module for other kinds of warnings using its
5918 carp() and cluck() functions.
5920 =item write FILEHANDLE
5926 Writes a formatted record (possibly multi-line) to the specified FILEHANDLE,
5927 using the format associated with that file. By default the format for
5928 a file is the one having the same name as the filehandle, but the
5929 format for the current output channel (see the C<select> function) may be set
5930 explicitly by assigning the name of the format to the C<$~> variable.
5932 Top of form processing is handled automatically: if there is
5933 insufficient room on the current page for the formatted record, the
5934 page is advanced by writing a form feed, a special top-of-page format
5935 is used to format the new page header, and then the record is written.
5936 By default the top-of-page format is the name of the filehandle with
5937 "_TOP" appended, but it may be dynamically set to the format of your
5938 choice by assigning the name to the C<$^> variable while the filehandle is
5939 selected. The number of lines remaining on the current page is in
5940 variable C<$->, which can be set to C<0> to force a new page.
5942 If FILEHANDLE is unspecified, output goes to the current default output
5943 channel, which starts out as STDOUT but may be changed by the
5944 C<select> operator. If the FILEHANDLE is an EXPR, then the expression
5945 is evaluated and the resulting string is used to look up the name of
5946 the FILEHANDLE at run time. For more on formats, see L<perlform>.
5948 Note that write is I<not> the opposite of C<read>. Unfortunately.
5952 The transliteration operator. Same as C<tr///>. See L<perlop>.