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 and 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.
21 In the syntax descriptions that follow, list operators that expect a
22 list (and provide list context for the elements of the list) are shown
23 with LIST as an argument. Such a list may consist of any combination
24 of scalar arguments or list values; the list values will be included
25 in the list as if each individual element were interpolated at that
26 point in the list, forming a longer single-dimensional list value.
27 Elements of the LIST should be separated by commas.
29 Any function in the list below may be used either with or without
30 parentheses around its arguments. (The syntax descriptions omit the
31 parentheses.) If you use the parentheses, the simple (but occasionally
32 surprising) rule is this: It I<LOOKS> like a function, therefore it I<IS> a
33 function, and precedence doesn't matter. Otherwise it's a list
34 operator or unary operator, and precedence does matter. And whitespace
35 between the function and left parenthesis doesn't count--so you need to
38 print 1+2+4; # Prints 7.
39 print(1+2) + 4; # Prints 3.
40 print (1+2)+4; # Also prints 3!
41 print +(1+2)+4; # Prints 7.
42 print ((1+2)+4); # Prints 7.
44 If you run Perl with the B<-w> switch it can warn you about this. For
45 example, the third line above produces:
47 print (...) interpreted as function at - line 1.
48 Useless use of integer addition in void context at - line 1.
50 For functions that can be used in either a scalar or list context,
51 nonabortive failure is generally indicated in a scalar context by
52 returning the undefined value, and in a list context by returning the
55 Remember the following important rule: There is B<no rule> that relates
56 the behavior of an expression in list context to its behavior in scalar
57 context, or vice versa. It might do two totally different things.
58 Each operator and function decides which sort of value it would be most
59 appropriate to return in a scalar context. Some operators return the
60 length of the list that would have been returned in list context. Some
61 operators return the first value in the list. Some operators return the
62 last value in the list. Some operators return a count of successful
63 operations. In general, they do what you want, unless you want
66 An named array in scalar context is quite different from what would at
67 first glance appear to be a list in scalar context. You can't get a list
68 like C<(1,2,3)> into being in scalar context, because the compiler knows
69 the context at compile time. It would generate the scalar comma operator
70 there, not the list construction version of the comma. That means it
71 was never a list to start with.
73 In general, functions in Perl that serve as wrappers for system calls
74 of the same name (like chown(2), fork(2), closedir(2), etc.) all return
75 true when they succeed and C<undef> otherwise, as is usually mentioned
76 in the descriptions below. This is different from the C interfaces,
77 which return C<-1> on failure. Exceptions to this rule are C<wait()>,
78 C<waitpid()>, and C<syscall()>. System calls also set the special C<$!>
79 variable on failure. Other functions do not, except accidentally.
81 =head2 Perl Functions by Category
83 Here are Perl's functions (including things that look like
84 functions, like some keywords and named operators)
85 arranged by category. Some functions appear in more
90 =item Functions for SCALARs or strings
92 C<chomp>, C<chop>, C<chr>, C<crypt>, C<hex>, C<index>, C<lc>, C<lcfirst>,
93 C<length>, C<oct>, C<ord>, C<pack>, C<q/STRING/>, C<qq/STRING/>, C<reverse>,
94 C<rindex>, C<sprintf>, C<substr>, C<tr///>, C<uc>, C<ucfirst>, C<y///>
96 =item Regular expressions and pattern matching
98 C<m//>, C<pos>, C<quotemeta>, C<s///>, C<split>, C<study>, C<qr//>
100 =item Numeric functions
102 C<abs>, C<atan2>, C<cos>, C<exp>, C<hex>, C<int>, C<log>, C<oct>, C<rand>,
103 C<sin>, C<sqrt>, C<srand>
105 =item Functions for real @ARRAYs
107 C<pop>, C<push>, C<shift>, C<splice>, C<unshift>
109 =item Functions for list data
111 C<grep>, C<join>, C<map>, C<qw/STRING/>, C<reverse>, C<sort>, C<unpack>
113 =item Functions for real %HASHes
115 C<delete>, C<each>, C<exists>, C<keys>, C<values>
117 =item Input and output functions
119 C<binmode>, C<close>, C<closedir>, C<dbmclose>, C<dbmopen>, C<die>, C<eof>,
120 C<fileno>, C<flock>, C<format>, C<getc>, C<print>, C<printf>, C<read>,
121 C<readdir>, C<rewinddir>, C<seek>, C<seekdir>, C<select>, C<syscall>,
122 C<sysread>, C<sysseek>, C<syswrite>, C<tell>, C<telldir>, C<truncate>,
125 =item Functions for fixed length data or records
127 C<pack>, C<read>, C<syscall>, C<sysread>, C<syswrite>, C<unpack>, C<vec>
129 =item Functions for filehandles, files, or directories
131 C<-I<X>>, C<chdir>, C<chmod>, C<chown>, C<chroot>, C<fcntl>, C<glob>,
132 C<ioctl>, C<link>, C<lstat>, C<mkdir>, C<open>, C<opendir>,
133 C<readlink>, C<rename>, C<rmdir>, C<stat>, C<symlink>, C<umask>,
136 =item Keywords related to the control flow of your perl program
138 C<caller>, C<continue>, C<die>, C<do>, C<dump>, C<eval>, C<exit>,
139 C<goto>, C<last>, C<next>, C<redo>, C<return>, C<sub>, C<wantarray>
141 =item Keywords related to scoping
143 C<caller>, C<import>, C<local>, C<my>, C<package>, C<use>
145 =item Miscellaneous functions
147 C<defined>, C<dump>, C<eval>, C<formline>, C<local>, C<my>, C<reset>,
148 C<scalar>, C<undef>, C<wantarray>
150 =item Functions for processes and process groups
152 C<alarm>, C<exec>, C<fork>, C<getpgrp>, C<getppid>, C<getpriority>, C<kill>,
153 C<pipe>, C<qx/STRING/>, C<setpgrp>, C<setpriority>, C<sleep>, C<system>,
154 C<times>, C<wait>, C<waitpid>
156 =item Keywords related to perl modules
158 C<do>, C<import>, C<no>, C<package>, C<require>, C<use>
160 =item Keywords related to classes and object-orientedness
162 C<bless>, C<dbmclose>, C<dbmopen>, C<package>, C<ref>, C<tie>, C<tied>,
165 =item Low-level socket functions
167 C<accept>, C<bind>, C<connect>, C<getpeername>, C<getsockname>,
168 C<getsockopt>, C<listen>, C<recv>, C<send>, C<setsockopt>, C<shutdown>,
169 C<socket>, C<socketpair>
171 =item System V interprocess communication functions
173 C<msgctl>, C<msgget>, C<msgrcv>, C<msgsnd>, C<semctl>, C<semget>, C<semop>,
174 C<shmctl>, C<shmget>, C<shmread>, C<shmwrite>
176 =item Fetching user and group info
178 C<endgrent>, C<endhostent>, C<endnetent>, C<endpwent>, C<getgrent>,
179 C<getgrgid>, C<getgrnam>, C<getlogin>, C<getpwent>, C<getpwnam>,
180 C<getpwuid>, C<setgrent>, C<setpwent>
182 =item Fetching network info
184 C<endprotoent>, C<endservent>, C<gethostbyaddr>, C<gethostbyname>,
185 C<gethostent>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
186 C<getprotobyname>, C<getprotobynumber>, C<getprotoent>,
187 C<getservbyname>, C<getservbyport>, C<getservent>, C<sethostent>,
188 C<setnetent>, C<setprotoent>, C<setservent>
190 =item Time-related functions
192 C<gmtime>, C<localtime>, C<time>, C<times>
194 =item Functions new in perl5
196 C<abs>, C<bless>, C<chomp>, C<chr>, C<exists>, C<formline>, C<glob>,
197 C<import>, C<lc>, C<lcfirst>, C<map>, C<my>, C<no>, C<prototype>, C<qx>,
198 C<qw>, C<readline>, C<readpipe>, C<ref>, C<sub*>, C<sysopen>, C<tie>,
199 C<tied>, C<uc>, C<ucfirst>, C<untie>, C<use>
201 * - C<sub> was a keyword in perl4, but in perl5 it is an
202 operator, which can be used in expressions.
204 =item Functions obsoleted in perl5
206 C<dbmclose>, C<dbmopen>
212 Perl was born in UNIX and therefore it can access all the common UNIX
213 system calls. In non-UNIX environments the functionality of many
214 UNIX system calls may not be available or the details of the available
215 functionality may be slightly different. The Perl functions affected
218 C<-X>, C<binmode>, C<chmod>, C<chown>, C<chroot>, C<crypt>,
219 C<dbmclose>, C<dbmopen>, C<dump>, C<endgrent>, C<endhostent>,
220 C<endnetent>, C<endprotoent>, C<endpwent>, C<endservent>, C<exec>,
221 C<fcntl>, C<flock>, C<fork>, C<getgrent>, C<getgrgid>, C<gethostent>,
222 C<getlogin>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
223 C<getppid>, C<getprgp>, C<getpriority>, C<getprotobynumber>,
224 C<getprotoent>, C<getpwent>, C<getpwnam>, C<getpwuid>,
225 C<getservbyport>, C<getservent>, C<getsockopt>, C<glob>, C<ioctl>,
226 C<kill>, C<link>, C<lstat>, C<msgctl>, C<msgget>, C<msgrcv>,
227 C<msgsnd>, C<open>, C<pipe>, C<readlink>, C<select>, C<semctl>,
228 C<semget>, C<semop>, C<setgrent>, C<sethostent>, C<setnetent>,
229 C<setpgrp>, C<setpriority>, C<setprotoent>, C<setpwent>,
230 C<setservent>, C<setsockopt>, C<shmctl>, C<shmget>, C<shmread>,
231 C<shmwrite>, C<socketpair>, C<stat>, C<symlink>, C<syscall>,
232 C<sysopen>, C<system>, C<times>, C<truncate>, C<umask>, C<utime>,
235 For more information about the portability of these functions, see
236 L<perlport> and other available platform-specific documentation.
238 =head2 Alphabetical Listing of Perl Functions
242 =item I<-X> FILEHANDLE
248 A file test, where X is one of the letters listed below. This unary
249 operator takes one argument, either a filename or a filehandle, and
250 tests the associated file to see if something is true about it. If the
251 argument is omitted, tests C<$_>, except for C<-t>, which tests STDIN.
252 Unless otherwise documented, it returns C<1> for TRUE and C<''> for FALSE, or
253 the undefined value if the file doesn't exist. Despite the funny
254 names, precedence is the same as any other named unary operator, and
255 the argument may be parenthesized like any other unary operator. The
256 operator may be any of:
257 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>
258 X<-S>X<-b>X<-c>X<-t>X<-u>X<-g>X<-k>X<-T>X<-B>X<-M>X<-A>X<-C>
260 -r File is readable by effective uid/gid.
261 -w File is writable by effective uid/gid.
262 -x File is executable by effective uid/gid.
263 -o File is owned by effective uid.
265 -R File is readable by real uid/gid.
266 -W File is writable by real uid/gid.
267 -X File is executable by real uid/gid.
268 -O File is owned by real uid.
271 -z File has zero size.
272 -s File has nonzero size (returns size).
274 -f File is a plain file.
275 -d File is a directory.
276 -l File is a symbolic link.
277 -p File is a named pipe (FIFO), or Filehandle is a pipe.
279 -b File is a block special file.
280 -c File is a character special file.
281 -t Filehandle is opened to a tty.
283 -u File has setuid bit set.
284 -g File has setgid bit set.
285 -k File has sticky bit set.
287 -T File is a text file.
288 -B File is a binary file (opposite of -T).
290 -M Age of file in days when script started.
291 -A Same for access time.
292 -C Same for inode change time.
298 next unless -f $_; # ignore specials
302 The interpretation of the file permission operators C<-r>, C<-R>,
303 C<-w>, C<-W>, C<-x>, and C<-X> is by default based solely on the mode
304 of the file and the uids and gids of the user. There may be other
305 reasons you can't actually read, write, or execute the file. Such
306 reasons may be for example network filesystem access controls, ACLs
307 (access control lists), read-only filesystems, and unrecognized
310 Also note that, for the superuser on the local filesystems, C<-r>,
311 C<-R>, C<-w>, and C<-W> always return 1, and C<-x> and C<-X> return 1
312 if any execute bit is set in the mode. Scripts run by the superuser
313 may thus need to do a stat() to determine the actual mode of the file,
314 or temporarily set the uid to something else.
316 If you are using ACLs, there is a pragma called C<filetest> that may
317 produce more accurate results than the bare stat() mode bits.
319 When under the C<use filetest 'access'> the above-mentioned filetests
320 will test whether the permission can (not) be granted using the
321 access() family of system calls. Also note that the C<-x> and C<-X> may
322 under this pragma return true even if there are no execute permission
323 bits set (nor any extra execute permission ACLs). This strangeness is
324 due to the underlying system calls' definitions. Read the
325 documentation for the C<filetest> pragma for more information.
327 Note that C<-s/a/b/> does not do a negated substitution. Saying
328 C<-exp($foo)> still works as expected, however--only single letters
329 following a minus are interpreted as file tests.
331 The C<-T> and C<-B> switches work as follows. The first block or so of the
332 file is examined for odd characters such as strange control codes or
333 characters with the high bit set. If too many strange characters (E<gt>30%)
334 are found, it's a C<-B> file, otherwise it's a C<-T> file. Also, any file
335 containing null in the first block is considered a binary file. If C<-T>
336 or C<-B> is used on a filehandle, the current stdio buffer is examined
337 rather than the first block. Both C<-T> and C<-B> return TRUE on a null
338 file, or a file at EOF when testing a filehandle. Because you have to
339 read a file to do the C<-T> test, on most occasions you want to use a C<-f>
340 against the file first, as in C<next unless -f $file && -T $file>.
342 If any of the file tests (or either the C<stat()> or C<lstat()> operators) are given
343 the special filehandle consisting of a solitary underline, then the stat
344 structure of the previous file test (or stat operator) is used, saving
345 a system call. (This doesn't work with C<-t>, and you need to remember
346 that lstat() and C<-l> will leave values in the stat structure for the
347 symbolic link, not the real file.) Example:
349 print "Can do.\n" if -r $a || -w _ || -x _;
352 print "Readable\n" if -r _;
353 print "Writable\n" if -w _;
354 print "Executable\n" if -x _;
355 print "Setuid\n" if -u _;
356 print "Setgid\n" if -g _;
357 print "Sticky\n" if -k _;
358 print "Text\n" if -T _;
359 print "Binary\n" if -B _;
365 Returns the absolute value of its argument.
366 If VALUE is omitted, uses C<$_>.
368 =item accept NEWSOCKET,GENERICSOCKET
370 Accepts an incoming socket connect, just as the accept(2) system call
371 does. Returns the packed address if it succeeded, FALSE otherwise.
372 See example in L<perlipc/"Sockets: Client/Server Communication">.
378 Arranges to have a SIGALRM delivered to this process after the
379 specified number of seconds have elapsed. If SECONDS is not specified,
380 the value stored in C<$_> is used. (On some machines,
381 unfortunately, the elapsed time may be up to one second less than you
382 specified because of how seconds are counted.) Only one timer may be
383 counting at once. Each call disables the previous timer, and an
384 argument of C<0> may be supplied to cancel the previous timer without
385 starting a new one. The returned value is the amount of time remaining
386 on the previous timer.
388 For delays of finer granularity than one second, you may use Perl's
389 C<syscall()> interface to access setitimer(2) if your system supports it,
390 or else see L</select()>. It is usually a mistake to intermix C<alarm()>
391 and C<sleep()> calls.
393 If you want to use C<alarm()> to time out a system call you need to use an
394 C<eval()>/C<die()> pair. You can't rely on the alarm causing the system call to
395 fail with C<$!> set to C<EINTR> because Perl sets up signal handlers to
396 restart system calls on some systems. Using C<eval()>/C<die()> always works,
397 modulo the caveats given in L<perlipc/"Signals">.
400 local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
402 $nread = sysread SOCKET, $buffer, $size;
406 die unless $@ eq "alarm\n"; # propagate unexpected errors
415 Returns the arctangent of Y/X in the range -PI to PI.
417 For the tangent operation, you may use the C<POSIX::tan()>
418 function, or use the familiar relation:
420 sub tan { sin($_[0]) / cos($_[0]) }
422 =item bind SOCKET,NAME
424 Binds a network address to a socket, just as the bind system call
425 does. Returns TRUE if it succeeded, FALSE otherwise. NAME should be a
426 packed address of the appropriate type for the socket. See the examples in
427 L<perlipc/"Sockets: Client/Server Communication">.
429 =item binmode FILEHANDLE
431 Arranges for the file to be read or written in "binary" mode in operating
432 systems that distinguish between binary and text files. Files that are
433 not in binary mode have CR LF sequences translated to LF on input and LF
434 translated to CR LF on output. Binmode has no effect under Unix; in MS-DOS
435 and similarly archaic systems, it may be imperative--otherwise your
436 MS-DOS-damaged C library may mangle your file. The key distinction between
437 systems that need C<binmode()> and those that don't is their text file
438 formats. Systems like Unix, MacOS, and Plan9 that delimit lines with a single
439 character, and that encode that character in C as C<"\n">, do not need
440 C<binmode()>. The rest need it. If FILEHANDLE is an expression, the value
441 is taken as the name of the filehandle.
443 =item bless REF,CLASSNAME
447 This function tells the thingy referenced by REF that it is now
448 an object in the CLASSNAME package--or the current package if no CLASSNAME
449 is specified, which is often the case. It returns the reference for
450 convenience, because a C<bless()> is often the last thing in a constructor.
451 Always use the two-argument version if the function doing the blessing
452 might be inherited by a derived class. See L<perltoot> and L<perlobj>
453 for more about the blessing (and blessings) of objects.
455 Consider always blessing objects in CLASSNAMEs that are mixed case.
456 Namespaces with all lowercase names are considered reserved for Perl
457 pragmata. Builtin types have all uppercase names, so to prevent confusion,
458 it is best to avoid such package names as well.
460 See L<perlmod/"Perl Modules">.
466 Returns the context of the current subroutine call. In scalar context,
467 returns the caller's package name if there is a caller, that is, if
468 we're in a subroutine or C<eval()> or C<require()>, and the undefined value
469 otherwise. In list context, returns
471 ($package, $filename, $line) = caller;
473 With EXPR, it returns some extra information that the debugger uses to
474 print a stack trace. The value of EXPR indicates how many call frames
475 to go back before the current one.
477 ($package, $filename, $line, $subroutine,
478 $hasargs, $wantarray, $evaltext, $is_require) = caller($i);
480 Here C<$subroutine> may be C<"(eval)"> if the frame is not a subroutine
481 call, but an C<eval()>. In such a case additional elements C<$evaltext> and
482 C<$is_require> are set: C<$is_require> is true if the frame is created by a
483 C<require> or C<use> statement, C<$evaltext> contains the text of the
484 C<eval EXPR> statement. In particular, for a C<eval BLOCK> statement,
485 C<$filename> is C<"(eval)">, but C<$evaltext> is undefined. (Note also that
486 each C<use> statement creates a C<require> frame inside an C<eval EXPR>)
489 Furthermore, when called from within the DB package, caller returns more
490 detailed information: it sets the list variable C<@DB::args> to be the
491 arguments with which the subroutine was invoked.
493 Be aware that the optimizer might have optimized call frames away before
494 C<caller()> had a chance to get the information. That means that C<caller(N)>
495 might not return information about the call frame you expect it do, for
496 C<N E<gt> 1>. In particular, C<@DB::args> might have information from the
497 previous time C<caller()> was called.
501 Changes the working directory to EXPR, if possible. If EXPR is
502 omitted, changes to home directory. Returns TRUE upon success, FALSE
503 otherwise. See example under C<die()>.
507 Changes the permissions of a list of files. The first element of the
508 list must be the numerical mode, which should probably be an octal
509 number, and which definitely should I<not> a string of octal digits:
510 C<0644> is okay, C<'0644'> is not. Returns the number of files
511 successfully changed. See also L</oct>, if all you have is a string.
513 $cnt = chmod 0755, 'foo', 'bar';
514 chmod 0755, @executables;
515 $mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to
517 $mode = '0644'; chmod oct($mode), 'foo'; # this is better
518 $mode = 0644; chmod $mode, 'foo'; # this is best
526 This is a slightly safer version of L</chop>. It removes any
527 line ending that corresponds to the current value of C<$/> (also known as
528 $INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
529 number of characters removed from all its arguments. It's often used to
530 remove the newline from the end of an input record when you're worried
531 that the final record may be missing its newline. When in paragraph mode
532 (C<$/ = "">), it removes all trailing newlines from the string. If
533 VARIABLE is omitted, it chomps C<$_>. Example:
536 chomp; # avoid \n on last field
541 You can actually chomp anything that's an lvalue, including an assignment:
544 chomp($answer = <STDIN>);
546 If you chomp a list, each element is chomped, and the total number of
547 characters removed is returned.
555 Chops off the last character of a string and returns the character
556 chopped. It's used primarily to remove the newline from the end of an
557 input record, but is much more efficient than C<s/\n//> because it neither
558 scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
562 chop; # avoid \n on last field
567 You can actually chop anything that's an lvalue, including an assignment:
570 chop($answer = <STDIN>);
572 If you chop a list, each element is chopped. Only the value of the
573 last C<chop()> is returned.
575 Note that C<chop()> returns the last character. To return all but the last
576 character, use C<substr($string, 0, -1)>.
580 Changes the owner (and group) of a list of files. The first two
581 elements of the list must be the I<NUMERICAL> uid and gid, in that order.
582 Returns the number of files successfully changed.
584 $cnt = chown $uid, $gid, 'foo', 'bar';
585 chown $uid, $gid, @filenames;
587 Here's an example that looks up nonnumeric uids in the passwd file:
590 chop($user = <STDIN>);
592 chop($pattern = <STDIN>);
594 ($login,$pass,$uid,$gid) = getpwnam($user)
595 or die "$user not in passwd file";
597 @ary = glob($pattern); # expand filenames
598 chown $uid, $gid, @ary;
600 On most systems, you are not allowed to change the ownership of the
601 file unless you're the superuser, although you should be able to change
602 the group to any of your secondary groups. On insecure systems, these
603 restrictions may be relaxed, but this is not a portable assumption.
609 Returns the character represented by that NUMBER in the character set.
610 For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
611 chr(0x263a) is a Unicode smiley face (but only within the scope of a
612 C<use utf8>). For the reverse, use L</ord>.
614 If NUMBER is omitted, uses C<$_>.
616 =item chroot FILENAME
620 This function works like the system call by the same name: it makes the
621 named directory the new root directory for all further pathnames that
622 begin with a C<"/"> by your process and all its children. (It doesn't
623 change your current working directory, which is unaffected.) For security
624 reasons, this call is restricted to the superuser. If FILENAME is
625 omitted, does a C<chroot()> to C<$_>.
627 =item close FILEHANDLE
631 Closes the file or pipe associated with the file handle, returning TRUE
632 only if stdio successfully flushes buffers and closes the system file
633 descriptor. Closes the currently selected filehandle if the argument
636 You don't have to close FILEHANDLE if you are immediately going to do
637 another C<open()> on it, because C<open()> will close it for you. (See
638 C<open()>.) However, an explicit C<close()> on an input file resets the line
639 counter (C<$.>), while the implicit close done by C<open()> does not.
641 If the file handle came from a piped open C<close()> will additionally
642 return FALSE if one of the other system calls involved fails or if the
643 program exits with non-zero status. (If the only problem was that the
644 program exited non-zero C<$!> will be set to C<0>.) Also, closing a pipe
645 waits for the process executing on the pipe to complete, in case you
646 want to look at the output of the pipe afterwards. Closing a pipe
647 explicitly also puts the exit status value of the command into C<$?>.
651 open(OUTPUT, '|sort >foo') # pipe to sort
652 or die "Can't start sort: $!";
653 #... # print stuff to output
654 close OUTPUT # wait for sort to finish
655 or warn $! ? "Error closing sort pipe: $!"
656 : "Exit status $? from sort";
657 open(INPUT, 'foo') # get sort's results
658 or die "Can't open 'foo' for input: $!";
660 FILEHANDLE may be an expression whose value can be used as an indirect
661 filehandle, usually the real filehandle name.
663 =item closedir DIRHANDLE
665 Closes a directory opened by C<opendir()> and returns the success of that
668 DIRHANDLE may be an expression whose value can be used as an indirect
669 dirhandle, usually the real dirhandle name.
671 =item connect SOCKET,NAME
673 Attempts to connect to a remote socket, just as the connect system call
674 does. Returns TRUE if it succeeded, FALSE otherwise. NAME should be a
675 packed address of the appropriate type for the socket. See the examples in
676 L<perlipc/"Sockets: Client/Server Communication">.
680 Actually a flow control statement rather than a function. If there is a
681 C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
682 C<foreach>), it is always executed just before the conditional is about to
683 be evaluated again, just like the third part of a C<for> loop in C. Thus
684 it can be used to increment a loop variable, even when the loop has been
685 continued via the C<next> statement (which is similar to the C C<continue>
688 C<last>, C<next>, or C<redo> may appear within a C<continue>
689 block. C<last> and C<redo> will behave as if they had been executed within
690 the main block. So will C<next>, but since it will execute a C<continue>
691 block, it may be more entertaining.
694 ### redo always comes here
697 ### next always comes here
699 # then back the top to re-check EXPR
701 ### last always comes here
703 Omitting the C<continue> section is semantically equivalent to using an
704 empty one, logically enough. In that case, C<next> goes directly back
705 to check the condition at the top of the loop.
709 Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
710 takes cosine of C<$_>.
712 For the inverse cosine operation, you may use the C<POSIX::acos()>
713 function, or use this relation:
715 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
717 =item crypt PLAINTEXT,SALT
719 Encrypts a string exactly like the crypt(3) function in the C library
720 (assuming that you actually have a version there that has not been
721 extirpated as a potential munition). This can prove useful for checking
722 the password file for lousy passwords, amongst other things. Only the
723 guys wearing white hats should do this.
725 Note that C<crypt()> is intended to be a one-way function, much like breaking
726 eggs to make an omelette. There is no (known) corresponding decrypt
727 function. As a result, this function isn't all that useful for
728 cryptography. (For that, see your nearby CPAN mirror.)
730 When verifying an existing encrypted string you should use the encrypted
731 text as the salt (like C<crypt($plain, $crypted) eq $crypted>). This
732 allows your code to work with the standard C<crypt()> and with more
733 exotic implementations. When choosing a new salt create a random two
734 character string whose characters come from the set C<[./0-9A-Za-z]>
735 (like C<join '', ('.', '/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>).
737 Here's an example that makes sure that whoever runs this program knows
740 $pwd = (getpwuid($<))[1];
744 chomp($word = <STDIN>);
748 if (crypt($word, $pwd) ne $pwd) {
754 Of course, typing in your own password to whoever asks you
759 [This function has been superseded by the C<untie()> function.]
761 Breaks the binding between a DBM file and a hash.
763 =item dbmopen HASH,DBNAME,MODE
765 [This function has been superseded by the C<tie()> function.]
767 This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
768 hash. HASH is the name of the hash. (Unlike normal C<open()>, the first
769 argument is I<NOT> a filehandle, even though it looks like one). DBNAME
770 is the name of the database (without the F<.dir> or F<.pag> extension if
771 any). If the database does not exist, it is created with protection
772 specified by MODE (as modified by the C<umask()>). If your system supports
773 only the older DBM functions, you may perform only one C<dbmopen()> in your
774 program. In older versions of Perl, if your system had neither DBM nor
775 ndbm, calling C<dbmopen()> produced a fatal error; it now falls back to
778 If you don't have write access to the DBM file, you can only read hash
779 variables, not set them. If you want to test whether you can write,
780 either use file tests or try setting a dummy hash entry inside an C<eval()>,
781 which will trap the error.
783 Note that functions such as C<keys()> and C<values()> may return huge lists
784 when used on large DBM files. You may prefer to use the C<each()>
785 function to iterate over large DBM files. Example:
787 # print out history file offsets
788 dbmopen(%HIST,'/usr/lib/news/history',0666);
789 while (($key,$val) = each %HIST) {
790 print $key, ' = ', unpack('L',$val), "\n";
794 See also L<AnyDBM_File> for a more general description of the pros and
795 cons of the various dbm approaches, as well as L<DB_File> for a particularly
802 Returns a Boolean value telling whether EXPR has a value other than
803 the undefined value C<undef>. If EXPR is not present, C<$_> will be
806 Many operations return C<undef> to indicate failure, end of file,
807 system error, uninitialized variable, and other exceptional
808 conditions. This function allows you to distinguish C<undef> from
809 other values. (A simple Boolean test will not distinguish among
810 C<undef>, zero, the empty string, and C<"0">, which are all equally
811 false.) Note that since C<undef> is a valid scalar, its presence
812 doesn't I<necessarily> indicate an exceptional condition: C<pop()>
813 returns C<undef> when its argument is an empty array, I<or> when the
814 element to return happens to be C<undef>.
816 You may also use C<defined()> to check whether a subroutine exists, by
817 saying C<defined &func> without parentheses. On the other hand, use
818 of C<defined()> upon aggregates (hashes and arrays) is not guaranteed to
819 produce intuitive results, and should probably be avoided.
821 When used on a hash element, it tells you whether the value is defined,
822 not whether the key exists in the hash. Use L</exists> for the latter
827 print if defined $switch{'D'};
828 print "$val\n" while defined($val = pop(@ary));
829 die "Can't readlink $sym: $!"
830 unless defined($value = readlink $sym);
831 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
832 $debugging = 0 unless defined $debugging;
834 Note: Many folks tend to overuse C<defined()>, and then are surprised to
835 discover that the number C<0> and C<""> (the zero-length string) are, in fact,
836 defined values. For example, if you say
840 The pattern match succeeds, and C<$1> is defined, despite the fact that it
841 matched "nothing". But it didn't really match nothing--rather, it
842 matched something that happened to be C<0> characters long. This is all
843 very above-board and honest. When a function returns an undefined value,
844 it's an admission that it couldn't give you an honest answer. So you
845 should use C<defined()> only when you're questioning the integrity of what
846 you're trying to do. At other times, a simple comparison to C<0> or C<""> is
849 Currently, using C<defined()> on an entire array or hash reports whether
850 memory for that aggregate has ever been allocated. So an array you set
851 to the empty list appears undefined initially, and one that once was full
852 and that you then set to the empty list still appears defined. You
853 should instead use a simple test for size:
855 if (@an_array) { print "has array elements\n" }
856 if (%a_hash) { print "has hash members\n" }
858 Using C<undef()> on these, however, does clear their memory and then report
859 them as not defined anymore, but you shouldn't do that unless you don't
860 plan to use them again, because it saves time when you load them up
861 again to have memory already ready to be filled. The normal way to
862 free up space used by an aggregate is to assign the empty list.
864 This counterintuitive behavior of C<defined()> on aggregates may be
865 changed, fixed, or broken in a future release of Perl.
867 See also L</undef>, L</exists>, L</ref>.
871 Deletes the specified key(s) and their associated values from a hash.
872 For each key, returns the deleted value associated with that key, or
873 the undefined value if there was no such key. Deleting from C<$ENV{}>
874 modifies the environment. Deleting from a hash tied to a DBM file
875 deletes the entry from the DBM file. (But deleting from a C<tie()>d hash
876 doesn't necessarily return anything.)
878 The following deletes all the values of a hash:
880 foreach $key (keys %HASH) {
886 delete @HASH{keys %HASH}
888 (But both of these are slower than just assigning the empty list, or
889 using C<undef()>.) Note that the EXPR can be arbitrarily complicated as
890 long as the final operation is a hash element lookup or hash slice:
892 delete $ref->[$x][$y]{$key};
893 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
897 Outside an C<eval()>, prints the value of LIST to C<STDERR> and exits with
898 the current value of C<$!> (errno). If C<$!> is C<0>, exits with the value of
899 C<($? E<gt>E<gt> 8)> (backtick `command` status). If C<($? E<gt>E<gt> 8)>
900 is C<0>, exits with C<255>. Inside an C<eval(),> the error message is stuffed into
901 C<$@> and the C<eval()> is terminated with the undefined value. This makes
902 C<die()> the way to raise an exception.
906 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
907 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
909 If the value of EXPR does not end in a newline, the current script line
910 number and input line number (if any) are also printed, and a newline
911 is supplied. Hint: sometimes appending C<", stopped"> to your message
912 will cause it to make better sense when the string C<"at foo line 123"> is
913 appended. Suppose you are running script "canasta".
915 die "/etc/games is no good";
916 die "/etc/games is no good, stopped";
918 produce, respectively
920 /etc/games is no good at canasta line 123.
921 /etc/games is no good, stopped at canasta line 123.
923 See also C<exit()> and C<warn()>.
925 If LIST is empty and C<$@> already contains a value (typically from a
926 previous eval) that value is reused after appending C<"\t...propagated">.
927 This is useful for propagating exceptions:
930 die unless $@ =~ /Expected exception/;
932 If C<$@> is empty then the string C<"Died"> is used.
934 You can arrange for a callback to be run just before the C<die()> does
935 its deed, by setting the C<$SIG{__DIE__}> hook. The associated handler
936 will be called with the error text and can change the error message, if
937 it sees fit, by calling C<die()> again. See L<perlvar/$SIG{expr}> for details on
938 setting C<%SIG> entries, and L<"eval BLOCK"> for some examples.
940 Note that the C<$SIG{__DIE__}> hook is called even inside eval()ed
941 blocks/strings. If one wants the hook to do nothing in such
946 as the first line of the handler (see L<perlvar/$^S>).
950 Not really a function. Returns the value of the last command in the
951 sequence of commands indicated by BLOCK. When modified by a loop
952 modifier, executes the BLOCK once before testing the loop condition.
953 (On other statements the loop modifiers test the conditional first.)
955 C<do BLOCK> does I<not> count as a loop, so the loop control statements
956 C<next>, C<last> or C<redo> cannot be used to leave or restart the block.
958 =item do SUBROUTINE(LIST)
960 A deprecated form of subroutine call. See L<perlsub>.
964 Uses the value of EXPR as a filename and executes the contents of the
965 file as a Perl script. Its primary use is to include subroutines
966 from a Perl subroutine library.
972 scalar eval `cat stat.pl`;
974 except that it's more efficient and concise, keeps track of the
975 current filename for error messages, and searches all the B<-I>
976 libraries if the file isn't in the current directory (see also the @INC
977 array in L<perlvar/Predefined Names>). It is also different in how
978 code evaluated with C<do FILENAME> doesn't see lexicals in the enclosing
979 scope like C<eval STRING> does. It's the same, however, in that it does
980 reparse the file every time you call it, so you probably don't want to
981 do this inside a loop.
983 If C<do> cannot read the file, it returns undef and sets C<$!> to the
984 error. If C<do> can read the file but cannot compile it, it
985 returns undef and sets an error message in C<$@>. If the file is
986 successfully compiled, C<do> returns the value of the last expression
989 Note that inclusion of library modules is better done with the
990 C<use()> and C<require()> operators, which also do automatic error checking
991 and raise an exception if there's a problem.
993 You might like to use C<do> to read in a program configuration
994 file. Manual error checking can be done this way:
996 # read in config files: system first, then user
997 for $file ("/share/prog/defaults.rc",
998 "$ENV{HOME}/.someprogrc") {
999 unless ($return = do $file) {
1000 warn "couldn't parse $file: $@" if $@;
1001 warn "couldn't do $file: $!" unless defined $return;
1002 warn "couldn't run $file" unless $return;
1008 This causes an immediate core dump. Primarily this is so that you can
1009 use the B<undump> program to turn your core dump into an executable binary
1010 after having initialized all your variables at the beginning of the
1011 program. When the new binary is executed it will begin by executing a
1012 C<goto LABEL> (with all the restrictions that C<goto> suffers). Think of
1013 it as a goto with an intervening core dump and reincarnation. If C<LABEL>
1014 is omitted, restarts the program from the top. WARNING: Any files
1015 opened at the time of the dump will NOT be open any more when the
1016 program is reincarnated, with possible resulting confusion on the part
1017 of Perl. See also B<-u> option in L<perlrun>.
1022 require 'getopt.pl';
1034 dump QUICKSTART if $ARGV[0] eq '-d';
1039 This operator is largely obsolete, partly because it's very hard to
1040 convert a core file into an executable, and because the real perl-to-C
1041 compiler has superseded it.
1045 When called in list context, returns a 2-element list consisting of the
1046 key and value for the next element of a hash, so that you can iterate over
1047 it. When called in scalar context, returns the key for only the "next"
1048 element in the hash. (Note: Keys may be C<"0"> or C<"">, which are logically
1049 false; you may wish to avoid constructs like C<while ($k = each %foo) {}>
1052 Entries are returned in an apparently random order. The actual random
1053 order is subject to change in future versions of perl, but it is guaranteed
1054 to be in the same order as either the C<keys()> or C<values()> function
1055 would produce on the same (unmodified) hash.
1057 When the hash is entirely read, a null array is returned in list context
1058 (which when assigned produces a FALSE (C<0>) value), and C<undef> in
1059 scalar context. The next call to C<each()> after that will start iterating
1060 again. There is a single iterator for each hash, shared by all C<each()>,
1061 C<keys()>, and C<values()> function calls in the program; it can be reset by
1062 reading all the elements from the hash, or by evaluating C<keys HASH> or
1063 C<values HASH>. If you add or delete elements of a hash while you're
1064 iterating over it, you may get entries skipped or duplicated, so don't.
1066 The following prints out your environment like the printenv(1) program,
1067 only in a different order:
1069 while (($key,$value) = each %ENV) {
1070 print "$key=$value\n";
1073 See also C<keys()>, C<values()> and C<sort()>.
1075 =item eof FILEHANDLE
1081 Returns 1 if the next read on FILEHANDLE will return end of file, or if
1082 FILEHANDLE is not open. FILEHANDLE may be an expression whose value
1083 gives the real filehandle. (Note that this function actually
1084 reads a character and then C<ungetc()>s it, so isn't very useful in an
1085 interactive context.) Do not read from a terminal file (or call
1086 C<eof(FILEHANDLE)> on it) after end-of-file is reached. Filetypes such
1087 as terminals may lose the end-of-file condition if you do.
1089 An C<eof> without an argument uses the last file read as argument.
1090 Using C<eof()> with empty parentheses is very different. It indicates the pseudo file formed of
1091 the files listed on the command line, i.e., C<eof()> is reasonable to
1092 use inside a C<while (E<lt>E<gt>)> loop to detect the end of only the
1093 last file. Use C<eof(ARGV)> or eof without the parentheses to test
1094 I<EACH> file in a while (E<lt>E<gt>) loop. Examples:
1096 # reset line numbering on each input file
1098 next if /^\s*#/; # skip comments
1101 close ARGV if eof; # Not eof()!
1104 # insert dashes just before last line of last file
1106 if (eof()) { # check for end of current file
1107 print "--------------\n";
1108 close(ARGV); # close or break; is needed if we
1109 # are reading from the terminal
1114 Practical hint: you almost never need to use C<eof> in Perl, because the
1115 input operators return false values when they run out of data, or if there
1122 In the first form, the return value of EXPR is parsed and executed as if it
1123 were a little Perl program. The value of the expression (which is itself
1124 determined within scalar context) is first parsed, and if there weren't any
1125 errors, executed in the context of the current Perl program, so that any
1126 variable settings or subroutine and format definitions remain afterwards.
1127 Note that the value is parsed every time the eval executes. If EXPR is
1128 omitted, evaluates C<$_>. This form is typically used to delay parsing
1129 and subsequent execution of the text of EXPR until run time.
1131 In the second form, the code within the BLOCK is parsed only once--at the
1132 same time the code surrounding the eval itself was parsed--and executed
1133 within the context of the current Perl program. This form is typically
1134 used to trap exceptions more efficiently than the first (see below), while
1135 also providing the benefit of checking the code within BLOCK at compile
1138 The final semicolon, if any, may be omitted from the value of EXPR or within
1141 In both forms, the value returned is the value of the last expression
1142 evaluated inside the mini-program; a return statement may be also used, just
1143 as with subroutines. The expression providing the return value is evaluated
1144 in void, scalar, or list context, depending on the context of the eval itself.
1145 See L</wantarray> for more on how the evaluation context can be determined.
1147 If there is a syntax error or runtime error, or a C<die()> statement is
1148 executed, an undefined value is returned by C<eval()>, and C<$@> is set to the
1149 error message. If there was no error, C<$@> is guaranteed to be a null
1150 string. Beware that using C<eval()> neither silences perl from printing
1151 warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
1152 To do either of those, you have to use the C<$SIG{__WARN__}> facility. See
1153 L</warn> and L<perlvar>.
1155 Note that, because C<eval()> traps otherwise-fatal errors, it is useful for
1156 determining whether a particular feature (such as C<socket()> or C<symlink()>)
1157 is implemented. It is also Perl's exception trapping mechanism, where
1158 the die operator is used to raise exceptions.
1160 If the code to be executed doesn't vary, you may use the eval-BLOCK
1161 form to trap run-time errors without incurring the penalty of
1162 recompiling each time. The error, if any, is still returned in C<$@>.
1165 # make divide-by-zero nonfatal
1166 eval { $answer = $a / $b; }; warn $@ if $@;
1168 # same thing, but less efficient
1169 eval '$answer = $a / $b'; warn $@ if $@;
1171 # a compile-time error
1172 eval { $answer = }; # WRONG
1175 eval '$answer ='; # sets $@
1177 When using the C<eval{}> form as an exception trap in libraries, you may
1178 wish not to trigger any C<__DIE__> hooks that user code may have
1179 installed. You can use the C<local $SIG{__DIE__}> construct for this
1180 purpose, as shown in this example:
1182 # a very private exception trap for divide-by-zero
1183 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1186 This is especially significant, given that C<__DIE__> hooks can call
1187 C<die()> again, which has the effect of changing their error messages:
1189 # __DIE__ hooks may modify error messages
1191 local $SIG{'__DIE__'} =
1192 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
1193 eval { die "foo lives here" };
1194 print $@ if $@; # prints "bar lives here"
1197 With an C<eval()>, you should be especially careful to remember what's
1198 being looked at when:
1204 eval { $x }; # CASE 4
1206 eval "\$$x++"; # CASE 5
1209 Cases 1 and 2 above behave identically: they run the code contained in
1210 the variable C<$x>. (Although case 2 has misleading double quotes making
1211 the reader wonder what else might be happening (nothing is).) Cases 3
1212 and 4 likewise behave in the same way: they run the code C<'$x'>, which
1213 does nothing but return the value of C<$x>. (Case 4 is preferred for
1214 purely visual reasons, but it also has the advantage of compiling at
1215 compile-time instead of at run-time.) Case 5 is a place where
1216 normally you I<WOULD> like to use double quotes, except that in this
1217 particular situation, you can just use symbolic references instead, as
1220 C<eval BLOCK> does I<not> count as a loop, so the loop control statements
1221 C<next>, C<last> or C<redo> cannot be used to leave or restart the block.
1226 =item exec PROGRAM LIST
1228 The C<exec()> function executes a system command I<AND NEVER RETURNS> -
1229 use C<system()> instead of C<exec()> if you want it to return. It fails and
1230 returns FALSE only if the command does not exist I<and> it is executed
1231 directly instead of via your system's command shell (see below).
1233 Since it's a common mistake to use C<exec()> instead of C<system()>, Perl
1234 warns you if there is a following statement which isn't C<die()>, C<warn()>,
1235 or C<exit()> (if C<-w> is set - but you always do that). If you
1236 I<really> want to follow an C<exec()> with some other statement, you
1237 can use one of these styles to avoid the warning:
1239 exec ('foo') or print STDERR "couldn't exec foo: $!";
1240 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
1242 If there is more than one argument in LIST, or if LIST is an array
1243 with more than one value, calls execvp(3) with the arguments in LIST.
1244 If there is only one scalar argument or an array with one element in it,
1245 the argument is checked for shell metacharacters, and if there are any,
1246 the entire argument is passed to the system's command shell for parsing
1247 (this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1248 If there are no shell metacharacters in the argument, it is split into
1249 words and passed directly to C<execvp()>, which is more efficient. Note:
1250 C<exec()> and C<system()> do not flush your output buffer, so you may need to
1251 set C<$|> to avoid lost output. Examples:
1253 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1254 exec "sort $outfile | uniq";
1256 If you don't really want to execute the first argument, but want to lie
1257 to the program you are executing about its own name, you can specify
1258 the program you actually want to run as an "indirect object" (without a
1259 comma) in front of the LIST. (This always forces interpretation of the
1260 LIST as a multivalued list, even if there is only a single scalar in
1263 $shell = '/bin/csh';
1264 exec $shell '-sh'; # pretend it's a login shell
1268 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1270 When the arguments get executed via the system shell, results will
1271 be subject to its quirks and capabilities. See L<perlop/"`STRING`">
1274 Using an indirect object with C<exec()> or C<system()> is also more secure.
1275 This usage forces interpretation of the arguments as a multivalued list,
1276 even if the list had just one argument. That way you're safe from the
1277 shell expanding wildcards or splitting up words with whitespace in them.
1279 @args = ( "echo surprise" );
1281 system @args; # subject to shell escapes
1283 system { $args[0] } @args; # safe even with one-arg list
1285 The first version, the one without the indirect object, ran the I<echo>
1286 program, passing it C<"surprise"> an argument. The second version
1287 didn't--it tried to run a program literally called I<"echo surprise">,
1288 didn't find it, and set C<$?> to a non-zero value indicating failure.
1290 Note that C<exec()> will not call your C<END> blocks, nor will it call
1291 any C<DESTROY> methods in your objects.
1295 Returns TRUE if the specified hash key exists in its hash array, even
1296 if the corresponding value is undefined.
1298 print "Exists\n" if exists $array{$key};
1299 print "Defined\n" if defined $array{$key};
1300 print "True\n" if $array{$key};
1302 A hash element can be TRUE only if it's defined, and defined if
1303 it exists, but the reverse doesn't necessarily hold true.
1305 Note that the EXPR can be arbitrarily complicated as long as the final
1306 operation is a hash key lookup:
1308 if (exists $ref->{"A"}{"B"}{$key}) { ... }
1310 Although the last element will not spring into existence just because its
1311 existence was tested, intervening ones will. Thus C<$ref-E<gt>{"A"}>
1312 C<$ref-E<gt>{"B"}> will spring into existence due to the existence
1313 test for a $key element. This autovivification may be fixed in a later
1318 Evaluates EXPR and exits immediately with that value. (Actually, it
1319 calls any defined C<END> routines first, but the C<END> routines may not
1320 abort the exit. Likewise any object destructors that need to be called
1321 are called before exit.) Example:
1324 exit 0 if $ans =~ /^[Xx]/;
1326 See also C<die()>. If EXPR is omitted, exits with C<0> status. The only
1327 universally portable values for EXPR are C<0> for success and C<1> for error;
1328 all other values are subject to unpredictable interpretation depending
1329 on the environment in which the Perl program is running.
1331 You shouldn't use C<exit()> to abort a subroutine if there's any chance that
1332 someone might want to trap whatever error happened. Use C<die()> instead,
1333 which can be trapped by an C<eval()>.
1335 All C<END{}> blocks are run at exit time. See L<perlsub> for details.
1341 Returns I<e> (the natural logarithm base) to the power of EXPR.
1342 If EXPR is omitted, gives C<exp($_)>.
1344 =item fcntl FILEHANDLE,FUNCTION,SCALAR
1346 Implements the fcntl(2) function. You'll probably have to say
1350 first to get the correct constant definitions. Argument processing and
1351 value return works just like C<ioctl()> below.
1355 fcntl($filehandle, F_GETFL, $packed_return_buffer)
1356 or die "can't fcntl F_GETFL: $!";
1358 You don't have to check for C<defined()> on the return from
1359 C<fnctl()>. Like C<ioctl()>, it maps a C<0> return from the system
1360 call into "C<0> but true" in Perl. This string is true in
1361 boolean context and C<0> in numeric context. It is also
1362 exempt from the normal B<-w> warnings on improper numeric
1365 Note that C<fcntl()> will produce a fatal error if used on a machine that
1366 doesn't implement fcntl(2).
1368 =item fileno FILEHANDLE
1370 Returns the file descriptor for a filehandle. This is useful for
1371 constructing bitmaps for C<select()> and low-level POSIX tty-handling
1372 operations. If FILEHANDLE is an expression, the value is taken as
1373 an indirect filehandle, generally its name.
1375 You can use this to find out whether two handles refer to the
1376 same underlying descriptor:
1378 if (fileno(THIS) == fileno(THAT)) {
1379 print "THIS and THAT are dups\n";
1382 =item flock FILEHANDLE,OPERATION
1384 Calls flock(2), or an emulation of it, on FILEHANDLE. Returns TRUE for
1385 success, FALSE on failure. Produces a fatal error if used on a machine
1386 that doesn't implement flock(2), fcntl(2) locking, or lockf(3). C<flock()>
1387 is Perl's portable file locking interface, although it locks only entire
1390 On many platforms (including most versions or clones of Unix), locks
1391 established by C<flock()> are B<merely advisory>. Such discretionary locks
1392 are more flexible, but offer fewer guarantees. This means that files
1393 locked with C<flock()> may be modified by programs that do not also use
1394 C<flock()>. Windows NT and OS/2 are among the platforms which
1395 enforce mandatory locking. See your local documentation for details.
1397 OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1398 LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
1399 you can use the symbolic names if import them from the Fcntl module,
1400 either individually, or as a group using the ':flock' tag. LOCK_SH
1401 requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
1402 releases a previously requested lock. If LOCK_NB is added to LOCK_SH or
1403 LOCK_EX then C<flock()> will return immediately rather than blocking
1404 waiting for the lock (check the return status to see if you got it).
1406 To avoid the possibility of mis-coordination, Perl flushes FILEHANDLE
1407 before (un)locking it.
1409 Note that the emulation built with lockf(3) doesn't provide shared
1410 locks, and it requires that FILEHANDLE be open with write intent. These
1411 are the semantics that lockf(3) implements. Most (all?) systems
1412 implement lockf(3) in terms of fcntl(2) locking, though, so the
1413 differing semantics shouldn't bite too many people.
1415 Note also that some versions of C<flock()> cannot lock things over the
1416 network; you would need to use the more system-specific C<fcntl()> for
1417 that. If you like you can force Perl to ignore your system's flock(2)
1418 function, and so provide its own fcntl(2)-based emulation, by passing
1419 the switch C<-Ud_flock> to the F<Configure> program when you configure
1422 Here's a mailbox appender for BSD systems.
1424 use Fcntl ':flock'; # import LOCK_* constants
1427 flock(MBOX,LOCK_EX);
1428 # and, in case someone appended
1429 # while we were waiting...
1434 flock(MBOX,LOCK_UN);
1437 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1438 or die "Can't open mailbox: $!";
1441 print MBOX $msg,"\n\n";
1444 See also L<DB_File> for other flock() examples.
1448 Does a fork(2) system call. Returns the child pid to the parent process,
1449 C<0> to the child process, or C<undef> if the fork is unsuccessful.
1451 Note: unflushed buffers remain unflushed in both processes, which means
1452 you may need to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()>
1453 method of C<IO::Handle> to avoid duplicate output.
1455 If you C<fork()> without ever waiting on your children, you will accumulate
1458 $SIG{CHLD} = sub { wait };
1460 There's also the double-fork trick (error checking on
1461 C<fork()> returns omitted);
1463 unless ($pid = fork) {
1465 exec "what you really wanna do";
1468 ## (some_perl_code_here)
1475 See also L<perlipc> for more examples of forking and reaping
1478 Note that if your forked child inherits system file descriptors like
1479 STDIN and STDOUT that are actually connected by a pipe or socket, even
1480 if you exit, then the remote server (such as, say, httpd or rsh) won't think
1481 you're done. You should reopen those to F</dev/null> if it's any issue.
1485 Declare a picture format for use by the C<write()> function. For
1489 Test: @<<<<<<<< @||||| @>>>>>
1490 $str, $%, '$' . int($num)
1494 $num = $cost/$quantity;
1498 See L<perlform> for many details and examples.
1500 =item formline PICTURE,LIST
1502 This is an internal function used by C<format>s, though you may call it,
1503 too. It formats (see L<perlform>) a list of values according to the
1504 contents of PICTURE, placing the output into the format output
1505 accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
1506 Eventually, when a C<write()> is done, the contents of
1507 C<$^A> are written to some filehandle, but you could also read C<$^A>
1508 yourself and then set C<$^A> back to C<"">. Note that a format typically
1509 does one C<formline()> per line of form, but the C<formline()> function itself
1510 doesn't care how many newlines are embedded in the PICTURE. This means
1511 that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
1512 You may therefore need to use multiple formlines to implement a single
1513 record format, just like the format compiler.
1515 Be careful if you put double quotes around the picture, because an "C<@>"
1516 character may be taken to mean the beginning of an array name.
1517 C<formline()> always returns TRUE. See L<perlform> for other examples.
1519 =item getc FILEHANDLE
1523 Returns the next character from the input file attached to FILEHANDLE,
1524 or the undefined value at end of file, or if there was an error. If
1525 FILEHANDLE is omitted, reads from STDIN. This is not particularly
1526 efficient. It cannot be used to get unbuffered single-characters,
1527 however. For that, try something more like:
1530 system "stty cbreak </dev/tty >/dev/tty 2>&1";
1533 system "stty", '-icanon', 'eol', "\001";
1539 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
1542 system "stty", 'icanon', 'eol', '^@'; # ASCII null
1546 Determination of whether $BSD_STYLE should be set
1547 is left as an exercise to the reader.
1549 The C<POSIX::getattr()> function can do this more portably on systems
1550 purporting POSIX compliance.
1551 See also the C<Term::ReadKey> module from your nearest CPAN site;
1552 details on CPAN can be found on L<perlmodlib/CPAN>.
1556 Implements the C library function of the same name, which on most
1557 systems returns the current login from F</etc/utmp>, if any. If null,
1560 $login = getlogin || getpwuid($<) || "Kilroy";
1562 Do not consider C<getlogin()> for authentication: it is not as
1563 secure as C<getpwuid()>.
1565 =item getpeername SOCKET
1567 Returns the packed sockaddr address of other end of the SOCKET connection.
1570 $hersockaddr = getpeername(SOCK);
1571 ($port, $iaddr) = unpack_sockaddr_in($hersockaddr);
1572 $herhostname = gethostbyaddr($iaddr, AF_INET);
1573 $herstraddr = inet_ntoa($iaddr);
1577 Returns the current process group for the specified PID. Use
1578 a PID of C<0> to get the current process group for the
1579 current process. Will raise an exception if used on a machine that
1580 doesn't implement getpgrp(2). If PID is omitted, returns process
1581 group of current process. Note that the POSIX version of C<getpgrp()>
1582 does not accept a PID argument, so only C<PID==0> is truly portable.
1586 Returns the process id of the parent process.
1588 =item getpriority WHICH,WHO
1590 Returns the current priority for a process, a process group, or a user.
1591 (See L<getpriority(2)>.) Will raise a fatal exception if used on a
1592 machine that doesn't implement getpriority(2).
1598 =item gethostbyname NAME
1600 =item getnetbyname NAME
1602 =item getprotobyname NAME
1608 =item getservbyname NAME,PROTO
1610 =item gethostbyaddr ADDR,ADDRTYPE
1612 =item getnetbyaddr ADDR,ADDRTYPE
1614 =item getprotobynumber NUMBER
1616 =item getservbyport PORT,PROTO
1634 =item sethostent STAYOPEN
1636 =item setnetent STAYOPEN
1638 =item setprotoent STAYOPEN
1640 =item setservent STAYOPEN
1654 These routines perform the same functions as their counterparts in the
1655 system library. In list context, the return values from the
1656 various get routines are as follows:
1658 ($name,$passwd,$uid,$gid,
1659 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
1660 ($name,$passwd,$gid,$members) = getgr*
1661 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
1662 ($name,$aliases,$addrtype,$net) = getnet*
1663 ($name,$aliases,$proto) = getproto*
1664 ($name,$aliases,$port,$proto) = getserv*
1666 (If the entry doesn't exist you get a null list.)
1668 In scalar context, you get the name, unless the function was a
1669 lookup by name, in which case you get the other thing, whatever it is.
1670 (If the entry doesn't exist you get the undefined value.) For example:
1672 $uid = getpwnam($name);
1673 $name = getpwuid($num);
1675 $gid = getgrnam($name);
1676 $name = getgrgid($num;
1680 In I<getpw*()> the fields C<$quota>, C<$comment>, and C<$expire> are special
1681 cases in the sense that in many systems they are unsupported. If the
1682 C<$quota> is unsupported, it is an empty scalar. If it is supported, it
1683 usually encodes the disk quota. If the C<$comment> field is unsupported,
1684 it is an empty scalar. If it is supported it usually encodes some
1685 administrative comment about the user. In some systems the $quota
1686 field may be C<$change> or C<$age>, fields that have to do with password
1687 aging. In some systems the C<$comment> field may be C<$class>. The C<$expire>
1688 field, if present, encodes the expiration period of the account or the
1689 password. For the availability and the exact meaning of these fields
1690 in your system, please consult your getpwnam(3) documentation and your
1691 F<pwd.h> file. You can also find out from within Perl which meaning
1692 your C<$quota> and C<$comment> fields have and whether you have the C<$expire>
1693 field by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
1694 C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>.
1696 The C<$members> value returned by I<getgr*()> is a space separated list of
1697 the login names of the members of the group.
1699 For the I<gethost*()> functions, if the C<h_errno> variable is supported in
1700 C, it will be returned to you via C<$?> if the function call fails. The
1701 C<@addrs> value returned by a successful call is a list of the raw
1702 addresses returned by the corresponding system library call. In the
1703 Internet domain, each address is four bytes long and you can unpack it
1704 by saying something like:
1706 ($a,$b,$c,$d) = unpack('C4',$addr[0]);
1708 If you get tired of remembering which element of the return list contains
1709 which return value, by-name interfaces are also provided in modules:
1710 C<File::stat>, C<Net::hostent>, C<Net::netent>, C<Net::protoent>, C<Net::servent>,
1711 C<Time::gmtime>, C<Time::localtime>, and C<User::grent>. These override the
1712 normal built-in, replacing them with versions that return objects with
1713 the appropriate names for each field. For example:
1717 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
1719 Even though it looks like they're the same method calls (uid),
1720 they aren't, because a C<File::stat> object is different from a C<User::pwent> object.
1722 =item getsockname SOCKET
1724 Returns the packed sockaddr address of this end of the SOCKET connection.
1727 $mysockaddr = getsockname(SOCK);
1728 ($port, $myaddr) = unpack_sockaddr_in($mysockaddr);
1730 =item getsockopt SOCKET,LEVEL,OPTNAME
1732 Returns the socket option requested, or undef if there is an error.
1738 Returns the value of EXPR with filename expansions such as the standard Unix shell F</bin/sh> would
1739 do. This is the internal function implementing the C<E<lt>*.cE<gt>>
1740 operator, but you can use it directly. If EXPR is omitted, C<$_> is used.
1741 The C<E<lt>*.cE<gt>> operator is discussed in more detail in
1742 L<perlop/"I/O Operators">.
1746 Converts a time as returned by the time function to a 9-element array
1747 with the time localized for the standard Greenwich time zone.
1748 Typically used as follows:
1751 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
1754 All array elements are numeric, and come straight out of a struct tm.
1755 In particular this means that C<$mon> has the range C<0..11> and C<$wday> has
1756 the range C<0..6> with sunday as day C<0>. Also, C<$year> is the number of
1757 years since 1900, that is, C<$year> is C<123> in year 2023, I<not> simply the last two digits of the year.
1759 If EXPR is omitted, does C<gmtime(time())>.
1761 In scalar context, returns the ctime(3) value:
1763 $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
1765 Also see the C<timegm()> function provided by the C<Time::Local> module,
1766 and the strftime(3) function available via the POSIX module.
1768 This scalar value is B<not> locale dependent, see L<perllocale>, but
1769 instead a Perl builtin. Also see the C<Time::Local> module, and the
1770 strftime(3) and mktime(3) function available via the POSIX module. To
1771 get somewhat similar but locale dependent date strings, set up your
1772 locale environment variables appropriately (please see L<perllocale>)
1773 and try for example:
1775 use POSIX qw(strftime);
1776 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
1778 Note that the C<%a> and C<%b>, the short forms of the day of the week
1779 and the month of the year, may not necessarily be three characters wide.
1787 The C<goto-LABEL> form finds the statement labeled with LABEL and resumes
1788 execution there. It may not be used to go into any construct that
1789 requires initialization, such as a subroutine or a C<foreach> loop. It
1790 also can't be used to go into a construct that is optimized away,
1791 or to get out of a block or subroutine given to C<sort()>.
1792 It can be used to go almost anywhere else within the dynamic scope,
1793 including out of subroutines, but it's usually better to use some other
1794 construct such as C<last> or C<die()>. The author of Perl has never felt the
1795 need to use this form of C<goto> (in Perl, that is--C is another matter).
1797 The C<goto-EXPR> form expects a label name, whose scope will be resolved
1798 dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
1799 necessarily recommended if you're optimizing for maintainability:
1801 goto ("FOO", "BAR", "GLARCH")[$i];
1803 The C<goto-&NAME> form is highly magical, and substitutes a call to the
1804 named subroutine for the currently running subroutine. This is used by
1805 C<AUTOLOAD> subroutines that wish to load another subroutine and then
1806 pretend that the other subroutine had been called in the first place
1807 (except that any modifications to C<@_> in the current subroutine are
1808 propagated to the other subroutine.) After the C<goto>, not even C<caller()>
1809 will be able to tell that this routine was called first.
1811 =item grep BLOCK LIST
1813 =item grep EXPR,LIST
1815 This is similar in spirit to, but not the same as, grep(1)
1816 and its relatives. In particular, it is not limited to using
1817 regular expressions.
1819 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
1820 C<$_> to each element) and returns the list value consisting of those
1821 elements for which the expression evaluated to TRUE. In a scalar
1822 context, returns the number of times the expression was TRUE.
1824 @foo = grep(!/^#/, @bar); # weed out comments
1828 @foo = grep {!/^#/} @bar; # weed out comments
1830 Note that, because C<$_> is a reference into the list value, it can be used
1831 to modify the elements of the array. While this is useful and
1832 supported, it can cause bizarre results if the LIST is not a named
1833 array. Similarly, grep returns aliases into the original list,
1834 much like the way that a for loop's index variable aliases the list
1835 elements. That is, modifying an element of a list returned by grep
1836 (for example, in a C<foreach>, C<map()> or another C<grep()>)
1837 actually modifies the element in the original list.
1839 See also L</map> for an array composed of the results of the BLOCK or EXPR.
1845 Interprets EXPR as a hex string and returns the corresponding
1846 value. (To convert strings that might start with either 0 or 0x
1847 see L</oct>.) If EXPR is omitted, uses C<$_>.
1849 print hex '0xAf'; # prints '175'
1850 print hex 'aF'; # same
1854 There is no builtin C<import()> function. It is just an ordinary
1855 method (subroutine) defined (or inherited) by modules that wish to export
1856 names to another module. The C<use()> function calls the C<import()> method
1857 for the package used. See also L</use()>, L<perlmod>, and L<Exporter>.
1859 =item index STR,SUBSTR,POSITION
1861 =item index STR,SUBSTR
1863 Returns the position of the first occurrence of SUBSTR in STR at or after
1864 POSITION. If POSITION is omitted, starts searching from the beginning of
1865 the string. The return value is based at C<0> (or whatever you've set the C<$[>
1866 variable to--but don't do that). If the substring is not found, returns
1867 one less than the base, ordinarily C<-1>.
1873 Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
1874 You should not use this for rounding, because it truncates
1875 towards C<0>, and because machine representations of floating point
1876 numbers can sometimes produce counterintuitive results. Usually C<sprintf()> or C<printf()>,
1877 or the C<POSIX::floor> or C<POSIX::ceil> functions, would serve you better.
1879 =item ioctl FILEHANDLE,FUNCTION,SCALAR
1881 Implements the ioctl(2) function. You'll probably have to say
1883 require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph
1885 first to get the correct function definitions. If F<ioctl.ph> doesn't
1886 exist or doesn't have the correct definitions you'll have to roll your
1887 own, based on your C header files such as F<E<lt>sys/ioctl.hE<gt>>.
1888 (There is a Perl script called B<h2ph> that comes with the Perl kit that
1889 may help you in this, but it's nontrivial.) SCALAR will be read and/or
1890 written depending on the FUNCTION--a pointer to the string value of SCALAR
1891 will be passed as the third argument of the actual C<ioctl()> call. (If SCALAR
1892 has no string value but does have a numeric value, that value will be
1893 passed rather than a pointer to the string value. To guarantee this to be
1894 TRUE, add a C<0> to the scalar before using it.) The C<pack()> and C<unpack()>
1895 functions are useful for manipulating the values of structures used by
1896 C<ioctl()>. The following example sets the erase character to DEL.
1900 die "NO TIOCGETP" if $@ || !$getp;
1901 $sgttyb_t = "ccccs"; # 4 chars and a short
1902 if (ioctl(STDIN,$getp,$sgttyb)) {
1903 @ary = unpack($sgttyb_t,$sgttyb);
1905 $sgttyb = pack($sgttyb_t,@ary);
1906 ioctl(STDIN,&TIOCSETP,$sgttyb)
1907 || die "Can't ioctl: $!";
1910 The return value of C<ioctl()> (and C<fcntl()>) is as follows:
1912 if OS returns: then Perl returns:
1914 0 string "0 but true"
1915 anything else that number
1917 Thus Perl returns TRUE on success and FALSE on failure, yet you can
1918 still easily determine the actual value returned by the operating
1921 ($retval = ioctl(...)) || ($retval = -1);
1922 printf "System returned %d\n", $retval;
1924 The special string "C<0> but true" is excempt from B<-w> complaints
1925 about improper numeric conversions.
1927 =item join EXPR,LIST
1929 Joins the separate strings of LIST into a single string with
1930 fields separated by the value of EXPR, and returns the string.
1933 $_ = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
1939 Returns a list consisting of all the keys of the named hash. (In a
1940 scalar context, returns the number of keys.) The keys are returned in
1941 an apparently random order. The actual random order is subject to
1942 change in future versions of perl, but it is guaranteed to be the same
1943 order as either the C<values()> or C<each()> function produces (given
1944 that the hash has not been modified). As a side effect, it resets
1947 Here is yet another way to print your environment:
1950 @values = values %ENV;
1951 while ($#keys >= 0) {
1952 print pop(@keys), '=', pop(@values), "\n";
1955 or how about sorted by key:
1957 foreach $key (sort(keys %ENV)) {
1958 print $key, '=', $ENV{$key}, "\n";
1961 To sort a hash by value, you'll need to use a C<sort()> function.
1962 Here's a descending numeric sort of a hash by its values:
1964 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
1965 printf "%4d %s\n", $hash{$key}, $key;
1968 As an lvalue C<keys()> allows you to increase the number of hash buckets
1969 allocated for the given hash. This can gain you a measure of efficiency if
1970 you know the hash is going to get big. (This is similar to pre-extending
1971 an array by assigning a larger number to $#array.) If you say
1975 then C<%hash> will have at least 200 buckets allocated for it--256 of them,
1976 in fact, since it rounds up to the next power of two. These
1977 buckets will be retained even if you do C<%hash = ()>, use C<undef
1978 %hash> if you want to free the storage while C<%hash> is still in scope.
1979 You can't shrink the number of buckets allocated for the hash using
1980 C<keys()> in this way (but you needn't worry about doing this by accident,
1981 as trying has no effect).
1983 See also C<each()>, C<values()> and C<sort()>.
1987 Sends a signal to a list of processes. The first element of
1988 the list must be the signal to send. Returns the number of
1989 processes successfully signaled.
1991 $cnt = kill 1, $child1, $child2;
1994 Unlike in the shell, in Perl if the I<SIGNAL> is negative, it kills
1995 process groups instead of processes. (On System V, a negative I<PROCESS>
1996 number will also kill process groups, but that's not portable.) That
1997 means you usually want to use positive not negative signals. You may also
1998 use a signal name in quotes. See L<perlipc/"Signals"> for details.
2004 The C<last> command is like the C<break> statement in C (as used in
2005 loops); it immediately exits the loop in question. If the LABEL is
2006 omitted, the command refers to the innermost enclosing loop. The
2007 C<continue> block, if any, is not executed:
2009 LINE: while (<STDIN>) {
2010 last LINE if /^$/; # exit when done with header
2014 C<last> cannot be used to exit a block which returns a value such as
2015 C<eval {}>, C<sub {}> or C<do {}>.
2017 See also L</continue> for an illustration of how C<last>, C<next>, and
2024 Returns an lowercased version of EXPR. This is the internal function
2025 implementing the C<\L> escape in double-quoted strings.
2026 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
2028 If EXPR is omitted, uses C<$_>.
2034 Returns the value of EXPR with the first character lowercased. This is
2035 the internal function implementing the C<\l> escape in double-quoted strings.
2036 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
2038 If EXPR is omitted, uses C<$_>.
2044 Returns the length in characters of the value of EXPR. If EXPR is
2045 omitted, returns length of C<$_>.
2047 =item link OLDFILE,NEWFILE
2049 Creates a new filename linked to the old filename. Returns TRUE for
2050 success, FALSE otherwise.
2052 =item listen SOCKET,QUEUESIZE
2054 Does the same thing that the listen system call does. Returns TRUE if
2055 it succeeded, FALSE otherwise. See example in L<perlipc/"Sockets: Client/Server Communication">.
2059 A local modifies the listed variables to be local to the enclosing
2060 block, file, or eval. If more than one value is listed, the list must
2061 be placed in parentheses. See L<perlsub/"Temporary Values via local()">
2062 for details, including issues with tied arrays and hashes.
2064 You really probably want to be using C<my()> instead, because C<local()> isn't
2065 what most people think of as "local". See L<perlsub/"Private Variables
2066 via my()"> for details.
2068 =item localtime EXPR
2070 Converts a time as returned by the time function to a 9-element array
2071 with the time analyzed for the local time zone. Typically used as
2075 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
2078 All array elements are numeric, and come straight out of a struct tm.
2079 In particular this means that C<$mon> has the range C<0..11> and C<$wday> has
2080 the range C<0..6> with sunday as day C<0>. Also, C<$year> is the number of
2081 years since 1900, that is, C<$year> is C<123> in year 2023, and I<not> simply the last two digits of the year.
2083 If EXPR is omitted, uses the current time (C<localtime(time)>).
2085 In scalar context, returns the ctime(3) value:
2087 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
2089 This scalar value is B<not> locale dependent, see L<perllocale>, but
2090 instead a Perl builtin. Also see the C<Time::Local> module, and the
2091 strftime(3) and mktime(3) function available via the POSIX module. To
2092 get somewhat similar but locale dependent date strings, set up your
2093 locale environment variables appropriately (please see L<perllocale>)
2094 and try for example:
2096 use POSIX qw(strftime);
2097 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
2099 Note that the C<%a> and C<%b>, the short forms of the day of the week
2100 and the month of the year, may not necessarily be three characters wide.
2106 Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted, returns log
2109 =item lstat FILEHANDLE
2115 Does the same thing as the C<stat()> function (including setting the
2116 special C<_> filehandle) but stats a symbolic link instead of the file
2117 the symbolic link points to. If symbolic links are unimplemented on
2118 your system, a normal C<stat()> is done.
2120 If EXPR is omitted, stats C<$_>.
2124 The match operator. See L<perlop>.
2126 =item map BLOCK LIST
2130 Evaluates the BLOCK or EXPR for each element of LIST (locally setting C<$_> to each
2131 element) and returns the list value composed of the results of each such
2132 evaluation. Evaluates BLOCK or EXPR in a list context, so each element of LIST
2133 may produce zero, one, or more elements in the returned value.
2135 @chars = map(chr, @nums);
2137 translates a list of numbers to the corresponding characters. And
2139 %hash = map { getkey($_) => $_ } @array;
2141 is just a funny way to write
2144 foreach $_ (@array) {
2145 $hash{getkey($_)} = $_;
2148 Note that, because C<$_> is a reference into the list value, it can be used
2149 to modify the elements of the array. While this is useful and
2150 supported, it can cause bizarre results if the LIST is not a named
2151 array. See also L</grep> for an array composed of those items of the
2152 original list for which the BLOCK or EXPR evaluates to true.
2154 =item mkdir FILENAME,MODE
2156 Creates the directory specified by FILENAME, with permissions
2157 specified by MODE (as modified by C<umask>). If it succeeds it
2158 returns TRUE, otherwise it returns FALSE and sets C<$!> (errno).
2160 In general, it is better to create directories with permissive MODEs,
2161 and let the user modify that with their C<umask>, than it is to supply
2162 a restrictive MODE and give the user no way to be more permissive.
2163 The exceptions to this rule are when the file or directory should be
2164 kept private (mail files, for instance). The perlfunc(1) entry on
2165 C<umask> discusses the choice of MODE in more detail.
2167 =item msgctl ID,CMD,ARG
2169 Calls the System V IPC function msgctl(2). You'll probably have to say
2173 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
2174 then ARG must be a variable which will hold the returned C<msqid_ds>
2175 structure. Returns like C<ioctl()>: the undefined value for error, "C<0> but
2176 true" for zero, or the actual return value otherwise. See also
2177 C<IPC::SysV> and C<IPC::Semaphore::Msg> documentation.
2179 =item msgget KEY,FLAGS
2181 Calls the System V IPC function msgget(2). Returns the message queue
2182 id, or the undefined value if there is an error. See also C<IPC::SysV>
2183 and C<IPC::SysV::Msg> documentation.
2185 =item msgsnd ID,MSG,FLAGS
2187 Calls the System V IPC function msgsnd to send the message MSG to the
2188 message queue ID. MSG must begin with the long integer message type,
2189 which may be created with C<pack("l", $type)>. Returns TRUE if
2190 successful, or FALSE if there is an error. See also C<IPC::SysV>
2191 and C<IPC::SysV::Msg> documentation.
2193 =item msgrcv ID,VAR,SIZE,TYPE,FLAGS
2195 Calls the System V IPC function msgrcv to receive a message from
2196 message queue ID into variable VAR with a maximum message size of
2197 SIZE. Note that if a message is received, the message type will be
2198 the first thing in VAR, and the maximum length of VAR is SIZE plus the
2199 size of the message type. Returns TRUE if successful, or FALSE if
2200 there is an error. See also C<IPC::SysV> and C<IPC::SysV::Msg> documentation.
2204 A C<my()> declares the listed variables to be local (lexically) to the
2205 enclosing block, file, or C<eval()>. If
2206 more than one value is listed, the list must be placed in parentheses. See
2207 L<perlsub/"Private Variables via my()"> for details.
2213 The C<next> command is like the C<continue> statement in C; it starts
2214 the next iteration of the loop:
2216 LINE: while (<STDIN>) {
2217 next LINE if /^#/; # discard comments
2221 Note that if there were a C<continue> block on the above, it would get
2222 executed even on discarded lines. If the LABEL is omitted, the command
2223 refers to the innermost enclosing loop.
2225 C<next> cannot be used to exit a block which returns a value such as
2226 C<eval {}>, C<sub {}> or C<do {}>.
2228 See also L</continue> for an illustration of how C<last>, C<next>, and
2231 =item no Module LIST
2233 See the L</use> function, which C<no> is the opposite of.
2239 Interprets EXPR as an octal string and returns the corresponding
2240 value. (If EXPR happens to start off with C<0x>, interprets it as a
2241 hex string. If EXPR starts off with C<0b>, it is interpreted as a
2242 binary string.) The following will handle decimal, binary, octal, and
2243 hex in the standard Perl or C notation:
2245 $val = oct($val) if $val =~ /^0/;
2247 If EXPR is omitted, uses C<$_>. This function is commonly used when
2248 a string such as C<644> needs to be converted into a file mode, for
2249 example. (Although perl will automatically convert strings into
2250 numbers as needed, this automatic conversion assumes base 10.)
2252 =item open FILEHANDLE,EXPR
2254 =item open FILEHANDLE
2256 Opens the file whose filename is given by EXPR, and associates it with
2257 FILEHANDLE. If FILEHANDLE is an expression, its value is used as the
2258 name of the real filehandle wanted. If EXPR is omitted, the scalar
2259 variable of the same name as the FILEHANDLE contains the filename.
2260 (Note that lexical variables--those declared with C<my()>--will not work
2261 for this purpose; so if you're using C<my()>, specify EXPR in your call
2264 If the filename begins with C<'E<lt>'> or nothing, the file is opened for input.
2265 If the filename begins with C<'E<gt>'>, the file is truncated and opened for
2266 output, being created if necessary. If the filename begins with C<'E<gt>E<gt>'>,
2267 the file is opened for appending, again being created if necessary.
2268 You can put a C<'+'> in front of the C<'E<gt>'> or C<'E<lt>'> to indicate that
2269 you want both read and write access to the file; thus C<'+E<lt>'> is almost
2270 always preferred for read/write updates--the C<'+E<gt>'> mode would clobber the
2271 file first. You can't usually use either read-write mode for updating
2272 textfiles, since they have variable length records. See the B<-i>
2273 switch in L<perlrun> for a better approach. The file is created with
2274 permissions of C<0666> modified by the process' C<umask> value.
2276 The prefix and the filename may be separated with spaces.
2277 These various prefixes correspond to the fopen(3) modes of C<'r'>, C<'r+'>, C<'w'>,
2278 C<'w+'>, C<'a'>, and C<'a+'>.
2280 If the filename begins with C<'|'>, the filename is interpreted as a
2281 command to which output is to be piped, and if the filename ends with a
2282 C<'|'>, the filename is interpreted as a command which pipes output to
2283 us. See L<perlipc/"Using open() for IPC">
2284 for more examples of this. (You are not allowed to C<open()> to a command
2285 that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
2286 and L<perlipc/"Bidirectional Communication"> for alternatives.)
2288 Opening C<'-'> opens STDIN and opening C<'E<gt>-'> opens STDOUT. Open returns
2289 nonzero upon success, the undefined value otherwise. If the C<open()>
2290 involved a pipe, the return value happens to be the pid of the
2293 If you're unfortunate enough to be running Perl on a system that
2294 distinguishes between text files and binary files (modern operating
2295 systems don't care), then you should check out L</binmode> for tips for
2296 dealing with this. The key distinction between systems that need C<binmode()>
2297 and those that don't is their text file formats. Systems like Unix, MacOS, and
2298 Plan9, which delimit lines with a single character, and which encode that
2299 character in C as C<"\n">, do not need C<binmode()>. The rest need it.
2301 When opening a file, it's usually a bad idea to continue normal execution
2302 if the request failed, so C<open()> is frequently used in connection with
2303 C<die()>. Even if C<die()> won't do what you want (say, in a CGI script,
2304 where you want to make a nicely formatted error message (but there are
2305 modules that can help with that problem)) you should always check
2306 the return value from opening a file. The infrequent exception is when
2307 working with an unopened filehandle is actually what you want to do.
2312 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
2313 while (<ARTICLE>) {...
2315 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
2316 # if the open fails, output is discarded
2318 open(DBASE, '+<dbase.mine') # open for update
2319 or die "Can't open 'dbase.mine' for update: $!";
2321 open(ARTICLE, "caesar <$article |") # decrypt article
2322 or die "Can't start caesar: $!";
2324 open(EXTRACT, "|sort >/tmp/Tmp$$") # $$ is our process id
2325 or die "Can't start sort: $!";
2327 # process argument list of files along with any includes
2329 foreach $file (@ARGV) {
2330 process($file, 'fh00');
2334 my($filename, $input) = @_;
2335 $input++; # this is a string increment
2336 unless (open($input, $filename)) {
2337 print STDERR "Can't open $filename: $!\n";
2342 while (<$input>) { # note use of indirection
2343 if (/^#include "(.*)"/) {
2344 process($1, $input);
2351 You may also, in the Bourne shell tradition, specify an EXPR beginning
2352 with C<'E<gt>&'>, in which case the rest of the string is interpreted as the
2353 name of a filehandle (or file descriptor, if numeric) to be
2354 duped and opened. You may use C<&> after C<E<gt>>, C<E<gt>E<gt>>, C<E<lt>>, C<+E<gt>>,
2355 C<+E<gt>E<gt>>, and C<+E<lt>>. The
2356 mode you specify should match the mode of the original filehandle.
2357 (Duping a filehandle does not take into account any existing contents of
2359 Here is a script that saves, redirects, and restores STDOUT and
2363 open(OLDOUT, ">&STDOUT");
2364 open(OLDERR, ">&STDERR");
2366 open(STDOUT, ">foo.out") || die "Can't redirect stdout";
2367 open(STDERR, ">&STDOUT") || die "Can't dup stdout";
2369 select(STDERR); $| = 1; # make unbuffered
2370 select(STDOUT); $| = 1; # make unbuffered
2372 print STDOUT "stdout 1\n"; # this works for
2373 print STDERR "stderr 1\n"; # subprocesses too
2378 open(STDOUT, ">&OLDOUT");
2379 open(STDERR, ">&OLDERR");
2381 print STDOUT "stdout 2\n";
2382 print STDERR "stderr 2\n";
2385 If you specify C<'E<lt>&=N'>, where C<N> is a number, then Perl will do an
2386 equivalent of C's C<fdopen()> of that file descriptor; this is more
2387 parsimonious of file descriptors. For example:
2389 open(FILEHANDLE, "<&=$fd")
2391 If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'>, then
2392 there is an implicit fork done, and the return value of open is the pid
2393 of the child within the parent process, and C<0> within the child
2394 process. (Use C<defined($pid)> to determine whether the open was successful.)
2395 The filehandle behaves normally for the parent, but i/o to that
2396 filehandle is piped from/to the STDOUT/STDIN of the child process.
2397 In the child process the filehandle isn't opened--i/o happens from/to
2398 the new STDOUT or STDIN. Typically this is used like the normal
2399 piped open when you want to exercise more control over just how the
2400 pipe command gets executed, such as when you are running setuid, and
2401 don't want to have to scan shell commands for metacharacters.
2402 The following pairs are more or less equivalent:
2404 open(FOO, "|tr '[a-z]' '[A-Z]'");
2405 open(FOO, "|-") || exec 'tr', '[a-z]', '[A-Z]';
2407 open(FOO, "cat -n '$file'|");
2408 open(FOO, "-|") || exec 'cat', '-n', $file;
2410 See L<perlipc/"Safe Pipe Opens"> for more examples of this.
2412 NOTE: On any operation that may do a fork, any unflushed buffers remain
2413 unflushed in both processes, which means you may need to set C<$|> to
2414 avoid duplicate output. On systems that support a close-on-exec flag on
2415 files, the flag will be set for the newly opened file descriptor as
2416 determined by the value of $^F. See L<perlvar/$^F>.
2418 Closing any piped filehandle causes the parent process to wait for the
2419 child to finish, and returns the status value in C<$?>.
2421 The filename passed to open will have leading and trailing
2422 whitespace deleted, and the normal redirection characters
2423 honored. This property, known as "magic open",
2424 can often be used to good effect. A user could specify a filename of
2425 F<"rsh cat file |">, or you could change certain filenames as needed:
2427 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
2428 open(FH, $filename) or die "Can't open $filename: $!";
2430 However, to open a file with arbitrary weird characters in it, it's
2431 necessary to protect any leading and trailing whitespace:
2433 $file =~ s#^(\s)#./$1#;
2434 open(FOO, "< $file\0");
2436 If you want a "real" C C<open()> (see L<open(2)> on your system), then you
2437 should use the C<sysopen()> function, which involves no such magic. This is
2438 another way to protect your filenames from interpretation. For example:
2441 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
2442 or die "sysopen $path: $!";
2443 $oldfh = select(HANDLE); $| = 1; select($oldfh);
2444 print HANDLE "stuff $$\n");
2446 print "File contains: ", <HANDLE>;
2448 Using the constructor from the C<IO::Handle> package (or one of its
2449 subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous
2450 filehandles that have the scope of whatever variables hold references to
2451 them, and automatically close whenever and however you leave that scope:
2455 sub read_myfile_munged {
2457 my $handle = new IO::File;
2458 open($handle, "myfile") or die "myfile: $!";
2460 or return (); # Automatically closed here.
2461 mung $first or die "mung failed"; # Or here.
2462 return $first, <$handle> if $ALL; # Or here.
2466 See L</seek> for some details about mixing reading and writing.
2468 =item opendir DIRHANDLE,EXPR
2470 Opens a directory named EXPR for processing by C<readdir()>, C<telldir()>,
2471 C<seekdir()>, C<rewinddir()>, and C<closedir()>. Returns TRUE if successful.
2472 DIRHANDLEs have their own namespace separate from FILEHANDLEs.
2478 Returns the numeric (ASCII or Unicode) value of the first character of EXPR. If
2479 EXPR is omitted, uses C<$_>. For the reverse, see L</chr>.
2481 =item pack TEMPLATE,LIST
2483 Takes an array or list of values and packs it into a binary structure,
2484 returning the string containing the structure. The TEMPLATE is a
2485 sequence of characters that give the order and type of values, as
2488 A An ascii string, will be space padded.
2489 a An ascii string, will be null padded.
2490 b A bit string (ascending bit order, like vec()).
2491 B A bit string (descending bit order).
2492 h A hex string (low nybble first).
2493 H A hex string (high nybble first).
2495 c A signed char value.
2496 C An unsigned char value. Only does bytes. See U for Unicode.
2498 s A signed short value.
2499 S An unsigned short value.
2500 (This 'short' is _exactly_ 16 bits, which may differ from
2501 what a local C compiler calls 'short'.)
2503 i A signed integer value.
2504 I An unsigned integer value.
2505 (This 'integer' is _at_least_ 32 bits wide. Its exact
2506 size depends on what a local C compiler calls 'int',
2507 and may even be larger than the 'long' described in
2510 l A signed long value.
2511 L An unsigned long value.
2512 (This 'long' is _exactly_ 32 bits, which may differ from
2513 what a local C compiler calls 'long'.)
2515 n A short in "network" (big-endian) order.
2516 N A long in "network" (big-endian) order.
2517 v A short in "VAX" (little-endian) order.
2518 V A long in "VAX" (little-endian) order.
2519 (These 'shorts' and 'longs' are _exactly_ 16 bits and
2520 _exactly_ 32 bits, respectively.)
2522 q A signed quad (64-bit) value.
2523 Q An unsigned quad value.
2524 (Available only if your system supports 64-bit integer values
2525 _and_ if Perl has been compiled to support those.
2526 Causes a fatal error otherwise.)
2528 f A single-precision float in the native format.
2529 d A double-precision float in the native format.
2531 p A pointer to a null-terminated string.
2532 P A pointer to a structure (fixed-length string).
2534 u A uuencoded string.
2535 U A Unicode character number. Encodes to UTF-8 internally.
2536 Works even if C<use utf8> is not in effect.
2538 w A BER compressed integer. Its bytes represent an unsigned
2539 integer in base 128, most significant digit first, with as
2540 few digits as possible. Bit eight (the high bit) is set
2541 on each byte except the last.
2545 @ Null fill to absolute position.
2547 Each letter may optionally be followed by a number giving a repeat
2548 count. With all types except C<"a">, C<"A">, C<"b">, C<"B">, C<"h">, C<"H">, and C<"P"> the
2549 pack function will gobble up that many values from the LIST. A C<*> for the
2550 repeat count means to use however many items are left. The C<"a"> and C<"A">
2551 types gobble just one value, but pack it as a string of length count,
2552 padding with nulls or spaces as necessary. (When unpacking, C<"A"> strips
2553 trailing spaces and nulls, but C<"a"> does not.) Likewise, the C<"b"> and C<"B">
2554 fields pack a string that many bits long. The C<"h"> and C<"H"> fields pack a
2555 string that many nybbles long. The C<"p"> type packs a pointer to a null-
2556 terminated string. You are responsible for ensuring the string is not a
2557 temporary value (which can potentially get deallocated before you get
2558 around to using the packed result). The C<"P"> packs a pointer to a structure
2559 of the size indicated by the length. A NULL pointer is created if the
2560 corresponding value for C<"p"> or C<"P"> is C<undef>.
2561 Real numbers (floats and doubles) are
2562 in the native machine format only; due to the multiplicity of floating
2563 formats around, and the lack of a standard "network" representation, no
2564 facility for interchange has been made. This means that packed floating
2565 point data written on one machine may not be readable on another - even if
2566 both use IEEE floating point arithmetic (as the endian-ness of the memory
2567 representation is not part of the IEEE spec). Note that Perl uses doubles
2568 internally for all numeric calculation, and converting from double into
2569 float and thence back to double again will lose precision (i.e.,
2570 C<unpack("f", pack("f", $foo)>) will not in general equal C<$foo>).
2574 $foo = pack("CCCC",65,66,67,68);
2576 $foo = pack("C4",65,66,67,68);
2578 $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);
2579 # same thing with Unicode circled letters
2581 $foo = pack("ccxxcc",65,66,67,68);
2584 $foo = pack("s2",1,2);
2585 # "\1\0\2\0" on little-endian
2586 # "\0\1\0\2" on big-endian
2588 $foo = pack("a4","abcd","x","y","z");
2591 $foo = pack("aaaa","abcd","x","y","z");
2594 $foo = pack("a14","abcdefg");
2595 # "abcdefg\0\0\0\0\0\0\0"
2597 $foo = pack("i9pl", gmtime);
2598 # a real struct tm (on my system anyway)
2601 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
2604 The same template may generally also be used in the unpack function.
2608 =item package NAMESPACE
2610 Declares the compilation unit as being in the given namespace. The scope
2611 of the package declaration is from the declaration itself through the end of
2612 the enclosing block (the same scope as the C<local()> operator). All further
2613 unqualified dynamic identifiers will be in this namespace. A package
2614 statement affects only dynamic variables--including those you've used
2615 C<local()> on--but I<not> lexical variables created with C<my()>. Typically it
2616 would be the first declaration in a file to be included by the C<require>
2617 or C<use> operator. You can switch into a package in more than one place;
2618 it merely influences which symbol table is used by the compiler for the
2619 rest of that block. You can refer to variables and filehandles in other
2620 packages by prefixing the identifier with the package name and a double
2621 colon: C<$Package::Variable>. If the package name is null, the C<main>
2622 package as assumed. That is, C<$::sail> is equivalent to C<$main::sail>.
2624 If NAMESPACE is omitted, then there is no current package, and all
2625 identifiers must be fully qualified or lexicals. This is stricter
2626 than C<use strict>, since it also extends to function names.
2628 See L<perlmod/"Packages"> for more information about packages, modules,
2629 and classes. See L<perlsub> for other scoping issues.
2631 =item pipe READHANDLE,WRITEHANDLE
2633 Opens a pair of connected pipes like the corresponding system call.
2634 Note that if you set up a loop of piped processes, deadlock can occur
2635 unless you are very careful. In addition, note that Perl's pipes use
2636 stdio buffering, so you may need to set C<$|> to flush your WRITEHANDLE
2637 after each command, depending on the application.
2639 See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
2640 for examples of such things.
2642 On systems that support a close-on-exec flag on files, the flag will be set
2643 for the newly opened file descriptors as determined by the value of $^F.
2650 Pops and returns the last value of the array, shortening the array by
2651 1. Has a similar effect to
2653 $tmp = $ARRAY[$#ARRAY--];
2655 If there are no elements in the array, returns the undefined value.
2656 If ARRAY is omitted, pops the
2657 C<@ARGV> array in the main program, and the C<@_> array in subroutines, just
2664 Returns the offset of where the last C<m//g> search left off for the variable
2665 is in question (C<$_> is used when the variable is not specified). May be
2666 modified to change that offset. Such modification will also influence
2667 the C<\G> zero-width assertion in regular expressions. See L<perlre> and
2670 =item print FILEHANDLE LIST
2676 Prints a string or a comma-separated list of strings. Returns TRUE
2677 if successful. FILEHANDLE may be a scalar variable name, in which case
2678 the variable contains the name of or a reference to the filehandle, thus introducing one
2679 level of indirection. (NOTE: If FILEHANDLE is a variable and the next
2680 token is a term, it may be misinterpreted as an operator unless you
2681 interpose a C<+> or put parentheses around the arguments.) If FILEHANDLE is
2682 omitted, prints by default to standard output (or to the last selected
2683 output channel--see L</select>). If LIST is also omitted, prints C<$_> to
2684 the currently selected output channel. To set the default output channel to something other than
2685 STDOUT use the select operation. Note that, because print takes a
2686 LIST, anything in the LIST is evaluated in list context, and any
2687 subroutine that you call will have one or more of its expressions
2688 evaluated in list context. Also be careful not to follow the print
2689 keyword with a left parenthesis unless you want the corresponding right
2690 parenthesis to terminate the arguments to the print--interpose a C<+> or
2691 put parentheses around all the arguments.
2693 Note that if you're storing FILEHANDLES in an array or other expression,
2694 you will have to use a block returning its value instead:
2696 print { $files[$i] } "stuff\n";
2697 print { $OK ? STDOUT : STDERR } "stuff\n";
2699 =item printf FILEHANDLE FORMAT, LIST
2701 =item printf FORMAT, LIST
2703 Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that C<$\>
2704 (the output record separator) is not appended. The first argument
2705 of the list will be interpreted as the C<printf()> format. If C<use locale> is
2706 in effect, the character used for the decimal point in formatted real numbers
2707 is affected by the LC_NUMERIC locale. See L<perllocale>.
2709 Don't fall into the trap of using a C<printf()> when a simple
2710 C<print()> would do. The C<print()> is more efficient and less
2713 =item prototype FUNCTION
2715 Returns the prototype of a function as a string (or C<undef> if the
2716 function has no prototype). FUNCTION is a reference to, or the name of,
2717 the function whose prototype you want to retrieve.
2719 If FUNCTION is a string starting with C<CORE::>, the rest is taken as
2720 a name for Perl builtin. If builtin is not I<overridable> (such as
2721 C<qw//>) or its arguments cannot be expressed by a prototype (such as
2722 C<system()>) - in other words, the builtin does not behave like a Perl
2723 function - returns C<undef>. Otherwise, the string describing the
2724 equivalent prototype is returned.
2726 =item push ARRAY,LIST
2728 Treats ARRAY as a stack, and pushes the values of LIST
2729 onto the end of ARRAY. The length of ARRAY increases by the length of
2730 LIST. Has the same effect as
2733 $ARRAY[++$#ARRAY] = $value;
2736 but is more efficient. Returns the new number of elements in the array.
2748 Generalized quotes. See L<perlop/"Regexp Quote-Like Operators">.
2750 =item quotemeta EXPR
2754 Returns the value of EXPR with all non-alphanumeric
2755 characters backslashed. (That is, all characters not matching
2756 C</[A-Za-z_0-9]/> will be preceded by a backslash in the
2757 returned string, regardless of any locale settings.)
2758 This is the internal function implementing
2759 the C<\Q> escape in double-quoted strings.
2761 If EXPR is omitted, uses C<$_>.
2767 Returns a random fractional number greater than or equal to C<0> and less
2768 than the value of EXPR. (EXPR should be positive.) If EXPR is
2769 omitted, the value C<1> is used. Automatically calls C<srand()> unless
2770 C<srand()> has already been called. See also C<srand()>.
2772 (Note: If your rand function consistently returns numbers that are too
2773 large or too small, then your version of Perl was probably compiled
2774 with the wrong number of RANDBITS.)
2776 =item read FILEHANDLE,SCALAR,LENGTH,OFFSET
2778 =item read FILEHANDLE,SCALAR,LENGTH
2780 Attempts to read LENGTH bytes of data into variable SCALAR from the
2781 specified FILEHANDLE. Returns the number of bytes actually read,
2782 C<0> at end of file, or undef if there was an error. SCALAR will be grown
2783 or shrunk to the length actually read. An OFFSET may be specified to
2784 place the read data at some other place than the beginning of the
2785 string. This call is actually implemented in terms of stdio's fread(3)
2786 call. To get a true read(2) system call, see C<sysread()>.
2788 =item readdir DIRHANDLE
2790 Returns the next directory entry for a directory opened by C<opendir()>.
2791 If used in list context, returns all the rest of the entries in the
2792 directory. If there are no more entries, returns an undefined value in
2793 scalar context or a null list in list context.
2795 If you're planning to filetest the return values out of a C<readdir()>, you'd
2796 better prepend the directory in question. Otherwise, because we didn't
2797 C<chdir()> there, it would have been testing the wrong file.
2799 opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!";
2800 @dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR);
2805 Reads from the filehandle whose typeglob is contained in EXPR. In scalar context, a single line
2806 is read and returned. In list context, reads until end-of-file is
2807 reached and returns a list of lines (however you've defined lines
2808 with C<$/> or C<$INPUT_RECORD_SEPARATOR>).
2809 This is the internal function implementing the C<E<lt>EXPRE<gt>>
2810 operator, but you can use it directly. The C<E<lt>EXPRE<gt>>
2811 operator is discussed in more detail in L<perlop/"I/O Operators">.
2814 $line = readline(*STDIN); # same thing
2820 Returns the value of a symbolic link, if symbolic links are
2821 implemented. If not, gives a fatal error. If there is some system
2822 error, returns the undefined value and sets C<$!> (errno). If EXPR is
2823 omitted, uses C<$_>.
2827 EXPR is executed as a system command.
2828 The collected standard output of the command is returned.
2829 In scalar context, it comes back as a single (potentially
2830 multi-line) string. In list context, returns a list of lines
2831 (however you've defined lines with C<$/> or C<$INPUT_RECORD_SEPARATOR>).
2832 This is the internal function implementing the C<qx/EXPR/>
2833 operator, but you can use it directly. The C<qx/EXPR/>
2834 operator is discussed in more detail in L<perlop/"I/O Operators">.
2836 =item recv SOCKET,SCALAR,LEN,FLAGS
2838 Receives a message on a socket. Attempts to receive LENGTH bytes of
2839 data into variable SCALAR from the specified SOCKET filehandle.
2840 Actually does a C C<recvfrom()>, so that it can return the address of the
2841 sender. Returns the undefined value if there's an error. SCALAR will
2842 be grown or shrunk to the length actually read. Takes the same flags
2843 as the system call of the same name.
2844 See L<perlipc/"UDP: Message Passing"> for examples.
2850 The C<redo> command restarts the loop block without evaluating the
2851 conditional again. The C<continue> block, if any, is not executed. If
2852 the LABEL is omitted, the command refers to the innermost enclosing
2853 loop. This command is normally used by programs that want to lie to
2854 themselves about what was just input:
2856 # a simpleminded Pascal comment stripper
2857 # (warning: assumes no { or } in strings)
2858 LINE: while (<STDIN>) {
2859 while (s|({.*}.*){.*}|$1 |) {}
2864 if (/}/) { # end of comment?
2873 C<redo> cannot be used to retry a block which returns a value such as
2874 C<eval {}>, C<sub {}> or C<do {}>.
2876 See also L</continue> for an illustration of how C<last>, C<next>, and
2883 Returns a TRUE value if EXPR is a reference, FALSE otherwise. If EXPR
2884 is not specified, C<$_> will be used. The value returned depends on the
2885 type of thing the reference is a reference to.
2886 Builtin types include:
2895 If the referenced object has been blessed into a package, then that package
2896 name is returned instead. You can think of C<ref()> as a C<typeof()> operator.
2898 if (ref($r) eq "HASH") {
2899 print "r is a reference to a hash.\n";
2902 print "r is not a reference at all.\n";
2905 See also L<perlref>.
2907 =item rename OLDNAME,NEWNAME
2909 Changes the name of a file. Returns C<1> for success, C<0> otherwise. Will
2910 not work across file system boundaries.
2916 Demands some semantics specified by EXPR, or by C<$_> if EXPR is not
2917 supplied. If EXPR is numeric, demands that the current version of Perl
2918 (C<$]> or $PERL_VERSION) be equal or greater than EXPR.
2920 Otherwise, demands that a library file be included if it hasn't already
2921 been included. The file is included via the do-FILE mechanism, which is
2922 essentially just a variety of C<eval()>. Has semantics similar to the following
2927 return 1 if $INC{$filename};
2928 my($realfilename,$result);
2930 foreach $prefix (@INC) {
2931 $realfilename = "$prefix/$filename";
2932 if (-f $realfilename) {
2933 $result = do $realfilename;
2937 die "Can't find $filename in \@INC";
2940 die "$filename did not return true value" unless $result;
2941 $INC{$filename} = $realfilename;
2945 Note that the file will not be included twice under the same specified
2946 name. The file must return TRUE as the last statement to indicate
2947 successful execution of any initialization code, so it's customary to
2948 end such a file with "C<1;>" unless you're sure it'll return TRUE
2949 otherwise. But it's better just to put the "C<1;>", in case you add more
2952 If EXPR is a bareword, the require assumes a "F<.pm>" extension and
2953 replaces "F<::>" with "F</>" in the filename for you,
2954 to make it easy to load standard modules. This form of loading of
2955 modules does not risk altering your namespace.
2957 In other words, if you try this:
2959 require Foo::Bar; # a splendid bareword
2961 The require function will actually look for the "F<Foo/Bar.pm>" file in the
2962 directories specified in the C<@INC> array.
2964 But if you try this:
2966 $class = 'Foo::Bar';
2967 require $class; # $class is not a bareword
2969 require "Foo::Bar"; # not a bareword because of the ""
2971 The require function will look for the "F<Foo::Bar>" file in the @INC array and
2972 will complain about not finding "F<Foo::Bar>" there. In this case you can do:
2974 eval "require $class";
2976 For a yet-more-powerful import facility, see L</use> and L<perlmod>.
2982 Generally used in a C<continue> block at the end of a loop to clear
2983 variables and reset C<??> searches so that they work again. The
2984 expression is interpreted as a list of single characters (hyphens
2985 allowed for ranges). All variables and arrays beginning with one of
2986 those letters are reset to their pristine state. If the expression is
2987 omitted, one-match searches (C<?pattern?>) are reset to match again. Resets
2988 only variables or searches in the current package. Always returns
2991 reset 'X'; # reset all X variables
2992 reset 'a-z'; # reset lower case variables
2993 reset; # just reset ?? searches
2995 Resetting C<"A-Z"> is not recommended because you'll wipe out your
2996 C<@ARGV> and C<@INC> arrays and your C<%ENV> hash. Resets only package variables--lexical variables
2997 are unaffected, but they clean themselves up on scope exit anyway,
2998 so you'll probably want to use them instead. See L</my>.
3004 Returns from a subroutine, C<eval()>, or C<do FILE> with the value
3005 given in EXPR. Evaluation of EXPR may be in list, scalar, or void
3006 context, depending on how the return value will be used, and the context
3007 may vary from one execution to the next (see C<wantarray()>). If no EXPR
3008 is given, returns an empty list in list context, an undefined value in
3009 scalar context, or nothing in a void context.
3011 (Note that in the absence of a return, a subroutine, eval, or do FILE
3012 will automatically return the value of the last expression evaluated.)
3016 In list context, returns a list value consisting of the elements
3017 of LIST in the opposite order. In scalar context, concatenates the
3018 elements of LIST, and returns a string value with all the characters
3019 in the opposite order.
3021 print reverse <>; # line tac, last line first
3023 undef $/; # for efficiency of <>
3024 print scalar reverse <>; # character tac, last line tsrif
3026 This operator is also handy for inverting a hash, although there are some
3027 caveats. If a value is duplicated in the original hash, only one of those
3028 can be represented as a key in the inverted hash. Also, this has to
3029 unwind one hash and build a whole new one, which may take some time
3032 %by_name = reverse %by_address; # Invert the hash
3034 =item rewinddir DIRHANDLE
3036 Sets the current position to the beginning of the directory for the
3037 C<readdir()> routine on DIRHANDLE.
3039 =item rindex STR,SUBSTR,POSITION
3041 =item rindex STR,SUBSTR
3043 Works just like index except that it returns the position of the LAST
3044 occurrence of SUBSTR in STR. If POSITION is specified, returns the
3045 last occurrence at or before that position.
3047 =item rmdir FILENAME
3051 Deletes the directory specified by FILENAME if that directory is empty. If it
3052 succeeds it returns TRUE, otherwise it returns FALSE and sets C<$!> (errno). If
3053 FILENAME is omitted, uses C<$_>.
3057 The substitution operator. See L<perlop>.
3061 Forces EXPR to be interpreted in scalar context and returns the value
3064 @counts = ( scalar @a, scalar @b, scalar @c );
3066 There is no equivalent operator to force an expression to
3067 be interpolated in list context because it's in practice never
3068 needed. If you really wanted to do so, however, you could use
3069 the construction C<@{[ (some expression) ]}>, but usually a simple
3070 C<(some expression)> suffices.
3072 Though C<scalar> can be considered in general to be a unary operator,
3073 EXPR is also allowed to be a parenthesized list. The list in fact
3074 behaves as a scalar comma expression, evaluating all but the last
3075 element in void context and returning the final element evaluated in
3078 The following single statement:
3080 print uc(scalar(&foo,$bar)),$baz;
3082 is the moral equivalent of these two:
3085 print(uc($bar),$baz);
3087 See L<perlop> for more details on unary operators and the comma operator.
3089 =item seek FILEHANDLE,POSITION,WHENCE
3091 Sets FILEHANDLE's position, just like the C<fseek()> call of C<stdio()>.
3092 FILEHANDLE may be an expression whose value gives the name of the
3093 filehandle. The values for WHENCE are C<0> to set the new position to
3094 POSITION, C<1> to set it to the current position plus POSITION, and C<2> to
3095 set it to EOF plus POSITION (typically negative). For WHENCE you may
3096 use the constants C<SEEK_SET>, C<SEEK_CUR>, and C<SEEK_END> from either the
3097 C<IO::Seekable> or the POSIX module. Returns C<1> upon success, C<0> otherwise.
3099 If you want to position file for C<sysread()> or C<syswrite()>, don't use
3100 C<seek()> -- buffering makes its effect on the file's system position
3101 unpredictable and non-portable. Use C<sysseek()> instead.
3103 On some systems you have to do a seek whenever you switch between reading
3104 and writing. Amongst other things, this may have the effect of calling
3105 stdio's clearerr(3). A WHENCE of C<1> (C<SEEK_CUR>) is useful for not moving
3110 This is also useful for applications emulating C<tail -f>. Once you hit
3111 EOF on your read, and then sleep for a while, you might have to stick in a
3112 seek() to reset things. The C<seek()> doesn't change the current position,
3113 but it I<does> clear the end-of-file condition on the handle, so that the
3114 next C<E<lt>FILEE<gt>> makes Perl try again to read something. We hope.
3116 If that doesn't work (some stdios are particularly cantankerous), then
3117 you may need something more like this:
3120 for ($curpos = tell(FILE); $_ = <FILE>;
3121 $curpos = tell(FILE)) {
3122 # search for some stuff and put it into files
3124 sleep($for_a_while);
3125 seek(FILE, $curpos, 0);
3128 =item seekdir DIRHANDLE,POS
3130 Sets the current position for the C<readdir()> routine on DIRHANDLE. POS
3131 must be a value returned by C<telldir()>. Has the same caveats about
3132 possible directory compaction as the corresponding system library
3135 =item select FILEHANDLE
3139 Returns the currently selected filehandle. Sets the current default
3140 filehandle for output, if FILEHANDLE is supplied. This has two
3141 effects: first, a C<write()> or a C<print()> without a filehandle will
3142 default to this FILEHANDLE. Second, references to variables related to
3143 output will refer to this output channel. For example, if you have to
3144 set the top of form format for more than one output channel, you might
3152 FILEHANDLE may be an expression whose value gives the name of the
3153 actual filehandle. Thus:
3155 $oldfh = select(STDERR); $| = 1; select($oldfh);
3157 Some programmers may prefer to think of filehandles as objects with
3158 methods, preferring to write the last example as:
3161 STDERR->autoflush(1);
3163 =item select RBITS,WBITS,EBITS,TIMEOUT
3165 This calls the select(2) system call with the bit masks specified, which
3166 can be constructed using C<fileno()> and C<vec()>, along these lines:
3168 $rin = $win = $ein = '';
3169 vec($rin,fileno(STDIN),1) = 1;
3170 vec($win,fileno(STDOUT),1) = 1;
3173 If you want to select on many filehandles you might wish to write a
3177 my(@fhlist) = split(' ',$_[0]);
3180 vec($bits,fileno($_),1) = 1;
3184 $rin = fhbits('STDIN TTY SOCK');
3188 ($nfound,$timeleft) =
3189 select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
3191 or to block until something becomes ready just do this
3193 $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
3195 Most systems do not bother to return anything useful in C<$timeleft>, so
3196 calling select() in scalar context just returns C<$nfound>.
3198 Any of the bit masks can also be undef. The timeout, if specified, is
3199 in seconds, which may be fractional. Note: not all implementations are
3200 capable of returning theC<$timeleft>. If not, they always return
3201 C<$timeleft> equal to the supplied C<$timeout>.
3203 You can effect a sleep of 250 milliseconds this way:
3205 select(undef, undef, undef, 0.25);
3207 B<WARNING>: One should not attempt to mix buffered I/O (like C<read()>
3208 or E<lt>FHE<gt>) with C<select()>, except as permitted by POSIX, and even
3209 then only on POSIX systems. You have to use C<sysread()> instead.
3211 =item semctl ID,SEMNUM,CMD,ARG
3213 Calls the System V IPC function C<semctl()>. You'll probably have to say
3217 first to get the correct constant definitions. If CMD is IPC_STAT or
3218 GETALL, then ARG must be a variable which will hold the returned
3219 semid_ds structure or semaphore value array. Returns like C<ioctl()>: the
3220 undefined value for error, "C<0> but true" for zero, or the actual return
3221 value otherwise. See also C<IPC::SysV> and C<IPC::Semaphore> documentation.
3223 =item semget KEY,NSEMS,FLAGS
3225 Calls the System V IPC function semget. Returns the semaphore id, or
3226 the undefined value if there is an error. See also C<IPC::SysV> and
3227 C<IPC::SysV::Semaphore> documentation.
3229 =item semop KEY,OPSTRING
3231 Calls the System V IPC function semop to perform semaphore operations
3232 such as signaling and waiting. OPSTRING must be a packed array of
3233 semop structures. Each semop structure can be generated with
3234 C<pack("sss", $semnum, $semop, $semflag)>. The number of semaphore
3235 operations is implied by the length of OPSTRING. Returns TRUE if
3236 successful, or FALSE if there is an error. As an example, the
3237 following code waits on semaphore C<$semnum> of semaphore id C<$semid>:
3239 $semop = pack("sss", $semnum, -1, 0);
3240 die "Semaphore trouble: $!\n" unless semop($semid, $semop);
3242 To signal the semaphore, replace C<-1> with C<1>. See also C<IPC::SysV>
3243 and C<IPC::SysV::Semaphore> documentation.
3245 =item send SOCKET,MSG,FLAGS,TO
3247 =item send SOCKET,MSG,FLAGS
3249 Sends a message on a socket. Takes the same flags as the system call
3250 of the same name. On unconnected sockets you must specify a
3251 destination to send TO, in which case it does a C C<sendto()>. Returns
3252 the number of characters sent, or the undefined value if there is an
3254 See L<perlipc/"UDP: Message Passing"> for examples.
3256 =item setpgrp PID,PGRP
3258 Sets the current process group for the specified PID, C<0> for the current
3259 process. Will produce a fatal error if used on a machine that doesn't
3260 implement setpgrp(2). If the arguments are omitted, it defaults to
3261 C<0,0>. Note that the POSIX version of C<setpgrp()> does not accept any
3262 arguments, so only setpgrp C<0,0> is portable.
3264 =item setpriority WHICH,WHO,PRIORITY
3266 Sets the current priority for a process, a process group, or a user.
3267 (See setpriority(2).) Will produce a fatal error if used on a machine
3268 that doesn't implement setpriority(2).
3270 =item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
3272 Sets the socket option requested. Returns undefined if there is an
3273 error. OPTVAL may be specified as C<undef> if you don't want to pass an
3280 Shifts the first value of the array off and returns it, shortening the
3281 array by 1 and moving everything down. If there are no elements in the
3282 array, returns the undefined value. If ARRAY is omitted, shifts the
3283 C<@_> array within the lexical scope of subroutines and formats, and the
3284 C<@ARGV> array at file scopes or within the lexical scopes established by
3285 the C<eval ''>, C<BEGIN {}>, C<END {}>, and C<INIT {}> constructs.
3286 See also C<unshift()>, C<push()>, and C<pop()>. C<Shift()> and C<unshift()> do the
3287 same thing to the left end of an array that C<pop()> and C<push()> do to the
3290 =item shmctl ID,CMD,ARG
3292 Calls the System V IPC function shmctl. You'll probably have to say
3296 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
3297 then ARG must be a variable which will hold the returned C<shmid_ds>
3298 structure. Returns like ioctl: the undefined value for error, "C<0> but
3299 true" for zero, or the actual return value otherwise.
3300 See also C<IPC::SysV> documentation.
3302 =item shmget KEY,SIZE,FLAGS
3304 Calls the System V IPC function shmget. Returns the shared memory
3305 segment id, or the undefined value if there is an error.
3306 See also C<IPC::SysV> documentation.
3308 =item shmread ID,VAR,POS,SIZE
3310 =item shmwrite ID,STRING,POS,SIZE
3312 Reads or writes the System V shared memory segment ID starting at
3313 position POS for size SIZE by attaching to it, copying in/out, and
3314 detaching from it. When reading, VAR must be a variable that will
3315 hold the data read. When writing, if STRING is too long, only SIZE
3316 bytes are used; if STRING is too short, nulls are written to fill out
3317 SIZE bytes. Return TRUE if successful, or FALSE if there is an error.
3318 See also C<IPC::SysV> documentation.
3320 =item shutdown SOCKET,HOW
3322 Shuts down a socket connection in the manner indicated by HOW, which
3323 has the same interpretation as in the system call of the same name.
3325 shutdown(SOCKET, 0); # I/we have stopped reading data
3326 shutdown(SOCKET, 1); # I/we have stopped writing data
3327 shutdown(SOCKET, 2); # I/we have stopped using this socket
3329 This is useful with sockets when you want to tell the other
3330 side you're done writing but not done reading, or vice versa.
3331 It's also a more insistent form of close because it also
3332 disables the filedescriptor in any forked copies in other
3339 Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
3340 returns sine of C<$_>.
3342 For the inverse sine operation, you may use the C<POSIX::asin()>
3343 function, or use this relation:
3345 sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
3351 Causes the script to sleep for EXPR seconds, or forever if no EXPR.
3352 May be interrupted if the process receives a signal such as C<SIGALRM>.
3353 Returns the number of seconds actually slept. You probably cannot
3354 mix C<alarm()> and C<sleep()> calls, because C<sleep()> is often implemented
3357 On some older systems, it may sleep up to a full second less than what
3358 you requested, depending on how it counts seconds. Most modern systems
3359 always sleep the full amount. They may appear to sleep longer than that,
3360 however, because your process might not be scheduled right away in a
3361 busy multitasking system.
3363 For delays of finer granularity than one second, you may use Perl's
3364 C<syscall()> interface to access setitimer(2) if your system supports it,
3365 or else see L</select> above.
3367 See also the POSIX module's C<sigpause()> function.
3369 =item socket SOCKET,DOMAIN,TYPE,PROTOCOL
3371 Opens a socket of the specified kind and attaches it to filehandle
3372 SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for the
3373 system call of the same name. You should "C<use Socket;>" first to get
3374 the proper definitions imported. See the example in L<perlipc/"Sockets: Client/Server Communication">.
3376 =item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
3378 Creates an unnamed pair of sockets in the specified domain, of the
3379 specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as
3380 for the system call of the same name. If unimplemented, yields a fatal
3381 error. Returns TRUE if successful.
3383 Some systems defined C<pipe()> in terms of C<socketpair()>, in which a call
3384 to C<pipe(Rdr, Wtr)> is essentially:
3387 socketpair(Rdr, Wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC);
3388 shutdown(Rdr, 1); # no more writing for reader
3389 shutdown(Wtr, 0); # no more reading for writer
3391 See L<perlipc> for an example of socketpair use.
3393 =item sort SUBNAME LIST
3395 =item sort BLOCK LIST
3399 Sorts the LIST and returns the sorted list value. If SUBNAME or BLOCK
3400 is omitted, C<sort()>s in standard string comparison order. If SUBNAME is
3401 specified, it gives the name of a subroutine that returns an integer
3402 less than, equal to, or greater than C<0>, depending on how the elements
3403 of the array are to be ordered. (The C<E<lt>=E<gt>> and C<cmp>
3404 operators are extremely useful in such routines.) SUBNAME may be a
3405 scalar variable name (unsubscripted), in which case the value provides
3406 the name of (or a reference to) the actual subroutine to use. In place
3407 of a SUBNAME, you can provide a BLOCK as an anonymous, in-line sort
3410 In the interests of efficiency the normal calling code for subroutines is
3411 bypassed, with the following effects: the subroutine may not be a
3412 recursive subroutine, and the two elements to be compared are passed into
3413 the subroutine not via C<@_> but as the package global variables C<$a> and
3414 C<$b> (see example below). They are passed by reference, so don't
3415 modify C<$a> and C<$b>. And don't try to declare them as lexicals either.
3417 You also cannot exit out of the sort block or subroutine using any of the
3418 loop control operators described in L<perlsyn> or with C<goto()>.
3420 When C<use locale> is in effect, C<sort LIST> sorts LIST according to the
3421 current collation locale. See L<perllocale>.
3426 @articles = sort @files;
3428 # same thing, but with explicit sort routine
3429 @articles = sort {$a cmp $b} @files;
3431 # now case-insensitively
3432 @articles = sort {uc($a) cmp uc($b)} @files;
3434 # same thing in reversed order
3435 @articles = sort {$b cmp $a} @files;
3437 # sort numerically ascending
3438 @articles = sort {$a <=> $b} @files;
3440 # sort numerically descending
3441 @articles = sort {$b <=> $a} @files;
3443 # sort using explicit subroutine name
3445 $age{$a} <=> $age{$b}; # presuming numeric
3447 @sortedclass = sort byage @class;
3449 # this sorts the %age hash by value instead of key
3450 # using an in-line function
3451 @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
3453 sub backwards { $b cmp $a; }
3454 @harry = ('dog','cat','x','Cain','Abel');
3455 @george = ('gone','chased','yz','Punished','Axed');
3457 # prints AbelCaincatdogx
3458 print sort backwards @harry;
3459 # prints xdogcatCainAbel
3460 print sort @george, 'to', @harry;
3461 # prints AbelAxedCainPunishedcatchaseddoggonetoxyz
3463 # inefficiently sort by descending numeric compare using
3464 # the first integer after the first = sign, or the
3465 # whole record case-insensitively otherwise
3468 ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
3473 # same thing, but much more efficiently;
3474 # we'll build auxiliary indices instead
3478 push @nums, /=(\d+)/;
3483 $nums[$b] <=> $nums[$a]
3485 $caps[$a] cmp $caps[$b]
3489 # same thing using a Schwartzian Transform (no temps)
3490 @new = map { $_->[0] }
3491 sort { $b->[1] <=> $a->[1]
3494 } map { [$_, /=(\d+)/, uc($_)] } @old;
3496 If you're using strict, you I<MUST NOT> declare C<$a>
3497 and C<$b> as lexicals. They are package globals. That means
3498 if you're in the C<main> package, it's
3500 @articles = sort {$main::b <=> $main::a} @files;
3504 @articles = sort {$::b <=> $::a} @files;
3506 but if you're in the C<FooPack> package, it's
3508 @articles = sort {$FooPack::b <=> $FooPack::a} @files;
3510 The comparison function is required to behave. If it returns
3511 inconsistent results (sometimes saying C<$x[1]> is less than C<$x[2]> and
3512 sometimes saying the opposite, for example) the results are not
3515 =item splice ARRAY,OFFSET,LENGTH,LIST
3517 =item splice ARRAY,OFFSET,LENGTH
3519 =item splice ARRAY,OFFSET
3521 Removes the elements designated by OFFSET and LENGTH from an array, and
3522 replaces them with the elements of LIST, if any. In list context,
3523 returns the elements removed from the array. In scalar context,
3524 returns the last element removed, or C<undef> if no elements are
3525 removed. The array grows or shrinks as necessary.
3526 If OFFSET is negative then it start that far from the end of the array.
3527 If LENGTH is omitted, removes everything from OFFSET onward.
3528 If LENGTH is negative, leave that many elements off the end of the array.
3529 The following equivalences hold (assuming C<$[ == 0>):
3531 push(@a,$x,$y) splice(@a,@a,0,$x,$y)
3532 pop(@a) splice(@a,-1)
3533 shift(@a) splice(@a,0,1)
3534 unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
3535 $a[$x] = $y splice(@a,$x,1,$y)
3537 Example, assuming array lengths are passed before arrays:
3539 sub aeq { # compare two list values
3540 my(@a) = splice(@_,0,shift);
3541 my(@b) = splice(@_,0,shift);
3542 return 0 unless @a == @b; # same len?
3544 return 0 if pop(@a) ne pop(@b);
3548 if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
3550 =item split /PATTERN/,EXPR,LIMIT
3552 =item split /PATTERN/,EXPR
3554 =item split /PATTERN/
3558 Splits a string into an array of strings, and returns it. By default,
3559 empty leading fields are preserved, and empty trailing ones are deleted.
3561 If not in list context, returns the number of fields found and splits into
3562 the C<@_> array. (In list context, you can force the split into C<@_> by
3563 using C<??> as the pattern delimiters, but it still returns the list
3564 value.) The use of implicit split to C<@_> is deprecated, however, because
3565 it clobbers your subroutine arguments.
3567 If EXPR is omitted, splits the C<$_> string. If PATTERN is also omitted,
3568 splits on whitespace (after skipping any leading whitespace). Anything
3569 matching PATTERN is taken to be a delimiter separating the fields. (Note
3570 that the delimiter may be longer than one character.)
3572 If LIMIT is specified and positive, splits into no more than that
3573 many fields (though it may split into fewer). If LIMIT is unspecified
3574 or zero, trailing null fields are stripped (which potential users
3575 of C<pop()> would do well to remember). If LIMIT is negative, it is
3576 treated as if an arbitrarily large LIMIT had been specified.
3578 A pattern matching the null string (not to be confused with
3579 a null pattern C<//>, which is just one member of the set of patterns
3580 matching a null string) will split the value of EXPR into separate
3581 characters at each point it matches that way. For example:
3583 print join(':', split(/ */, 'hi there'));
3585 produces the output 'h:i:t:h:e:r:e'.
3587 The LIMIT parameter can be used to split a line partially
3589 ($login, $passwd, $remainder) = split(/:/, $_, 3);
3591 When assigning to a list, if LIMIT is omitted, Perl supplies a LIMIT
3592 one larger than the number of variables in the list, to avoid
3593 unnecessary work. For the list above LIMIT would have been 4 by
3594 default. In time critical applications it behooves you not to split
3595 into more fields than you really need.
3597 If the PATTERN contains parentheses, additional array elements are
3598 created from each matching substring in the delimiter.
3600 split(/([,-])/, "1-10,20", 3);
3602 produces the list value
3604 (1, '-', 10, ',', 20)
3606 If you had the entire header of a normal Unix email message in C<$header>,
3607 you could split it up into fields and their values this way:
3609 $header =~ s/\n\s+/ /g; # fix continuation lines
3610 %hdrs = (UNIX_FROM => split /^(\S*?):\s*/m, $header);
3612 The pattern C</PATTERN/> may be replaced with an expression to specify
3613 patterns that vary at runtime. (To do runtime compilation only once,
3614 use C</$variable/o>.)
3616 As a special case, specifying a PATTERN of space (C<' '>) will split on
3617 white space just as C<split()> with no arguments does. Thus, C<split(' ')> can
3618 be used to emulate B<awk>'s default behavior, whereas C<split(/ /)>
3619 will give you as many null initial fields as there are leading spaces.
3620 A C<split()> on C</\s+/> is like a C<split(' ')> except that any leading
3621 whitespace produces a null first field. A C<split()> with no arguments
3622 really does a C<split(' ', $_)> internally.
3626 open(PASSWD, '/etc/passwd');
3628 ($login, $passwd, $uid, $gid,
3629 $gcos, $home, $shell) = split(/:/);
3633 (Note that C<$shell> above will still have a newline on it. See L</chop>,
3634 L</chomp>, and L</join>.)
3636 =item sprintf FORMAT, LIST
3638 Returns a string formatted by the usual C<printf()> conventions of the
3639 C library function C<sprintf()>. See L<sprintf(3)> or L<printf(3)>
3640 on your system for an explanation of the general principles.
3642 Perl does its own C<sprintf()> formatting -- it emulates the C
3643 function C<sprintf()>, but it doesn't use it (except for floating-point
3644 numbers, and even then only the standard modifiers are allowed). As a
3645 result, any non-standard extensions in your local C<sprintf()> are not
3646 available from Perl.
3648 Perl's C<sprintf()> permits the following universally-known conversions:
3651 %c a character with the given number
3653 %d a signed integer, in decimal
3654 %u an unsigned integer, in decimal
3655 %o an unsigned integer, in octal
3656 %x an unsigned integer, in hexadecimal
3657 %e a floating-point number, in scientific notation
3658 %f a floating-point number, in fixed decimal notation
3659 %g a floating-point number, in %e or %f notation
3661 In addition, Perl permits the following widely-supported conversions:
3663 %X like %x, but using upper-case letters
3664 %E like %e, but using an upper-case "E"
3665 %G like %g, but with an upper-case "E" (if applicable)
3666 %b an unsigned integer, in binary
3667 %p a pointer (outputs the Perl value's address in hexadecimal)
3668 %n special: *stores* the number of characters output so far
3669 into the next variable in the parameter list
3671 Finally, for backward (and we do mean "backward") compatibility, Perl
3672 permits these unnecessary but widely-supported conversions:
3675 %D a synonym for %ld
3676 %U a synonym for %lu
3677 %O a synonym for %lo
3680 Perl permits the following universally-known flags between the C<%>
3681 and the conversion letter:
3683 space prefix positive number with a space
3684 + prefix positive number with a plus sign
3685 - left-justify within the field
3686 0 use zeros, not spaces, to right-justify
3687 # prefix non-zero octal with "0", non-zero hex with "0x"
3688 number minimum field width
3689 .number "precision": digits after decimal point for
3690 floating-point, max length for string, minimum length
3692 l interpret integer as C type "long" or "unsigned long"
3693 h interpret integer as C type "short" or "unsigned short"
3695 There is also one Perl-specific flag:
3697 V interpret integer as Perl's standard integer type
3699 Where a number would appear in the flags, an asterisk ("C<*>") may be
3700 used instead, in which case Perl uses the next item in the parameter
3701 list as the given number (that is, as the field width or precision).
3702 If a field width obtained through "C<*>" is negative, it has the same
3703 effect as the "C<->" flag: left-justification.
3705 If C<use locale> is in effect, the character used for the decimal
3706 point in formatted real numbers is affected by the LC_NUMERIC locale.
3713 Return the square root of EXPR. If EXPR is omitted, returns square
3720 Sets the random number seed for the C<rand()> operator. If EXPR is
3721 omitted, uses a semi-random value supplied by the kernel (if it supports
3722 the F</dev/urandom> device) or based on the current time and process
3723 ID, among other things. In versions of Perl prior to 5.004 the default
3724 seed was just the current C<time()>. This isn't a particularly good seed,
3725 so many old programs supply their own seed value (often C<time ^ $$> or
3726 C<time ^ ($$ + ($$ E<lt>E<lt> 15))>), but that isn't necessary any more.
3728 In fact, it's usually not necessary to call C<srand()> at all, because if
3729 it is not called explicitly, it is called implicitly at the first use of
3730 the C<rand()> operator. However, this was not the case in version of Perl
3731 before 5.004, so if your script will run under older Perl versions, it
3732 should call C<srand()>.
3734 Note that you need something much more random than the default seed for
3735 cryptographic purposes. Checksumming the compressed output of one or more
3736 rapidly changing operating system status programs is the usual method. For
3739 srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip`);
3741 If you're particularly concerned with this, see the C<Math::TrulyRandom>
3744 Do I<not> call C<srand()> multiple times in your program unless you know
3745 exactly what you're doing and why you're doing it. The point of the
3746 function is to "seed" the C<rand()> function so that C<rand()> can produce
3747 a different sequence each time you run your program. Just do it once at the
3748 top of your program, or you I<won't> get random numbers out of C<rand()>!
3750 Frequently called programs (like CGI scripts) that simply use
3754 for a seed can fall prey to the mathematical property that
3758 one-third of the time. So don't do that.
3760 =item stat FILEHANDLE
3766 Returns a 13-element list giving the status info for a file, either
3767 the file opened via FILEHANDLE, or named by EXPR. If EXPR is omitted,
3768 it stats C<$_>. Returns a null list if the stat fails. Typically used
3771 ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
3772 $atime,$mtime,$ctime,$blksize,$blocks)
3775 Not all fields are supported on all filesystem types. Here are the
3776 meaning of the fields:
3778 0 dev device number of filesystem
3780 2 mode file mode (type and permissions)
3781 3 nlink number of (hard) links to the file
3782 4 uid numeric user ID of file's owner
3783 5 gid numeric group ID of file's owner
3784 6 rdev the device identifier (special files only)
3785 7 size total size of file, in bytes
3786 8 atime last access time since the epoch
3787 9 mtime last modify time since the epoch
3788 10 ctime inode change time (NOT creation time!) since the epoch
3789 11 blksize preferred block size for file system I/O
3790 12 blocks actual number of blocks allocated
3792 (The epoch was at 00:00 January 1, 1970 GMT.)
3794 If stat is passed the special filehandle consisting of an underline, no
3795 stat is done, but the current contents of the stat structure from the
3796 last stat or filetest are returned. Example:
3798 if (-x $file && (($d) = stat(_)) && $d < 0) {
3799 print "$file is executable NFS file\n";
3802 (This works on machines only for which the device number is negative under NFS.)
3804 In scalar context, C<stat()> returns a boolean value indicating success
3805 or failure, and, if successful, sets the information associated with
3806 the special filehandle C<_>.
3812 Takes extra time to study SCALAR (C<$_> if unspecified) in anticipation of
3813 doing many pattern matches on the string before it is next modified.
3814 This may or may not save time, depending on the nature and number of
3815 patterns you are searching on, and on the distribution of character
3816 frequencies in the string to be searched -- you probably want to compare
3817 run times with and without it to see which runs faster. Those loops
3818 which scan for many short constant strings (including the constant
3819 parts of more complex patterns) will benefit most. You may have only
3820 one C<study()> active at a time -- if you study a different scalar the first
3821 is "unstudied". (The way C<study()> works is this: a linked list of every
3822 character in the string to be searched is made, so we know, for
3823 example, where all the C<'k'> characters are. From each search string,
3824 the rarest character is selected, based on some static frequency tables
3825 constructed from some C programs and English text. Only those places
3826 that contain this "rarest" character are examined.)
3828 For example, here is a loop that inserts index producing entries
3829 before any line containing a certain pattern:
3833 print ".IX foo\n" if /\bfoo\b/;
3834 print ".IX bar\n" if /\bbar\b/;
3835 print ".IX blurfl\n" if /\bblurfl\b/;
3840 In searching for C</\bfoo\b/>, only those locations in C<$_> that contain C<"f">
3841 will be looked at, because C<"f"> is rarer than C<"o">. In general, this is
3842 a big win except in pathological cases. The only question is whether
3843 it saves you more time than it took to build the linked list in the
3846 Note that if you have to look for strings that you don't know till
3847 runtime, you can build an entire loop as a string and C<eval()> that to
3848 avoid recompiling all your patterns all the time. Together with
3849 undefining C<$/> to input entire files as one record, this can be very
3850 fast, often faster than specialized programs like fgrep(1). The following
3851 scans a list of files (C<@files>) for a list of words (C<@words>), and prints
3852 out the names of those files that contain a match:
3854 $search = 'while (<>) { study;';
3855 foreach $word (@words) {
3856 $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
3861 eval $search; # this screams
3862 $/ = "\n"; # put back to normal input delimiter
3863 foreach $file (sort keys(%seen)) {
3871 =item sub NAME BLOCK
3873 This is subroutine definition, not a real function I<per se>. With just a
3874 NAME (and possibly prototypes), it's just a forward declaration. Without
3875 a NAME, it's an anonymous function declaration, and does actually return a
3876 value: the CODE ref of the closure you just created. See L<perlsub> and
3877 L<perlref> for details.
3879 =item substr EXPR,OFFSET,LEN,REPLACEMENT
3881 =item substr EXPR,OFFSET,LEN
3883 =item substr EXPR,OFFSET
3885 Extracts a substring out of EXPR and returns it. First character is at
3886 offset C<0>, or whatever you've set C<$[> to (but don't do that).
3887 If OFFSET is negative (or more precisely, less than C<$[>), starts
3888 that far from the end of the string. If LEN is omitted, returns
3889 everything to the end of the string. If LEN is negative, leaves that
3890 many characters off the end of the string.
3892 If you specify a substring that is partly outside the string, the part
3893 within the string is returned. If the substring is totally outside
3894 the string a warning is produced.
3896 You can use the C<substr()> function
3897 as an lvalue, in which case EXPR must be an lvalue. If you assign
3898 something shorter than LEN, the string will shrink, and if you assign
3899 something longer than LEN, the string will grow to accommodate it. To
3900 keep the string the same length you may need to pad or chop your value
3903 An alternative to using C<substr()> as an lvalue is to specify the
3904 replacement string as the 4th argument. This allows you to replace
3905 parts of the EXPR and return what was there before in one operation.
3907 =item symlink OLDFILE,NEWFILE
3909 Creates a new filename symbolically linked to the old filename.
3910 Returns C<1> for success, C<0> otherwise. On systems that don't support
3911 symbolic links, produces a fatal error at run time. To check for that,
3914 $symlink_exists = eval { symlink("",""); 1 };
3918 Calls the system call specified as the first element of the list,
3919 passing the remaining elements as arguments to the system call. If
3920 unimplemented, produces a fatal error. The arguments are interpreted
3921 as follows: if a given argument is numeric, the argument is passed as
3922 an int. If not, the pointer to the string value is passed. You are
3923 responsible to make sure a string is pre-extended long enough to
3924 receive any result that might be written into a string. You can't use a
3925 string literal (or other read-only string) as an argument to C<syscall()>
3926 because Perl has to assume that any string pointer might be written
3928 integer arguments are not literals and have never been interpreted in a
3929 numeric context, you may need to add C<0> to them to force them to look
3930 like numbers. This emulates the C<syswrite()> function (or vice versa):
3932 require 'syscall.ph'; # may need to run h2ph
3934 syscall(&SYS_write, fileno(STDOUT), $s, length $s);
3936 Note that Perl supports passing of up to only 14 arguments to your system call,
3937 which in practice should usually suffice.
3939 Syscall returns whatever value returned by the system call it calls.
3940 If the system call fails, C<syscall()> returns C<-1> and sets C<$!> (errno).
3941 Note that some system calls can legitimately return C<-1>. The proper
3942 way to handle such calls is to assign C<$!=0;> before the call and
3943 check the value of C<$!> if syscall returns C<-1>.
3945 There's a problem with C<syscall(&SYS_pipe)>: it returns the file
3946 number of the read end of the pipe it creates. There is no way
3947 to retrieve the file number of the other end. You can avoid this
3948 problem by using C<pipe()> instead.
3950 =item sysopen FILEHANDLE,FILENAME,MODE
3952 =item sysopen FILEHANDLE,FILENAME,MODE,PERMS
3954 Opens the file whose filename is given by FILENAME, and associates it
3955 with FILEHANDLE. If FILEHANDLE is an expression, its value is used as
3956 the name of the real filehandle wanted. This function calls the
3957 underlying operating system's C<open()> function with the parameters
3958 FILENAME, MODE, PERMS.
3960 The possible values and flag bits of the MODE parameter are
3961 system-dependent; they are available via the standard module C<Fcntl>.
3962 For historical reasons, some values work on almost every system
3963 supported by perl: zero means read-only, one means write-only, and two
3964 means read/write. We know that these values do I<not> work under
3965 OS/390 & VM/ESA Unix and on the Macintosh; you probably don't want to
3966 use them in new code.
3968 If the file named by FILENAME does not exist and the C<open()> call creates
3969 it (typically because MODE includes the C<O_CREAT> flag), then the value of
3970 PERMS specifies the permissions of the newly created file. If you omit
3971 the PERMS argument to C<sysopen()>, Perl uses the octal value C<0666>.
3972 These permission values need to be in octal, and are modified by your
3973 process's current C<umask>.
3975 Seldom if ever use C<0644> as argument to C<sysopen()> because that
3976 takes away the user's option to have a more permissive umask. Better
3977 to omit it. See the perlfunc(1) entry on C<umask> for more on this.
3979 The C<IO::File> module provides a more object-oriented approach, if you're
3980 into that kind of thing.
3982 =item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
3984 =item sysread FILEHANDLE,SCALAR,LENGTH
3986 Attempts to read LENGTH bytes of data into variable SCALAR from the
3987 specified FILEHANDLE, using the system call read(2). It bypasses stdio,
3988 so mixing this with other kinds of reads, C<print()>, C<write()>,
3989 C<seek()>, C<tell()>, or C<eof()> can cause confusion because stdio
3990 usually buffers data. Returns the number of bytes actually read, C<0>
3991 at end of file, or undef if there was an error. SCALAR will be grown or
3992 shrunk so that the last byte actually read is the last byte of the
3993 scalar after the read.
3995 An OFFSET may be specified to place the read data at some place in the
3996 string other than the beginning. A negative OFFSET specifies
3997 placement at that many bytes counting backwards from the end of the
3998 string. A positive OFFSET greater than the length of SCALAR results
3999 in the string being padded to the required size with C<"\0"> bytes before
4000 the result of the read is appended.
4002 =item sysseek FILEHANDLE,POSITION,WHENCE
4004 Sets FILEHANDLE's system position using the system call lseek(2). It
4005 bypasses stdio, so mixing this with reads (other than C<sysread()>),
4006 C<print()>, C<write()>, C<seek()>, C<tell()>, or C<eof()> may cause
4007 confusion. FILEHANDLE may be an expression whose value gives the name
4008 of the filehandle. The values for WHENCE are C<0> to set the new
4009 position to POSITION, C<1> to set the it to the current position plus
4010 POSITION, and C<2> to set it to EOF plus POSITION (typically negative).
4011 For WHENCE, you may use the constants C<SEEK_SET>, C<SEEK_CUR>, and
4012 C<SEEK_END> from either the C<IO::Seekable> or the POSIX module.
4014 Returns the new position, or the undefined value on failure. A position
4015 of zero is returned as the string "C<0> but true"; thus C<sysseek()> returns
4016 TRUE on success and FALSE on failure, yet you can still easily determine
4021 =item system PROGRAM LIST
4023 Does exactly the same thing as "C<exec LIST>" except that a fork is done
4024 first, and the parent process waits for the child process to complete.
4025 Note that argument processing varies depending on the number of
4026 arguments. If there is more than one argument in LIST, or if LIST is
4027 an array with more than one value, starts the program given by the
4028 first element of the list with arguments given by the rest of the list.
4029 If there is only one scalar argument, the argument is
4030 checked for shell metacharacters, and if there are any, the entire
4031 argument is passed to the system's command shell for parsing (this is
4032 C</bin/sh -c> on Unix platforms, but varies on other platforms). If
4033 there are no shell metacharacters in the argument, it is split into
4034 words and passed directly to C<execvp()>, which is more efficient.
4036 The return value is the exit status of the program as
4037 returned by the C<wait()> call. To get the actual exit value divide by
4038 256. See also L</exec>. This is I<NOT> what you want to use to capture
4039 the output from a command, for that you should use merely backticks or
4040 C<qx//>, as described in L<perlop/"`STRING`">.
4042 Like C<exec()>, C<system()> allows you to lie to a program about its name if
4043 you use the "C<system PROGRAM LIST>" syntax. Again, see L</exec>.
4045 Because C<system()> and backticks block C<SIGINT> and C<SIGQUIT>, killing the
4046 program they're running doesn't actually interrupt your program.
4048 @args = ("command", "arg1", "arg2");
4050 or die "system @args failed: $?"
4052 You can check all the failure possibilities by inspecting
4055 $exit_value = $? >> 8;
4056 $signal_num = $? & 127;
4057 $dumped_core = $? & 128;
4059 When the arguments get executed via the system shell, results
4060 and return codes will be subject to its quirks and capabilities.
4061 See L<perlop/"`STRING`"> and L</exec> for details.
4063 =item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
4065 =item syswrite FILEHANDLE,SCALAR,LENGTH
4067 =item syswrite FILEHANDLE,SCALAR
4069 Attempts to write LENGTH bytes of data from variable SCALAR to the
4070 specified FILEHANDLE, using the system call write(2). If LENGTH is
4071 not specified, writes whole SCALAR. It bypasses
4072 stdio, so mixing this with reads (other than C<sysread())>, C<print()>,
4073 C<write()>, C<seek()>, C<tell()>, or C<eof()> may cause confusion
4074 because stdio usually buffers data. Returns the number of bytes
4075 actually written, or C<undef> if there was an error. If the LENGTH is
4076 greater than the available data in the SCALAR after the OFFSET, only as
4077 much data as is available will be written.
4079 An OFFSET may be specified to write the data from some part of the
4080 string other than the beginning. A negative OFFSET specifies writing
4081 that many bytes counting backwards from the end of the string. In the
4082 case the SCALAR is empty you can use OFFSET but only zero offset.
4084 =item tell FILEHANDLE
4088 Returns the current position for FILEHANDLE. FILEHANDLE may be an
4089 expression whose value gives the name of the actual filehandle. If
4090 FILEHANDLE is omitted, assumes the file last read.
4092 =item telldir DIRHANDLE
4094 Returns the current position of the C<readdir()> routines on DIRHANDLE.
4095 Value may be given to C<seekdir()> to access a particular location in a
4096 directory. Has the same caveats about possible directory compaction as
4097 the corresponding system library routine.
4099 =item tie VARIABLE,CLASSNAME,LIST
4101 This function binds a variable to a package class that will provide the
4102 implementation for the variable. VARIABLE is the name of the variable
4103 to be enchanted. CLASSNAME is the name of a class implementing objects
4104 of correct type. Any additional arguments are passed to the "C<new()>"
4105 method of the class (meaning C<TIESCALAR>, C<TIEHANDLE>, C<TIEARRAY>,
4106 or C<TIEHASH>). Typically these are arguments such as might be passed
4107 to the C<dbm_open()> function of C. The object returned by the "C<new()>"
4108 method is also returned by the C<tie()> function, which would be useful
4109 if you want to access other methods in CLASSNAME.
4111 Note that functions such as C<keys()> and C<values()> may return huge lists
4112 when used on large objects, like DBM files. You may prefer to use the
4113 C<each()> function to iterate over such. Example:
4115 # print out history file offsets
4117 tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
4118 while (($key,$val) = each %HIST) {
4119 print $key, ' = ', unpack('L',$val), "\n";
4123 A class implementing a hash should have the following methods:
4125 TIEHASH classname, LIST
4127 STORE this, key, value
4132 NEXTKEY this, lastkey
4135 A class implementing an ordinary array should have the following methods:
4137 TIEARRAY classname, LIST
4139 STORE this, key, value
4141 STORESIZE this, count
4147 SPLICE this, offset, length, LIST
4151 A class implementing a file handle should have the following methods:
4153 TIEHANDLE classname, LIST
4154 READ this, scalar, length, offset
4157 WRITE this, scalar, length, offset
4159 PRINTF this, format, LIST
4163 A class implementing a scalar should have the following methods:
4165 TIESCALAR classname, LIST
4170 Not all methods indicated above need be implemented. See L<perltie>,
4171 L<Tie::Hash>, L<Tie::Array>, L<Tie::Scalar> and L<Tie::Handle>.
4173 Unlike C<dbmopen()>, the C<tie()> function will not use or require a module
4174 for you--you need to do that explicitly yourself. See L<DB_File>
4175 or the F<Config> module for interesting C<tie()> implementations.
4177 For further details see L<perltie>, L<"tied VARIABLE">.
4181 Returns a reference to the object underlying VARIABLE (the same value
4182 that was originally returned by the C<tie()> call that bound the variable
4183 to a package.) Returns the undefined value if VARIABLE isn't tied to a
4188 Returns the number of non-leap seconds since whatever time the system
4189 considers to be the epoch (that's 00:00:00, January 1, 1904 for MacOS,
4190 and 00:00:00 UTC, January 1, 1970 for most other systems).
4191 Suitable for feeding to C<gmtime()> and C<localtime()>.
4195 Returns a four-element list giving the user and system times, in
4196 seconds, for this process and the children of this process.
4198 ($user,$system,$cuser,$csystem) = times;
4202 The transliteration operator. Same as C<y///>. See L<perlop>.
4204 =item truncate FILEHANDLE,LENGTH
4206 =item truncate EXPR,LENGTH
4208 Truncates the file opened on FILEHANDLE, or named by EXPR, to the
4209 specified length. Produces a fatal error if truncate isn't implemented
4210 on your system. Returns TRUE if successful, the undefined value
4217 Returns an uppercased version of EXPR. This is the internal function
4218 implementing the C<\U> escape in double-quoted strings.
4219 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
4220 Under Unicode (C<use utf8>) it uses the standard Unicode uppercase mappings. (It
4221 does not attempt to do titlecase mapping on initial letters. See C<ucfirst()> for that.)
4223 If EXPR is omitted, uses C<$_>.
4229 Returns the value of EXPR with the first character
4230 in uppercase (titlecase in Unicode). This is
4231 the internal function implementing the C<\u> escape in double-quoted strings.
4232 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
4234 If EXPR is omitted, uses C<$_>.
4240 Sets the umask for the process to EXPR and returns the previous value.
4241 If EXPR is omitted, merely returns the current umask.
4243 The Unix permission C<rwxr-x---> is represented as three sets of three
4244 bits, or three octal digits: C<0750> (the leading 0 indicates octal
4245 and isn't one of the digits). The C<umask> value is such a number
4246 representing disabled permissions bits. The permission (or "mode")
4247 values you pass C<mkdir> or C<sysopen> are modified by your umask, so
4248 even if you tell C<sysopen> to create a file with permissions C<0777>,
4249 if your umask is C<0022> then the file will actually be created with
4250 permissions C<0755>. If your C<umask> were C<0027> (group can't
4251 write; others can't read, write, or execute), then passing
4252 C<sysopen()> C<0666> would create a file with mode C<0640> (C<0666 &~
4255 Here's some advice: supply a creation mode of C<0666> for regular
4256 files (in C<sysopen()>) and one of C<0777> for directories (in
4257 C<mkdir()>) and executable files. This gives users the freedom of
4258 choice: if they want protected files, they might choose process umasks
4259 of C<022>, C<027>, or even the particularly antisocial mask of C<077>.
4260 Programs should rarely if ever make policy decisions better left to
4261 the user. The exception to this is when writing files that should be
4262 kept private: mail files, web browser cookies, I<.rhosts> files, and
4265 If umask(2) is not implemented on your system and you are trying to
4266 restrict access for I<yourself> (i.e., (EXPR & 0700) > 0), produces a
4267 fatal error at run time. If umask(2) is not implemented and you are
4268 not trying to restrict access for yourself, returns C<undef>.
4270 Remember that a umask is a number, usually given in octal; it is I<not> a
4271 string of octal digits. See also L</oct>, if all you have is a string.
4279 Undefines the value of EXPR, which must be an lvalue. Use only on a
4280 scalar value, an array (using "C<@>"), a hash (using "C<%>"), a subroutine
4281 (using "C<&>"), or a typeglob (using "<*>"). (Saying C<undef $hash{$key}>
4282 will probably not do what you expect on most predefined variables or
4283 DBM list values, so don't do that; see L<delete>.) Always returns the
4284 undefined value. You can omit the EXPR, in which case nothing is
4285 undefined, but you still get an undefined value that you could, for
4286 instance, return from a subroutine, assign to a variable or pass as a
4287 parameter. Examples:
4290 undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'};
4294 undef *xyz; # destroys $xyz, @xyz, %xyz, &xyz, etc.
4295 return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it;
4296 select undef, undef, undef, 0.25;
4297 ($a, $b, undef, $c) = &foo; # Ignore third value returned
4299 Note that this is a unary operator, not a list operator.
4305 Deletes a list of files. Returns the number of files successfully
4308 $cnt = unlink 'a', 'b', 'c';
4312 Note: C<unlink()> will not delete directories unless you are superuser and
4313 the B<-U> flag is supplied to Perl. Even if these conditions are
4314 met, be warned that unlinking a directory can inflict damage on your
4315 filesystem. Use C<rmdir()> instead.
4317 If LIST is omitted, uses C<$_>.
4319 =item unpack TEMPLATE,EXPR
4321 C<Unpack()> does the reverse of C<pack()>: it takes a string representing a
4322 structure and expands it out into a list value, returning the array
4323 value. (In scalar context, it returns merely the first value
4324 produced.) The TEMPLATE has the same format as in the C<pack()> function.
4325 Here's a subroutine that does substring:
4328 my($what,$where,$howmuch) = @_;
4329 unpack("x$where a$howmuch", $what);
4334 sub ordinal { unpack("c",$_[0]); } # same as ord()
4336 In addition, you may prefix a field with a %E<lt>numberE<gt> to indicate that
4337 you want a E<lt>numberE<gt>-bit checksum of the items instead of the items
4338 themselves. Default is a 16-bit checksum. For example, the following
4339 computes the same number as the System V sum program:
4342 $checksum += unpack("%32C*", $_);
4346 The following efficiently counts the number of set bits in a bit vector:
4348 $setbits = unpack("%32b*", $selectmask);
4350 =item untie VARIABLE
4352 Breaks the binding between a variable and a package. (See C<tie()>.)
4354 =item unshift ARRAY,LIST
4356 Does the opposite of a C<shift()>. Or the opposite of a C<push()>,
4357 depending on how you look at it. Prepends list to the front of the
4358 array, and returns the new number of elements in the array.
4360 unshift(ARGV, '-e') unless $ARGV[0] =~ /^-/;
4362 Note the LIST is prepended whole, not one element at a time, so the
4363 prepended elements stay in the same order. Use C<reverse()> to do the
4366 =item use Module LIST
4370 =item use Module VERSION LIST
4374 Imports some semantics into the current package from the named module,
4375 generally by aliasing certain subroutine or variable names into your
4376 package. It is exactly equivalent to
4378 BEGIN { require Module; import Module LIST; }
4380 except that Module I<must> be a bareword.
4382 If the first argument to C<use> is a number, it is treated as a version
4383 number instead of a module name. If the version of the Perl interpreter
4384 is less than VERSION, then an error message is printed and Perl exits
4385 immediately. This is often useful if you need to check the current
4386 Perl version before C<use>ing library modules that have changed in
4387 incompatible ways from older versions of Perl. (We try not to do
4388 this more than we have to.)
4390 The C<BEGIN> forces the C<require> and C<import()> to happen at compile time. The
4391 C<require> makes sure the module is loaded into memory if it hasn't been
4392 yet. The C<import()> is not a builtin--it's just an ordinary static method
4393 call into the "C<Module>" package to tell the module to import the list of
4394 features back into the current package. The module can implement its
4395 C<import()> method any way it likes, though most modules just choose to
4396 derive their C<import()> method via inheritance from the C<Exporter> class that
4397 is defined in the C<Exporter> module. See L<Exporter>. If no C<import()>
4398 method can be found then the error is currently silently ignored. This
4399 may change to a fatal error in a future version.
4401 If you don't want your namespace altered, explicitly supply an empty list:
4405 That is exactly equivalent to
4407 BEGIN { require Module }
4409 If the VERSION argument is present between Module and LIST, then the
4410 C<use> will call the VERSION method in class Module with the given
4411 version as an argument. The default VERSION method, inherited from
4412 the Universal class, croaks if the given version is larger than the
4413 value of the variable C<$Module::VERSION>. (Note that there is not a
4414 comma after VERSION!)
4416 Because this is a wide-open interface, pragmas (compiler directives)
4417 are also implemented this way. Currently implemented pragmas are:
4421 use sigtrap qw(SEGV BUS);
4422 use strict qw(subs vars refs);
4423 use subs qw(afunc blurfl);
4425 Some of these these pseudo-modules import semantics into the current
4426 block scope (like C<strict> or C<integer>, unlike ordinary modules,
4427 which import symbols into the current package (which are effective
4428 through the end of the file).
4430 There's a corresponding "C<no>" command that unimports meanings imported
4431 by C<use>, i.e., it calls C<unimport Module LIST> instead of C<import()>.
4436 If no C<unimport()> method can be found the call fails with a fatal error.
4438 See L<perlmod> for a list of standard modules and pragmas.
4442 Changes the access and modification times on each file of a list of
4443 files. The first two elements of the list must be the NUMERICAL access
4444 and modification times, in that order. Returns the number of files
4445 successfully changed. The inode modification time of each file is set
4446 to the current time. This code has the same effect as the "C<touch>"
4447 command if the files already exist:
4451 utime $now, $now, @ARGV;
4455 Returns a list consisting of all the values of the named hash. (In a
4456 scalar context, returns the number of values.) The values are
4457 returned in an apparently random order. The actual random order is
4458 subject to change in future versions of perl, but it is guaranteed to
4459 be the same order as either the C<keys()> or C<each()> function would
4460 produce on the same (unmodified) hash.
4462 As a side effect, it resets HASH's iterator. See also C<keys()>, C<each()>,
4465 =item vec EXPR,OFFSET,BITS
4467 Treats the string in EXPR as a vector of unsigned integers, and
4468 returns the value of the bit field specified by OFFSET. BITS specifies
4469 the number of bits that are reserved for each entry in the bit
4470 vector. This must be a power of two from 1 to 32. C<vec()> may also be
4471 assigned to, in which case parentheses are needed to give the expression
4472 the correct precedence as in
4474 vec($image, $max_x * $x + $y, 8) = 3;
4476 Vectors created with C<vec()> can also be manipulated with the logical
4477 operators C<|>, C<&>, and C<^>, which will assume a bit vector operation is
4478 desired when both operands are strings.
4480 The following code will build up an ASCII string saying C<'PerlPerlPerl'>.
4481 The comments show the string after each step. Note that this code works
4482 in the same way on big-endian or little-endian machines.
4485 vec($foo, 0, 32) = 0x5065726C; # 'Perl'
4486 vec($foo, 2, 16) = 0x5065; # 'PerlPe'
4487 vec($foo, 3, 16) = 0x726C; # 'PerlPerl'
4488 vec($foo, 8, 8) = 0x50; # 'PerlPerlP'
4489 vec($foo, 9, 8) = 0x65; # 'PerlPerlPe'
4490 vec($foo, 20, 4) = 2; # 'PerlPerlPe' . "\x02"
4491 vec($foo, 21, 4) = 7; # 'PerlPerlPer'
4493 vec($foo, 45, 2) = 3; # 'PerlPerlPer' . "\x0c"
4494 vec($foo, 93, 1) = 1; # 'PerlPerlPer' . "\x2c"
4495 vec($foo, 94, 1) = 1; # 'PerlPerlPerl'
4498 To transform a bit vector into a string or array of 0's and 1's, use these:
4500 $bits = unpack("b*", $vector);
4501 @bits = split(//, unpack("b*", $vector));
4503 If you know the exact length in bits, it can be used in place of the C<*>.
4507 Waits for a child process to terminate and returns the pid of the
4508 deceased process, or C<-1> if there are no child processes. The status is
4509 returned in C<$?>. Note that a return value of C<-1> could mean that
4510 child processes are being automatically reaped, as described in L<perlipc>.
4512 =item waitpid PID,FLAGS
4514 Waits for a particular child process to terminate and returns the pid
4515 of the deceased process, or C<-1> if there is no such child process. The
4516 status is returned in C<$?>. If you say
4518 use POSIX ":sys_wait_h";
4520 waitpid(-1,&WNOHANG);
4522 then you can do a non-blocking wait for any process. Non-blocking wait
4523 is available on machines supporting either the waitpid(2) or
4524 wait4(2) system calls. However, waiting for a particular pid with
4525 FLAGS of C<0> is implemented everywhere. (Perl emulates the system call
4526 by remembering the status values of processes that have exited but have
4527 not been harvested by the Perl script yet.)
4529 Note that a return value of C<-1> could mean that child processes are being
4530 automatically reaped. See L<perlipc> for details, and for other examples.
4534 Returns TRUE if the context of the currently executing subroutine is
4535 looking for a list value. Returns FALSE if the context is looking
4536 for a scalar. Returns the undefined value if the context is looking
4537 for no value (void context).
4539 return unless defined wantarray; # don't bother doing more
4540 my @a = complex_calculation();
4541 return wantarray ? @a : "@a";
4545 Produces a message on STDERR just like C<die()>, but doesn't exit or throw
4548 If LIST is empty and C<$@> already contains a value (typically from a
4549 previous eval) that value is used after appending C<"\t...caught">
4550 to C<$@>. This is useful for staying almost, but not entirely similar to
4553 If C<$@> is empty then the string C<"Warning: Something's wrong"> is used.
4555 No message is printed if there is a C<$SIG{__WARN__}> handler
4556 installed. It is the handler's responsibility to deal with the message
4557 as it sees fit (like, for instance, converting it into a C<die()>). Most
4558 handlers must therefore make arrangements to actually display the
4559 warnings that they are not prepared to deal with, by calling C<warn()>
4560 again in the handler. Note that this is quite safe and will not
4561 produce an endless loop, since C<__WARN__> hooks are not called from
4564 You will find this behavior is slightly different from that of
4565 C<$SIG{__DIE__}> handlers (which don't suppress the error text, but can
4566 instead call C<die()> again to change it).
4568 Using a C<__WARN__> handler provides a powerful way to silence all
4569 warnings (even the so-called mandatory ones). An example:
4571 # wipe out *all* compile-time warnings
4572 BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } }
4574 my $foo = 20; # no warning about duplicate my $foo,
4575 # but hey, you asked for it!
4576 # no compile-time or run-time warnings before here
4579 # run-time warnings enabled after here
4580 warn "\$foo is alive and $foo!"; # does show up
4582 See L<perlvar> for details on setting C<%SIG> entries, and for more
4585 =item write FILEHANDLE
4591 Writes a formatted record (possibly multi-line) to the specified FILEHANDLE,
4592 using the format associated with that file. By default the format for
4593 a file is the one having the same name as the filehandle, but the
4594 format for the current output channel (see the C<select()> function) may be set
4595 explicitly by assigning the name of the format to the C<$~> variable.
4597 Top of form processing is handled automatically: if there is
4598 insufficient room on the current page for the formatted record, the
4599 page is advanced by writing a form feed, a special top-of-page format
4600 is used to format the new page header, and then the record is written.
4601 By default the top-of-page format is the name of the filehandle with
4602 "_TOP" appended, but it may be dynamically set to the format of your
4603 choice by assigning the name to the C<$^> variable while the filehandle is
4604 selected. The number of lines remaining on the current page is in
4605 variable C<$->, which can be set to C<0> to force a new page.
4607 If FILEHANDLE is unspecified, output goes to the current default output
4608 channel, which starts out as STDOUT but may be changed by the
4609 C<select()> operator. If the FILEHANDLE is an EXPR, then the expression
4610 is evaluated and the resulting string is used to look up the name of
4611 the FILEHANDLE at run time. For more on formats, see L<perlform>.
4613 Note that write is I<NOT> the opposite of C<read()>. Unfortunately.
4617 The transliteration operator. Same as C<tr///>. See L<perlop>.