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<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<socket>, C<socketpair>, C<stat>, C<symlink>, C<syscall>,
238 C<sysopen>, C<system>, C<times>, C<truncate>, C<umask>, C<unlink>,
239 C<utime>, C<wait>, C<waitpid>
241 For more information about the portability of these functions, see
242 L<perlport> and other available platform-specific documentation.
244 =head2 Alphabetical Listing of Perl Functions
248 =item I<-X> FILEHANDLE
254 A file test, where X is one of the letters listed below. This unary
255 operator takes one argument, either a filename or a filehandle, and
256 tests the associated file to see if something is true about it. If the
257 argument is omitted, tests C<$_>, except for C<-t>, which tests STDIN.
258 Unless otherwise documented, it returns C<1> for true and C<''> for false, or
259 the undefined value if the file doesn't exist. Despite the funny
260 names, precedence is the same as any other named unary operator, and
261 the argument may be parenthesized like any other unary operator. The
262 operator may be any of:
263 X<-r>X<-w>X<-x>X<-o>X<-R>X<-W>X<-X>X<-O>X<-e>X<-z>X<-s>X<-f>X<-d>X<-l>X<-p>
264 X<-S>X<-b>X<-c>X<-t>X<-u>X<-g>X<-k>X<-T>X<-B>X<-M>X<-A>X<-C>
266 -r File is readable by effective uid/gid.
267 -w File is writable by effective uid/gid.
268 -x File is executable by effective uid/gid.
269 -o File is owned by effective uid.
271 -R File is readable by real uid/gid.
272 -W File is writable by real uid/gid.
273 -X File is executable by real uid/gid.
274 -O File is owned by real uid.
277 -z File has zero size.
278 -s File has nonzero size (returns size).
280 -f File is a plain file.
281 -d File is a directory.
282 -l File is a symbolic link.
283 -p File is a named pipe (FIFO), or Filehandle is a pipe.
285 -b File is a block special file.
286 -c File is a character special file.
287 -t Filehandle is opened to a tty.
289 -u File has setuid bit set.
290 -g File has setgid bit set.
291 -k File has sticky bit set.
293 -T File is an ASCII text file.
294 -B File is a "binary" file (opposite of -T).
296 -M Age of file in days when script started.
297 -A Same for access time.
298 -C Same for inode change time.
304 next unless -f $_; # ignore specials
308 The interpretation of the file permission operators C<-r>, C<-R>,
309 C<-w>, C<-W>, C<-x>, and C<-X> is by default based solely on the mode
310 of the file and the uids and gids of the user. There may be other
311 reasons you can't actually read, write, or execute the file. Such
312 reasons may be for example network filesystem access controls, ACLs
313 (access control lists), read-only filesystems, and unrecognized
316 Also note that, for the superuser on the local filesystems, the C<-r>,
317 C<-R>, C<-w>, and C<-W> tests always return 1, and C<-x> and C<-X> return 1
318 if any execute bit is set in the mode. Scripts run by the superuser
319 may thus need to do a stat() to determine the actual mode of the file,
320 or temporarily set their effective uid to something else.
322 If you are using ACLs, there is a pragma called C<filetest> that may
323 produce more accurate results than the bare stat() mode bits.
324 When under the C<use filetest 'access'> the above-mentioned filetests
325 will test whether the permission can (not) be granted using the
326 access() family of system calls. Also note that the C<-x> and C<-X> may
327 under this pragma return true even if there are no execute permission
328 bits set (nor any extra execute permission ACLs). This strangeness is
329 due to the underlying system calls' definitions. Read the
330 documentation for the C<filetest> pragma for more information.
332 Note that C<-s/a/b/> does not do a negated substitution. Saying
333 C<-exp($foo)> still works as expected, however--only single letters
334 following a minus are interpreted as file tests.
336 The C<-T> and C<-B> switches work as follows. The first block or so of the
337 file is examined for odd characters such as strange control codes or
338 characters with the high bit set. If too many strange characters (>30%)
339 are found, it's a C<-B> file, otherwise it's a C<-T> file. Also, any file
340 containing null in the first block is considered a binary file. If C<-T>
341 or C<-B> is used on a filehandle, the current stdio buffer is examined
342 rather than the first block. Both C<-T> and C<-B> return true on a null
343 file, or a file at EOF when testing a filehandle. Because you have to
344 read a file to do the C<-T> test, on most occasions you want to use a C<-f>
345 against the file first, as in C<next unless -f $file && -T $file>.
347 If any of the file tests (or either the C<stat> or C<lstat> operators) are given
348 the special filehandle consisting of a solitary underline, then the stat
349 structure of the previous file test (or stat operator) is used, saving
350 a system call. (This doesn't work with C<-t>, and you need to remember
351 that lstat() and C<-l> will leave values in the stat structure for the
352 symbolic link, not the real file.) Example:
354 print "Can do.\n" if -r $a || -w _ || -x _;
357 print "Readable\n" if -r _;
358 print "Writable\n" if -w _;
359 print "Executable\n" if -x _;
360 print "Setuid\n" if -u _;
361 print "Setgid\n" if -g _;
362 print "Sticky\n" if -k _;
363 print "Text\n" if -T _;
364 print "Binary\n" if -B _;
370 Returns the absolute value of its argument.
371 If VALUE is omitted, uses C<$_>.
373 =item accept NEWSOCKET,GENERICSOCKET
375 Accepts an incoming socket connect, just as the accept(2) system call
376 does. Returns the packed address if it succeeded, false otherwise.
377 See the example in L<perlipc/"Sockets: Client/Server Communication">.
379 On systems that support a close-on-exec flag on files, the flag will
380 be set for the newly opened file descriptor, as determined by the
381 value of $^F. See L<perlvar/$^F>.
387 Arranges to have a SIGALRM delivered to this process after the
388 specified number of seconds have elapsed. If SECONDS is not specified,
389 the value stored in C<$_> is used. (On some machines,
390 unfortunately, the elapsed time may be up to one second less than you
391 specified because of how seconds are counted.) Only one timer may be
392 counting at once. Each call disables the previous timer, and an
393 argument of C<0> may be supplied to cancel the previous timer without
394 starting a new one. The returned value is the amount of time remaining
395 on the previous timer.
397 For delays of finer granularity than one second, you may use Perl's
398 four-argument version of select() leaving the first three arguments
399 undefined, or you might be able to use the C<syscall> interface to
400 access setitimer(2) if your system supports it. The Time::HiRes module
401 from CPAN may also prove useful.
403 It is usually a mistake to intermix C<alarm> and C<sleep> calls.
404 (C<sleep> may be internally implemented in your system with C<alarm>)
406 If you want to use C<alarm> to time out a system call you need to use an
407 C<eval>/C<die> pair. You can't rely on the alarm causing the system call to
408 fail with C<$!> set to C<EINTR> because Perl sets up signal handlers to
409 restart system calls on some systems. Using C<eval>/C<die> always works,
410 modulo the caveats given in L<perlipc/"Signals">.
413 local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
415 $nread = sysread SOCKET, $buffer, $size;
419 die unless $@ eq "alarm\n"; # propagate unexpected errors
428 Returns the arctangent of Y/X in the range -PI to PI.
430 For the tangent operation, you may use the C<Math::Trig::tan>
431 function, or use the familiar relation:
433 sub tan { sin($_[0]) / cos($_[0]) }
435 =item bind SOCKET,NAME
437 Binds a network address to a socket, just as the bind system call
438 does. Returns true if it succeeded, false otherwise. NAME should be a
439 packed address of the appropriate type for the socket. See the examples in
440 L<perlipc/"Sockets: Client/Server Communication">.
442 =item binmode FILEHANDLE, DISCIPLINE
444 =item binmode FILEHANDLE
446 Arranges for FILEHANDLE to be read or written in "binary" or "text" mode
447 on systems where the run-time libraries distinguish between binary and
448 text files. If FILEHANDLE is an expression, the value is taken as the
449 name of the filehandle. DISCIPLINE can be either of C<":raw"> for
450 binary mode or C<":crlf"> for "text" mode. If the DISCIPLINE is
451 omitted, it defaults to C<":raw">.
453 binmode() should be called after open() but before any I/O is done on
456 On many systems binmode() currently has no effect, but in future, it
457 will be extended to support user-defined input and output disciplines.
458 On some systems binmode() is necessary when you're not working with a
459 text file. For the sake of portability it is a good idea to always use
460 it when appropriate, and to never use it when it isn't appropriate.
462 In other words: Regardless of platform, use binmode() on binary
463 files, and do not use binmode() on text files.
465 The C<open> pragma can be used to establish default disciplines.
468 The operating system, device drivers, C libraries, and Perl run-time
469 system all work together to let the programmer treat a single
470 character (C<\n>) as the line terminator, irrespective of the external
471 representation. On many operating systems, the native text file
472 representation matches the internal representation, but on some
473 platforms the external representation of C<\n> is made up of more than
476 Mac OS and all variants of Unix use a single character to end each line
477 in the external representation of text (even though that single
478 character is not necessarily the same across these platforms).
479 Consequently binmode() has no effect on these operating systems. In
480 other systems like VMS, MS-DOS and the various flavors of MS-Windows
481 your program sees a C<\n> as a simple C<\cJ>, but what's stored in text
482 files are the two characters C<\cM\cJ>. That means that, if you don't
483 use binmode() on these systems, C<\cM\cJ> sequences on disk will be
484 converted to C<\n> on input, and any C<\n> in your program will be
485 converted back to C<\cM\cJ> on output. This is what you want for text
486 files, but it can be disastrous for binary files.
488 Another consequence of using binmode() (on some systems) is that
489 special end-of-file markers will be seen as part of the data stream.
490 For systems from the Microsoft family this means that if your binary
491 data contains C<\cZ>, the I/O subsystem will regard it as the end of
492 the file, unless you use binmode().
494 binmode() is not only important for readline() and print() operations,
495 but also when using read(), seek(), sysread(), syswrite() and tell()
496 (see L<perlport> for more details). See the C<$/> and C<$\> variables
497 in L<perlvar> for how to manually set your input and output
498 line-termination sequences.
500 =item bless REF,CLASSNAME
504 This function tells the thingy referenced by REF that it is now an object
505 in the CLASSNAME package. If CLASSNAME is omitted, the current package
506 is used. Because a C<bless> is often the last thing in a constructor,
507 it returns the reference for convenience. Always use the two-argument
508 version if the function doing the blessing might be inherited by a
509 derived class. See L<perltoot> and L<perlobj> for more about the blessing
510 (and blessings) of objects.
512 Consider always blessing objects in CLASSNAMEs that are mixed case.
513 Namespaces with all lowercase names are considered reserved for
514 Perl pragmata. Builtin types have all uppercase names, so to prevent
515 confusion, you may wish to avoid such package names as well. Make sure
516 that CLASSNAME is a true value.
518 See L<perlmod/"Perl Modules">.
524 Returns the context of the current subroutine call. In scalar context,
525 returns the caller's package name if there is a caller, that is, if
526 we're in a subroutine or C<eval> or C<require>, and the undefined value
527 otherwise. In list context, returns
529 ($package, $filename, $line) = caller;
531 With EXPR, it returns some extra information that the debugger uses to
532 print a stack trace. The value of EXPR indicates how many call frames
533 to go back before the current one.
535 ($package, $filename, $line, $subroutine, $hasargs,
536 $wantarray, $evaltext, $is_require, $hints, $bitmask) = caller($i);
538 Here $subroutine may be C<(eval)> if the frame is not a subroutine
539 call, but an C<eval>. In such a case additional elements $evaltext and
540 C<$is_require> are set: C<$is_require> is true if the frame is created by a
541 C<require> or C<use> statement, $evaltext contains the text of the
542 C<eval EXPR> statement. In particular, for an C<eval BLOCK> statement,
543 $filename is C<(eval)>, but $evaltext is undefined. (Note also that
544 each C<use> statement creates a C<require> frame inside an C<eval EXPR>)
545 frame. C<$hasargs> is true if a new instance of C<@_> was set up for the
546 frame. C<$hints> and C<$bitmask> contain pragmatic hints that the caller
547 was compiled with. The C<$hints> and C<$bitmask> values are subject to
548 change between versions of Perl, and are not meant for external use.
550 Furthermore, when called from within the DB package, caller returns more
551 detailed information: it sets the list variable C<@DB::args> to be the
552 arguments with which the subroutine was invoked.
554 Be aware that the optimizer might have optimized call frames away before
555 C<caller> had a chance to get the information. That means that C<caller(N)>
556 might not return information about the call frame you expect it do, for
557 C<< N > 1 >>. In particular, C<@DB::args> might have information from the
558 previous time C<caller> was called.
562 Changes the working directory to EXPR, if possible. If EXPR is omitted,
563 changes to the directory specified by C<$ENV{HOME}>, if set; if not,
564 changes to the directory specified by C<$ENV{LOGDIR}>. If neither is
565 set, C<chdir> does nothing. It returns true upon success, false
566 otherwise. See the example under C<die>.
570 Changes the permissions of a list of files. The first element of the
571 list must be the numerical mode, which should probably be an octal
572 number, and which definitely should I<not> a string of octal digits:
573 C<0644> is okay, C<'0644'> is not. Returns the number of files
574 successfully changed. See also L</oct>, if all you have is a string.
576 $cnt = chmod 0755, 'foo', 'bar';
577 chmod 0755, @executables;
578 $mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to
580 $mode = '0644'; chmod oct($mode), 'foo'; # this is better
581 $mode = 0644; chmod $mode, 'foo'; # this is best
583 You can also import the symbolic C<S_I*> constants from the Fcntl
588 chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
589 # This is identical to the chmod 0755 of the above example.
597 This safer version of L</chop> removes any trailing string
598 that corresponds to the current value of C<$/> (also known as
599 $INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
600 number of characters removed from all its arguments. It's often used to
601 remove the newline from the end of an input record when you're worried
602 that the final record may be missing its newline. When in paragraph
603 mode (C<$/ = "">), it removes all trailing newlines from the string.
604 When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is
605 a reference to an integer or the like, see L<perlvar>) chomp() won't
607 If VARIABLE is omitted, it chomps C<$_>. Example:
610 chomp; # avoid \n on last field
615 You can actually chomp anything that's an lvalue, including an assignment:
618 chomp($answer = <STDIN>);
620 If you chomp a list, each element is chomped, and the total number of
621 characters removed is returned.
629 Chops off the last character of a string and returns the character
630 chopped. It's used primarily to remove the newline from the end of an
631 input record, but is much more efficient than C<s/\n//> because it neither
632 scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
636 chop; # avoid \n on last field
641 You can actually chop anything that's an lvalue, including an assignment:
644 chop($answer = <STDIN>);
646 If you chop a list, each element is chopped. Only the value of the
647 last C<chop> is returned.
649 Note that C<chop> returns the last character. To return all but the last
650 character, use C<substr($string, 0, -1)>.
654 Changes the owner (and group) of a list of files. The first two
655 elements of the list must be the I<numeric> uid and gid, in that
656 order. A value of -1 in either position is interpreted by most
657 systems to leave that value unchanged. Returns the number of files
658 successfully changed.
660 $cnt = chown $uid, $gid, 'foo', 'bar';
661 chown $uid, $gid, @filenames;
663 Here's an example that looks up nonnumeric uids in the passwd file:
666 chomp($user = <STDIN>);
668 chomp($pattern = <STDIN>);
670 ($login,$pass,$uid,$gid) = getpwnam($user)
671 or die "$user not in passwd file";
673 @ary = glob($pattern); # expand filenames
674 chown $uid, $gid, @ary;
676 On most systems, you are not allowed to change the ownership of the
677 file unless you're the superuser, although you should be able to change
678 the group to any of your secondary groups. On insecure systems, these
679 restrictions may be relaxed, but this is not a portable assumption.
680 On POSIX systems, you can detect this condition this way:
682 use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
683 $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
689 Returns the character represented by that NUMBER in the character set.
690 For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
691 chr(0x263a) is a Unicode smiley face. Within the scope of C<use utf8>,
692 characters higher than 127 are encoded in Unicode; if you don't want
693 this, temporarily C<use bytes> or use C<pack("C*",...)>
695 For the reverse, use L</ord>.
696 See L<utf8> for more about Unicode.
698 If NUMBER is omitted, uses C<$_>.
700 =item chroot FILENAME
704 This function works like the system call by the same name: it makes the
705 named directory the new root directory for all further pathnames that
706 begin with a C</> by your process and all its children. (It doesn't
707 change your current working directory, which is unaffected.) For security
708 reasons, this call is restricted to the superuser. If FILENAME is
709 omitted, does a C<chroot> to C<$_>.
711 =item close FILEHANDLE
715 Closes the file or pipe associated with the file handle, returning true
716 only if stdio successfully flushes buffers and closes the system file
717 descriptor. Closes the currently selected filehandle if the argument
720 You don't have to close FILEHANDLE if you are immediately going to do
721 another C<open> on it, because C<open> will close it for you. (See
722 C<open>.) However, an explicit C<close> on an input file resets the line
723 counter (C<$.>), while the implicit close done by C<open> does not.
725 If the file handle came from a piped open C<close> will additionally
726 return false if one of the other system calls involved fails or if the
727 program exits with non-zero status. (If the only problem was that the
728 program exited non-zero C<$!> will be set to C<0>.) Closing a pipe
729 also waits for the process executing on the pipe to complete, in case you
730 want to look at the output of the pipe afterwards, and
731 implicitly puts the exit status value of that command into C<$?>.
733 Prematurely closing the read end of a pipe (i.e. before the process
734 writing to it at the other end has closed it) will result in a
735 SIGPIPE being delivered to the writer. If the other end can't
736 handle that, be sure to read all the data before closing the pipe.
740 open(OUTPUT, '|sort >foo') # pipe to sort
741 or die "Can't start sort: $!";
742 #... # print stuff to output
743 close OUTPUT # wait for sort to finish
744 or warn $! ? "Error closing sort pipe: $!"
745 : "Exit status $? from sort";
746 open(INPUT, 'foo') # get sort's results
747 or die "Can't open 'foo' for input: $!";
749 FILEHANDLE may be an expression whose value can be used as an indirect
750 filehandle, usually the real filehandle name.
752 =item closedir DIRHANDLE
754 Closes a directory opened by C<opendir> and returns the success of that
757 DIRHANDLE may be an expression whose value can be used as an indirect
758 dirhandle, usually the real dirhandle name.
760 =item connect SOCKET,NAME
762 Attempts to connect to a remote socket, just as the connect system call
763 does. Returns true if it succeeded, false otherwise. NAME should be a
764 packed address of the appropriate type for the socket. See the examples in
765 L<perlipc/"Sockets: Client/Server Communication">.
769 Actually a flow control statement rather than a function. If there is a
770 C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
771 C<foreach>), it is always executed just before the conditional is about to
772 be evaluated again, just like the third part of a C<for> loop in C. Thus
773 it can be used to increment a loop variable, even when the loop has been
774 continued via the C<next> statement (which is similar to the C C<continue>
777 C<last>, C<next>, or C<redo> may appear within a C<continue>
778 block. C<last> and C<redo> will behave as if they had been executed within
779 the main block. So will C<next>, but since it will execute a C<continue>
780 block, it may be more entertaining.
783 ### redo always comes here
786 ### next always comes here
788 # then back the top to re-check EXPR
790 ### last always comes here
792 Omitting the C<continue> section is semantically equivalent to using an
793 empty one, logically enough. In that case, C<next> goes directly back
794 to check the condition at the top of the loop.
800 Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
801 takes cosine of C<$_>.
803 For the inverse cosine operation, you may use the C<Math::Trig::acos()>
804 function, or use this relation:
806 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
808 =item crypt PLAINTEXT,SALT
810 Encrypts a string exactly like the crypt(3) function in the C library
811 (assuming that you actually have a version there that has not been
812 extirpated as a potential munition). This can prove useful for checking
813 the password file for lousy passwords, amongst other things. Only the
814 guys wearing white hats should do this.
816 Note that C<crypt> is intended to be a one-way function, much like breaking
817 eggs to make an omelette. There is no (known) corresponding decrypt
818 function. As a result, this function isn't all that useful for
819 cryptography. (For that, see your nearby CPAN mirror.)
821 When verifying an existing encrypted string you should use the encrypted
822 text as the salt (like C<crypt($plain, $crypted) eq $crypted>). This
823 allows your code to work with the standard C<crypt> and with more
824 exotic implementations. When choosing a new salt create a random two
825 character string whose characters come from the set C<[./0-9A-Za-z]>
826 (like C<join '', ('.', '/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>).
828 Here's an example that makes sure that whoever runs this program knows
831 $pwd = (getpwuid($<))[1];
835 chomp($word = <STDIN>);
839 if (crypt($word, $pwd) ne $pwd) {
845 Of course, typing in your own password to whoever asks you
848 The L<crypt> function is unsuitable for encrypting large quantities
849 of data, not least of all because you can't get the information
850 back. Look at the F<by-module/Crypt> and F<by-module/PGP> directories
851 on your favorite CPAN mirror for a slew of potentially useful
856 [This function has been largely superseded by the C<untie> function.]
858 Breaks the binding between a DBM file and a hash.
860 =item dbmopen HASH,DBNAME,MASK
862 [This function has been largely superseded by the C<tie> function.]
864 This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
865 hash. HASH is the name of the hash. (Unlike normal C<open>, the first
866 argument is I<not> a filehandle, even though it looks like one). DBNAME
867 is the name of the database (without the F<.dir> or F<.pag> extension if
868 any). If the database does not exist, it is created with protection
869 specified by MASK (as modified by the C<umask>). If your system supports
870 only the older DBM functions, you may perform only one C<dbmopen> in your
871 program. In older versions of Perl, if your system had neither DBM nor
872 ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
875 If you don't have write access to the DBM file, you can only read hash
876 variables, not set them. If you want to test whether you can write,
877 either use file tests or try setting a dummy hash entry inside an C<eval>,
878 which will trap the error.
880 Note that functions such as C<keys> and C<values> may return huge lists
881 when used on large DBM files. You may prefer to use the C<each>
882 function to iterate over large DBM files. Example:
884 # print out history file offsets
885 dbmopen(%HIST,'/usr/lib/news/history',0666);
886 while (($key,$val) = each %HIST) {
887 print $key, ' = ', unpack('L',$val), "\n";
891 See also L<AnyDBM_File> for a more general description of the pros and
892 cons of the various dbm approaches, as well as L<DB_File> for a particularly
895 You can control which DBM library you use by loading that library
896 before you call dbmopen():
899 dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
900 or die "Can't open netscape history file: $!";
906 Returns a Boolean value telling whether EXPR has a value other than
907 the undefined value C<undef>. If EXPR is not present, C<$_> will be
910 Many operations return C<undef> to indicate failure, end of file,
911 system error, uninitialized variable, and other exceptional
912 conditions. This function allows you to distinguish C<undef> from
913 other values. (A simple Boolean test will not distinguish among
914 C<undef>, zero, the empty string, and C<"0">, which are all equally
915 false.) Note that since C<undef> is a valid scalar, its presence
916 doesn't I<necessarily> indicate an exceptional condition: C<pop>
917 returns C<undef> when its argument is an empty array, I<or> when the
918 element to return happens to be C<undef>.
920 You may also use C<defined(&func)> to check whether subroutine C<&func>
921 has ever been defined. The return value is unaffected by any forward
922 declarations of C<&foo>.
924 Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
925 used to report whether memory for that aggregate has ever been
926 allocated. This behavior may disappear in future versions of Perl.
927 You should instead use a simple test for size:
929 if (@an_array) { print "has array elements\n" }
930 if (%a_hash) { print "has hash members\n" }
932 When used on a hash element, it tells you whether the value is defined,
933 not whether the key exists in the hash. Use L</exists> for the latter
938 print if defined $switch{'D'};
939 print "$val\n" while defined($val = pop(@ary));
940 die "Can't readlink $sym: $!"
941 unless defined($value = readlink $sym);
942 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
943 $debugging = 0 unless defined $debugging;
945 Note: Many folks tend to overuse C<defined>, and then are surprised to
946 discover that the number C<0> and C<""> (the zero-length string) are, in fact,
947 defined values. For example, if you say
951 The pattern match succeeds, and C<$1> is defined, despite the fact that it
952 matched "nothing". But it didn't really match nothing--rather, it
953 matched something that happened to be zero characters long. This is all
954 very above-board and honest. When a function returns an undefined value,
955 it's an admission that it couldn't give you an honest answer. So you
956 should use C<defined> only when you're questioning the integrity of what
957 you're trying to do. At other times, a simple comparison to C<0> or C<""> is
960 See also L</undef>, L</exists>, L</ref>.
964 Given an expression that specifies a hash element, array element, hash slice,
965 or array slice, deletes the specified element(s) from the hash or array.
966 In the case of an array, if the array elements happen to be at the end,
967 the size of the array will shrink to the highest element that tests
968 true for exists() (or 0 if no such element exists).
970 Returns each element so deleted or the undefined value if there was no such
971 element. Deleting from C<$ENV{}> modifies the environment. Deleting from
972 a hash tied to a DBM file deletes the entry from the DBM file. Deleting
973 from a C<tie>d hash or array may not necessarily return anything.
975 Deleting an array element effectively returns that position of the array
976 to its initial, uninitialized state. Subsequently testing for the same
977 element with exists() will return false. Note that deleting array
978 elements in the middle of an array will not shift the index of the ones
979 after them down--use splice() for that. See L</exists>.
981 The following (inefficiently) deletes all the values of %HASH and @ARRAY:
983 foreach $key (keys %HASH) {
987 foreach $index (0 .. $#ARRAY) {
988 delete $ARRAY[$index];
993 delete @HASH{keys %HASH};
995 delete @ARRAY[0 .. $#ARRAY];
997 But both of these are slower than just assigning the empty list
998 or undefining %HASH or @ARRAY:
1000 %HASH = (); # completely empty %HASH
1001 undef %HASH; # forget %HASH ever existed
1003 @ARRAY = (); # completely empty @ARRAY
1004 undef @ARRAY; # forget @ARRAY ever existed
1006 Note that the EXPR can be arbitrarily complicated as long as the final
1007 operation is a hash element, array element, hash slice, or array slice
1010 delete $ref->[$x][$y]{$key};
1011 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
1013 delete $ref->[$x][$y][$index];
1014 delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
1018 Outside an C<eval>, prints the value of LIST to C<STDERR> and
1019 exits with the current value of C<$!> (errno). If C<$!> is C<0>,
1020 exits with the value of C<<< ($? >> 8) >>> (backtick `command`
1021 status). If C<<< ($? >> 8) >>> is C<0>, exits with C<255>. Inside
1022 an C<eval(),> the error message is stuffed into C<$@> and the
1023 C<eval> is terminated with the undefined value. This makes
1024 C<die> the way to raise an exception.
1026 Equivalent examples:
1028 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
1029 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
1031 If the value of EXPR does not end in a newline, the current script line
1032 number and input line number (if any) are also printed, and a newline
1033 is supplied. Note that the "input line number" (also known as "chunk")
1034 is subject to whatever notion of "line" happens to be currently in
1035 effect, and is also available as the special variable C<$.>.
1036 See L<perlvar/"$/"> and L<perlvar/"$.">.
1038 Hint: sometimes appending C<", stopped"> to your message
1039 will cause it to make better sense when the string C<"at foo line 123"> is
1040 appended. Suppose you are running script "canasta".
1042 die "/etc/games is no good";
1043 die "/etc/games is no good, stopped";
1045 produce, respectively
1047 /etc/games is no good at canasta line 123.
1048 /etc/games is no good, stopped at canasta line 123.
1050 See also exit(), warn(), and the Carp module.
1052 If LIST is empty and C<$@> already contains a value (typically from a
1053 previous eval) that value is reused after appending C<"\t...propagated">.
1054 This is useful for propagating exceptions:
1057 die unless $@ =~ /Expected exception/;
1059 If C<$@> is empty then the string C<"Died"> is used.
1061 die() can also be called with a reference argument. If this happens to be
1062 trapped within an eval(), $@ contains the reference. This behavior permits
1063 a more elaborate exception handling implementation using objects that
1064 maintain arbitrary state about the nature of the exception. Such a scheme
1065 is sometimes preferable to matching particular string values of $@ using
1066 regular expressions. Here's an example:
1068 eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
1070 if (ref($@) && UNIVERSAL::isa($@,"Some::Module::Exception")) {
1071 # handle Some::Module::Exception
1074 # handle all other possible exceptions
1078 Because perl will stringify uncaught exception messages before displaying
1079 them, you may want to overload stringification operations on such custom
1080 exception objects. See L<overload> for details about that.
1082 You can arrange for a callback to be run just before the C<die>
1083 does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
1084 handler will be called with the error text and can change the error
1085 message, if it sees fit, by calling C<die> again. See
1086 L<perlvar/$SIG{expr}> for details on setting C<%SIG> entries, and
1087 L<"eval BLOCK"> for some examples. Although this feature was meant
1088 to be run only right before your program was to exit, this is not
1089 currently the case--the C<$SIG{__DIE__}> hook is currently called
1090 even inside eval()ed blocks/strings! If one wants the hook to do
1091 nothing in such situations, put
1095 as the first line of the handler (see L<perlvar/$^S>). Because
1096 this promotes strange action at a distance, this counterintuitive
1097 behavior may be fixed in a future release.
1101 Not really a function. Returns the value of the last command in the
1102 sequence of commands indicated by BLOCK. When modified by a loop
1103 modifier, executes the BLOCK once before testing the loop condition.
1104 (On other statements the loop modifiers test the conditional first.)
1106 C<do BLOCK> does I<not> count as a loop, so the loop control statements
1107 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1108 See L<perlsyn> for alternative strategies.
1110 =item do SUBROUTINE(LIST)
1112 A deprecated form of subroutine call. See L<perlsub>.
1116 Uses the value of EXPR as a filename and executes the contents of the
1117 file as a Perl script. Its primary use is to include subroutines
1118 from a Perl subroutine library.
1124 scalar eval `cat stat.pl`;
1126 except that it's more efficient and concise, keeps track of the current
1127 filename for error messages, searches the @INC libraries, and updates
1128 C<%INC> if the file is found. See L<perlvar/Predefined Names> for these
1129 variables. It also differs in that code evaluated with C<do FILENAME>
1130 cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
1131 same, however, in that it does reparse the file every time you call it,
1132 so you probably don't want to do this inside a loop.
1134 If C<do> cannot read the file, it returns undef and sets C<$!> to the
1135 error. If C<do> can read the file but cannot compile it, it
1136 returns undef and sets an error message in C<$@>. If the file is
1137 successfully compiled, C<do> returns the value of the last expression
1140 Note that inclusion of library modules is better done with the
1141 C<use> and C<require> operators, which also do automatic error checking
1142 and raise an exception if there's a problem.
1144 You might like to use C<do> to read in a program configuration
1145 file. Manual error checking can be done this way:
1147 # read in config files: system first, then user
1148 for $file ("/share/prog/defaults.rc",
1149 "$ENV{HOME}/.someprogrc")
1151 unless ($return = do $file) {
1152 warn "couldn't parse $file: $@" if $@;
1153 warn "couldn't do $file: $!" unless defined $return;
1154 warn "couldn't run $file" unless $return;
1162 This function causes an immediate core dump. See also the B<-u>
1163 command-line switch in L<perlrun>, which does the same thing.
1164 Primarily this is so that you can use the B<undump> program (not
1165 supplied) to turn your core dump into an executable binary after
1166 having initialized all your variables at the beginning of the
1167 program. When the new binary is executed it will begin by executing
1168 a C<goto LABEL> (with all the restrictions that C<goto> suffers).
1169 Think of it as a goto with an intervening core dump and reincarnation.
1170 If C<LABEL> is omitted, restarts the program from the top.
1172 B<WARNING>: Any files opened at the time of the dump will I<not>
1173 be open any more when the program is reincarnated, with possible
1174 resulting confusion on the part of Perl.
1176 This function is now largely obsolete, partly because it's very
1177 hard to convert a core file into an executable, and because the
1178 real compiler backends for generating portable bytecode and compilable
1179 C code have superseded it.
1181 If you're looking to use L<dump> to speed up your program, consider
1182 generating bytecode or native C code as described in L<perlcc>. If
1183 you're just trying to accelerate a CGI script, consider using the
1184 C<mod_perl> extension to B<Apache>, or the CPAN module, Fast::CGI.
1185 You might also consider autoloading or selfloading, which at least
1186 make your program I<appear> to run faster.
1190 When called in list context, returns a 2-element list consisting of the
1191 key and value for the next element of a hash, so that you can iterate over
1192 it. When called in scalar context, returns the key for only the "next"
1193 element in the hash.
1195 Entries are returned in an apparently random order. The actual random
1196 order is subject to change in future versions of perl, but it is guaranteed
1197 to be in the same order as either the C<keys> or C<values> function
1198 would produce on the same (unmodified) hash.
1200 When the hash is entirely read, a null array is returned in list context
1201 (which when assigned produces a false (C<0>) value), and C<undef> in
1202 scalar context. The next call to C<each> after that will start iterating
1203 again. There is a single iterator for each hash, shared by all C<each>,
1204 C<keys>, and C<values> function calls in the program; it can be reset by
1205 reading all the elements from the hash, or by evaluating C<keys HASH> or
1206 C<values HASH>. If you add or delete elements of a hash while you're
1207 iterating over it, you may get entries skipped or duplicated, so don't.
1209 The following prints out your environment like the printenv(1) program,
1210 only in a different order:
1212 while (($key,$value) = each %ENV) {
1213 print "$key=$value\n";
1216 See also C<keys>, C<values> and C<sort>.
1218 =item eof FILEHANDLE
1224 Returns 1 if the next read on FILEHANDLE will return end of file, or if
1225 FILEHANDLE is not open. FILEHANDLE may be an expression whose value
1226 gives the real filehandle. (Note that this function actually
1227 reads a character and then C<ungetc>s it, so isn't very useful in an
1228 interactive context.) Do not read from a terminal file (or call
1229 C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
1230 as terminals may lose the end-of-file condition if you do.
1232 An C<eof> without an argument uses the last file read. Using C<eof()>
1233 with empty parentheses is very different. It refers to the pseudo file
1234 formed from the files listed on the command line and accessed via the
1235 C<< <> >> operator. Since C<< <> >> isn't explicitly opened,
1236 as a normal filehandle is, an C<eof()> before C<< <> >> has been
1237 used will cause C<@ARGV> to be examined to determine if input is
1240 In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to
1241 detect the end of each file, C<eof()> will only detect the end of the
1242 last file. Examples:
1244 # reset line numbering on each input file
1246 next if /^\s*#/; # skip comments
1249 close ARGV if eof; # Not eof()!
1252 # insert dashes just before last line of last file
1254 if (eof()) { # check for end of current file
1255 print "--------------\n";
1256 close(ARGV); # close or last; is needed if we
1257 # are reading from the terminal
1262 Practical hint: you almost never need to use C<eof> in Perl, because the
1263 input operators typically return C<undef> when they run out of data, or if
1270 In the first form, the return value of EXPR is parsed and executed as if it
1271 were a little Perl program. The value of the expression (which is itself
1272 determined within scalar context) is first parsed, and if there weren't any
1273 errors, executed in the lexical context of the current Perl program, so
1274 that any variable settings or subroutine and format definitions remain
1275 afterwards. Note that the value is parsed every time the eval executes.
1276 If EXPR is omitted, evaluates C<$_>. This form is typically used to
1277 delay parsing and subsequent execution of the text of EXPR until run time.
1279 In the second form, the code within the BLOCK is parsed only once--at the
1280 same time the code surrounding the eval itself was parsed--and executed
1281 within the context of the current Perl program. This form is typically
1282 used to trap exceptions more efficiently than the first (see below), while
1283 also providing the benefit of checking the code within BLOCK at compile
1286 The final semicolon, if any, may be omitted from the value of EXPR or within
1289 In both forms, the value returned is the value of the last expression
1290 evaluated inside the mini-program; a return statement may be also used, just
1291 as with subroutines. The expression providing the return value is evaluated
1292 in void, scalar, or list context, depending on the context of the eval itself.
1293 See L</wantarray> for more on how the evaluation context can be determined.
1295 If there is a syntax error or runtime error, or a C<die> statement is
1296 executed, an undefined value is returned by C<eval>, and C<$@> is set to the
1297 error message. If there was no error, C<$@> is guaranteed to be a null
1298 string. Beware that using C<eval> neither silences perl from printing
1299 warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
1300 To do either of those, you have to use the C<$SIG{__WARN__}> facility. See
1301 L</warn> and L<perlvar>.
1303 Note that, because C<eval> traps otherwise-fatal errors, it is useful for
1304 determining whether a particular feature (such as C<socket> or C<symlink>)
1305 is implemented. It is also Perl's exception trapping mechanism, where
1306 the die operator is used to raise exceptions.
1308 If the code to be executed doesn't vary, you may use the eval-BLOCK
1309 form to trap run-time errors without incurring the penalty of
1310 recompiling each time. The error, if any, is still returned in C<$@>.
1313 # make divide-by-zero nonfatal
1314 eval { $answer = $a / $b; }; warn $@ if $@;
1316 # same thing, but less efficient
1317 eval '$answer = $a / $b'; warn $@ if $@;
1319 # a compile-time error
1320 eval { $answer = }; # WRONG
1323 eval '$answer ='; # sets $@
1325 Due to the current arguably broken state of C<__DIE__> hooks, when using
1326 the C<eval{}> form as an exception trap in libraries, you may wish not
1327 to trigger any C<__DIE__> hooks that user code may have installed.
1328 You can use the C<local $SIG{__DIE__}> construct for this purpose,
1329 as shown in this example:
1331 # a very private exception trap for divide-by-zero
1332 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1335 This is especially significant, given that C<__DIE__> hooks can call
1336 C<die> again, which has the effect of changing their error messages:
1338 # __DIE__ hooks may modify error messages
1340 local $SIG{'__DIE__'} =
1341 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
1342 eval { die "foo lives here" };
1343 print $@ if $@; # prints "bar lives here"
1346 Because this promotes action at a distance, this counterintuitive behavior
1347 may be fixed in a future release.
1349 With an C<eval>, you should be especially careful to remember what's
1350 being looked at when:
1356 eval { $x }; # CASE 4
1358 eval "\$$x++"; # CASE 5
1361 Cases 1 and 2 above behave identically: they run the code contained in
1362 the variable $x. (Although case 2 has misleading double quotes making
1363 the reader wonder what else might be happening (nothing is).) Cases 3
1364 and 4 likewise behave in the same way: they run the code C<'$x'>, which
1365 does nothing but return the value of $x. (Case 4 is preferred for
1366 purely visual reasons, but it also has the advantage of compiling at
1367 compile-time instead of at run-time.) Case 5 is a place where
1368 normally you I<would> like to use double quotes, except that in this
1369 particular situation, you can just use symbolic references instead, as
1372 C<eval BLOCK> does I<not> count as a loop, so the loop control statements
1373 C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
1377 =item exec PROGRAM LIST
1379 The C<exec> function executes a system command I<and never returns>--
1380 use C<system> instead of C<exec> if you want it to return. It fails and
1381 returns false only if the command does not exist I<and> it is executed
1382 directly instead of via your system's command shell (see below).
1384 Since it's a common mistake to use C<exec> instead of C<system>, Perl
1385 warns you if there is a following statement which isn't C<die>, C<warn>,
1386 or C<exit> (if C<-w> is set - but you always do that). If you
1387 I<really> want to follow an C<exec> with some other statement, you
1388 can use one of these styles to avoid the warning:
1390 exec ('foo') or print STDERR "couldn't exec foo: $!";
1391 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
1393 If there is more than one argument in LIST, or if LIST is an array
1394 with more than one value, calls execvp(3) with the arguments in LIST.
1395 If there is only one scalar argument or an array with one element in it,
1396 the argument is checked for shell metacharacters, and if there are any,
1397 the entire argument is passed to the system's command shell for parsing
1398 (this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1399 If there are no shell metacharacters in the argument, it is split into
1400 words and passed directly to C<execvp>, which is more efficient.
1403 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1404 exec "sort $outfile | uniq";
1406 If you don't really want to execute the first argument, but want to lie
1407 to the program you are executing about its own name, you can specify
1408 the program you actually want to run as an "indirect object" (without a
1409 comma) in front of the LIST. (This always forces interpretation of the
1410 LIST as a multivalued list, even if there is only a single scalar in
1413 $shell = '/bin/csh';
1414 exec $shell '-sh'; # pretend it's a login shell
1418 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1420 When the arguments get executed via the system shell, results will
1421 be subject to its quirks and capabilities. See L<perlop/"`STRING`">
1424 Using an indirect object with C<exec> or C<system> is also more
1425 secure. This usage (which also works fine with system()) forces
1426 interpretation of the arguments as a multivalued list, even if the
1427 list had just one argument. That way you're safe from the shell
1428 expanding wildcards or splitting up words with whitespace in them.
1430 @args = ( "echo surprise" );
1432 exec @args; # subject to shell escapes
1434 exec { $args[0] } @args; # safe even with one-arg list
1436 The first version, the one without the indirect object, ran the I<echo>
1437 program, passing it C<"surprise"> an argument. The second version
1438 didn't--it tried to run a program literally called I<"echo surprise">,
1439 didn't find it, and set C<$?> to a non-zero value indicating failure.
1441 Beginning with v5.6.0, Perl will attempt to flush all files opened for
1442 output before the exec, but this may not be supported on some platforms
1443 (see L<perlport>). To be safe, you may need to set C<$|> ($AUTOFLUSH
1444 in English) or call the C<autoflush()> method of C<IO::Handle> on any
1445 open handles in order to avoid lost output.
1447 Note that C<exec> will not call your C<END> blocks, nor will it call
1448 any C<DESTROY> methods in your objects.
1452 Given an expression that specifies a hash element or array element,
1453 returns true if the specified element in the hash or array has ever
1454 been initialized, even if the corresponding value is undefined. The
1455 element is not autovivified if it doesn't exist.
1457 print "Exists\n" if exists $hash{$key};
1458 print "Defined\n" if defined $hash{$key};
1459 print "True\n" if $hash{$key};
1461 print "Exists\n" if exists $array[$index];
1462 print "Defined\n" if defined $array[$index];
1463 print "True\n" if $array[$index];
1465 A hash or array element can be true only if it's defined, and defined if
1466 it exists, but the reverse doesn't necessarily hold true.
1468 Given an expression that specifies the name of a subroutine,
1469 returns true if the specified subroutine has ever been declared, even
1470 if it is undefined. Mentioning a subroutine name for exists or defined
1471 does not count as declaring it.
1473 print "Exists\n" if exists &subroutine;
1474 print "Defined\n" if defined &subroutine;
1476 Note that the EXPR can be arbitrarily complicated as long as the final
1477 operation is a hash or array key lookup or subroutine name:
1479 if (exists $ref->{A}->{B}->{$key}) { }
1480 if (exists $hash{A}{B}{$key}) { }
1482 if (exists $ref->{A}->{B}->[$ix]) { }
1483 if (exists $hash{A}{B}[$ix]) { }
1485 if (exists &{$ref->{A}{B}{$key}}) { }
1487 Although the deepest nested array or hash will not spring into existence
1488 just because its existence was tested, any intervening ones will.
1489 Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
1490 into existence due to the existence test for the $key element above.
1491 This happens anywhere the arrow operator is used, including even:
1494 if (exists $ref->{"Some key"}) { }
1495 print $ref; # prints HASH(0x80d3d5c)
1497 This surprising autovivification in what does not at first--or even
1498 second--glance appear to be an lvalue context may be fixed in a future
1501 See L<perlref/"Pseudo-hashes: Using an array as a hash"> for specifics
1502 on how exists() acts when used on a pseudo-hash.
1504 Use of a subroutine call, rather than a subroutine name, as an argument
1505 to exists() is an error.
1508 exists &sub(); # Error
1512 Evaluates EXPR and exits immediately with that value. Example:
1515 exit 0 if $ans =~ /^[Xx]/;
1517 See also C<die>. If EXPR is omitted, exits with C<0> status. The only
1518 universally recognized values for EXPR are C<0> for success and C<1>
1519 for error; other values are subject to interpretation depending on the
1520 environment in which the Perl program is running. For example, exiting
1521 69 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
1522 the mailer to return the item undelivered, but that's not true everywhere.
1524 Don't use C<exit> to abort a subroutine if there's any chance that
1525 someone might want to trap whatever error happened. Use C<die> instead,
1526 which can be trapped by an C<eval>.
1528 The exit() function does not always exit immediately. It calls any
1529 defined C<END> routines first, but these C<END> routines may not
1530 themselves abort the exit. Likewise any object destructors that need to
1531 be called are called before the real exit. If this is a problem, you
1532 can call C<POSIX:_exit($status)> to avoid END and destructor processing.
1533 See L<perlmod> for details.
1539 Returns I<e> (the natural logarithm base) to the power of EXPR.
1540 If EXPR is omitted, gives C<exp($_)>.
1542 =item fcntl FILEHANDLE,FUNCTION,SCALAR
1544 Implements the fcntl(2) function. You'll probably have to say
1548 first to get the correct constant definitions. Argument processing and
1549 value return works just like C<ioctl> below.
1553 fcntl($filehandle, F_GETFL, $packed_return_buffer)
1554 or die "can't fcntl F_GETFL: $!";
1556 You don't have to check for C<defined> on the return from C<fnctl>.
1557 Like C<ioctl>, it maps a C<0> return from the system call into
1558 C<"0 but true"> in Perl. This string is true in boolean context and C<0>
1559 in numeric context. It is also exempt from the normal B<-w> warnings
1560 on improper numeric conversions.
1562 Note that C<fcntl> will produce a fatal error if used on a machine that
1563 doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
1564 manpage to learn what functions are available on your system.
1566 =item fileno FILEHANDLE
1568 Returns the file descriptor for a filehandle, or undefined if the
1569 filehandle is not open. This is mainly useful for constructing
1570 bitmaps for C<select> and low-level POSIX tty-handling operations.
1571 If FILEHANDLE is an expression, the value is taken as an indirect
1572 filehandle, generally its name.
1574 You can use this to find out whether two handles refer to the
1575 same underlying descriptor:
1577 if (fileno(THIS) == fileno(THAT)) {
1578 print "THIS and THAT are dups\n";
1581 =item flock FILEHANDLE,OPERATION
1583 Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
1584 for success, false on failure. Produces a fatal error if used on a
1585 machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
1586 C<flock> is Perl's portable file locking interface, although it locks
1587 only entire files, not records.
1589 Two potentially non-obvious but traditional C<flock> semantics are
1590 that it waits indefinitely until the lock is granted, and that its locks
1591 B<merely advisory>. Such discretionary locks are more flexible, but offer
1592 fewer guarantees. This means that files locked with C<flock> may be
1593 modified by programs that do not also use C<flock>. See L<perlport>,
1594 your port's specific documentation, or your system-specific local manpages
1595 for details. It's best to assume traditional behavior if you're writing
1596 portable programs. (But if you're not, you should as always feel perfectly
1597 free to write for your own system's idiosyncrasies (sometimes called
1598 "features"). Slavish adherence to portability concerns shouldn't get
1599 in the way of your getting your job done.)
1601 OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1602 LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
1603 you can use the symbolic names if you import them from the Fcntl module,
1604 either individually, or as a group using the ':flock' tag. LOCK_SH
1605 requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
1606 releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
1607 LOCK_SH or LOCK_EX then C<flock> will return immediately rather than blocking
1608 waiting for the lock (check the return status to see if you got it).
1610 To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
1611 before locking or unlocking it.
1613 Note that the emulation built with lockf(3) doesn't provide shared
1614 locks, and it requires that FILEHANDLE be open with write intent. These
1615 are the semantics that lockf(3) implements. Most if not all systems
1616 implement lockf(3) in terms of fcntl(2) locking, though, so the
1617 differing semantics shouldn't bite too many people.
1619 Note also that some versions of C<flock> cannot lock things over the
1620 network; you would need to use the more system-specific C<fcntl> for
1621 that. If you like you can force Perl to ignore your system's flock(2)
1622 function, and so provide its own fcntl(2)-based emulation, by passing
1623 the switch C<-Ud_flock> to the F<Configure> program when you configure
1626 Here's a mailbox appender for BSD systems.
1628 use Fcntl ':flock'; # import LOCK_* constants
1631 flock(MBOX,LOCK_EX);
1632 # and, in case someone appended
1633 # while we were waiting...
1638 flock(MBOX,LOCK_UN);
1641 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1642 or die "Can't open mailbox: $!";
1645 print MBOX $msg,"\n\n";
1648 On systems that support a real flock(), locks are inherited across fork()
1649 calls, whereas those that must resort to the more capricious fcntl()
1650 function lose the locks, making it harder to write servers.
1652 See also L<DB_File> for other flock() examples.
1656 Does a fork(2) system call to create a new process running the
1657 same program at the same point. It returns the child pid to the
1658 parent process, C<0> to the child process, or C<undef> if the fork is
1659 unsuccessful. File descriptors (and sometimes locks on those descriptors)
1660 are shared, while everything else is copied. On most systems supporting
1661 fork(), great care has gone into making it extremely efficient (for
1662 example, using copy-on-write technology on data pages), making it the
1663 dominant paradigm for multitasking over the last few decades.
1665 Beginning with v5.6.0, Perl will attempt to flush all files opened for
1666 output before forking the child process, but this may not be supported
1667 on some platforms (see L<perlport>). To be safe, you may need to set
1668 C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
1669 C<IO::Handle> on any open handles in order to avoid duplicate output.
1671 If you C<fork> without ever waiting on your children, you will
1672 accumulate zombies. On some systems, you can avoid this by setting
1673 C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
1674 forking and reaping moribund children.
1676 Note that if your forked child inherits system file descriptors like
1677 STDIN and STDOUT that are actually connected by a pipe or socket, even
1678 if you exit, then the remote server (such as, say, a CGI script or a
1679 backgrounded job launched from a remote shell) won't think you're done.
1680 You should reopen those to F</dev/null> if it's any issue.
1684 Declare a picture format for use by the C<write> function. For
1688 Test: @<<<<<<<< @||||| @>>>>>
1689 $str, $%, '$' . int($num)
1693 $num = $cost/$quantity;
1697 See L<perlform> for many details and examples.
1699 =item formline PICTURE,LIST
1701 This is an internal function used by C<format>s, though you may call it,
1702 too. It formats (see L<perlform>) a list of values according to the
1703 contents of PICTURE, placing the output into the format output
1704 accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
1705 Eventually, when a C<write> is done, the contents of
1706 C<$^A> are written to some filehandle, but you could also read C<$^A>
1707 yourself and then set C<$^A> back to C<"">. Note that a format typically
1708 does one C<formline> per line of form, but the C<formline> function itself
1709 doesn't care how many newlines are embedded in the PICTURE. This means
1710 that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
1711 You may therefore need to use multiple formlines to implement a single
1712 record format, just like the format compiler.
1714 Be careful if you put double quotes around the picture, because an C<@>
1715 character may be taken to mean the beginning of an array name.
1716 C<formline> always returns true. See L<perlform> for other examples.
1718 =item getc FILEHANDLE
1722 Returns the next character from the input file attached to FILEHANDLE,
1723 or the undefined value at end of file, or if there was an error.
1724 If FILEHANDLE is omitted, reads from STDIN. This is not particularly
1725 efficient. However, it cannot be used by itself to fetch single
1726 characters without waiting for the user to hit enter. For that, try
1727 something more like:
1730 system "stty cbreak </dev/tty >/dev/tty 2>&1";
1733 system "stty", '-icanon', 'eol', "\001";
1739 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
1742 system "stty", 'icanon', 'eol', '^@'; # ASCII null
1746 Determination of whether $BSD_STYLE should be set
1747 is left as an exercise to the reader.
1749 The C<POSIX::getattr> function can do this more portably on
1750 systems purporting POSIX compliance. See also the C<Term::ReadKey>
1751 module from your nearest CPAN site; details on CPAN can be found on
1756 Implements the C library function of the same name, which on most
1757 systems returns the current login from F</etc/utmp>, if any. If null,
1760 $login = getlogin || getpwuid($<) || "Kilroy";
1762 Do not consider C<getlogin> for authentication: it is not as
1763 secure as C<getpwuid>.
1765 =item getpeername SOCKET
1767 Returns the packed sockaddr address of other end of the SOCKET connection.
1770 $hersockaddr = getpeername(SOCK);
1771 ($port, $iaddr) = sockaddr_in($hersockaddr);
1772 $herhostname = gethostbyaddr($iaddr, AF_INET);
1773 $herstraddr = inet_ntoa($iaddr);
1777 Returns the current process group for the specified PID. Use
1778 a PID of C<0> to get the current process group for the
1779 current process. Will raise an exception if used on a machine that
1780 doesn't implement getpgrp(2). If PID is omitted, returns process
1781 group of current process. Note that the POSIX version of C<getpgrp>
1782 does not accept a PID argument, so only C<PID==0> is truly portable.
1786 Returns the process id of the parent process.
1788 =item getpriority WHICH,WHO
1790 Returns the current priority for a process, a process group, or a user.
1791 (See L<getpriority(2)>.) Will raise a fatal exception if used on a
1792 machine that doesn't implement getpriority(2).
1798 =item gethostbyname NAME
1800 =item getnetbyname NAME
1802 =item getprotobyname NAME
1808 =item getservbyname NAME,PROTO
1810 =item gethostbyaddr ADDR,ADDRTYPE
1812 =item getnetbyaddr ADDR,ADDRTYPE
1814 =item getprotobynumber NUMBER
1816 =item getservbyport PORT,PROTO
1834 =item sethostent STAYOPEN
1836 =item setnetent STAYOPEN
1838 =item setprotoent STAYOPEN
1840 =item setservent STAYOPEN
1854 These routines perform the same functions as their counterparts in the
1855 system library. In list context, the return values from the
1856 various get routines are as follows:
1858 ($name,$passwd,$uid,$gid,
1859 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
1860 ($name,$passwd,$gid,$members) = getgr*
1861 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
1862 ($name,$aliases,$addrtype,$net) = getnet*
1863 ($name,$aliases,$proto) = getproto*
1864 ($name,$aliases,$port,$proto) = getserv*
1866 (If the entry doesn't exist you get a null list.)
1868 The exact meaning of the $gcos field varies but it usually contains
1869 the real name of the user (as opposed to the login name) and other
1870 information pertaining to the user. Beware, however, that in many
1871 system users are able to change this information and therefore it
1872 cannot be trusted and therefore the $gcos is tainted (see
1873 L<perlsec>). The $passwd and $shell, user's encrypted password and
1874 login shell, are also tainted, because of the same reason.
1876 In scalar context, you get the name, unless the function was a
1877 lookup by name, in which case you get the other thing, whatever it is.
1878 (If the entry doesn't exist you get the undefined value.) For example:
1880 $uid = getpwnam($name);
1881 $name = getpwuid($num);
1883 $gid = getgrnam($name);
1884 $name = getgrgid($num;
1888 In I<getpw*()> the fields $quota, $comment, and $expire are special
1889 cases in the sense that in many systems they are unsupported. If the
1890 $quota is unsupported, it is an empty scalar. If it is supported, it
1891 usually encodes the disk quota. If the $comment field is unsupported,
1892 it is an empty scalar. If it is supported it usually encodes some
1893 administrative comment about the user. In some systems the $quota
1894 field may be $change or $age, fields that have to do with password
1895 aging. In some systems the $comment field may be $class. The $expire
1896 field, if present, encodes the expiration period of the account or the
1897 password. For the availability and the exact meaning of these fields
1898 in your system, please consult your getpwnam(3) documentation and your
1899 F<pwd.h> file. You can also find out from within Perl what your
1900 $quota and $comment fields mean and whether you have the $expire field
1901 by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
1902 C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password
1903 files are only supported if your vendor has implemented them in the
1904 intuitive fashion that calling the regular C library routines gets the
1905 shadow versions if you're running under privilege or if there exists
1906 the shadow(3) functions as found in System V ( this includes Solaris
1907 and Linux.) Those systems which implement a proprietary shadow password
1908 facility are unlikely to be supported.
1910 The $members value returned by I<getgr*()> is a space separated list of
1911 the login names of the members of the group.
1913 For the I<gethost*()> functions, if the C<h_errno> variable is supported in
1914 C, it will be returned to you via C<$?> if the function call fails. The
1915 C<@addrs> value returned by a successful call is a list of the raw
1916 addresses returned by the corresponding system library call. In the
1917 Internet domain, each address is four bytes long and you can unpack it
1918 by saying something like:
1920 ($a,$b,$c,$d) = unpack('C4',$addr[0]);
1922 The Socket library makes this slightly easier:
1925 $iaddr = inet_aton("127.1"); # or whatever address
1926 $name = gethostbyaddr($iaddr, AF_INET);
1928 # or going the other way
1929 $straddr = inet_ntoa($iaddr);
1931 If you get tired of remembering which element of the return list
1932 contains which return value, by-name interfaces are provided
1933 in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
1934 C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
1935 and C<User::grent>. These override the normal built-ins, supplying
1936 versions that return objects with the appropriate names
1937 for each field. For example:
1941 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
1943 Even though it looks like they're the same method calls (uid),
1944 they aren't, because a C<File::stat> object is different from
1945 a C<User::pwent> object.
1947 =item getsockname SOCKET
1949 Returns the packed sockaddr address of this end of the SOCKET connection,
1950 in case you don't know the address because you have several different
1951 IPs that the connection might have come in on.
1954 $mysockaddr = getsockname(SOCK);
1955 ($port, $myaddr) = sockaddr_in($mysockaddr);
1956 printf "Connect to %s [%s]\n",
1957 scalar gethostbyaddr($myaddr, AF_INET),
1960 =item getsockopt SOCKET,LEVEL,OPTNAME
1962 Returns the socket option requested, or undef if there is an error.
1968 Returns the value of EXPR with filename expansions such as the
1969 standard Unix shell F</bin/csh> would do. This is the internal function
1970 implementing the C<< <*.c> >> operator, but you can use it directly.
1971 If EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is
1972 discussed in more detail in L<perlop/"I/O Operators">.
1974 Beginning with v5.6.0, this operator is implemented using the standard
1975 C<File::Glob> extension. See L<File::Glob> for details.
1979 Converts a time as returned by the time function to a 8-element list
1980 with the time localized for the standard Greenwich time zone.
1981 Typically used as follows:
1984 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday) =
1987 All list elements are numeric, and come straight out of the C `struct
1988 tm'. $sec, $min, and $hour are the seconds, minutes, and hours of the
1989 specified time. $mday is the day of the month, and $mon is the month
1990 itself, in the range C<0..11> with 0 indicating January and 11
1991 indicating December. $year is the number of years since 1900. That
1992 is, $year is C<123> in year 2023. $wday is the day of the week, with
1993 0 indicating Sunday and 3 indicating Wednesday. $yday is the day of
1994 the year, in the range C<0..364> (or C<0..365> in leap years.)
1996 Note that the $year element is I<not> simply the last two digits of
1997 the year. If you assume it is, then you create non-Y2K-compliant
1998 programs--and you wouldn't want to do that, would you?
2000 The proper way to get a complete 4-digit year is simply:
2004 And to get the last two digits of the year (e.g., '01' in 2001) do:
2006 $year = sprintf("%02d", $year % 100);
2008 If EXPR is omitted, C<gmtime()> uses the current time (C<gmtime(time)>).
2010 In scalar context, C<gmtime()> returns the ctime(3) value:
2012 $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
2014 Also see the C<timegm> function provided by the C<Time::Local> module,
2015 and the strftime(3) function available via the POSIX module.
2017 This scalar value is B<not> locale dependent (see L<perllocale>), but
2018 is instead a Perl builtin. Also see the C<Time::Local> module, and the
2019 strftime(3) and mktime(3) functions available via the POSIX module. To
2020 get somewhat similar but locale dependent date strings, set up your
2021 locale environment variables appropriately (please see L<perllocale>)
2022 and try for example:
2024 use POSIX qw(strftime);
2025 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
2027 Note that the C<%a> and C<%b> escapes, which represent the short forms
2028 of the day of the week and the month of the year, may not necessarily
2029 be three characters wide in all locales.
2037 The C<goto-LABEL> form finds the statement labeled with LABEL and resumes
2038 execution there. It may not be used to go into any construct that
2039 requires initialization, such as a subroutine or a C<foreach> loop. It
2040 also can't be used to go into a construct that is optimized away,
2041 or to get out of a block or subroutine given to C<sort>.
2042 It can be used to go almost anywhere else within the dynamic scope,
2043 including out of subroutines, but it's usually better to use some other
2044 construct such as C<last> or C<die>. The author of Perl has never felt the
2045 need to use this form of C<goto> (in Perl, that is--C is another matter).
2047 The C<goto-EXPR> form expects a label name, whose scope will be resolved
2048 dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
2049 necessarily recommended if you're optimizing for maintainability:
2051 goto ("FOO", "BAR", "GLARCH")[$i];
2053 The C<goto-&NAME> form is quite different from the other forms of C<goto>.
2054 In fact, it isn't a goto in the normal sense at all, and doesn't have
2055 the stigma associated with other gotos. Instead, it
2056 substitutes a call to the named subroutine for the currently running
2057 subroutine. This is used by C<AUTOLOAD> subroutines that wish to load
2058 another subroutine and then pretend that the other subroutine had been
2059 called in the first place (except that any modifications to C<@_>
2060 in the current subroutine are propagated to the other subroutine.)
2061 After the C<goto>, not even C<caller> will be able to tell that this
2062 routine was called first.
2064 NAME needn't be the name of a subroutine; it can be a scalar variable
2065 containing a code reference, or a block which evaluates to a code
2068 =item grep BLOCK LIST
2070 =item grep EXPR,LIST
2072 This is similar in spirit to, but not the same as, grep(1) and its
2073 relatives. In particular, it is not limited to using regular expressions.
2075 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2076 C<$_> to each element) and returns the list value consisting of those
2077 elements for which the expression evaluated to true. In scalar
2078 context, returns the number of times the expression was true.
2080 @foo = grep(!/^#/, @bar); # weed out comments
2084 @foo = grep {!/^#/} @bar; # weed out comments
2086 Note that C<$_> is an alias to the list value, so it can be used to
2087 modify the elements of the LIST. While this is useful and supported,
2088 it can cause bizarre results if the elements of LIST are not variables.
2089 Similarly, grep returns aliases into the original list, much as a for
2090 loop's index variable aliases the list elements. That is, modifying an
2091 element of a list returned by grep (for example, in a C<foreach>, C<map>
2092 or another C<grep>) actually modifies the element in the original list.
2093 This is usually something to be avoided when writing clear code.
2095 See also L</map> for a list composed of the results of the BLOCK or EXPR.
2101 Interprets EXPR as a hex string and returns the corresponding value.
2102 (To convert strings that might start with either 0, 0x, or 0b, see
2103 L</oct>.) If EXPR is omitted, uses C<$_>.
2105 print hex '0xAf'; # prints '175'
2106 print hex 'aF'; # same
2108 Hex strings may only represent integers. Strings that would cause
2109 integer overflow trigger a warning.
2113 There is no builtin C<import> function. It is just an ordinary
2114 method (subroutine) defined (or inherited) by modules that wish to export
2115 names to another module. The C<use> function calls the C<import> method
2116 for the package used. See also L</use()>, L<perlmod>, and L<Exporter>.
2118 =item index STR,SUBSTR,POSITION
2120 =item index STR,SUBSTR
2122 The index function searches for one string within another, but without
2123 the wildcard-like behavior of a full regular-expression pattern match.
2124 It returns the position of the first occurrence of SUBSTR in STR at
2125 or after POSITION. If POSITION is omitted, starts searching from the
2126 beginning of the string. The return value is based at C<0> (or whatever
2127 you've set the C<$[> variable to--but don't do that). If the substring
2128 is not found, returns one less than the base, ordinarily C<-1>.
2134 Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
2135 You should not use this function for rounding: one because it truncates
2136 towards C<0>, and two because machine representations of floating point
2137 numbers can sometimes produce counterintuitive results. For example,
2138 C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
2139 because it's really more like -268.99999999999994315658 instead. Usually,
2140 the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
2141 functions will serve you better than will int().
2143 =item ioctl FILEHANDLE,FUNCTION,SCALAR
2145 Implements the ioctl(2) function. You'll probably first have to say
2147 require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph
2149 to get the correct function definitions. If F<ioctl.ph> doesn't
2150 exist or doesn't have the correct definitions you'll have to roll your
2151 own, based on your C header files such as F<< <sys/ioctl.h> >>.
2152 (There is a Perl script called B<h2ph> that comes with the Perl kit that
2153 may help you in this, but it's nontrivial.) SCALAR will be read and/or
2154 written depending on the FUNCTION--a pointer to the string value of SCALAR
2155 will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
2156 has no string value but does have a numeric value, that value will be
2157 passed rather than a pointer to the string value. To guarantee this to be
2158 true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
2159 functions may be needed to manipulate the values of structures used by
2162 The return value of C<ioctl> (and C<fcntl>) is as follows:
2164 if OS returns: then Perl returns:
2166 0 string "0 but true"
2167 anything else that number
2169 Thus Perl returns true on success and false on failure, yet you can
2170 still easily determine the actual value returned by the operating
2173 $retval = ioctl(...) || -1;
2174 printf "System returned %d\n", $retval;
2176 The special string "C<0> but true" is exempt from B<-w> complaints
2177 about improper numeric conversions.
2179 Here's an example of setting a filehandle named C<REMOTE> to be
2180 non-blocking at the system level. You'll have to negotiate C<$|>
2181 on your own, though.
2183 use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
2185 $flags = fcntl(REMOTE, F_GETFL, 0)
2186 or die "Can't get flags for the socket: $!\n";
2188 $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
2189 or die "Can't set flags for the socket: $!\n";
2191 =item join EXPR,LIST
2193 Joins the separate strings of LIST into a single string with fields
2194 separated by the value of EXPR, and returns that new string. Example:
2196 $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
2198 Beware that unlike C<split>, C<join> doesn't take a pattern as its
2199 first argument. Compare L</split>.
2203 Returns a list consisting of all the keys of the named hash. (In
2204 scalar context, returns the number of keys.) The keys are returned in
2205 an apparently random order. The actual random order is subject to
2206 change in future versions of perl, but it is guaranteed to be the same
2207 order as either the C<values> or C<each> function produces (given
2208 that the hash has not been modified). As a side effect, it resets
2211 Here is yet another way to print your environment:
2214 @values = values %ENV;
2216 print pop(@keys), '=', pop(@values), "\n";
2219 or how about sorted by key:
2221 foreach $key (sort(keys %ENV)) {
2222 print $key, '=', $ENV{$key}, "\n";
2225 The returned values are copies of the original keys in the hash, so
2226 modifying them will not affect the original hash. Compare L</values>.
2228 To sort a hash by value, you'll need to use a C<sort> function.
2229 Here's a descending numeric sort of a hash by its values:
2231 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
2232 printf "%4d %s\n", $hash{$key}, $key;
2235 As an lvalue C<keys> allows you to increase the number of hash buckets
2236 allocated for the given hash. This can gain you a measure of efficiency if
2237 you know the hash is going to get big. (This is similar to pre-extending
2238 an array by assigning a larger number to $#array.) If you say
2242 then C<%hash> will have at least 200 buckets allocated for it--256 of them,
2243 in fact, since it rounds up to the next power of two. These
2244 buckets will be retained even if you do C<%hash = ()>, use C<undef
2245 %hash> if you want to free the storage while C<%hash> is still in scope.
2246 You can't shrink the number of buckets allocated for the hash using
2247 C<keys> in this way (but you needn't worry about doing this by accident,
2248 as trying has no effect).
2250 See also C<each>, C<values> and C<sort>.
2252 =item kill SIGNAL, LIST
2254 Sends a signal to a list of processes. Returns the number of
2255 processes successfully signaled (which is not necessarily the
2256 same as the number actually killed).
2258 $cnt = kill 1, $child1, $child2;
2261 If SIGNAL is zero, no signal is sent to the process. This is a
2262 useful way to check that the process is alive and hasn't changed
2263 its UID. See L<perlport> for notes on the portability of this
2266 Unlike in the shell, if SIGNAL is negative, it kills
2267 process groups instead of processes. (On System V, a negative I<PROCESS>
2268 number will also kill process groups, but that's not portable.) That
2269 means you usually want to use positive not negative signals. You may also
2270 use a signal name in quotes. See L<perlipc/"Signals"> for details.
2276 The C<last> command is like the C<break> statement in C (as used in
2277 loops); it immediately exits the loop in question. If the LABEL is
2278 omitted, the command refers to the innermost enclosing loop. The
2279 C<continue> block, if any, is not executed:
2281 LINE: while (<STDIN>) {
2282 last LINE if /^$/; # exit when done with header
2286 C<last> cannot be used to exit a block which returns a value such as
2287 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2288 a grep() or map() operation.
2290 Note that a block by itself is semantically identical to a loop
2291 that executes once. Thus C<last> can be used to effect an early
2292 exit out of such a block.
2294 See also L</continue> for an illustration of how C<last>, C<next>, and
2301 Returns an lowercased version of EXPR. This is the internal function
2302 implementing the C<\L> escape in double-quoted strings.
2303 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
2306 If EXPR is omitted, uses C<$_>.
2312 Returns the value of EXPR with the first character lowercased. This is
2313 the internal function implementing the C<\l> escape in double-quoted strings.
2314 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
2316 If EXPR is omitted, uses C<$_>.
2322 Returns the length in characters of the value of EXPR. If EXPR is
2323 omitted, returns length of C<$_>. Note that this cannot be used on
2324 an entire array or hash to find out how many elements these have.
2325 For that, use C<scalar @array> and C<scalar keys %hash> respectively.
2327 =item link OLDFILE,NEWFILE
2329 Creates a new filename linked to the old filename. Returns true for
2330 success, false otherwise.
2332 =item listen SOCKET,QUEUESIZE
2334 Does the same thing that the listen system call does. Returns true if
2335 it succeeded, false otherwise. See the example in L<perlipc/"Sockets: Client/Server Communication">.
2339 You really probably want to be using C<my> instead, because C<local> isn't
2340 what most people think of as "local". See L<perlsub/"Private Variables
2341 via my()"> for details.
2343 A local modifies the listed variables to be local to the enclosing
2344 block, file, or eval. If more than one value is listed, the list must
2345 be placed in parentheses. See L<perlsub/"Temporary Values via local()">
2346 for details, including issues with tied arrays and hashes.
2348 =item localtime EXPR
2350 Converts a time as returned by the time function to a 9-element list
2351 with the time analyzed for the local time zone. Typically used as
2355 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
2358 All list elements are numeric, and come straight out of the C `struct
2359 tm'. $sec, $min, and $hour are the seconds, minutes, and hours of the
2360 specified time. $mday is the day of the month, and $mon is the month
2361 itself, in the range C<0..11> with 0 indicating January and 11
2362 indicating December. $year is the number of years since 1900. That
2363 is, $year is C<123> in year 2023. $wday is the day of the week, with
2364 0 indicating Sunday and 3 indicating Wednesday. $yday is the day of
2365 the year, in the range C<0..364> (or C<0..365> in leap years.) $isdst
2366 is true if the specified time occurs during daylight savings time,
2369 Note that the $year element is I<not> simply the last two digits of
2370 the year. If you assume it is, then you create non-Y2K-compliant
2371 programs--and you wouldn't want to do that, would you?
2373 The proper way to get a complete 4-digit year is simply:
2377 And to get the last two digits of the year (e.g., '01' in 2001) do:
2379 $year = sprintf("%02d", $year % 100);
2381 If EXPR is omitted, C<localtime()> uses the current time (C<localtime(time)>).
2383 In scalar context, C<localtime()> returns the ctime(3) value:
2385 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
2387 This scalar value is B<not> locale dependent, see L<perllocale>, but
2388 instead a Perl builtin. Also see the C<Time::Local> module
2389 (to convert the second, minutes, hours, ... back to seconds since the
2390 stroke of midnight the 1st of January 1970, the value returned by
2391 time()), and the strftime(3) and mktime(3) functions available via the
2392 POSIX module. To get somewhat similar but locale dependent date
2393 strings, set up your locale environment variables appropriately
2394 (please see L<perllocale>) and try for example:
2396 use POSIX qw(strftime);
2397 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
2399 Note that the C<%a> and C<%b>, the short forms of the day of the week
2400 and the month of the year, may not necessarily be three characters wide.
2406 This function places an advisory lock on a variable, subroutine,
2407 or referenced object contained in I<THING> until the lock goes out
2408 of scope. This is a built-in function only if your version of Perl
2409 was built with threading enabled, and if you've said C<use Threads>.
2410 Otherwise a user-defined function by this name will be called. See
2417 Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
2418 returns log of C<$_>. To get the log of another base, use basic algebra:
2419 The base-N log of a number is equal to the natural log of that number
2420 divided by the natural log of N. For example:
2424 return log($n)/log(10);
2427 See also L</exp> for the inverse operation.
2433 Does the same thing as the C<stat> function (including setting the
2434 special C<_> filehandle) but stats a symbolic link instead of the file
2435 the symbolic link points to. If symbolic links are unimplemented on
2436 your system, a normal C<stat> is done.
2438 If EXPR is omitted, stats C<$_>.
2442 The match operator. See L<perlop>.
2444 =item map BLOCK LIST
2448 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
2449 C<$_> to each element) and returns the list value composed of the
2450 results of each such evaluation. In scalar context, returns the
2451 total number of elements so generated. Evaluates BLOCK or EXPR in
2452 list context, so each element of LIST may produce zero, one, or
2453 more elements in the returned value.
2455 @chars = map(chr, @nums);
2457 translates a list of numbers to the corresponding characters. And
2459 %hash = map { getkey($_) => $_ } @array;
2461 is just a funny way to write
2464 foreach $_ (@array) {
2465 $hash{getkey($_)} = $_;
2468 Note that C<$_> is an alias to the list value, so it can be used to
2469 modify the elements of the LIST. While this is useful and supported,
2470 it can cause bizarre results if the elements of LIST are not variables.
2471 Using a regular C<foreach> loop for this purpose would be clearer in
2472 most cases. See also L</grep> for an array composed of those items of
2473 the original list for which the BLOCK or EXPR evaluates to true.
2475 =item mkdir FILENAME,MASK
2477 =item mkdir FILENAME
2479 Creates the directory specified by FILENAME, with permissions
2480 specified by MASK (as modified by C<umask>). If it succeeds it
2481 returns true, otherwise it returns false and sets C<$!> (errno).
2482 If omitted, MASK defaults to 0777.
2484 In general, it is better to create directories with permissive MASK,
2485 and let the user modify that with their C<umask>, than it is to supply
2486 a restrictive MASK and give the user no way to be more permissive.
2487 The exceptions to this rule are when the file or directory should be
2488 kept private (mail files, for instance). The perlfunc(1) entry on
2489 C<umask> discusses the choice of MASK in more detail.
2491 =item msgctl ID,CMD,ARG
2493 Calls the System V IPC function msgctl(2). You'll probably have to say
2497 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
2498 then ARG must be a variable which will hold the returned C<msqid_ds>
2499 structure. Returns like C<ioctl>: the undefined value for error,
2500 C<"0 but true"> for zero, or the actual return value otherwise. See also
2501 L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::Semaphore> documentation.
2503 =item msgget KEY,FLAGS
2505 Calls the System V IPC function msgget(2). Returns the message queue
2506 id, or the undefined value if there is an error. See also
2507 L<perlipc/"SysV IPC"> and C<IPC::SysV> and C<IPC::Msg> documentation.
2509 =item msgrcv ID,VAR,SIZE,TYPE,FLAGS
2511 Calls the System V IPC function msgrcv to receive a message from
2512 message queue ID into variable VAR with a maximum message size of
2513 SIZE. Note that when a message is received, the message type as a
2514 native long integer will be the first thing in VAR, followed by the
2515 actual message. This packing may be opened with C<unpack("l! a*")>.
2516 Taints the variable. Returns true if successful, or false if there is
2517 an error. See also L<perlipc/"SysV IPC">, C<IPC::SysV>, and
2518 C<IPC::SysV::Msg> documentation.
2520 =item msgsnd ID,MSG,FLAGS
2522 Calls the System V IPC function msgsnd to send the message MSG to the
2523 message queue ID. MSG must begin with the native long integer message
2524 type, and be followed by the length of the actual message, and finally
2525 the message itself. This kind of packing can be achieved with
2526 C<pack("l! a*", $type, $message)>. Returns true if successful,
2527 or false if there is an error. See also C<IPC::SysV>
2528 and C<IPC::SysV::Msg> documentation.
2532 =item my EXPR : ATTRIBUTES
2534 A C<my> declares the listed variables to be local (lexically) to the
2535 enclosing block, file, or C<eval>. If
2536 more than one value is listed, the list must be placed in parentheses. See
2537 L<perlsub/"Private Variables via my()"> for details.
2543 The C<next> command is like the C<continue> statement in C; it starts
2544 the next iteration of the loop:
2546 LINE: while (<STDIN>) {
2547 next LINE if /^#/; # discard comments
2551 Note that if there were a C<continue> block on the above, it would get
2552 executed even on discarded lines. If the LABEL is omitted, the command
2553 refers to the innermost enclosing loop.
2555 C<next> cannot be used to exit a block which returns a value such as
2556 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
2557 a grep() or map() operation.
2559 Note that a block by itself is semantically identical to a loop
2560 that executes once. Thus C<next> will exit such a block early.
2562 See also L</continue> for an illustration of how C<last>, C<next>, and
2565 =item no Module LIST
2567 See the L</use> function, which C<no> is the opposite of.
2573 Interprets EXPR as an octal string and returns the corresponding
2574 value. (If EXPR happens to start off with C<0x>, interprets it as a
2575 hex string. If EXPR starts off with C<0b>, it is interpreted as a
2576 binary string.) The following will handle decimal, binary, octal, and
2577 hex in the standard Perl or C notation:
2579 $val = oct($val) if $val =~ /^0/;
2581 If EXPR is omitted, uses C<$_>. To go the other way (produce a number
2582 in octal), use sprintf() or printf():
2584 $perms = (stat("filename"))[2] & 07777;
2585 $oct_perms = sprintf "%lo", $perms;
2587 The oct() function is commonly used when a string such as C<644> needs
2588 to be converted into a file mode, for example. (Although perl will
2589 automatically convert strings into numbers as needed, this automatic
2590 conversion assumes base 10.)
2592 =item open FILEHANDLE,MODE,LIST
2594 =item open FILEHANDLE,EXPR
2596 =item open FILEHANDLE
2598 Opens the file whose filename is given by EXPR, and associates it with
2599 FILEHANDLE. If FILEHANDLE is an expression, its value is used as the
2600 name of the real filehandle wanted. (This is considered a symbolic
2601 reference, so C<use strict 'refs'> should I<not> be in effect.)
2603 If EXPR is omitted, the scalar
2604 variable of the same name as the FILEHANDLE contains the filename.
2605 (Note that lexical variables--those declared with C<my>--will not work
2606 for this purpose; so if you're using C<my>, specify EXPR in your call
2607 to open.) See L<perlopentut> for a kinder, gentler explanation of opening
2610 If MODE is C<< '<' >> or nothing, the file is opened for input.
2611 If MODE is C<< '>' >>, the file is truncated and opened for
2612 output, being created if necessary. If MODE is C<<< '>>' >>>,
2613 the file is opened for appending, again being created if necessary.
2614 You can put a C<'+'> in front of the C<< '>' >> or C<< '<' >> to indicate that
2615 you want both read and write access to the file; thus C<< '+<' >> is almost
2616 always preferred for read/write updates--the C<< '+>' >> mode would clobber the
2617 file first. You can't usually use either read-write mode for updating
2618 textfiles, since they have variable length records. See the B<-i>
2619 switch in L<perlrun> for a better approach. The file is created with
2620 permissions of C<0666> modified by the process' C<umask> value.
2622 These various prefixes correspond to the fopen(3) modes of C<'r'>, C<'r+'>,
2623 C<'w'>, C<'w+'>, C<'a'>, and C<'a+'>.
2625 In the 2-arguments (and 1-argument) form of the call the mode and
2626 filename should be concatenated (in this order), possibly separated by
2627 spaces. It is possible to omit the mode if the mode is C<< '<' >>.
2629 If the filename begins with C<'|'>, the filename is interpreted as a
2630 command to which output is to be piped, and if the filename ends with a
2631 C<'|'>, the filename is interpreted as a command which pipes output to
2632 us. See L<perlipc/"Using open() for IPC">
2633 for more examples of this. (You are not allowed to C<open> to a command
2634 that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
2635 and L<perlipc/"Bidirectional Communication with Another Process">
2638 If MODE is C<'|-'>, the filename is interpreted as a
2639 command to which output is to be piped, and if MODE is
2640 C<'-|'>, the filename is interpreted as a command which pipes output to
2641 us. In the 2-arguments (and 1-argument) form one should replace dash
2642 (C<'-'>) with the command. See L<perlipc/"Using open() for IPC">
2643 for more examples of this. (You are not allowed to C<open> to a command
2644 that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
2645 and L<perlipc/"Bidirectional Communication"> for alternatives.)
2647 In the 2-arguments (and 1-argument) form opening C<'-'> opens STDIN
2648 and opening C<< '>-' >> opens STDOUT.
2651 nonzero upon success, the undefined value otherwise. If the C<open>
2652 involved a pipe, the return value happens to be the pid of the
2655 If you're unfortunate enough to be running Perl on a system that
2656 distinguishes between text files and binary files (modern operating
2657 systems don't care), then you should check out L</binmode> for tips for
2658 dealing with this. The key distinction between systems that need C<binmode>
2659 and those that don't is their text file formats. Systems like Unix, MacOS, and
2660 Plan9, which delimit lines with a single character, and which encode that
2661 character in C as C<"\n">, do not need C<binmode>. The rest need it.
2663 When opening a file, it's usually a bad idea to continue normal execution
2664 if the request failed, so C<open> is frequently used in connection with
2665 C<die>. Even if C<die> won't do what you want (say, in a CGI script,
2666 where you want to make a nicely formatted error message (but there are
2667 modules that can help with that problem)) you should always check
2668 the return value from opening a file. The infrequent exception is when
2669 working with an unopened filehandle is actually what you want to do.
2674 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
2675 while (<ARTICLE>) {...
2677 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
2678 # if the open fails, output is discarded
2680 open(DBASE, '+<', 'dbase.mine') # open for update
2681 or die "Can't open 'dbase.mine' for update: $!";
2683 open(DBASE, '+<dbase.mine') # ditto
2684 or die "Can't open 'dbase.mine' for update: $!";
2686 open(ARTICLE, '-|', "caesar <$article") # decrypt article
2687 or die "Can't start caesar: $!";
2689 open(ARTICLE, "caesar <$article |") # ditto
2690 or die "Can't start caesar: $!";
2692 open(EXTRACT, "|sort >/tmp/Tmp$$") # $$ is our process id
2693 or die "Can't start sort: $!";
2695 # process argument list of files along with any includes
2697 foreach $file (@ARGV) {
2698 process($file, 'fh00');
2702 my($filename, $input) = @_;
2703 $input++; # this is a string increment
2704 unless (open($input, $filename)) {
2705 print STDERR "Can't open $filename: $!\n";
2710 while (<$input>) { # note use of indirection
2711 if (/^#include "(.*)"/) {
2712 process($1, $input);
2719 You may also, in the Bourne shell tradition, specify an EXPR beginning
2720 with C<< '>&' >>, in which case the rest of the string is interpreted as the
2721 name of a filehandle (or file descriptor, if numeric) to be
2722 duped and opened. You may use C<&> after C<< > >>, C<<< >> >>>,
2723 C<< < >>, C<< +> >>, C<<< +>> >>>, and C<< +< >>. The
2724 mode you specify should match the mode of the original filehandle.
2725 (Duping a filehandle does not take into account any existing contents of
2726 stdio buffers.) Duping file handles is not yet supported for 3-argument
2729 Here is a script that saves, redirects, and restores STDOUT and
2733 open(OLDOUT, ">&STDOUT");
2734 open(OLDERR, ">&STDERR");
2736 open(STDOUT, '>', "foo.out") || die "Can't redirect stdout";
2737 open(STDERR, ">&STDOUT") || die "Can't dup stdout";
2739 select(STDERR); $| = 1; # make unbuffered
2740 select(STDOUT); $| = 1; # make unbuffered
2742 print STDOUT "stdout 1\n"; # this works for
2743 print STDERR "stderr 1\n"; # subprocesses too
2748 open(STDOUT, ">&OLDOUT");
2749 open(STDERR, ">&OLDERR");
2751 print STDOUT "stdout 2\n";
2752 print STDERR "stderr 2\n";
2754 If you specify C<< '<&=N' >>, where C<N> is a number, then Perl will do an
2755 equivalent of C's C<fdopen> of that file descriptor; this is more
2756 parsimonious of file descriptors. For example:
2758 open(FILEHANDLE, "<&=$fd")
2760 Note that this feature depends on the fdopen() C library function.
2761 On many UNIX systems, fdopen() is known to fail when file descriptors
2762 exceed a certain value, typically 255. If you need more file
2763 descriptors than that, consider rebuilding Perl to use the C<sfio>
2766 If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'>
2767 with 2-arguments (or 1-argument) form of open(), then
2768 there is an implicit fork done, and the return value of open is the pid
2769 of the child within the parent process, and C<0> within the child
2770 process. (Use C<defined($pid)> to determine whether the open was successful.)
2771 The filehandle behaves normally for the parent, but i/o to that
2772 filehandle is piped from/to the STDOUT/STDIN of the child process.
2773 In the child process the filehandle isn't opened--i/o happens from/to
2774 the new STDOUT or STDIN. Typically this is used like the normal
2775 piped open when you want to exercise more control over just how the
2776 pipe command gets executed, such as when you are running setuid, and
2777 don't want to have to scan shell commands for metacharacters.
2778 The following triples are more or less equivalent:
2780 open(FOO, "|tr '[a-z]' '[A-Z]'");
2781 open(FOO, '|-', "tr '[a-z]' '[A-Z]'");
2782 open(FOO, '|-') || exec 'tr', '[a-z]', '[A-Z]';
2784 open(FOO, "cat -n '$file'|");
2785 open(FOO, '-|', "cat -n '$file'");
2786 open(FOO, '-|') || exec 'cat', '-n', $file;
2788 See L<perlipc/"Safe Pipe Opens"> for more examples of this.
2790 Beginning with v5.6.0, Perl will attempt to flush all files opened for
2791 output before any operation that may do a fork, but this may not be
2792 supported on some platforms (see L<perlport>). To be safe, you may need
2793 to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
2794 of C<IO::Handle> on any open handles.
2796 On systems that support a
2797 close-on-exec flag on files, the flag will be set for the newly opened
2798 file descriptor as determined by the value of $^F. See L<perlvar/$^F>.
2800 Closing any piped filehandle causes the parent process to wait for the
2801 child to finish, and returns the status value in C<$?>.
2803 The filename passed to 2-argument (or 1-argument) form of open()
2804 will have leading and trailing
2805 whitespace deleted, and the normal redirection characters
2806 honored. This property, known as "magic open",
2807 can often be used to good effect. A user could specify a filename of
2808 F<"rsh cat file |">, or you could change certain filenames as needed:
2810 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
2811 open(FH, $filename) or die "Can't open $filename: $!";
2813 Use 3-argument form to open a file with arbitrary weird characters in it,
2815 open(FOO, '<', $file);
2817 otherwise it's necessary to protect any leading and trailing whitespace:
2819 $file =~ s#^(\s)#./$1#;
2820 open(FOO, "< $file\0");
2822 (this may not work on some bizzare filesystems). One should
2823 conscientiously choose between the I<magic> and 3-arguments form
2828 will allow the user to specify an argument of the form C<"rsh cat file |">,
2829 but will not work on a filename which happens to have a trailing space, while
2831 open IN, '<', $ARGV[0];
2833 will have exactly the opposite restrictions.
2835 If you want a "real" C C<open> (see L<open(2)> on your system), then you
2836 should use the C<sysopen> function, which involves no such magic (but
2837 may use subtly different filemodes than Perl open(), which is mapped
2838 to C fopen()). This is
2839 another way to protect your filenames from interpretation. For example:
2842 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
2843 or die "sysopen $path: $!";
2844 $oldfh = select(HANDLE); $| = 1; select($oldfh);
2845 print HANDLE "stuff $$\n");
2847 print "File contains: ", <HANDLE>;
2849 Using the constructor from the C<IO::Handle> package (or one of its
2850 subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous
2851 filehandles that have the scope of whatever variables hold references to
2852 them, and automatically close whenever and however you leave that scope:
2856 sub read_myfile_munged {
2858 my $handle = new IO::File;
2859 open($handle, "myfile") or die "myfile: $!";
2861 or return (); # Automatically closed here.
2862 mung $first or die "mung failed"; # Or here.
2863 return $first, <$handle> if $ALL; # Or here.
2867 See L</seek> for some details about mixing reading and writing.
2869 =item opendir DIRHANDLE,EXPR
2871 Opens a directory named EXPR for processing by C<readdir>, C<telldir>,
2872 C<seekdir>, C<rewinddir>, and C<closedir>. Returns true if successful.
2873 DIRHANDLEs have their own namespace separate from FILEHANDLEs.
2879 Returns the numeric (ASCII or Unicode) value of the first character of EXPR. If
2880 EXPR is omitted, uses C<$_>. For the reverse, see L</chr>.
2881 See L<utf8> for more about Unicode.
2885 An C<our> declares the listed variables to be valid globals within
2886 the enclosing block, file, or C<eval>. That is, it has the same
2887 scoping rules as a "my" declaration, but does not create a local
2888 variable. If more than one value is listed, the list must be placed
2889 in parentheses. The C<our> declaration has no semantic effect unless
2890 "use strict vars" is in effect, in which case it lets you use the
2891 declared global variable without qualifying it with a package name.
2892 (But only within the lexical scope of the C<our> declaration. In this
2893 it differs from "use vars", which is package scoped.)
2895 An C<our> declaration declares a global variable that will be visible
2896 across its entire lexical scope, even across package boundaries. The
2897 package in which the variable is entered is determined at the point
2898 of the declaration, not at the point of use. This means the following
2902 our $bar; # declares $Foo::bar for rest of lexical scope
2906 print $bar; # prints 20
2908 Multiple C<our> declarations in the same lexical scope are allowed
2909 if they are in different packages. If they happened to be in the same
2910 package, Perl will emit warnings if you have asked for them.
2914 our $bar; # declares $Foo::bar for rest of lexical scope
2918 our $bar = 30; # declares $Bar::bar for rest of lexical scope
2919 print $bar; # prints 30
2921 our $bar; # emits warning
2923 =item pack TEMPLATE,LIST
2925 Takes a LIST of values and converts it into a string using the rules
2926 given by the TEMPLATE. The resulting string is the concatenation of
2927 the converted values. Typically, each converted value looks
2928 like its machine-level representation. For example, on 32-bit machines
2929 a converted integer may be represented by a sequence of 4 bytes.
2932 sequence of characters that give the order and type of values, as
2935 a A string with arbitrary binary data, will be null padded.
2936 A An ASCII string, will be space padded.
2937 Z A null terminated (asciz) string, will be null padded.
2939 b A bit string (ascending bit order inside each byte, like vec()).
2940 B A bit string (descending bit order inside each byte).
2941 h A hex string (low nybble first).
2942 H A hex string (high nybble first).
2944 c A signed char value.
2945 C An unsigned char value. Only does bytes. See U for Unicode.
2947 s A signed short value.
2948 S An unsigned short value.
2949 (This 'short' is _exactly_ 16 bits, which may differ from
2950 what a local C compiler calls 'short'. If you want
2951 native-length shorts, use the '!' suffix.)
2953 i A signed integer value.
2954 I An unsigned integer value.
2955 (This 'integer' is _at_least_ 32 bits wide. Its exact
2956 size depends on what a local C compiler calls 'int',
2957 and may even be larger than the 'long' described in
2960 l A signed long value.
2961 L An unsigned long value.
2962 (This 'long' is _exactly_ 32 bits, which may differ from
2963 what a local C compiler calls 'long'. If you want
2964 native-length longs, use the '!' suffix.)
2966 n An unsigned short in "network" (big-endian) order.
2967 N An unsigned long in "network" (big-endian) order.
2968 v An unsigned short in "VAX" (little-endian) order.
2969 V An unsigned long in "VAX" (little-endian) order.
2970 (These 'shorts' and 'longs' are _exactly_ 16 bits and
2971 _exactly_ 32 bits, respectively.)
2973 q A signed quad (64-bit) value.
2974 Q An unsigned quad value.
2975 (Quads are available only if your system supports 64-bit
2976 integer values _and_ if Perl has been compiled to support those.
2977 Causes a fatal error otherwise.)
2979 f A single-precision float in the native format.
2980 d A double-precision float in the native format.
2982 p A pointer to a null-terminated string.
2983 P A pointer to a structure (fixed-length string).
2985 u A uuencoded string.
2986 U A Unicode character number. Encodes to UTF-8 internally.
2987 Works even if C<use utf8> is not in effect.
2989 w A BER compressed integer. Its bytes represent an unsigned
2990 integer in base 128, most significant digit first, with as
2991 few digits as possible. Bit eight (the high bit) is set
2992 on each byte except the last.
2996 @ Null fill to absolute position.
2998 The following rules apply:
3004 Each letter may optionally be followed by a number giving a repeat
3005 count. With all types except C<a>, C<A>, C<Z>, C<b>, C<B>, C<h>,
3006 C<H>, and C<P> the pack function will gobble up that many values from
3007 the LIST. A C<*> for the repeat count means to use however many items are
3008 left, except for C<@>, C<x>, C<X>, where it is equivalent
3009 to C<0>, and C<u>, where it is equivalent to 1 (or 45, what is the
3012 When used with C<Z>, C<*> results in the addition of a trailing null
3013 byte (so the packed result will be one longer than the byte C<length>
3016 The repeat count for C<u> is interpreted as the maximal number of bytes
3017 to encode per line of output, with 0 and 1 replaced by 45.
3021 The C<a>, C<A>, and C<Z> types gobble just one value, but pack it as a
3022 string of length count, padding with nulls or spaces as necessary. When
3023 unpacking, C<A> strips trailing spaces and nulls, C<Z> strips everything
3024 after the first null, and C<a> returns data verbatim. When packing,
3025 C<a>, and C<Z> are equivalent.
3027 If the value-to-pack is too long, it is truncated. If too long and an
3028 explicit count is provided, C<Z> packs only C<$count-1> bytes, followed
3029 by a null byte. Thus C<Z> always packs a trailing null byte under
3034 Likewise, the C<b> and C<B> fields pack a string that many bits long.
3035 Each byte of the input field of pack() generates 1 bit of the result.
3036 Each result bit is based on the least-significant bit of the corresponding
3037 input byte, i.e., on C<ord($byte)%2>. In particular, bytes C<"0"> and
3038 C<"1"> generate bits 0 and 1, as do bytes C<"\0"> and C<"\1">.
3040 Starting from the beginning of the input string of pack(), each 8-tuple
3041 of bytes is converted to 1 byte of output. With format C<b>
3042 the first byte of the 8-tuple determines the least-significant bit of a
3043 byte, and with format C<B> it determines the most-significant bit of
3046 If the length of the input string is not exactly divisible by 8, the
3047 remainder is packed as if the input string were padded by null bytes
3048 at the end. Similarly, during unpack()ing the "extra" bits are ignored.
3050 If the input string of pack() is longer than needed, extra bytes are ignored.
3051 A C<*> for the repeat count of pack() means to use all the bytes of
3052 the input field. On unpack()ing the bits are converted to a string
3053 of C<"0">s and C<"1">s.
3057 The C<h> and C<H> fields pack a string that many nybbles (4-bit groups,
3058 representable as hexadecimal digits, 0-9a-f) long.
3060 Each byte of the input field of pack() generates 4 bits of the result.
3061 For non-alphabetical bytes the result is based on the 4 least-significant
3062 bits of the input byte, i.e., on C<ord($byte)%16>. In particular,
3063 bytes C<"0"> and C<"1"> generate nybbles 0 and 1, as do bytes
3064 C<"\0"> and C<"\1">. For bytes C<"a".."f"> and C<"A".."F"> the result
3065 is compatible with the usual hexadecimal digits, so that C<"a"> and
3066 C<"A"> both generate the nybble C<0xa==10>. The result for bytes
3067 C<"g".."z"> and C<"G".."Z"> is not well-defined.
3069 Starting from the beginning of the input string of pack(), each pair
3070 of bytes is converted to 1 byte of output. With format C<h> the
3071 first byte of the pair determines the least-significant nybble of the
3072 output byte, and with format C<H> it determines the most-significant
3075 If the length of the input string is not even, it behaves as if padded
3076 by a null byte at the end. Similarly, during unpack()ing the "extra"
3077 nybbles are ignored.
3079 If the input string of pack() is longer than needed, extra bytes are ignored.
3080 A C<*> for the repeat count of pack() means to use all the bytes of
3081 the input field. On unpack()ing the bits are converted to a string
3082 of hexadecimal digits.
3086 The C<p> type packs a pointer to a null-terminated string. You are
3087 responsible for ensuring the string is not a temporary value (which can
3088 potentially get deallocated before you get around to using the packed result).
3089 The C<P> type packs a pointer to a structure of the size indicated by the
3090 length. A NULL pointer is created if the corresponding value for C<p> or
3091 C<P> is C<undef>, similarly for unpack().
3095 The C</> template character allows packing and unpacking of strings where
3096 the packed structure contains a byte count followed by the string itself.
3097 You write I<length-item>C</>I<string-item>.
3099 The I<length-item> can be any C<pack> template letter,
3100 and describes how the length value is packed.
3101 The ones likely to be of most use are integer-packing ones like
3102 C<n> (for Java strings), C<w> (for ASN.1 or SNMP)
3103 and C<N> (for Sun XDR).
3105 The I<string-item> must, at present, be C<"A*">, C<"a*"> or C<"Z*">.
3106 For C<unpack> the length of the string is obtained from the I<length-item>,
3107 but if you put in the '*' it will be ignored.
3109 unpack 'C/a', "\04Gurusamy"; gives 'Guru'
3110 unpack 'a3/A* A*', '007 Bond J '; gives (' Bond','J')
3111 pack 'n/a* w/a*','hello,','world'; gives "\000\006hello,\005world"
3113 The I<length-item> is not returned explicitly from C<unpack>.
3115 Adding a count to the I<length-item> letter is unlikely to do anything
3116 useful, unless that letter is C<A>, C<a> or C<Z>. Packing with a
3117 I<length-item> of C<a> or C<Z> may introduce C<"\000"> characters,
3118 which Perl does not regard as legal in numeric strings.
3122 The integer types C<s>, C<S>, C<l>, and C<L> may be
3123 immediately followed by a C<!> suffix to signify native shorts or
3124 longs--as you can see from above for example a bare C<l> does mean
3125 exactly 32 bits, the native C<long> (as seen by the local C compiler)
3126 may be larger. This is an issue mainly in 64-bit platforms. You can
3127 see whether using C<!> makes any difference by
3129 print length(pack("s")), " ", length(pack("s!")), "\n";
3130 print length(pack("l")), " ", length(pack("l!")), "\n";
3132 C<i!> and C<I!> also work but only because of completeness;
3133 they are identical to C<i> and C<I>.
3135 The actual sizes (in bytes) of native shorts, ints, longs, and long
3136 longs on the platform where Perl was built are also available via
3140 print $Config{shortsize}, "\n";
3141 print $Config{intsize}, "\n";
3142 print $Config{longsize}, "\n";
3143 print $Config{longlongsize}, "\n";
3145 (The C<$Config{longlongsize}> will be undefine if your system does
3146 not support long longs.)
3150 The integer formats C<s>, C<S>, C<i>, C<I>, C<l>, and C<L>
3151 are inherently non-portable between processors and operating systems
3152 because they obey the native byteorder and endianness. For example a
3153 4-byte integer 0x12345678 (305419896 decimal) be ordered natively
3154 (arranged in and handled by the CPU registers) into bytes as
3156 0x12 0x34 0x56 0x78 # big-endian
3157 0x78 0x56 0x34 0x12 # little-endian
3159 Basically, the Intel, Alpha, and VAX CPUs are little-endian, while
3160 everybody else, for example Motorola m68k/88k, PPC, Sparc, HP PA,
3161 Power, and Cray are big-endian. MIPS can be either: Digital used it
3162 in little-endian mode; SGI uses it in big-endian mode.
3164 The names `big-endian' and `little-endian' are comic references to
3165 the classic "Gulliver's Travels" (via the paper "On Holy Wars and a
3166 Plea for Peace" by Danny Cohen, USC/ISI IEN 137, April 1, 1980) and
3167 the egg-eating habits of the Lilliputians.
3169 Some systems may have even weirder byte orders such as
3174 You can see your system's preference with
3176 print join(" ", map { sprintf "%#02x", $_ }
3177 unpack("C*",pack("L",0x12345678))), "\n";
3179 The byteorder on the platform where Perl was built is also available
3183 print $Config{byteorder}, "\n";
3185 Byteorders C<'1234'> and C<'12345678'> are little-endian, C<'4321'>
3186 and C<'87654321'> are big-endian.
3188 If you want portable packed integers use the formats C<n>, C<N>,
3189 C<v>, and C<V>, their byte endianness and size is known.
3190 See also L<perlport>.
3194 Real numbers (floats and doubles) are in the native machine format only;
3195 due to the multiplicity of floating formats around, and the lack of a
3196 standard "network" representation, no facility for interchange has been
3197 made. This means that packed floating point data written on one machine
3198 may not be readable on another - even if both use IEEE floating point
3199 arithmetic (as the endian-ness of the memory representation is not part
3200 of the IEEE spec). See also L<perlport>.
3202 Note that Perl uses doubles internally for all numeric calculation, and
3203 converting from double into float and thence back to double again will
3204 lose precision (i.e., C<unpack("f", pack("f", $foo)>) will not in general
3209 If the pattern begins with a C<U>, the resulting string will be treated
3210 as Unicode-encoded. You can force UTF8 encoding on in a string with an
3211 initial C<U0>, and the bytes that follow will be interpreted as Unicode
3212 characters. If you don't want this to happen, you can begin your pattern
3213 with C<C0> (or anything else) to force Perl not to UTF8 encode your
3214 string, and then follow this with a C<U*> somewhere in your pattern.
3218 You must yourself do any alignment or padding by inserting for example
3219 enough C<'x'>es while packing. There is no way to pack() and unpack()
3220 could know where the bytes are going to or coming from. Therefore
3221 C<pack> (and C<unpack>) handle their output and input as flat
3226 A comment in a TEMPLATE starts with C<#> and goes to the end of line.
3230 If TEMPLATE requires more arguments to pack() than actually given, pack()
3231 assumes additional C<""> arguments. If TEMPLATE requires less arguments
3232 to pack() than actually given, extra arguments are ignored.
3238 $foo = pack("CCCC",65,66,67,68);
3240 $foo = pack("C4",65,66,67,68);
3242 $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);
3243 # same thing with Unicode circled letters
3245 $foo = pack("ccxxcc",65,66,67,68);
3248 # note: the above examples featuring "C" and "c" are true
3249 # only on ASCII and ASCII-derived systems such as ISO Latin 1
3250 # and UTF-8. In EBCDIC the first example would be
3251 # $foo = pack("CCCC",193,194,195,196);
3253 $foo = pack("s2",1,2);
3254 # "\1\0\2\0" on little-endian
3255 # "\0\1\0\2" on big-endian
3257 $foo = pack("a4","abcd","x","y","z");
3260 $foo = pack("aaaa","abcd","x","y","z");
3263 $foo = pack("a14","abcdefg");
3264 # "abcdefg\0\0\0\0\0\0\0"
3266 $foo = pack("i9pl", gmtime);
3267 # a real struct tm (on my system anyway)
3269 $utmp_template = "Z8 Z8 Z16 L";
3270 $utmp = pack($utmp_template, @utmp1);
3271 # a struct utmp (BSDish)
3273 @utmp2 = unpack($utmp_template, $utmp);
3274 # "@utmp1" eq "@utmp2"
3277 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
3280 $foo = pack('sx2l', 12, 34);
3281 # short 12, two zero bytes padding, long 34
3282 $bar = pack('s@4l', 12, 34);
3283 # short 12, zero fill to position 4, long 34
3286 The same template may generally also be used in unpack().
3288 =item package NAMESPACE
3292 Declares the compilation unit as being in the given namespace. The scope
3293 of the package declaration is from the declaration itself through the end
3294 of the enclosing block, file, or eval (the same as the C<my> operator).
3295 All further unqualified dynamic identifiers will be in this namespace.
3296 A package statement affects only dynamic variables--including those
3297 you've used C<local> on--but I<not> lexical variables, which are created
3298 with C<my>. Typically it would be the first declaration in a file to
3299 be included by the C<require> or C<use> operator. You can switch into a
3300 package in more than one place; it merely influences which symbol table
3301 is used by the compiler for the rest of that block. You can refer to
3302 variables and filehandles in other packages by prefixing the identifier
3303 with the package name and a double colon: C<$Package::Variable>.
3304 If the package name is null, the C<main> package as assumed. That is,
3305 C<$::sail> is equivalent to C<$main::sail> (as well as to C<$main'sail>,
3306 still seen in older code).
3308 If NAMESPACE is omitted, then there is no current package, and all
3309 identifiers must be fully qualified or lexicals. This is stricter
3310 than C<use strict>, since it also extends to function names.
3312 See L<perlmod/"Packages"> for more information about packages, modules,
3313 and classes. See L<perlsub> for other scoping issues.
3315 =item pipe READHANDLE,WRITEHANDLE
3317 Opens a pair of connected pipes like the corresponding system call.
3318 Note that if you set up a loop of piped processes, deadlock can occur
3319 unless you are very careful. In addition, note that Perl's pipes use
3320 stdio buffering, so you may need to set C<$|> to flush your WRITEHANDLE
3321 after each command, depending on the application.
3323 See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
3324 for examples of such things.
3326 On systems that support a close-on-exec flag on files, the flag will be set
3327 for the newly opened file descriptors as determined by the value of $^F.
3334 Pops and returns the last value of the array, shortening the array by
3335 one element. Has an effect similar to
3339 If there are no elements in the array, returns the undefined value
3340 (although this may happen at other times as well). If ARRAY is
3341 omitted, pops the C<@ARGV> array in the main program, and the C<@_>
3342 array in subroutines, just like C<shift>.
3348 Returns the offset of where the last C<m//g> search left off for the variable
3349 in question (C<$_> is used when the variable is not specified). May be
3350 modified to change that offset. Such modification will also influence
3351 the C<\G> zero-width assertion in regular expressions. See L<perlre> and
3354 =item print FILEHANDLE LIST
3360 Prints a string or a list of strings. Returns true if successful.
3361 FILEHANDLE may be a scalar variable name, in which case the variable
3362 contains the name of or a reference to the filehandle, thus introducing
3363 one level of indirection. (NOTE: If FILEHANDLE is a variable and
3364 the next token is a term, it may be misinterpreted as an operator
3365 unless you interpose a C<+> or put parentheses around the arguments.)
3366 If FILEHANDLE is omitted, prints by default to standard output (or
3367 to the last selected output channel--see L</select>). If LIST is
3368 also omitted, prints C<$_> to the currently selected output channel.
3369 To set the default output channel to something other than STDOUT
3370 use the select operation. The current value of C<$,> (if any) is
3371 printed between each LIST item. The current value of C<$\> (if
3372 any) is printed after the entire LIST has been printed. Because
3373 print takes a LIST, anything in the LIST is evaluated in list
3374 context, and any subroutine that you call will have one or more of
3375 its expressions evaluated in list context. Also be careful not to
3376 follow the print keyword with a left parenthesis unless you want
3377 the corresponding right parenthesis to terminate the arguments to
3378 the print--interpose a C<+> or put parentheses around all the
3381 Note that if you're storing FILEHANDLES in an array or other expression,
3382 you will have to use a block returning its value instead:
3384 print { $files[$i] } "stuff\n";
3385 print { $OK ? STDOUT : STDERR } "stuff\n";
3387 =item printf FILEHANDLE FORMAT, LIST
3389 =item printf FORMAT, LIST
3391 Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that C<$\>
3392 (the output record separator) is not appended. The first argument
3393 of the list will be interpreted as the C<printf> format. If C<use locale> is
3394 in effect, the character used for the decimal point in formatted real numbers
3395 is affected by the LC_NUMERIC locale. See L<perllocale>.
3397 Don't fall into the trap of using a C<printf> when a simple
3398 C<print> would do. The C<print> is more efficient and less
3401 =item prototype FUNCTION
3403 Returns the prototype of a function as a string (or C<undef> if the
3404 function has no prototype). FUNCTION is a reference to, or the name of,
3405 the function whose prototype you want to retrieve.
3407 If FUNCTION is a string starting with C<CORE::>, the rest is taken as a
3408 name for Perl builtin. If the builtin is not I<overridable> (such as
3409 C<qw//>) or its arguments cannot be expressed by a prototype (such as
3410 C<system>) returns C<undef> because the builtin does not really behave
3411 like a Perl function. Otherwise, the string describing the equivalent
3412 prototype is returned.
3414 =item push ARRAY,LIST
3416 Treats ARRAY as a stack, and pushes the values of LIST
3417 onto the end of ARRAY. The length of ARRAY increases by the length of
3418 LIST. Has the same effect as
3421 $ARRAY[++$#ARRAY] = $value;
3424 but is more efficient. Returns the new number of elements in the array.
3436 Generalized quotes. See L<perlop/"Regexp Quote-Like Operators">.
3438 =item quotemeta EXPR
3442 Returns the value of EXPR with all non-"word"
3443 characters backslashed. (That is, all characters not matching
3444 C</[A-Za-z_0-9]/> will be preceded by a backslash in the
3445 returned string, regardless of any locale settings.)
3446 This is the internal function implementing
3447 the C<\Q> escape in double-quoted strings.
3449 If EXPR is omitted, uses C<$_>.
3455 Returns a random fractional number greater than or equal to C<0> and less
3456 than the value of EXPR. (EXPR should be positive.) If EXPR is
3457 omitted, the value C<1> is used. Automatically calls C<srand> unless
3458 C<srand> has already been called. See also C<srand>.
3460 (Note: If your rand function consistently returns numbers that are too
3461 large or too small, then your version of Perl was probably compiled
3462 with the wrong number of RANDBITS.)
3464 =item read FILEHANDLE,SCALAR,LENGTH,OFFSET
3466 =item read FILEHANDLE,SCALAR,LENGTH
3468 Attempts to read LENGTH bytes of data into variable SCALAR from the
3469 specified FILEHANDLE. Returns the number of bytes actually read,
3470 C<0> at end of file, or undef if there was an error. SCALAR will be grown
3471 or shrunk to the length actually read. An OFFSET may be specified to
3472 place the read data at some other place than the beginning of the
3473 string. This call is actually implemented in terms of stdio's fread(3)
3474 call. To get a true read(2) system call, see C<sysread>.
3476 =item readdir DIRHANDLE
3478 Returns the next directory entry for a directory opened by C<opendir>.
3479 If used in list context, returns all the rest of the entries in the
3480 directory. If there are no more entries, returns an undefined value in
3481 scalar context or a null list in list context.
3483 If you're planning to filetest the return values out of a C<readdir>, you'd
3484 better prepend the directory in question. Otherwise, because we didn't
3485 C<chdir> there, it would have been testing the wrong file.
3487 opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!";
3488 @dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR);
3493 Reads from the filehandle whose typeglob is contained in EXPR. In scalar
3494 context, each call reads and returns the next line, until end-of-file is
3495 reached, whereupon the subsequent call returns undef. In list context,
3496 reads until end-of-file is reached and returns a list of lines. Note that
3497 the notion of "line" used here is however you may have defined it
3498 with C<$/> or C<$INPUT_RECORD_SEPARATOR>). See L<perlvar/"$/">.
3500 When C<$/> is set to C<undef>, when readline() is in scalar
3501 context (i.e. file slurp mode), and when an empty file is read, it
3502 returns C<''> the first time, followed by C<undef> subsequently.
3504 This is the internal function implementing the C<< <EXPR> >>
3505 operator, but you can use it directly. The C<< <EXPR> >>
3506 operator is discussed in more detail in L<perlop/"I/O Operators">.
3509 $line = readline(*STDIN); # same thing
3515 Returns the value of a symbolic link, if symbolic links are
3516 implemented. If not, gives a fatal error. If there is some system
3517 error, returns the undefined value and sets C<$!> (errno). If EXPR is
3518 omitted, uses C<$_>.
3522 EXPR is executed as a system command.
3523 The collected standard output of the command is returned.
3524 In scalar context, it comes back as a single (potentially
3525 multi-line) string. In list context, returns a list of lines
3526 (however you've defined lines with C<$/> or C<$INPUT_RECORD_SEPARATOR>).
3527 This is the internal function implementing the C<qx/EXPR/>
3528 operator, but you can use it directly. The C<qx/EXPR/>
3529 operator is discussed in more detail in L<perlop/"I/O Operators">.
3531 =item recv SOCKET,SCALAR,LENGTH,FLAGS
3533 Receives a message on a socket. Attempts to receive LENGTH bytes of
3534 data into variable SCALAR from the specified SOCKET filehandle. SCALAR
3535 will be grown or shrunk to the length actually read. Takes the same
3536 flags as the system call of the same name. Returns the address of the
3537 sender if SOCKET's protocol supports this; returns an empty string
3538 otherwise. If there's an error, returns the undefined value. This call
3539 is actually implemented in terms of recvfrom(2) system call. See
3540 L<perlipc/"UDP: Message Passing"> for examples.
3546 The C<redo> command restarts the loop block without evaluating the
3547 conditional again. The C<continue> block, if any, is not executed. If
3548 the LABEL is omitted, the command refers to the innermost enclosing
3549 loop. This command is normally used by programs that want to lie to
3550 themselves about what was just input:
3552 # a simpleminded Pascal comment stripper
3553 # (warning: assumes no { or } in strings)
3554 LINE: while (<STDIN>) {
3555 while (s|({.*}.*){.*}|$1 |) {}
3560 if (/}/) { # end of comment?
3569 C<redo> cannot be used to retry a block which returns a value such as
3570 C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
3571 a grep() or map() operation.
3573 Note that a block by itself is semantically identical to a loop
3574 that executes once. Thus C<redo> inside such a block will effectively
3575 turn it into a looping construct.
3577 See also L</continue> for an illustration of how C<last>, C<next>, and
3584 Returns a true value if EXPR is a reference, false otherwise. If EXPR
3585 is not specified, C<$_> will be used. The value returned depends on the
3586 type of thing the reference is a reference to.
3587 Builtin types include:
3597 If the referenced object has been blessed into a package, then that package
3598 name is returned instead. You can think of C<ref> as a C<typeof> operator.
3600 if (ref($r) eq "HASH") {
3601 print "r is a reference to a hash.\n";
3604 print "r is not a reference at all.\n";
3606 if (UNIVERSAL::isa($r, "HASH")) { # for subclassing
3607 print "r is a reference to something that isa hash.\n";
3610 See also L<perlref>.
3612 =item rename OLDNAME,NEWNAME
3614 Changes the name of a file; an existing file NEWNAME will be
3615 clobbered. Returns true for success, false otherwise.
3617 Behavior of this function varies wildly depending on your system
3618 implementation. For example, it will usually not work across file system
3619 boundaries, even though the system I<mv> command sometimes compensates
3620 for this. Other restrictions include whether it works on directories,
3621 open files, or pre-existing files. Check L<perlport> and either the
3622 rename(2) manpage or equivalent system documentation for details.
3624 =item require VERSION
3630 Demands some semantics specified by EXPR, or by C<$_> if EXPR is not
3633 If a VERSION is specified as a literal of the form v5.6.1,
3634 demands that the current version of Perl (C<$^V> or $PERL_VERSION) be
3635 at least as recent as that version, at run time. (For compatibility
3636 with older versions of Perl, a numeric argument will also be interpreted
3637 as VERSION.) Compare with L</use>, which can do a similar check at
3640 require v5.6.1; # run time version check
3641 require 5.6.1; # ditto
3642 require 5.005_03; # float version allowed for compatibility
3644 Otherwise, demands that a library file be included if it hasn't already
3645 been included. The file is included via the do-FILE mechanism, which is
3646 essentially just a variety of C<eval>. Has semantics similar to the following
3651 return 1 if $INC{$filename};
3652 my($realfilename,$result);
3654 foreach $prefix (@INC) {
3655 $realfilename = "$prefix/$filename";
3656 if (-f $realfilename) {
3657 $INC{$filename} = $realfilename;
3658 $result = do $realfilename;
3662 die "Can't find $filename in \@INC";
3664 delete $INC{$filename} if $@ || !$result;
3666 die "$filename did not return true value" unless $result;
3670 Note that the file will not be included twice under the same specified
3671 name. The file must return true as the last statement to indicate
3672 successful execution of any initialization code, so it's customary to
3673 end such a file with C<1;> unless you're sure it'll return true
3674 otherwise. But it's better just to put the C<1;>, in case you add more
3677 If EXPR is a bareword, the require assumes a "F<.pm>" extension and
3678 replaces "F<::>" with "F</>" in the filename for you,
3679 to make it easy to load standard modules. This form of loading of
3680 modules does not risk altering your namespace.
3682 In other words, if you try this:
3684 require Foo::Bar; # a splendid bareword
3686 The require function will actually look for the "F<Foo/Bar.pm>" file in the
3687 directories specified in the C<@INC> array.
3689 But if you try this:
3691 $class = 'Foo::Bar';
3692 require $class; # $class is not a bareword
3694 require "Foo::Bar"; # not a bareword because of the ""
3696 The require function will look for the "F<Foo::Bar>" file in the @INC array and
3697 will complain about not finding "F<Foo::Bar>" there. In this case you can do:
3699 eval "require $class";
3701 For a yet-more-powerful import facility, see L</use> and L<perlmod>.
3707 Generally used in a C<continue> block at the end of a loop to clear
3708 variables and reset C<??> searches so that they work again. The
3709 expression is interpreted as a list of single characters (hyphens
3710 allowed for ranges). All variables and arrays beginning with one of
3711 those letters are reset to their pristine state. If the expression is
3712 omitted, one-match searches (C<?pattern?>) are reset to match again. Resets
3713 only variables or searches in the current package. Always returns
3716 reset 'X'; # reset all X variables
3717 reset 'a-z'; # reset lower case variables
3718 reset; # just reset ?one-time? searches
3720 Resetting C<"A-Z"> is not recommended because you'll wipe out your
3721 C<@ARGV> and C<@INC> arrays and your C<%ENV> hash. Resets only package
3722 variables--lexical variables are unaffected, but they clean themselves
3723 up on scope exit anyway, so you'll probably want to use them instead.
3730 Returns from a subroutine, C<eval>, or C<do FILE> with the value
3731 given in EXPR. Evaluation of EXPR may be in list, scalar, or void
3732 context, depending on how the return value will be used, and the context
3733 may vary from one execution to the next (see C<wantarray>). If no EXPR
3734 is given, returns an empty list in list context, the undefined value in
3735 scalar context, and (of course) nothing at all in a void context.
3737 (Note that in the absence of a explicit C<return>, a subroutine, eval,
3738 or do FILE will automatically return the value of the last expression
3743 In list context, returns a list value consisting of the elements
3744 of LIST in the opposite order. In scalar context, concatenates the
3745 elements of LIST and returns a string value with all characters
3746 in the opposite order.
3748 print reverse <>; # line tac, last line first
3750 undef $/; # for efficiency of <>
3751 print scalar reverse <>; # character tac, last line tsrif
3753 This operator is also handy for inverting a hash, although there are some
3754 caveats. If a value is duplicated in the original hash, only one of those
3755 can be represented as a key in the inverted hash. Also, this has to
3756 unwind one hash and build a whole new one, which may take some time
3757 on a large hash, such as from a DBM file.
3759 %by_name = reverse %by_address; # Invert the hash
3761 =item rewinddir DIRHANDLE
3763 Sets the current position to the beginning of the directory for the
3764 C<readdir> routine on DIRHANDLE.
3766 =item rindex STR,SUBSTR,POSITION
3768 =item rindex STR,SUBSTR
3770 Works just like index() except that it returns the position of the LAST
3771 occurrence of SUBSTR in STR. If POSITION is specified, returns the
3772 last occurrence at or before that position.
3774 =item rmdir FILENAME
3778 Deletes the directory specified by FILENAME if that directory is empty. If it
3779 succeeds it returns true, otherwise it returns false and sets C<$!> (errno). If
3780 FILENAME is omitted, uses C<$_>.
3784 The substitution operator. See L<perlop>.
3788 Forces EXPR to be interpreted in scalar context and returns the value
3791 @counts = ( scalar @a, scalar @b, scalar @c );
3793 There is no equivalent operator to force an expression to
3794 be interpolated in list context because in practice, this is never
3795 needed. If you really wanted to do so, however, you could use
3796 the construction C<@{[ (some expression) ]}>, but usually a simple
3797 C<(some expression)> suffices.
3799 Because C<scalar> is unary operator, if you accidentally use for EXPR a
3800 parenthesized list, this behaves as a scalar comma expression, evaluating
3801 all but the last element in void context and returning the final element
3802 evaluated in scalar context. This is seldom what you want.
3804 The following single statement:
3806 print uc(scalar(&foo,$bar)),$baz;
3808 is the moral equivalent of these two:
3811 print(uc($bar),$baz);
3813 See L<perlop> for more details on unary operators and the comma operator.
3815 =item seek FILEHANDLE,POSITION,WHENCE
3817 Sets FILEHANDLE's position, just like the C<fseek> call of C<stdio>.
3818 FILEHANDLE may be an expression whose value gives the name of the
3819 filehandle. The values for WHENCE are C<0> to set the new position to
3820 POSITION, C<1> to set it to the current position plus POSITION, and
3821 C<2> to set it to EOF plus POSITION (typically negative). For WHENCE
3822 you may use the constants C<SEEK_SET>, C<SEEK_CUR>, and C<SEEK_END>
3823 (start of the file, current position, end of the file) from the Fcntl
3824 module. Returns C<1> upon success, C<0> otherwise.
3826 If you want to position file for C<sysread> or C<syswrite>, don't use
3827 C<seek>--buffering makes its effect on the file's system position
3828 unpredictable and non-portable. Use C<sysseek> instead.
3830 Due to the rules and rigors of ANSI C, on some systems you have to do a
3831 seek whenever you switch between reading and writing. Amongst other
3832 things, this may have the effect of calling stdio's clearerr(3).
3833 A WHENCE of C<1> (C<SEEK_CUR>) is useful for not moving the file position:
3837 This is also useful for applications emulating C<tail -f>. Once you hit
3838 EOF on your read, and then sleep for a while, you might have to stick in a
3839 seek() to reset things. The C<seek> doesn't change the current position,
3840 but it I<does> clear the end-of-file condition on the handle, so that the
3841 next C<< <FILE> >> makes Perl try again to read something. We hope.
3843 If that doesn't work (some stdios are particularly cantankerous), then
3844 you may need something more like this:
3847 for ($curpos = tell(FILE); $_ = <FILE>;
3848 $curpos = tell(FILE)) {
3849 # search for some stuff and put it into files
3851 sleep($for_a_while);
3852 seek(FILE, $curpos, 0);
3855 =item seekdir DIRHANDLE,POS
3857 Sets the current position for the C<readdir> routine on DIRHANDLE. POS
3858 must be a value returned by C<telldir>. Has the same caveats about
3859 possible directory compaction as the corresponding system library
3862 =item select FILEHANDLE
3866 Returns the currently selected filehandle. Sets the current default
3867 filehandle for output, if FILEHANDLE is supplied. This has two
3868 effects: first, a C<write> or a C<print> without a filehandle will
3869 default to this FILEHANDLE. Second, references to variables related to
3870 output will refer to this output channel. For example, if you have to
3871 set the top of form format for more than one output channel, you might
3879 FILEHANDLE may be an expression whose value gives the name of the
3880 actual filehandle. Thus:
3882 $oldfh = select(STDERR); $| = 1; select($oldfh);
3884 Some programmers may prefer to think of filehandles as objects with
3885 methods, preferring to write the last example as:
3888 STDERR->autoflush(1);
3890 =item select RBITS,WBITS,EBITS,TIMEOUT
3892 This calls the select(2) system call with the bit masks specified, which
3893 can be constructed using C<fileno> and C<vec>, along these lines:
3895 $rin = $win = $ein = '';
3896 vec($rin,fileno(STDIN),1) = 1;
3897 vec($win,fileno(STDOUT),1) = 1;
3900 If you want to select on many filehandles you might wish to write a
3904 my(@fhlist) = split(' ',$_[0]);
3907 vec($bits,fileno($_),1) = 1;
3911 $rin = fhbits('STDIN TTY SOCK');
3915 ($nfound,$timeleft) =
3916 select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
3918 or to block until something becomes ready just do this
3920 $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
3922 Most systems do not bother to return anything useful in $timeleft, so
3923 calling select() in scalar context just returns $nfound.
3925 Any of the bit masks can also be undef. The timeout, if specified, is
3926 in seconds, which may be fractional. Note: not all implementations are
3927 capable of returning the$timeleft. If not, they always return
3928 $timeleft equal to the supplied $timeout.
3930 You can effect a sleep of 250 milliseconds this way:
3932 select(undef, undef, undef, 0.25);
3934 B<WARNING>: One should not attempt to mix buffered I/O (like C<read>
3935 or <FH>) with C<select>, except as permitted by POSIX, and even
3936 then only on POSIX systems. You have to use C<sysread> instead.
3938 =item semctl ID,SEMNUM,CMD,ARG
3940 Calls the System V IPC function C<semctl>. You'll probably have to say
3944 first to get the correct constant definitions. If CMD is IPC_STAT or
3945 GETALL, then ARG must be a variable which will hold the returned
3946 semid_ds structure or semaphore value array. Returns like C<ioctl>:
3947 the undefined value for error, "C<0 but true>" for zero, or the actual
3948 return value otherwise. The ARG must consist of a vector of native
3949 short integers, which may be created with C<pack("s!",(0)x$nsem)>.
3950 See also L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::Semaphore>
3953 =item semget KEY,NSEMS,FLAGS
3955 Calls the System V IPC function semget. Returns the semaphore id, or
3956 the undefined value if there is an error. See also
3957 L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::SysV::Semaphore>
3960 =item semop KEY,OPSTRING
3962 Calls the System V IPC function semop to perform semaphore operations
3963 such as signaling and waiting. OPSTRING must be a packed array of
3964 semop structures. Each semop structure can be generated with
3965 C<pack("sss", $semnum, $semop, $semflag)>. The number of semaphore
3966 operations is implied by the length of OPSTRING. Returns true if
3967 successful, or false if there is an error. As an example, the
3968 following code waits on semaphore $semnum of semaphore id $semid:
3970 $semop = pack("sss", $semnum, -1, 0);
3971 die "Semaphore trouble: $!\n" unless semop($semid, $semop);
3973 To signal the semaphore, replace C<-1> with C<1>. See also
3974 L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::SysV::Semaphore>
3977 =item send SOCKET,MSG,FLAGS,TO
3979 =item send SOCKET,MSG,FLAGS
3981 Sends a message on a socket. Takes the same flags as the system call
3982 of the same name. On unconnected sockets you must specify a
3983 destination to send TO, in which case it does a C C<sendto>. Returns
3984 the number of characters sent, or the undefined value if there is an
3985 error. The C system call sendmsg(2) is currently unimplemented.
3986 See L<perlipc/"UDP: Message Passing"> for examples.
3988 =item setpgrp PID,PGRP
3990 Sets the current process group for the specified PID, C<0> for the current
3991 process. Will produce a fatal error if used on a machine that doesn't
3992 implement POSIX setpgid(2) or BSD setpgrp(2). If the arguments are omitted,
3993 it defaults to C<0,0>. Note that the BSD 4.2 version of C<setpgrp> does not
3994 accept any arguments, so only C<setpgrp(0,0)> is portable. See also
3997 =item setpriority WHICH,WHO,PRIORITY
3999 Sets the current priority for a process, a process group, or a user.
4000 (See setpriority(2).) Will produce a fatal error if used on a machine
4001 that doesn't implement setpriority(2).
4003 =item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
4005 Sets the socket option requested. Returns undefined if there is an
4006 error. OPTVAL may be specified as C<undef> if you don't want to pass an
4013 Shifts the first value of the array off and returns it, shortening the
4014 array by 1 and moving everything down. If there are no elements in the
4015 array, returns the undefined value. If ARRAY is omitted, shifts the
4016 C<@_> array within the lexical scope of subroutines and formats, and the
4017 C<@ARGV> array at file scopes or within the lexical scopes established by
4018 the C<eval ''>, C<BEGIN {}>, C<INIT {}>, C<CHECK {}>, and C<END {}>
4021 See also C<unshift>, C<push>, and C<pop>. C<shift> and C<unshift> do the
4022 same thing to the left end of an array that C<pop> and C<push> do to the
4025 =item shmctl ID,CMD,ARG
4027 Calls the System V IPC function shmctl. You'll probably have to say
4031 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
4032 then ARG must be a variable which will hold the returned C<shmid_ds>
4033 structure. Returns like ioctl: the undefined value for error, "C<0> but
4034 true" for zero, or the actual return value otherwise.
4035 See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation.
4037 =item shmget KEY,SIZE,FLAGS
4039 Calls the System V IPC function shmget. Returns the shared memory
4040 segment id, or the undefined value if there is an error.
4041 See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation.
4043 =item shmread ID,VAR,POS,SIZE
4045 =item shmwrite ID,STRING,POS,SIZE
4047 Reads or writes the System V shared memory segment ID starting at
4048 position POS for size SIZE by attaching to it, copying in/out, and
4049 detaching from it. When reading, VAR must be a variable that will
4050 hold the data read. When writing, if STRING is too long, only SIZE
4051 bytes are used; if STRING is too short, nulls are written to fill out
4052 SIZE bytes. Return true if successful, or false if there is an error.
4053 shmread() taints the variable. See also L<perlipc/"SysV IPC">,
4054 C<IPC::SysV> documentation, and the C<IPC::Shareable> module from CPAN.
4056 =item shutdown SOCKET,HOW
4058 Shuts down a socket connection in the manner indicated by HOW, which
4059 has the same interpretation as in the system call of the same name.
4061 shutdown(SOCKET, 0); # I/we have stopped reading data
4062 shutdown(SOCKET, 1); # I/we have stopped writing data
4063 shutdown(SOCKET, 2); # I/we have stopped using this socket
4065 This is useful with sockets when you want to tell the other
4066 side you're done writing but not done reading, or vice versa.
4067 It's also a more insistent form of close because it also
4068 disables the file descriptor in any forked copies in other
4075 Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
4076 returns sine of C<$_>.
4078 For the inverse sine operation, you may use the C<Math::Trig::asin>
4079 function, or use this relation:
4081 sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
4087 Causes the script to sleep for EXPR seconds, or forever if no EXPR.
4088 May be interrupted if the process receives a signal such as C<SIGALRM>.
4089 Returns the number of seconds actually slept. You probably cannot
4090 mix C<alarm> and C<sleep> calls, because C<sleep> is often implemented
4093 On some older systems, it may sleep up to a full second less than what
4094 you requested, depending on how it counts seconds. Most modern systems
4095 always sleep the full amount. They may appear to sleep longer than that,
4096 however, because your process might not be scheduled right away in a
4097 busy multitasking system.
4099 For delays of finer granularity than one second, you may use Perl's
4100 C<syscall> interface to access setitimer(2) if your system supports
4101 it, or else see L</select> above. The Time::HiRes module from CPAN
4104 See also the POSIX module's C<sigpause> function.
4106 =item socket SOCKET,DOMAIN,TYPE,PROTOCOL
4108 Opens a socket of the specified kind and attaches it to filehandle
4109 SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for
4110 the system call of the same name. You should C<use Socket> first
4111 to get the proper definitions imported. See the examples in
4112 L<perlipc/"Sockets: Client/Server Communication">.
4114 On systems that support a close-on-exec flag on files, the flag will
4115 be set for the newly opened file descriptor, as determined by the
4116 value of $^F. See L<perlvar/$^F>.
4118 =item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
4120 Creates an unnamed pair of sockets in the specified domain, of the
4121 specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as
4122 for the system call of the same name. If unimplemented, yields a fatal
4123 error. Returns true if successful.
4125 On systems that support a close-on-exec flag on files, the flag will
4126 be set for the newly opened file descriptors, as determined by the value
4127 of $^F. See L<perlvar/$^F>.
4129 Some systems defined C<pipe> in terms of C<socketpair>, in which a call
4130 to C<pipe(Rdr, Wtr)> is essentially:
4133 socketpair(Rdr, Wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC);
4134 shutdown(Rdr, 1); # no more writing for reader
4135 shutdown(Wtr, 0); # no more reading for writer
4137 See L<perlipc> for an example of socketpair use.
4139 =item sort SUBNAME LIST
4141 =item sort BLOCK LIST
4145 Sorts the LIST and returns the sorted list value. If SUBNAME or BLOCK
4146 is omitted, C<sort>s in standard string comparison order. If SUBNAME is
4147 specified, it gives the name of a subroutine that returns an integer
4148 less than, equal to, or greater than C<0>, depending on how the elements
4149 of the list are to be ordered. (The C<< <=> >> and C<cmp>
4150 operators are extremely useful in such routines.) SUBNAME may be a
4151 scalar variable name (unsubscripted), in which case the value provides
4152 the name of (or a reference to) the actual subroutine to use. In place
4153 of a SUBNAME, you can provide a BLOCK as an anonymous, in-line sort
4156 If the subroutine's prototype is C<($$)>, the elements to be compared
4157 are passed by reference in C<@_>, as for a normal subroutine. This is
4158 slower than unprototyped subroutines, where the elements to be
4159 compared are passed into the subroutine
4160 as the package global variables $a and $b (see example below). Note that
4161 in the latter case, it is usually counter-productive to declare $a and
4164 In either case, the subroutine may not be recursive. The values to be
4165 compared are always passed by reference, so don't modify them.
4167 You also cannot exit out of the sort block or subroutine using any of the
4168 loop control operators described in L<perlsyn> or with C<goto>.
4170 When C<use locale> is in effect, C<sort LIST> sorts LIST according to the
4171 current collation locale. See L<perllocale>.
4176 @articles = sort @files;
4178 # same thing, but with explicit sort routine
4179 @articles = sort {$a cmp $b} @files;
4181 # now case-insensitively
4182 @articles = sort {uc($a) cmp uc($b)} @files;
4184 # same thing in reversed order
4185 @articles = sort {$b cmp $a} @files;
4187 # sort numerically ascending
4188 @articles = sort {$a <=> $b} @files;
4190 # sort numerically descending
4191 @articles = sort {$b <=> $a} @files;
4193 # this sorts the %age hash by value instead of key
4194 # using an in-line function
4195 @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
4197 # sort using explicit subroutine name
4199 $age{$a} <=> $age{$b}; # presuming numeric
4201 @sortedclass = sort byage @class;
4203 sub backwards { $b cmp $a }
4204 @harry = qw(dog cat x Cain Abel);
4205 @george = qw(gone chased yz Punished Axed);
4207 # prints AbelCaincatdogx
4208 print sort backwards @harry;
4209 # prints xdogcatCainAbel
4210 print sort @george, 'to', @harry;
4211 # prints AbelAxedCainPunishedcatchaseddoggonetoxyz
4213 # inefficiently sort by descending numeric compare using
4214 # the first integer after the first = sign, or the
4215 # whole record case-insensitively otherwise
4218 ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
4223 # same thing, but much more efficiently;
4224 # we'll build auxiliary indices instead
4228 push @nums, /=(\d+)/;
4233 $nums[$b] <=> $nums[$a]
4235 $caps[$a] cmp $caps[$b]
4239 # same thing, but without any temps
4240 @new = map { $_->[0] }
4241 sort { $b->[1] <=> $a->[1]
4244 } map { [$_, /=(\d+)/, uc($_)] } @old;
4246 # using a prototype allows you to use any comparison subroutine
4247 # as a sort subroutine (including other package's subroutines)
4249 sub backwards ($$) { $_[1] cmp $_[0]; } # $a and $b are not set here
4252 @new = sort other::backwards @old;
4254 If you're using strict, you I<must not> declare $a
4255 and $b as lexicals. They are package globals. That means
4256 if you're in the C<main> package and type
4258 @articles = sort {$b <=> $a} @files;
4260 then C<$a> and C<$b> are C<$main::a> and C<$main::b> (or C<$::a> and C<$::b>),
4261 but if you're in the C<FooPack> package, it's the same as typing
4263 @articles = sort {$FooPack::b <=> $FooPack::a} @files;
4265 The comparison function is required to behave. If it returns
4266 inconsistent results (sometimes saying C<$x[1]> is less than C<$x[2]> and
4267 sometimes saying the opposite, for example) the results are not
4270 =item splice ARRAY,OFFSET,LENGTH,LIST
4272 =item splice ARRAY,OFFSET,LENGTH
4274 =item splice ARRAY,OFFSET
4278 Removes the elements designated by OFFSET and LENGTH from an array, and
4279 replaces them with the elements of LIST, if any. In list context,
4280 returns the elements removed from the array. In scalar context,
4281 returns the last element removed, or C<undef> if no elements are
4282 removed. The array grows or shrinks as necessary.
4283 If OFFSET is negative then it starts that far from the end of the array.
4284 If LENGTH is omitted, removes everything from OFFSET onward.
4285 If LENGTH is negative, leaves that many elements off the end of the array.
4286 If both OFFSET and LENGTH are omitted, removes everything.
4288 The following equivalences hold (assuming C<$[ == 0>):
4290 push(@a,$x,$y) splice(@a,@a,0,$x,$y)
4291 pop(@a) splice(@a,-1)
4292 shift(@a) splice(@a,0,1)
4293 unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
4294 $a[$x] = $y splice(@a,$x,1,$y)
4296 Example, assuming array lengths are passed before arrays:
4298 sub aeq { # compare two list values
4299 my(@a) = splice(@_,0,shift);
4300 my(@b) = splice(@_,0,shift);
4301 return 0 unless @a == @b; # same len?
4303 return 0 if pop(@a) ne pop(@b);
4307 if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
4309 =item split /PATTERN/,EXPR,LIMIT
4311 =item split /PATTERN/,EXPR
4313 =item split /PATTERN/
4317 Splits a string into a list of strings and returns that list. By default,
4318 empty leading fields are preserved, and empty trailing ones are deleted.
4320 In scalar context, returns the number of fields found and splits into
4321 the C<@_> array. Use of split in scalar context is deprecated, however,
4322 because it clobbers your subroutine arguments.
4324 If EXPR is omitted, splits the C<$_> string. If PATTERN is also omitted,
4325 splits on whitespace (after skipping any leading whitespace). Anything
4326 matching PATTERN is taken to be a delimiter separating the fields. (Note
4327 that the delimiter may be longer than one character.)
4329 If LIMIT is specified and positive, splits into no more than that
4330 many fields (though it may split into fewer). If LIMIT is unspecified
4331 or zero, trailing null fields are stripped (which potential users
4332 of C<pop> would do well to remember). If LIMIT is negative, it is
4333 treated as if an arbitrarily large LIMIT had been specified.
4335 A pattern matching the null string (not to be confused with
4336 a null pattern C<//>, which is just one member of the set of patterns
4337 matching a null string) will split the value of EXPR into separate
4338 characters at each point it matches that way. For example:
4340 print join(':', split(/ */, 'hi there'));
4342 produces the output 'h:i:t:h:e:r:e'.
4344 The LIMIT parameter can be used to split a line partially
4346 ($login, $passwd, $remainder) = split(/:/, $_, 3);
4348 When assigning to a list, if LIMIT is omitted, Perl supplies a LIMIT
4349 one larger than the number of variables in the list, to avoid
4350 unnecessary work. For the list above LIMIT would have been 4 by
4351 default. In time critical applications it behooves you not to split
4352 into more fields than you really need.
4354 If the PATTERN contains parentheses, additional list elements are
4355 created from each matching substring in the delimiter.
4357 split(/([,-])/, "1-10,20", 3);
4359 produces the list value
4361 (1, '-', 10, ',', 20)
4363 If you had the entire header of a normal Unix email message in $header,
4364 you could split it up into fields and their values this way:
4366 $header =~ s/\n\s+/ /g; # fix continuation lines
4367 %hdrs = (UNIX_FROM => split /^(\S*?):\s*/m, $header);
4369 The pattern C</PATTERN/> may be replaced with an expression to specify
4370 patterns that vary at runtime. (To do runtime compilation only once,
4371 use C</$variable/o>.)
4373 As a special case, specifying a PATTERN of space (C<' '>) will split on
4374 white space just as C<split> with no arguments does. Thus, C<split(' ')> can
4375 be used to emulate B<awk>'s default behavior, whereas C<split(/ /)>
4376 will give you as many null initial fields as there are leading spaces.
4377 A C<split> on C</\s+/> is like a C<split(' ')> except that any leading
4378 whitespace produces a null first field. A C<split> with no arguments
4379 really does a C<split(' ', $_)> internally.
4381 A PATTERN of C</^/> is treated as if it were C</^/m>, since it isn't
4386 open(PASSWD, '/etc/passwd');
4388 ($login, $passwd, $uid, $gid,
4389 $gcos, $home, $shell) = split(/:/);
4393 (Note that $shell above will still have a newline on it. See L</chop>,
4394 L</chomp>, and L</join>.)
4396 =item sprintf FORMAT, LIST
4398 Returns a string formatted by the usual C<printf> conventions of the C
4399 library function C<sprintf>. See below for more details
4400 and see L<sprintf(3)> or L<printf(3)> on your system for an explanation of
4401 the general principles.
4405 # Format number with up to 8 leading zeroes
4406 $result = sprintf("%08d", $number);
4408 # Round number to 3 digits after decimal point
4409 $rounded = sprintf("%.3f", $number);
4411 Perl does its own C<sprintf> formatting--it emulates the C
4412 function C<sprintf>, but it doesn't use it (except for floating-point
4413 numbers, and even then only the standard modifiers are allowed). As a
4414 result, any non-standard extensions in your local C<sprintf> are not
4415 available from Perl.
4417 Unlike C<printf>, C<sprintf> does not do what you probably mean when you
4418 pass it an array as your first argument. The array is given scalar context,
4419 and instead of using the 0th element of the array as the format, Perl will
4420 use the count of elements in the array as the format, which is almost never
4423 Perl's C<sprintf> permits the following universally-known conversions:
4426 %c a character with the given number
4428 %d a signed integer, in decimal
4429 %u an unsigned integer, in decimal
4430 %o an unsigned integer, in octal
4431 %x an unsigned integer, in hexadecimal
4432 %e a floating-point number, in scientific notation
4433 %f a floating-point number, in fixed decimal notation
4434 %g a floating-point number, in %e or %f notation
4436 In addition, Perl permits the following widely-supported conversions:
4438 %X like %x, but using upper-case letters
4439 %E like %e, but using an upper-case "E"
4440 %G like %g, but with an upper-case "E" (if applicable)
4441 %b an unsigned integer, in binary
4442 %p a pointer (outputs the Perl value's address in hexadecimal)
4443 %n special: *stores* the number of characters output so far
4444 into the next variable in the parameter list
4446 Finally, for backward (and we do mean "backward") compatibility, Perl
4447 permits these unnecessary but widely-supported conversions:
4450 %D a synonym for %ld
4451 %U a synonym for %lu
4452 %O a synonym for %lo
4455 Conversions to scientific notation by C<%e>, C<%E>, C<%g> and C<%G>
4456 always have a two-digit exponent unless the modulus of the exponent is
4459 Perl permits the following universally-known flags between the C<%>
4460 and the conversion letter:
4462 space prefix positive number with a space
4463 + prefix positive number with a plus sign
4464 - left-justify within the field
4465 0 use zeros, not spaces, to right-justify
4466 # prefix non-zero octal with "0", non-zero hex with "0x"
4467 number minimum field width
4468 .number "precision": digits after decimal point for
4469 floating-point, max length for string, minimum length
4471 l interpret integer as C type "long" or "unsigned long"
4472 h interpret integer as C type "short" or "unsigned short"
4473 If no flags, interpret integer as C type "int" or "unsigned"
4475 There are also two Perl-specific flags:
4477 V interpret integer as Perl's standard integer type
4478 v interpret string as a vector of integers, output as
4479 numbers separated either by dots, or by an arbitrary
4480 string received from the argument list when the flag
4483 Where a number would appear in the flags, an asterisk (C<*>) may be
4484 used instead, in which case Perl uses the next item in the parameter
4485 list as the given number (that is, as the field width or precision).
4486 If a field width obtained through C<*> is negative, it has the same
4487 effect as the C<-> flag: left-justification.
4489 The C<v> flag is useful for displaying ordinal values of characters
4490 in arbitrary strings:
4492 printf "version is v%vd\n", $^V; # Perl's version
4493 printf "address is %*vX\n", ":", $addr; # IPv6 address
4494 printf "bits are %*vb\n", " ", $bits; # random bitstring
4496 If C<use locale> is in effect, the character used for the decimal
4497 point in formatted real numbers is affected by the LC_NUMERIC locale.
4500 If Perl understands "quads" (64-bit integers) (this requires
4501 either that the platform natively support quads or that Perl
4502 be specifically compiled to support quads), the characters
4506 print quads, and they may optionally be preceded by
4514 You can find out whether your Perl supports quads via L<Config>:
4517 ($Config{use64bitint} eq 'define' || $Config{longsize} == 8) &&
4520 If Perl understands "long doubles" (this requires that the platform
4521 support long doubles), the flags
4525 may optionally be preceded by
4533 You can find out whether your Perl supports long doubles via L<Config>:
4536 $Config{d_longdbl} eq 'define' && print "long doubles\n";
4542 Return the square root of EXPR. If EXPR is omitted, returns square
4543 root of C<$_>. Only works on non-negative operands, unless you've
4544 loaded the standard Math::Complex module.
4547 print sqrt(-2); # prints 1.4142135623731i
4553 Sets the random number seed for the C<rand> operator. If EXPR is
4554 omitted, uses a semi-random value supplied by the kernel (if it supports
4555 the F</dev/urandom> device) or based on the current time and process
4556 ID, among other things. In versions of Perl prior to 5.004 the default
4557 seed was just the current C<time>. This isn't a particularly good seed,
4558 so many old programs supply their own seed value (often C<time ^ $$> or
4559 C<time ^ ($$ + ($$ << 15))>), but that isn't necessary any more.
4561 In fact, it's usually not necessary to call C<srand> at all, because if
4562 it is not called explicitly, it is called implicitly at the first use of
4563 the C<rand> operator. However, this was not the case in version of Perl
4564 before 5.004, so if your script will run under older Perl versions, it
4565 should call C<srand>.
4567 Note that you need something much more random than the default seed for
4568 cryptographic purposes. Checksumming the compressed output of one or more
4569 rapidly changing operating system status programs is the usual method. For
4572 srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip`);
4574 If you're particularly concerned with this, see the C<Math::TrulyRandom>
4577 Do I<not> call C<srand> multiple times in your program unless you know
4578 exactly what you're doing and why you're doing it. The point of the
4579 function is to "seed" the C<rand> function so that C<rand> can produce
4580 a different sequence each time you run your program. Just do it once at the
4581 top of your program, or you I<won't> get random numbers out of C<rand>!
4583 Frequently called programs (like CGI scripts) that simply use
4587 for a seed can fall prey to the mathematical property that
4591 one-third of the time. So don't do that.
4593 =item stat FILEHANDLE
4599 Returns a 13-element list giving the status info for a file, either
4600 the file opened via FILEHANDLE, or named by EXPR. If EXPR is omitted,
4601 it stats C<$_>. Returns a null list if the stat fails. Typically used
4604 ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
4605 $atime,$mtime,$ctime,$blksize,$blocks)
4608 Not all fields are supported on all filesystem types. Here are the
4609 meaning of the fields:
4611 0 dev device number of filesystem
4613 2 mode file mode (type and permissions)
4614 3 nlink number of (hard) links to the file
4615 4 uid numeric user ID of file's owner
4616 5 gid numeric group ID of file's owner
4617 6 rdev the device identifier (special files only)
4618 7 size total size of file, in bytes
4619 8 atime last access time in seconds since the epoch
4620 9 mtime last modify time in seconds since the epoch
4621 10 ctime inode change time (NOT creation time!) in seconds since the epoch
4622 11 blksize preferred block size for file system I/O
4623 12 blocks actual number of blocks allocated
4625 (The epoch was at 00:00 January 1, 1970 GMT.)
4627 If stat is passed the special filehandle consisting of an underline, no
4628 stat is done, but the current contents of the stat structure from the
4629 last stat or filetest are returned. Example:
4631 if (-x $file && (($d) = stat(_)) && $d < 0) {
4632 print "$file is executable NFS file\n";
4635 (This works on machines only for which the device number is negative
4638 Because the mode contains both the file type and its permissions, you
4639 should mask off the file type portion and (s)printf using a C<"%o">
4640 if you want to see the real permissions.
4642 $mode = (stat($filename))[2];
4643 printf "Permissions are %04o\n", $mode & 07777;
4645 In scalar context, C<stat> returns a boolean value indicating success
4646 or failure, and, if successful, sets the information associated with
4647 the special filehandle C<_>.
4649 The File::stat module provides a convenient, by-name access mechanism:
4652 $sb = stat($filename);
4653 printf "File is %s, size is %s, perm %04o, mtime %s\n",
4654 $filename, $sb->size, $sb->mode & 07777,
4655 scalar localtime $sb->mtime;
4657 You can import symbolic mode constants (C<S_IF*>) and functions
4658 (C<S_IS*>) from the Fcntl module:
4662 $mode = (stat($filename))[2];
4664 $user_rwx = ($mode & S_IRWXU) >> 6;
4665 $group_read = ($mode & S_IRGRP) >> 3;
4666 $other_execute = $mode & S_IXOTH;
4668 printf "Permissions are %04o\n", S_ISMODE($mode), "\n";
4670 $is_setuid = $mode & S_ISUID;
4671 $is_setgid = S_ISDIR($mode);
4673 You could write the last two using the C<-u> and C<-d> operators.
4674 The commonly available S_IF* constants are
4676 # Permissions: read, write, execute, for user, group, others.
4678 S_IRWXU S_IRUSR S_IWUSR S_IXUSR
4679 S_IRWXG S_IRGRP S_IWGRP S_IXGRP
4680 S_IRWXO S_IROTH S_IWOTH S_IXOTH
4682 # Setuid/Setgid/Stickiness.
4684 S_ISUID S_ISGID S_ISVTX S_ISTXT
4686 # File types. Not necessarily all are available on your system.
4688 S_IFREG S_IFDIR S_IFLNK S_IFBLK S_ISCHR S_IFIFO S_IFSOCK S_IFWHT S_ENFMT
4690 # The following are compatibility aliases for S_IRUSR, S_IWUSR, S_IXUSR.
4692 S_IREAD S_IWRITE S_IEXEC
4694 and the S_IF* functions are
4696 S_IFMODE($mode) the part of $mode containing the permission bits
4697 and the setuid/setgid/sticky bits
4699 S_IFMT($mode) the part of $mode containing the file type
4700 which can be bit-anded with e.g. S_IFREG
4701 or with the following functions
4703 # The operators -f, -d, -l, -b, -c, -p, and -s.
4705 S_ISREG($mode) S_ISDIR($mode) S_ISLNK($mode)
4706 S_ISBLK($mode) S_ISCHR($mode) S_ISFIFO($mode) S_ISSOCK($mode)
4708 # No direct -X operator counterpart, but for the first one
4709 # the -g operator is often equivalent. The ENFMT stands for
4710 # record flocking enforcement, a platform-dependent feature.
4712 S_ISENFMT($mode) S_ISWHT($mode)
4714 See your native chmod(2) and stat(2) documentation for more details
4715 about the S_* constants.
4721 Takes extra time to study SCALAR (C<$_> if unspecified) in anticipation of
4722 doing many pattern matches on the string before it is next modified.
4723 This may or may not save time, depending on the nature and number of
4724 patterns you are searching on, and on the distribution of character
4725 frequencies in the string to be searched--you probably want to compare
4726 run times with and without it to see which runs faster. Those loops
4727 which scan for many short constant strings (including the constant
4728 parts of more complex patterns) will benefit most. You may have only
4729 one C<study> active at a time--if you study a different scalar the first
4730 is "unstudied". (The way C<study> works is this: a linked list of every
4731 character in the string to be searched is made, so we know, for
4732 example, where all the C<'k'> characters are. From each search string,
4733 the rarest character is selected, based on some static frequency tables
4734 constructed from some C programs and English text. Only those places
4735 that contain this "rarest" character are examined.)
4737 For example, here is a loop that inserts index producing entries
4738 before any line containing a certain pattern:
4742 print ".IX foo\n" if /\bfoo\b/;
4743 print ".IX bar\n" if /\bbar\b/;
4744 print ".IX blurfl\n" if /\bblurfl\b/;
4749 In searching for C</\bfoo\b/>, only those locations in C<$_> that contain C<f>
4750 will be looked at, because C<f> is rarer than C<o>. In general, this is
4751 a big win except in pathological cases. The only question is whether
4752 it saves you more time than it took to build the linked list in the
4755 Note that if you have to look for strings that you don't know till
4756 runtime, you can build an entire loop as a string and C<eval> that to
4757 avoid recompiling all your patterns all the time. Together with
4758 undefining C<$/> to input entire files as one record, this can be very
4759 fast, often faster than specialized programs like fgrep(1). The following
4760 scans a list of files (C<@files>) for a list of words (C<@words>), and prints
4761 out the names of those files that contain a match:
4763 $search = 'while (<>) { study;';
4764 foreach $word (@words) {
4765 $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
4770 eval $search; # this screams
4771 $/ = "\n"; # put back to normal input delimiter
4772 foreach $file (sort keys(%seen)) {
4780 =item sub NAME BLOCK
4782 This is subroutine definition, not a real function I<per se>. With just a
4783 NAME (and possibly prototypes or attributes), it's just a forward declaration.
4784 Without a NAME, it's an anonymous function declaration, and does actually
4785 return a value: the CODE ref of the closure you just created. See L<perlsub>
4786 and L<perlref> for details.
4788 =item substr EXPR,OFFSET,LENGTH,REPLACEMENT
4790 =item substr EXPR,OFFSET,LENGTH
4792 =item substr EXPR,OFFSET
4794 Extracts a substring out of EXPR and returns it. First character is at
4795 offset C<0>, or whatever you've set C<$[> to (but don't do that).
4796 If OFFSET is negative (or more precisely, less than C<$[>), starts
4797 that far from the end of the string. If LENGTH is omitted, returns
4798 everything to the end of the string. If LENGTH is negative, leaves that
4799 many characters off the end of the string.
4801 You can use the substr() function as an lvalue, in which case EXPR
4802 must itself be an lvalue. If you assign something shorter than LENGTH,
4803 the string will shrink, and if you assign something longer than LENGTH,
4804 the string will grow to accommodate it. To keep the string the same
4805 length you may need to pad or chop your value using C<sprintf>.
4807 If OFFSET and LENGTH specify a substring that is partly outside the
4808 string, only the part within the string is returned. If the substring
4809 is beyond either end of the string, substr() returns the undefined
4810 value and produces a warning. When used as an lvalue, specifying a
4811 substring that is entirely outside the string is a fatal error.
4812 Here's an example showing the behavior for boundary cases:
4815 substr($name, 4) = 'dy'; # $name is now 'freddy'
4816 my $null = substr $name, 6, 2; # returns '' (no warning)
4817 my $oops = substr $name, 7; # returns undef, with warning
4818 substr($name, 7) = 'gap'; # fatal error
4820 An alternative to using substr() as an lvalue is to specify the
4821 replacement string as the 4th argument. This allows you to replace
4822 parts of the EXPR and return what was there before in one operation,
4823 just as you can with splice().
4825 =item symlink OLDFILE,NEWFILE
4827 Creates a new filename symbolically linked to the old filename.
4828 Returns C<1> for success, C<0> otherwise. On systems that don't support
4829 symbolic links, produces a fatal error at run time. To check for that,
4832 $symlink_exists = eval { symlink("",""); 1 };
4836 Calls the system call specified as the first element of the list,
4837 passing the remaining elements as arguments to the system call. If
4838 unimplemented, produces a fatal error. The arguments are interpreted
4839 as follows: if a given argument is numeric, the argument is passed as
4840 an int. If not, the pointer to the string value is passed. You are
4841 responsible to make sure a string is pre-extended long enough to
4842 receive any result that might be written into a string. You can't use a
4843 string literal (or other read-only string) as an argument to C<syscall>
4844 because Perl has to assume that any string pointer might be written
4846 integer arguments are not literals and have never been interpreted in a
4847 numeric context, you may need to add C<0> to them to force them to look
4848 like numbers. This emulates the C<syswrite> function (or vice versa):
4850 require 'syscall.ph'; # may need to run h2ph
4852 syscall(&SYS_write, fileno(STDOUT), $s, length $s);
4854 Note that Perl supports passing of up to only 14 arguments to your system call,
4855 which in practice should usually suffice.
4857 Syscall returns whatever value returned by the system call it calls.
4858 If the system call fails, C<syscall> returns C<-1> and sets C<$!> (errno).
4859 Note that some system calls can legitimately return C<-1>. The proper
4860 way to handle such calls is to assign C<$!=0;> before the call and
4861 check the value of C<$!> if syscall returns C<-1>.
4863 There's a problem with C<syscall(&SYS_pipe)>: it returns the file
4864 number of the read end of the pipe it creates. There is no way
4865 to retrieve the file number of the other end. You can avoid this
4866 problem by using C<pipe> instead.
4868 =item sysopen FILEHANDLE,FILENAME,MODE
4870 =item sysopen FILEHANDLE,FILENAME,MODE,PERMS
4872 Opens the file whose filename is given by FILENAME, and associates it
4873 with FILEHANDLE. If FILEHANDLE is an expression, its value is used as
4874 the name of the real filehandle wanted. This function calls the
4875 underlying operating system's C<open> function with the parameters
4876 FILENAME, MODE, PERMS.
4878 The possible values and flag bits of the MODE parameter are
4879 system-dependent; they are available via the standard module C<Fcntl>.
4880 See the documentation of your operating system's C<open> to see which
4881 values and flag bits are available. You may combine several flags
4882 using the C<|>-operator.
4884 Some of the most common values are C<O_RDONLY> for opening the file in
4885 read-only mode, C<O_WRONLY> for opening the file in write-only mode,
4886 and C<O_RDWR> for opening the file in read-write mode, and.
4888 For historical reasons, some values work on almost every system
4889 supported by perl: zero means read-only, one means write-only, and two
4890 means read/write. We know that these values do I<not> work under
4891 OS/390 & VM/ESA Unix and on the Macintosh; you probably don't want to
4892 use them in new code.
4894 If the file named by FILENAME does not exist and the C<open> call creates
4895 it (typically because MODE includes the C<O_CREAT> flag), then the value of
4896 PERMS specifies the permissions of the newly created file. If you omit
4897 the PERMS argument to C<sysopen>, Perl uses the octal value C<0666>.
4898 These permission values need to be in octal, and are modified by your
4899 process's current C<umask>.
4901 In many systems the C<O_EXCL> flag is available for opening files in
4902 exclusive mode. This is B<not> locking: exclusiveness means here that
4903 if the file already exists, sysopen() fails. The C<O_EXCL> wins
4906 Sometimes you may want to truncate an already-existing file: C<O_TRUNC>.
4908 You should seldom if ever use C<0644> as argument to C<sysopen>, because
4909 that takes away the user's option to have a more permissive umask.
4910 Better to omit it. See the perlfunc(1) entry on C<umask> for more
4913 Note that C<sysopen> depends on the fdopen() C library function.
4914 On many UNIX systems, fdopen() is known to fail when file descriptors
4915 exceed a certain value, typically 255. If you need more file
4916 descriptors than that, consider rebuilding Perl to use the C<sfio>
4917 library, or perhaps using the POSIX::open() function.
4919 See L<perlopentut> for a kinder, gentler explanation of opening files.
4921 =item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
4923 =item sysread FILEHANDLE,SCALAR,LENGTH
4925 Attempts to read LENGTH bytes of data into variable SCALAR from the
4926 specified FILEHANDLE, using the system call read(2). It bypasses stdio,
4927 so mixing this with other kinds of reads, C<print>, C<write>,
4928 C<seek>, C<tell>, or C<eof> can cause confusion because stdio
4929 usually buffers data. Returns the number of bytes actually read, C<0>
4930 at end of file, or undef if there was an error. SCALAR will be grown or
4931 shrunk so that the last byte actually read is the last byte of the
4932 scalar after the read.
4934 An OFFSET may be specified to place the read data at some place in the
4935 string other than the beginning. A negative OFFSET specifies
4936 placement at that many bytes counting backwards from the end of the
4937 string. A positive OFFSET greater than the length of SCALAR results
4938 in the string being padded to the required size with C<"\0"> bytes before
4939 the result of the read is appended.
4941 There is no syseof() function, which is ok, since eof() doesn't work
4942 very well on device files (like ttys) anyway. Use sysread() and check
4943 for a return value for 0 to decide whether you're done.
4945 =item sysseek FILEHANDLE,POSITION,WHENCE
4947 Sets FILEHANDLE's system position using the system call lseek(2). It
4948 bypasses stdio, so mixing this with reads (other than C<sysread>),
4949 C<print>, C<write>, C<seek>, C<tell>, or C<eof> may cause confusion.
4950 FILEHANDLE may be an expression whose value gives the name of the
4951 filehandle. The values for WHENCE are C<0> to set the new position to
4952 POSITION, C<1> to set the it to the current position plus POSITION,
4953 and C<2> to set it to EOF plus POSITION (typically negative). For
4954 WHENCE, you may also use the constants C<SEEK_SET>, C<SEEK_CUR>, and
4955 C<SEEK_END> (start of the file, current position, end of the file)
4956 from the Fcntl module.
4958 Returns the new position, or the undefined value on failure. A position
4959 of zero is returned as the string C<"0 but true">; thus C<sysseek> returns
4960 true on success and false on failure, yet you can still easily determine
4965 =item system PROGRAM LIST
4967 Does exactly the same thing as C<exec LIST>, except that a fork is
4968 done first, and the parent process waits for the child process to
4969 complete. Note that argument processing varies depending on the
4970 number of arguments. If there is more than one argument in LIST,
4971 or if LIST is an array with more than one value, starts the program
4972 given by the first element of the list with arguments given by the
4973 rest of the list. If there is only one scalar argument, the argument
4974 is checked for shell metacharacters, and if there are any, the
4975 entire argument is passed to the system's command shell for parsing
4976 (this is C</bin/sh -c> on Unix platforms, but varies on other
4977 platforms). If there are no shell metacharacters in the argument,
4978 it is split into words and passed directly to C<execvp>, which is
4981 Beginning with v5.6.0, Perl will attempt to flush all files opened for
4982 output before any operation that may do a fork, but this may not be
4983 supported on some platforms (see L<perlport>). To be safe, you may need
4984 to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
4985 of C<IO::Handle> on any open handles.
4987 The return value is the exit status of the program as
4988 returned by the C<wait> call. To get the actual exit value divide by
4989 256. See also L</exec>. This is I<not> what you want to use to capture
4990 the output from a command, for that you should use merely backticks or
4991 C<qx//>, as described in L<perlop/"`STRING`">. Return value of -1
4992 indicates a failure to start the program (inspect $! for the reason).
4994 Like C<exec>, C<system> allows you to lie to a program about its name if
4995 you use the C<system PROGRAM LIST> syntax. Again, see L</exec>.
4997 Because C<system> and backticks block C<SIGINT> and C<SIGQUIT>, killing the
4998 program they're running doesn't actually interrupt your program.
5000 @args = ("command", "arg1", "arg2");
5002 or die "system @args failed: $?"
5004 You can check all the failure possibilities by inspecting
5007 $exit_value = $? >> 8;
5008 $signal_num = $? & 127;
5009 $dumped_core = $? & 128;
5011 When the arguments get executed via the system shell, results
5012 and return codes will be subject to its quirks and capabilities.
5013 See L<perlop/"`STRING`"> and L</exec> for details.
5015 =item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
5017 =item syswrite FILEHANDLE,SCALAR,LENGTH
5019 =item syswrite FILEHANDLE,SCALAR
5021 Attempts to write LENGTH bytes of data from variable SCALAR to the
5022 specified FILEHANDLE, using the system call write(2). If LENGTH
5023 is not specified, writes whole SCALAR. It bypasses stdio, so mixing
5024 this with reads (other than C<sysread())>, C<print>, C<write>,
5025 C<seek>, C<tell>, or C<eof> may cause confusion because stdio
5026 usually buffers data. Returns the number of bytes actually written,
5027 or C<undef> if there was an error. If the LENGTH is greater than
5028 the available data in the SCALAR after the OFFSET, only as much
5029 data as is available will be written.
5031 An OFFSET may be specified to write the data from some part of the
5032 string other than the beginning. A negative OFFSET specifies writing
5033 that many bytes counting backwards from the end of the string. In the
5034 case the SCALAR is empty you can use OFFSET but only zero offset.
5036 =item tell FILEHANDLE
5040 Returns the current position for FILEHANDLE. FILEHANDLE may be an
5041 expression whose value gives the name of the actual filehandle. If
5042 FILEHANDLE is omitted, assumes the file last read.
5044 There is no C<systell> function. Use C<sysseek(FH, 0, 1)> for that.
5046 =item telldir DIRHANDLE
5048 Returns the current position of the C<readdir> routines on DIRHANDLE.
5049 Value may be given to C<seekdir> to access a particular location in a
5050 directory. Has the same caveats about possible directory compaction as
5051 the corresponding system library routine.
5053 =item tie VARIABLE,CLASSNAME,LIST
5055 This function binds a variable to a package class that will provide the
5056 implementation for the variable. VARIABLE is the name of the variable
5057 to be enchanted. CLASSNAME is the name of a class implementing objects
5058 of correct type. Any additional arguments are passed to the C<new>
5059 method of the class (meaning C<TIESCALAR>, C<TIEHANDLE>, C<TIEARRAY>,
5060 or C<TIEHASH>). Typically these are arguments such as might be passed
5061 to the C<dbm_open()> function of C. The object returned by the C<new>
5062 method is also returned by the C<tie> function, which would be useful
5063 if you want to access other methods in CLASSNAME.
5065 Note that functions such as C<keys> and C<values> may return huge lists
5066 when used on large objects, like DBM files. You may prefer to use the
5067 C<each> function to iterate over such. Example:
5069 # print out history file offsets
5071 tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
5072 while (($key,$val) = each %HIST) {
5073 print $key, ' = ', unpack('L',$val), "\n";
5077 A class implementing a hash should have the following methods:
5079 TIEHASH classname, LIST
5081 STORE this, key, value
5086 NEXTKEY this, lastkey
5089 A class implementing an ordinary array should have the following methods:
5091 TIEARRAY classname, LIST
5093 STORE this, key, value
5095 STORESIZE this, count
5101 SPLICE this, offset, length, LIST
5105 A class implementing a file handle should have the following methods:
5107 TIEHANDLE classname, LIST
5108 READ this, scalar, length, offset
5111 WRITE this, scalar, length, offset
5113 PRINTF this, format, LIST
5117 A class implementing a scalar should have the following methods:
5119 TIESCALAR classname, LIST
5124 Not all methods indicated above need be implemented. See L<perltie>,
5125 L<Tie::Hash>, L<Tie::Array>, L<Tie::Scalar>, and L<Tie::Handle>.
5127 Unlike C<dbmopen>, the C<tie> function will not use or require a module
5128 for you--you need to do that explicitly yourself. See L<DB_File>
5129 or the F<Config> module for interesting C<tie> implementations.
5131 For further details see L<perltie>, L<"tied VARIABLE">.
5135 Returns a reference to the object underlying VARIABLE (the same value
5136 that was originally returned by the C<tie> call that bound the variable
5137 to a package.) Returns the undefined value if VARIABLE isn't tied to a
5142 Returns the number of non-leap seconds since whatever time the system
5143 considers to be the epoch (that's 00:00:00, January 1, 1904 for MacOS,
5144 and 00:00:00 UTC, January 1, 1970 for most other systems).
5145 Suitable for feeding to C<gmtime> and C<localtime>.
5147 For measuring time in better granularity than one second,
5148 you may use either the Time::HiRes module from CPAN, or
5149 if you have gettimeofday(2), you may be able to use the
5150 C<syscall> interface of Perl, see L<perlfaq8> for details.
5154 Returns a four-element list giving the user and system times, in
5155 seconds, for this process and the children of this process.
5157 ($user,$system,$cuser,$csystem) = times;
5161 The transliteration operator. Same as C<y///>. See L<perlop>.
5163 =item truncate FILEHANDLE,LENGTH
5165 =item truncate EXPR,LENGTH
5167 Truncates the file opened on FILEHANDLE, or named by EXPR, to the
5168 specified length. Produces a fatal error if truncate isn't implemented
5169 on your system. Returns true if successful, the undefined value
5176 Returns an uppercased version of EXPR. This is the internal function
5177 implementing the C<\U> escape in double-quoted strings.
5178 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
5179 Under Unicode (C<use utf8>) it uses the standard Unicode uppercase mappings. (It
5180 does not attempt to do titlecase mapping on initial letters. See C<ucfirst> for that.)
5182 If EXPR is omitted, uses C<$_>.
5188 Returns the value of EXPR with the first character
5189 in uppercase (titlecase in Unicode). This is
5190 the internal function implementing the C<\u> escape in double-quoted strings.
5191 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>
5194 If EXPR is omitted, uses C<$_>.
5200 Sets the umask for the process to EXPR and returns the previous value.
5201 If EXPR is omitted, merely returns the current umask.
5203 The Unix permission C<rwxr-x---> is represented as three sets of three
5204 bits, or three octal digits: C<0750> (the leading 0 indicates octal
5205 and isn't one of the digits). The C<umask> value is such a number
5206 representing disabled permissions bits. The permission (or "mode")
5207 values you pass C<mkdir> or C<sysopen> are modified by your umask, so
5208 even if you tell C<sysopen> to create a file with permissions C<0777>,
5209 if your umask is C<0022> then the file will actually be created with
5210 permissions C<0755>. If your C<umask> were C<0027> (group can't
5211 write; others can't read, write, or execute), then passing
5212 C<sysopen> C<0666> would create a file with mode C<0640> (C<0666 &~
5215 Here's some advice: supply a creation mode of C<0666> for regular
5216 files (in C<sysopen>) and one of C<0777> for directories (in
5217 C<mkdir>) and executable files. This gives users the freedom of
5218 choice: if they want protected files, they might choose process umasks
5219 of C<022>, C<027>, or even the particularly antisocial mask of C<077>.
5220 Programs should rarely if ever make policy decisions better left to
5221 the user. The exception to this is when writing files that should be
5222 kept private: mail files, web browser cookies, I<.rhosts> files, and
5225 If umask(2) is not implemented on your system and you are trying to
5226 restrict access for I<yourself> (i.e., (EXPR & 0700) > 0), produces a
5227 fatal error at run time. If umask(2) is not implemented and you are
5228 not trying to restrict access for yourself, returns C<undef>.
5230 Remember that a umask is a number, usually given in octal; it is I<not> a
5231 string of octal digits. See also L</oct>, if all you have is a string.
5237 Undefines the value of EXPR, which must be an lvalue. Use only on a
5238 scalar value, an array (using C<@>), a hash (using C<%>), a subroutine
5239 (using C<&>), or a typeglob (using <*>). (Saying C<undef $hash{$key}>
5240 will probably not do what you expect on most predefined variables or
5241 DBM list values, so don't do that; see L<delete>.) Always returns the
5242 undefined value. You can omit the EXPR, in which case nothing is
5243 undefined, but you still get an undefined value that you could, for
5244 instance, return from a subroutine, assign to a variable or pass as a
5245 parameter. Examples:
5248 undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'};
5252 undef *xyz; # destroys $xyz, @xyz, %xyz, &xyz, etc.
5253 return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it;
5254 select undef, undef, undef, 0.25;
5255 ($a, $b, undef, $c) = &foo; # Ignore third value returned
5257 Note that this is a unary operator, not a list operator.
5263 Deletes a list of files. Returns the number of files successfully
5266 $cnt = unlink 'a', 'b', 'c';
5270 Note: C<unlink> will not delete directories unless you are superuser and
5271 the B<-U> flag is supplied to Perl. Even if these conditions are
5272 met, be warned that unlinking a directory can inflict damage on your
5273 filesystem. Use C<rmdir> instead.
5275 If LIST is omitted, uses C<$_>.
5277 =item unpack TEMPLATE,EXPR
5279 C<unpack> does the reverse of C<pack>: it takes a string
5280 and expands it out into a list of values.
5281 (In scalar context, it returns merely the first value produced.)
5283 The string is broken into chunks described by the TEMPLATE. Each chunk
5284 is converted separately to a value. Typically, either the string is a result
5285 of C<pack>, or the bytes of the string represent a C structure of some
5288 The TEMPLATE has the same format as in the C<pack> function.
5289 Here's a subroutine that does substring:
5292 my($what,$where,$howmuch) = @_;
5293 unpack("x$where a$howmuch", $what);
5298 sub ordinal { unpack("c",$_[0]); } # same as ord()
5300 In addition to fields allowed in pack(), you may prefix a field with
5301 a %<number> to indicate that
5302 you want a <number>-bit checksum of the items instead of the items
5303 themselves. Default is a 16-bit checksum. Checksum is calculated by
5304 summing numeric values of expanded values (for string fields the sum of
5305 C<ord($char)> is taken, for bit fields the sum of zeroes and ones).
5307 For example, the following
5308 computes the same number as the System V sum program:
5312 unpack("%32C*",<>) % 65535;
5315 The following efficiently counts the number of set bits in a bit vector:
5317 $setbits = unpack("%32b*", $selectmask);
5319 The C<p> and C<P> formats should be used with care. Since Perl
5320 has no way of checking whether the value passed to C<unpack()>
5321 corresponds to a valid memory location, passing a pointer value that's
5322 not known to be valid is likely to have disastrous consequences.
5324 If the repeat count of a field is larger than what the remainder of
5325 the input string allows, repeat count is decreased. If the input string
5326 is longer than one described by the TEMPLATE, the rest is ignored.
5328 See L</pack> for more examples and notes.
5330 =item untie VARIABLE
5332 Breaks the binding between a variable and a package. (See C<tie>.)
5334 =item unshift ARRAY,LIST
5336 Does the opposite of a C<shift>. Or the opposite of a C<push>,
5337 depending on how you look at it. Prepends list to the front of the
5338 array, and returns the new number of elements in the array.
5340 unshift(ARGV, '-e') unless $ARGV[0] =~ /^-/;
5342 Note the LIST is prepended whole, not one element at a time, so the
5343 prepended elements stay in the same order. Use C<reverse> to do the
5346 =item use Module VERSION LIST
5348 =item use Module VERSION
5350 =item use Module LIST
5356 Imports some semantics into the current package from the named module,
5357 generally by aliasing certain subroutine or variable names into your
5358 package. It is exactly equivalent to
5360 BEGIN { require Module; import Module LIST; }
5362 except that Module I<must> be a bareword.
5364 VERSION, which can be specified as a literal of the form v5.6.1, demands
5365 that the current version of Perl (C<$^V> or $PERL_VERSION) be at least
5366 as recent as that version. (For compatibility with older versions of Perl,
5367 a numeric literal will also be interpreted as VERSION.) If the version
5368 of the running Perl interpreter is less than VERSION, then an error
5369 message is printed and Perl exits immediately without attempting to
5370 parse the rest of the file. Compare with L</require>, which can do a
5371 similar check at run time.
5373 use v5.6.1; # compile time version check
5375 use 5.005_03; # float version allowed for compatibility
5377 This is often useful if you need to check the current Perl version before
5378 C<use>ing library modules that have changed in incompatible ways from
5379 older versions of Perl. (We try not to do this more than we have to.)
5381 The C<BEGIN> forces the C<require> and C<import> to happen at compile time. The
5382 C<require> makes sure the module is loaded into memory if it hasn't been
5383 yet. The C<import> is not a builtin--it's just an ordinary static method
5384 call into the C<Module> package to tell the module to import the list of
5385 features back into the current package. The module can implement its
5386 C<import> method any way it likes, though most modules just choose to
5387 derive their C<import> method via inheritance from the C<Exporter> class that
5388 is defined in the C<Exporter> module. See L<Exporter>. If no C<import>
5389 method can be found then the call is skipped.
5391 If you do not want to call the package's C<import> method (for instance,
5392 to stop your namespace from being altered), explicitly supply the empty list:
5396 That is exactly equivalent to
5398 BEGIN { require Module }
5400 If the VERSION argument is present between Module and LIST, then the
5401 C<use> will call the VERSION method in class Module with the given
5402 version as an argument. The default VERSION method, inherited from
5403 the UNIVERSAL class, croaks if the given version is larger than the
5404 value of the variable C<$Module::VERSION>.
5406 Again, there is a distinction between omitting LIST (C<import> called
5407 with no arguments) and an explicit empty LIST C<()> (C<import> not
5408 called). Note that there is no comma after VERSION!
5410 Because this is a wide-open interface, pragmas (compiler directives)
5411 are also implemented this way. Currently implemented pragmas are:
5416 use sigtrap qw(SEGV BUS);
5417 use strict qw(subs vars refs);
5418 use subs qw(afunc blurfl);
5419 use warnings qw(all);
5421 Some of these pseudo-modules import semantics into the current
5422 block scope (like C<strict> or C<integer>, unlike ordinary modules,
5423 which import symbols into the current package (which are effective
5424 through the end of the file).
5426 There's a corresponding C<no> command that unimports meanings imported
5427 by C<use>, i.e., it calls C<unimport Module LIST> instead of C<import>.
5433 If no C<unimport> method can be found the call fails with a fatal error.
5435 See L<perlmod> for a list of standard modules and pragmas. See L<perlrun>
5436 for the C<-M> and C<-m> command-line options to perl that give C<use>
5437 functionality from the command-line.
5441 Changes the access and modification times on each file of a list of
5442 files. The first two elements of the list must be the NUMERICAL access
5443 and modification times, in that order. Returns the number of files
5444 successfully changed. The inode change time of each file is set
5445 to the current time. This code has the same effect as the C<touch>
5446 command if the files already exist:
5450 utime $now, $now, @ARGV;
5454 Returns a list consisting of all the values of the named hash. (In a
5455 scalar context, returns the number of values.) The values are
5456 returned in an apparently random order. The actual random order is
5457 subject to change in future versions of perl, but it is guaranteed to
5458 be the same order as either the C<keys> or C<each> function would
5459 produce on the same (unmodified) hash.
5461 Note that the values are not copied, which means modifying them will
5462 modify the contents of the hash:
5464 for (values %hash) { s/foo/bar/g } # modifies %hash values
5465 for (@hash{keys %hash}) { s/foo/bar/g } # same
5467 As a side effect, calling values() resets the HASH's internal iterator.
5468 See also C<keys>, C<each>, and C<sort>.
5470 =item vec EXPR,OFFSET,BITS
5472 Treats the string in EXPR as a bit vector made up of elements of
5473 width BITS, and returns the value of the element specified by OFFSET
5474 as an unsigned integer. BITS therefore specifies the number of bits
5475 that are reserved for each element in the bit vector. This must
5476 be a power of two from 1 to 32 (or 64, if your platform supports
5479 If BITS is 8, "elements" coincide with bytes of the input string.
5481 If BITS is 16 or more, bytes of the input string are grouped into chunks
5482 of size BITS/8, and each group is converted to a number as with
5483 pack()/unpack() with big-endian formats C<n>/C<N> (and analoguously
5484 for BITS==64). See L<"pack"> for details.
5486 If bits is 4 or less, the string is broken into bytes, then the bits
5487 of each byte are broken into 8/BITS groups. Bits of a byte are
5488 numbered in a little-endian-ish way, as in C<0x01>, C<0x02>,
5489 C<0x04>, C<0x08>, C<0x10>, C<0x20>, C<0x40>, C<0x80>. For example,
5490 breaking the single input byte C<chr(0x36)> into two groups gives a list
5491 C<(0x6, 0x3)>; breaking it into 4 groups gives C<(0x2, 0x1, 0x3, 0x0)>.
5493 C<vec> may also be assigned to, in which case parentheses are needed
5494 to give the expression the correct precedence as in
5496 vec($image, $max_x * $x + $y, 8) = 3;
5498 If the selected element is off the end of the string, the value 0 is
5499 returned. If an element off the end of the string is written to,
5500 Perl will first extend the string with sufficiently many zero bytes.
5502 Strings created with C<vec> can also be manipulated with the logical
5503 operators C<|>, C<&>, C<^>, and C<~>. These operators will assume a bit
5504 vector operation is desired when both operands are strings.
5505 See L<perlop/"Bitwise String Operators">.
5507 The following code will build up an ASCII string saying C<'PerlPerlPerl'>.
5508 The comments show the string after each step. Note that this code works
5509 in the same way on big-endian or little-endian machines.
5512 vec($foo, 0, 32) = 0x5065726C; # 'Perl'
5514 # $foo eq "Perl" eq "\x50\x65\x72\x6C", 32 bits
5515 print vec($foo, 0, 8); # prints 80 == 0x50 == ord('P')
5517 vec($foo, 2, 16) = 0x5065; # 'PerlPe'
5518 vec($foo, 3, 16) = 0x726C; # 'PerlPerl'
5519 vec($foo, 8, 8) = 0x50; # 'PerlPerlP'
5520 vec($foo, 9, 8) = 0x65; # 'PerlPerlPe'
5521 vec($foo, 20, 4) = 2; # 'PerlPerlPe' . "\x02"
5522 vec($foo, 21, 4) = 7; # 'PerlPerlPer'
5524 vec($foo, 45, 2) = 3; # 'PerlPerlPer' . "\x0c"
5525 vec($foo, 93, 1) = 1; # 'PerlPerlPer' . "\x2c"
5526 vec($foo, 94, 1) = 1; # 'PerlPerlPerl'
5529 To transform a bit vector into a string or list of 0's and 1's, use these:
5531 $bits = unpack("b*", $vector);
5532 @bits = split(//, unpack("b*", $vector));
5534 If you know the exact length in bits, it can be used in place of the C<*>.
5536 Here is an example to illustrate how the bits actually fall in place:
5542 unpack("V",$_) 01234567890123456789012345678901
5543 ------------------------------------------------------------------
5548 for ($shift=0; $shift < $width; ++$shift) {
5549 for ($off=0; $off < 32/$width; ++$off) {
5550 $str = pack("B*", "0"x32);
5551 $bits = (1<<$shift);
5552 vec($str, $off, $width) = $bits;
5553 $res = unpack("b*",$str);
5554 $val = unpack("V", $str);
5561 vec($_,@#,@#) = @<< == @######### @>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
5562 $off, $width, $bits, $val, $res
5566 Regardless of the machine architecture on which it is run, the above
5567 example should print the following table:
5570 unpack("V",$_) 01234567890123456789012345678901
5571 ------------------------------------------------------------------
5572 vec($_, 0, 1) = 1 == 1 10000000000000000000000000000000
5573 vec($_, 1, 1) = 1 == 2 01000000000000000000000000000000
5574 vec($_, 2, 1) = 1 == 4 00100000000000000000000000000000
5575 vec($_, 3, 1) = 1 == 8 00010000000000000000000000000000
5576 vec($_, 4, 1) = 1 == 16 00001000000000000000000000000000
5577 vec($_, 5, 1) = 1 == 32 00000100000000000000000000000000
5578 vec($_, 6, 1) = 1 == 64 00000010000000000000000000000000
5579 vec($_, 7, 1) = 1 == 128 00000001000000000000000000000000
5580 vec($_, 8, 1) = 1 == 256 00000000100000000000000000000000
5581 vec($_, 9, 1) = 1 == 512 00000000010000000000000000000000
5582 vec($_,10, 1) = 1 == 1024 00000000001000000000000000000000
5583 vec($_,11, 1) = 1 == 2048 00000000000100000000000000000000
5584 vec($_,12, 1) = 1 == 4096 00000000000010000000000000000000
5585 vec($_,13, 1) = 1 == 8192 00000000000001000000000000000000
5586 vec($_,14, 1) = 1 == 16384 00000000000000100000000000000000
5587 vec($_,15, 1) = 1 == 32768 00000000000000010000000000000000
5588 vec($_,16, 1) = 1 == 65536 00000000000000001000000000000000
5589 vec($_,17, 1) = 1 == 131072 00000000000000000100000000000000
5590 vec($_,18, 1) = 1 == 262144 00000000000000000010000000000000
5591 vec($_,19, 1) = 1 == 524288 00000000000000000001000000000000
5592 vec($_,20, 1) = 1 == 1048576 00000000000000000000100000000000
5593 vec($_,21, 1) = 1 == 2097152 00000000000000000000010000000000
5594 vec($_,22, 1) = 1 == 4194304 00000000000000000000001000000000
5595 vec($_,23, 1) = 1 == 8388608 00000000000000000000000100000000
5596 vec($_,24, 1) = 1 == 16777216 00000000000000000000000010000000
5597 vec($_,25, 1) = 1 == 33554432 00000000000000000000000001000000
5598 vec($_,26, 1) = 1 == 67108864 00000000000000000000000000100000
5599 vec($_,27, 1) = 1 == 134217728 00000000000000000000000000010000
5600 vec($_,28, 1) = 1 == 268435456 00000000000000000000000000001000
5601 vec($_,29, 1) = 1 == 536870912 00000000000000000000000000000100
5602 vec($_,30, 1) = 1 == 1073741824 00000000000000000000000000000010
5603 vec($_,31, 1) = 1 == 2147483648 00000000000000000000000000000001
5604 vec($_, 0, 2) = 1 == 1 10000000000000000000000000000000
5605 vec($_, 1, 2) = 1 == 4 00100000000000000000000000000000
5606 vec($_, 2, 2) = 1 == 16 00001000000000000000000000000000
5607 vec($_, 3, 2) = 1 == 64 00000010000000000000000000000000
5608 vec($_, 4, 2) = 1 == 256 00000000100000000000000000000000
5609 vec($_, 5, 2) = 1 == 1024 00000000001000000000000000000000
5610 vec($_, 6, 2) = 1 == 4096 00000000000010000000000000000000
5611 vec($_, 7, 2) = 1 == 16384 00000000000000100000000000000000
5612 vec($_, 8, 2) = 1 == 65536 00000000000000001000000000000000
5613 vec($_, 9, 2) = 1 == 262144 00000000000000000010000000000000
5614 vec($_,10, 2) = 1 == 1048576 00000000000000000000100000000000
5615 vec($_,11, 2) = 1 == 4194304 00000000000000000000001000000000
5616 vec($_,12, 2) = 1 == 16777216 00000000000000000000000010000000
5617 vec($_,13, 2) = 1 == 67108864 00000000000000000000000000100000
5618 vec($_,14, 2) = 1 == 268435456 00000000000000000000000000001000
5619 vec($_,15, 2) = 1 == 1073741824 00000000000000000000000000000010
5620 vec($_, 0, 2) = 2 == 2 01000000000000000000000000000000
5621 vec($_, 1, 2) = 2 == 8 00010000000000000000000000000000
5622 vec($_, 2, 2) = 2 == 32 00000100000000000000000000000000
5623 vec($_, 3, 2) = 2 == 128 00000001000000000000000000000000
5624 vec($_, 4, 2) = 2 == 512 00000000010000000000000000000000
5625 vec($_, 5, 2) = 2 == 2048 00000000000100000000000000000000
5626 vec($_, 6, 2) = 2 == 8192 00000000000001000000000000000000
5627 vec($_, 7, 2) = 2 == 32768 00000000000000010000000000000000
5628 vec($_, 8, 2) = 2 == 131072 00000000000000000100000000000000
5629 vec($_, 9, 2) = 2 == 524288 00000000000000000001000000000000
5630 vec($_,10, 2) = 2 == 2097152 00000000000000000000010000000000
5631 vec($_,11, 2) = 2 == 8388608 00000000000000000000000100000000
5632 vec($_,12, 2) = 2 == 33554432 00000000000000000000000001000000
5633 vec($_,13, 2) = 2 == 134217728 00000000000000000000000000010000
5634 vec($_,14, 2) = 2 == 536870912 00000000000000000000000000000100
5635 vec($_,15, 2) = 2 == 2147483648 00000000000000000000000000000001
5636 vec($_, 0, 4) = 1 == 1 10000000000000000000000000000000
5637 vec($_, 1, 4) = 1 == 16 00001000000000000000000000000000
5638 vec($_, 2, 4) = 1 == 256 00000000100000000000000000000000
5639 vec($_, 3, 4) = 1 == 4096 00000000000010000000000000000000
5640 vec($_, 4, 4) = 1 == 65536 00000000000000001000000000000000
5641 vec($_, 5, 4) = 1 == 1048576 00000000000000000000100000000000
5642 vec($_, 6, 4) = 1 == 16777216 00000000000000000000000010000000
5643 vec($_, 7, 4) = 1 == 268435456 00000000000000000000000000001000
5644 vec($_, 0, 4) = 2 == 2 01000000000000000000000000000000
5645 vec($_, 1, 4) = 2 == 32 00000100000000000000000000000000
5646 vec($_, 2, 4) = 2 == 512 00000000010000000000000000000000
5647 vec($_, 3, 4) = 2 == 8192 00000000000001000000000000000000
5648 vec($_, 4, 4) = 2 == 131072 00000000000000000100000000000000
5649 vec($_, 5, 4) = 2 == 2097152 00000000000000000000010000000000
5650 vec($_, 6, 4) = 2 == 33554432 00000000000000000000000001000000
5651 vec($_, 7, 4) = 2 == 536870912 00000000000000000000000000000100
5652 vec($_, 0, 4) = 4 == 4 00100000000000000000000000000000
5653 vec($_, 1, 4) = 4 == 64 00000010000000000000000000000000
5654 vec($_, 2, 4) = 4 == 1024 00000000001000000000000000000000
5655 vec($_, 3, 4) = 4 == 16384 00000000000000100000000000000000
5656 vec($_, 4, 4) = 4 == 262144 00000000000000000010000000000000
5657 vec($_, 5, 4) = 4 == 4194304 00000000000000000000001000000000
5658 vec($_, 6, 4) = 4 == 67108864 00000000000000000000000000100000
5659 vec($_, 7, 4) = 4 == 1073741824 00000000000000000000000000000010
5660 vec($_, 0, 4) = 8 == 8 00010000000000000000000000000000
5661 vec($_, 1, 4) = 8 == 128 00000001000000000000000000000000
5662 vec($_, 2, 4) = 8 == 2048 00000000000100000000000000000000
5663 vec($_, 3, 4) = 8 == 32768 00000000000000010000000000000000
5664 vec($_, 4, 4) = 8 == 524288 00000000000000000001000000000000
5665 vec($_, 5, 4) = 8 == 8388608 00000000000000000000000100000000
5666 vec($_, 6, 4) = 8 == 134217728 00000000000000000000000000010000
5667 vec($_, 7, 4) = 8 == 2147483648 00000000000000000000000000000001
5668 vec($_, 0, 8) = 1 == 1 10000000000000000000000000000000
5669 vec($_, 1, 8) = 1 == 256 00000000100000000000000000000000
5670 vec($_, 2, 8) = 1 == 65536 00000000000000001000000000000000
5671 vec($_, 3, 8) = 1 == 16777216 00000000000000000000000010000000
5672 vec($_, 0, 8) = 2 == 2 01000000000000000000000000000000
5673 vec($_, 1, 8) = 2 == 512 00000000010000000000000000000000
5674 vec($_, 2, 8) = 2 == 131072 00000000000000000100000000000000
5675 vec($_, 3, 8) = 2 == 33554432 00000000000000000000000001000000
5676 vec($_, 0, 8) = 4 == 4 00100000000000000000000000000000
5677 vec($_, 1, 8) = 4 == 1024 00000000001000000000000000000000
5678 vec($_, 2, 8) = 4 == 262144 00000000000000000010000000000000
5679 vec($_, 3, 8) = 4 == 67108864 00000000000000000000000000100000
5680 vec($_, 0, 8) = 8 == 8 00010000000000000000000000000000
5681 vec($_, 1, 8) = 8 == 2048 00000000000100000000000000000000
5682 vec($_, 2, 8) = 8 == 524288 00000000000000000001000000000000
5683 vec($_, 3, 8) = 8 == 134217728 00000000000000000000000000010000
5684 vec($_, 0, 8) = 16 == 16 00001000000000000000000000000000
5685 vec($_, 1, 8) = 16 == 4096 00000000000010000000000000000000
5686 vec($_, 2, 8) = 16 == 1048576 00000000000000000000100000000000
5687 vec($_, 3, 8) = 16 == 268435456 00000000000000000000000000001000
5688 vec($_, 0, 8) = 32 == 32 00000100000000000000000000000000
5689 vec($_, 1, 8) = 32 == 8192 00000000000001000000000000000000
5690 vec($_, 2, 8) = 32 == 2097152 00000000000000000000010000000000
5691 vec($_, 3, 8) = 32 == 536870912 00000000000000000000000000000100
5692 vec($_, 0, 8) = 64 == 64 00000010000000000000000000000000
5693 vec($_, 1, 8) = 64 == 16384 00000000000000100000000000000000
5694 vec($_, 2, 8) = 64 == 4194304 00000000000000000000001000000000
5695 vec($_, 3, 8) = 64 == 1073741824 00000000000000000000000000000010
5696 vec($_, 0, 8) = 128 == 128 00000001000000000000000000000000
5697 vec($_, 1, 8) = 128 == 32768 00000000000000010000000000000000
5698 vec($_, 2, 8) = 128 == 8388608 00000000000000000000000100000000
5699 vec($_, 3, 8) = 128 == 2147483648 00000000000000000000000000000001
5703 Behaves like the wait(2) system call on your system: it waits for a child
5704 process to terminate and returns the pid of the deceased process, or
5705 C<-1> if there are no child processes. The status is returned in C<$?>.
5706 Note that a return value of C<-1> could mean that child processes are
5707 being automatically reaped, as described in L<perlipc>.
5709 =item waitpid PID,FLAGS
5711 Waits for a particular child process to terminate and returns the pid of
5712 the deceased process, or C<-1> if there is no such child process. On some
5713 systems, a value of 0 indicates that there are processes still running.
5714 The status is returned in C<$?>. If you say
5716 use POSIX ":sys_wait_h";
5719 $kid = waitpid(-1,&WNOHANG);
5722 then you can do a non-blocking wait for all pending zombie processes.
5723 Non-blocking wait is available on machines supporting either the
5724 waitpid(2) or wait4(2) system calls. However, waiting for a particular
5725 pid with FLAGS of C<0> is implemented everywhere. (Perl emulates the
5726 system call by remembering the status values of processes that have
5727 exited but have not been harvested by the Perl script yet.)
5729 Note that on some systems, a return value of C<-1> could mean that child
5730 processes are being automatically reaped. See L<perlipc> for details,
5731 and for other examples.
5735 Returns true if the context of the currently executing subroutine is
5736 looking for a list value. Returns false if the context is looking
5737 for a scalar. Returns the undefined value if the context is looking
5738 for no value (void context).
5740 return unless defined wantarray; # don't bother doing more
5741 my @a = complex_calculation();
5742 return wantarray ? @a : "@a";
5744 This function should have been named wantlist() instead.
5748 Produces a message on STDERR just like C<die>, but doesn't exit or throw
5751 If LIST is empty and C<$@> already contains a value (typically from a
5752 previous eval) that value is used after appending C<"\t...caught">
5753 to C<$@>. This is useful for staying almost, but not entirely similar to
5756 If C<$@> is empty then the string C<"Warning: Something's wrong"> is used.
5758 No message is printed if there is a C<$SIG{__WARN__}> handler
5759 installed. It is the handler's responsibility to deal with the message
5760 as it sees fit (like, for instance, converting it into a C<die>). Most
5761 handlers must therefore make arrangements to actually display the
5762 warnings that they are not prepared to deal with, by calling C<warn>
5763 again in the handler. Note that this is quite safe and will not
5764 produce an endless loop, since C<__WARN__> hooks are not called from
5767 You will find this behavior is slightly different from that of
5768 C<$SIG{__DIE__}> handlers (which don't suppress the error text, but can
5769 instead call C<die> again to change it).
5771 Using a C<__WARN__> handler provides a powerful way to silence all
5772 warnings (even the so-called mandatory ones). An example:
5774 # wipe out *all* compile-time warnings
5775 BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } }
5777 my $foo = 20; # no warning about duplicate my $foo,
5778 # but hey, you asked for it!
5779 # no compile-time or run-time warnings before here
5782 # run-time warnings enabled after here
5783 warn "\$foo is alive and $foo!"; # does show up
5785 See L<perlvar> for details on setting C<%SIG> entries, and for more
5786 examples. See the Carp module for other kinds of warnings using its
5787 carp() and cluck() functions.
5789 =item write FILEHANDLE
5795 Writes a formatted record (possibly multi-line) to the specified FILEHANDLE,
5796 using the format associated with that file. By default the format for
5797 a file is the one having the same name as the filehandle, but the
5798 format for the current output channel (see the C<select> function) may be set
5799 explicitly by assigning the name of the format to the C<$~> variable.
5801 Top of form processing is handled automatically: if there is
5802 insufficient room on the current page for the formatted record, the
5803 page is advanced by writing a form feed, a special top-of-page format
5804 is used to format the new page header, and then the record is written.
5805 By default the top-of-page format is the name of the filehandle with
5806 "_TOP" appended, but it may be dynamically set to the format of your
5807 choice by assigning the name to the C<$^> variable while the filehandle is
5808 selected. The number of lines remaining on the current page is in
5809 variable C<$->, which can be set to C<0> to force a new page.
5811 If FILEHANDLE is unspecified, output goes to the current default output
5812 channel, which starts out as STDOUT but may be changed by the
5813 C<select> operator. If the FILEHANDLE is an EXPR, then the expression
5814 is evaluated and the resulting string is used to look up the name of
5815 the FILEHANDLE at run time. For more on formats, see L<perlform>.
5817 Note that write is I<not> the opposite of C<read>. Unfortunately.
5821 The transliteration operator. Same as C<tr///>. See L<perlop>.