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: $!\n"
909 If the value of EXPR does not end in a newline, the current script line
910 number and input line number (if any) are also printed, and a newline
911 is supplied. Note that the "input line number" (also known as "chunk")
912 is subject to whatever notion of "line" happens to be currently in
913 effect, and is also available as the special variable C<$.>.
914 See L<perlvar/"$/"> and L<perlvar/"$.">.
916 Hint: sometimes appending C<", stopped"> to your message
917 will cause it to make better sense when the string C<"at foo line 123"> is
918 appended. Suppose you are running script "canasta".
920 die "/etc/games is no good";
921 die "/etc/games is no good, stopped";
923 produce, respectively
925 /etc/games is no good at canasta line 123.
926 /etc/games is no good, stopped at canasta line 123.
928 See also C<exit()> and C<warn()>.
930 If LIST is empty and C<$@> already contains a value (typically from a
931 previous eval) that value is reused after appending C<"\t...propagated">.
932 This is useful for propagating exceptions:
935 die unless $@ =~ /Expected exception/;
937 If C<$@> is empty then the string C<"Died"> is used.
939 You can arrange for a callback to be run just before the C<die()> does
940 its deed, by setting the C<$SIG{__DIE__}> hook. The associated handler
941 will be called with the error text and can change the error message, if
942 it sees fit, by calling C<die()> again. See L<perlvar/$SIG{expr}> for details on
943 setting C<%SIG> entries, and L<"eval BLOCK"> for some examples.
945 Note that the C<$SIG{__DIE__}> hook is called even inside eval()ed
946 blocks/strings. If one wants the hook to do nothing in such
951 as the first line of the handler (see L<perlvar/$^S>).
955 Not really a function. Returns the value of the last command in the
956 sequence of commands indicated by BLOCK. When modified by a loop
957 modifier, executes the BLOCK once before testing the loop condition.
958 (On other statements the loop modifiers test the conditional first.)
960 C<do BLOCK> does I<not> count as a loop, so the loop control statements
961 C<next>, C<last> or C<redo> cannot be used to leave or restart the block.
963 =item do SUBROUTINE(LIST)
965 A deprecated form of subroutine call. See L<perlsub>.
969 Uses the value of EXPR as a filename and executes the contents of the
970 file as a Perl script. Its primary use is to include subroutines
971 from a Perl subroutine library.
977 scalar eval `cat stat.pl`;
979 except that it's more efficient and concise, keeps track of the
980 current filename for error messages, and searches all the B<-I>
981 libraries if the file isn't in the current directory (see also the @INC
982 array in L<perlvar/Predefined Names>). It is also different in how
983 code evaluated with C<do FILENAME> doesn't see lexicals in the enclosing
984 scope like C<eval STRING> does. It's the same, however, in that it does
985 reparse the file every time you call it, so you probably don't want to
986 do this inside a loop.
988 If C<do> cannot read the file, it returns undef and sets C<$!> to the
989 error. If C<do> can read the file but cannot compile it, it
990 returns undef and sets an error message in C<$@>. If the file is
991 successfully compiled, C<do> returns the value of the last expression
994 Note that inclusion of library modules is better done with the
995 C<use()> and C<require()> operators, which also do automatic error checking
996 and raise an exception if there's a problem.
998 You might like to use C<do> to read in a program configuration
999 file. Manual error checking can be done this way:
1001 # read in config files: system first, then user
1002 for $file ("/share/prog/defaults.rc",
1003 "$ENV{HOME}/.someprogrc") {
1004 unless ($return = do $file) {
1005 warn "couldn't parse $file: $@" if $@;
1006 warn "couldn't do $file: $!" unless defined $return;
1007 warn "couldn't run $file" unless $return;
1015 This causes an immediate core dump. Primarily this is so that you can
1016 use the B<undump> program to turn your core dump into an executable binary
1017 after having initialized all your variables at the beginning of the
1018 program. When the new binary is executed it will begin by executing a
1019 C<goto LABEL> (with all the restrictions that C<goto> suffers). Think of
1020 it as a goto with an intervening core dump and reincarnation. If C<LABEL>
1021 is omitted, restarts the program from the top. WARNING: Any files
1022 opened at the time of the dump will NOT be open any more when the
1023 program is reincarnated, with possible resulting confusion on the part
1024 of Perl. See also B<-u> option in L<perlrun>.
1029 require 'getopt.pl';
1041 dump QUICKSTART if $ARGV[0] eq '-d';
1046 This operator is largely obsolete, partly because it's very hard to
1047 convert a core file into an executable, and because the real perl-to-C
1048 compiler has superseded it.
1052 When called in list context, returns a 2-element list consisting of the
1053 key and value for the next element of a hash, so that you can iterate over
1054 it. When called in scalar context, returns the key for only the "next"
1055 element in the hash. (Note: Keys may be C<"0"> or C<"">, which are logically
1056 false; you may wish to avoid constructs like C<while ($k = each %foo) {}>
1059 Entries are returned in an apparently random order. The actual random
1060 order is subject to change in future versions of perl, but it is guaranteed
1061 to be in the same order as either the C<keys()> or C<values()> function
1062 would produce on the same (unmodified) hash.
1064 When the hash is entirely read, a null array is returned in list context
1065 (which when assigned produces a FALSE (C<0>) value), and C<undef> in
1066 scalar context. The next call to C<each()> after that will start iterating
1067 again. There is a single iterator for each hash, shared by all C<each()>,
1068 C<keys()>, and C<values()> function calls in the program; it can be reset by
1069 reading all the elements from the hash, or by evaluating C<keys HASH> or
1070 C<values HASH>. If you add or delete elements of a hash while you're
1071 iterating over it, you may get entries skipped or duplicated, so don't.
1073 The following prints out your environment like the printenv(1) program,
1074 only in a different order:
1076 while (($key,$value) = each %ENV) {
1077 print "$key=$value\n";
1080 See also C<keys()>, C<values()> and C<sort()>.
1082 =item eof FILEHANDLE
1088 Returns 1 if the next read on FILEHANDLE will return end of file, or if
1089 FILEHANDLE is not open. FILEHANDLE may be an expression whose value
1090 gives the real filehandle. (Note that this function actually
1091 reads a character and then C<ungetc()>s it, so isn't very useful in an
1092 interactive context.) Do not read from a terminal file (or call
1093 C<eof(FILEHANDLE)> on it) after end-of-file is reached. Filetypes such
1094 as terminals may lose the end-of-file condition if you do.
1096 An C<eof> without an argument uses the last file read as argument.
1097 Using C<eof()> with empty parentheses is very different. It indicates the pseudo file formed of
1098 the files listed on the command line, i.e., C<eof()> is reasonable to
1099 use inside a C<while (E<lt>E<gt>)> loop to detect the end of only the
1100 last file. Use C<eof(ARGV)> or eof without the parentheses to test
1101 I<EACH> file in a while (E<lt>E<gt>) loop. Examples:
1103 # reset line numbering on each input file
1105 next if /^\s*#/; # skip comments
1108 close ARGV if eof; # Not eof()!
1111 # insert dashes just before last line of last file
1113 if (eof()) { # check for end of current file
1114 print "--------------\n";
1115 close(ARGV); # close or break; is needed if we
1116 # are reading from the terminal
1121 Practical hint: you almost never need to use C<eof> in Perl, because the
1122 input operators return false values when they run out of data, or if there
1129 In the first form, the return value of EXPR is parsed and executed as if it
1130 were a little Perl program. The value of the expression (which is itself
1131 determined within scalar context) is first parsed, and if there weren't any
1132 errors, executed in the context of the current Perl program, so that any
1133 variable settings or subroutine and format definitions remain afterwards.
1134 Note that the value is parsed every time the eval executes. If EXPR is
1135 omitted, evaluates C<$_>. This form is typically used to delay parsing
1136 and subsequent execution of the text of EXPR until run time.
1138 In the second form, the code within the BLOCK is parsed only once--at the
1139 same time the code surrounding the eval itself was parsed--and executed
1140 within the context of the current Perl program. This form is typically
1141 used to trap exceptions more efficiently than the first (see below), while
1142 also providing the benefit of checking the code within BLOCK at compile
1145 The final semicolon, if any, may be omitted from the value of EXPR or within
1148 In both forms, the value returned is the value of the last expression
1149 evaluated inside the mini-program; a return statement may be also used, just
1150 as with subroutines. The expression providing the return value is evaluated
1151 in void, scalar, or list context, depending on the context of the eval itself.
1152 See L</wantarray> for more on how the evaluation context can be determined.
1154 If there is a syntax error or runtime error, or a C<die()> statement is
1155 executed, an undefined value is returned by C<eval()>, and C<$@> is set to the
1156 error message. If there was no error, C<$@> is guaranteed to be a null
1157 string. Beware that using C<eval()> neither silences perl from printing
1158 warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
1159 To do either of those, you have to use the C<$SIG{__WARN__}> facility. See
1160 L</warn> and L<perlvar>.
1162 Note that, because C<eval()> traps otherwise-fatal errors, it is useful for
1163 determining whether a particular feature (such as C<socket()> or C<symlink()>)
1164 is implemented. It is also Perl's exception trapping mechanism, where
1165 the die operator is used to raise exceptions.
1167 If the code to be executed doesn't vary, you may use the eval-BLOCK
1168 form to trap run-time errors without incurring the penalty of
1169 recompiling each time. The error, if any, is still returned in C<$@>.
1172 # make divide-by-zero nonfatal
1173 eval { $answer = $a / $b; }; warn $@ if $@;
1175 # same thing, but less efficient
1176 eval '$answer = $a / $b'; warn $@ if $@;
1178 # a compile-time error
1179 eval { $answer = }; # WRONG
1182 eval '$answer ='; # sets $@
1184 When using the C<eval{}> form as an exception trap in libraries, you may
1185 wish not to trigger any C<__DIE__> hooks that user code may have
1186 installed. You can use the C<local $SIG{__DIE__}> construct for this
1187 purpose, as shown in this example:
1189 # a very private exception trap for divide-by-zero
1190 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1193 This is especially significant, given that C<__DIE__> hooks can call
1194 C<die()> again, which has the effect of changing their error messages:
1196 # __DIE__ hooks may modify error messages
1198 local $SIG{'__DIE__'} =
1199 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
1200 eval { die "foo lives here" };
1201 print $@ if $@; # prints "bar lives here"
1204 With an C<eval()>, you should be especially careful to remember what's
1205 being looked at when:
1211 eval { $x }; # CASE 4
1213 eval "\$$x++"; # CASE 5
1216 Cases 1 and 2 above behave identically: they run the code contained in
1217 the variable C<$x>. (Although case 2 has misleading double quotes making
1218 the reader wonder what else might be happening (nothing is).) Cases 3
1219 and 4 likewise behave in the same way: they run the code C<'$x'>, which
1220 does nothing but return the value of C<$x>. (Case 4 is preferred for
1221 purely visual reasons, but it also has the advantage of compiling at
1222 compile-time instead of at run-time.) Case 5 is a place where
1223 normally you I<WOULD> like to use double quotes, except that in this
1224 particular situation, you can just use symbolic references instead, as
1227 C<eval BLOCK> does I<not> count as a loop, so the loop control statements
1228 C<next>, C<last> or C<redo> cannot be used to leave or restart the block.
1233 =item exec PROGRAM LIST
1235 The C<exec()> function executes a system command I<AND NEVER RETURNS> -
1236 use C<system()> instead of C<exec()> if you want it to return. It fails and
1237 returns FALSE only if the command does not exist I<and> it is executed
1238 directly instead of via your system's command shell (see below).
1240 Since it's a common mistake to use C<exec()> instead of C<system()>, Perl
1241 warns you if there is a following statement which isn't C<die()>, C<warn()>,
1242 or C<exit()> (if C<-w> is set - but you always do that). If you
1243 I<really> want to follow an C<exec()> with some other statement, you
1244 can use one of these styles to avoid the warning:
1246 exec ('foo') or print STDERR "couldn't exec foo: $!";
1247 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
1249 If there is more than one argument in LIST, or if LIST is an array
1250 with more than one value, calls execvp(3) with the arguments in LIST.
1251 If there is only one scalar argument or an array with one element in it,
1252 the argument is checked for shell metacharacters, and if there are any,
1253 the entire argument is passed to the system's command shell for parsing
1254 (this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1255 If there are no shell metacharacters in the argument, it is split into
1256 words and passed directly to C<execvp()>, which is more efficient. Note:
1257 C<exec()> and C<system()> do not flush your output buffer, so you may need to
1258 set C<$|> to avoid lost output. Examples:
1260 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1261 exec "sort $outfile | uniq";
1263 If you don't really want to execute the first argument, but want to lie
1264 to the program you are executing about its own name, you can specify
1265 the program you actually want to run as an "indirect object" (without a
1266 comma) in front of the LIST. (This always forces interpretation of the
1267 LIST as a multivalued list, even if there is only a single scalar in
1270 $shell = '/bin/csh';
1271 exec $shell '-sh'; # pretend it's a login shell
1275 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1277 When the arguments get executed via the system shell, results will
1278 be subject to its quirks and capabilities. See L<perlop/"`STRING`">
1281 Using an indirect object with C<exec()> or C<system()> is also more secure.
1282 This usage forces interpretation of the arguments as a multivalued list,
1283 even if the list had just one argument. That way you're safe from the
1284 shell expanding wildcards or splitting up words with whitespace in them.
1286 @args = ( "echo surprise" );
1288 system @args; # subject to shell escapes
1290 system { $args[0] } @args; # safe even with one-arg list
1292 The first version, the one without the indirect object, ran the I<echo>
1293 program, passing it C<"surprise"> an argument. The second version
1294 didn't--it tried to run a program literally called I<"echo surprise">,
1295 didn't find it, and set C<$?> to a non-zero value indicating failure.
1297 Note that C<exec()> will not call your C<END> blocks, nor will it call
1298 any C<DESTROY> methods in your objects.
1302 Returns TRUE if the specified hash key exists in its hash array, even
1303 if the corresponding value is undefined.
1305 print "Exists\n" if exists $array{$key};
1306 print "Defined\n" if defined $array{$key};
1307 print "True\n" if $array{$key};
1309 A hash element can be TRUE only if it's defined, and defined if
1310 it exists, but the reverse doesn't necessarily hold true.
1312 Note that the EXPR can be arbitrarily complicated as long as the final
1313 operation is a hash key lookup:
1315 if (exists $ref->{"A"}{"B"}{$key}) { ... }
1317 Although the last element will not spring into existence just because its
1318 existence was tested, intervening ones will. Thus C<$ref-E<gt>{"A"}>
1319 C<$ref-E<gt>{"B"}> will spring into existence due to the existence
1320 test for a $key element. This autovivification may be fixed in a later
1325 Evaluates EXPR and exits immediately with that value. (Actually, it
1326 calls any defined C<END> routines first, but the C<END> routines may not
1327 abort the exit. Likewise any object destructors that need to be called
1328 are called before exit.) Example:
1331 exit 0 if $ans =~ /^[Xx]/;
1333 See also C<die()>. If EXPR is omitted, exits with C<0> status. The only
1334 universally portable values for EXPR are C<0> for success and C<1> for error;
1335 all other values are subject to unpredictable interpretation depending
1336 on the environment in which the Perl program is running.
1338 You shouldn't use C<exit()> to abort a subroutine if there's any chance that
1339 someone might want to trap whatever error happened. Use C<die()> instead,
1340 which can be trapped by an C<eval()>.
1342 All C<END{}> blocks are run at exit time. See L<perlsub> for details.
1348 Returns I<e> (the natural logarithm base) to the power of EXPR.
1349 If EXPR is omitted, gives C<exp($_)>.
1351 =item fcntl FILEHANDLE,FUNCTION,SCALAR
1353 Implements the fcntl(2) function. You'll probably have to say
1357 first to get the correct constant definitions. Argument processing and
1358 value return works just like C<ioctl()> below.
1362 fcntl($filehandle, F_GETFL, $packed_return_buffer)
1363 or die "can't fcntl F_GETFL: $!";
1365 You don't have to check for C<defined()> on the return from
1366 C<fnctl()>. Like C<ioctl()>, it maps a C<0> return from the system
1367 call into "C<0> but true" in Perl. This string is true in
1368 boolean context and C<0> in numeric context. It is also
1369 exempt from the normal B<-w> warnings on improper numeric
1372 Note that C<fcntl()> will produce a fatal error if used on a machine that
1373 doesn't implement fcntl(2).
1375 =item fileno FILEHANDLE
1377 Returns the file descriptor for a filehandle. This is useful for
1378 constructing bitmaps for C<select()> and low-level POSIX tty-handling
1379 operations. If FILEHANDLE is an expression, the value is taken as
1380 an indirect filehandle, generally its name.
1382 You can use this to find out whether two handles refer to the
1383 same underlying descriptor:
1385 if (fileno(THIS) == fileno(THAT)) {
1386 print "THIS and THAT are dups\n";
1389 =item flock FILEHANDLE,OPERATION
1391 Calls flock(2), or an emulation of it, on FILEHANDLE. Returns TRUE for
1392 success, FALSE on failure. Produces a fatal error if used on a machine
1393 that doesn't implement flock(2), fcntl(2) locking, or lockf(3). C<flock()>
1394 is Perl's portable file locking interface, although it locks only entire
1397 On many platforms (including most versions or clones of Unix), locks
1398 established by C<flock()> are B<merely advisory>. Such discretionary locks
1399 are more flexible, but offer fewer guarantees. This means that files
1400 locked with C<flock()> may be modified by programs that do not also use
1401 C<flock()>. Windows NT and OS/2 are among the platforms which
1402 enforce mandatory locking. See your local documentation for details.
1404 OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1405 LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
1406 you can use the symbolic names if import them from the Fcntl module,
1407 either individually, or as a group using the ':flock' tag. LOCK_SH
1408 requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
1409 releases a previously requested lock. If LOCK_NB is added to LOCK_SH or
1410 LOCK_EX then C<flock()> will return immediately rather than blocking
1411 waiting for the lock (check the return status to see if you got it).
1413 To avoid the possibility of mis-coordination, Perl flushes FILEHANDLE
1414 before (un)locking it.
1416 Note that the emulation built with lockf(3) doesn't provide shared
1417 locks, and it requires that FILEHANDLE be open with write intent. These
1418 are the semantics that lockf(3) implements. Most (all?) systems
1419 implement lockf(3) in terms of fcntl(2) locking, though, so the
1420 differing semantics shouldn't bite too many people.
1422 Note also that some versions of C<flock()> cannot lock things over the
1423 network; you would need to use the more system-specific C<fcntl()> for
1424 that. If you like you can force Perl to ignore your system's flock(2)
1425 function, and so provide its own fcntl(2)-based emulation, by passing
1426 the switch C<-Ud_flock> to the F<Configure> program when you configure
1429 Here's a mailbox appender for BSD systems.
1431 use Fcntl ':flock'; # import LOCK_* constants
1434 flock(MBOX,LOCK_EX);
1435 # and, in case someone appended
1436 # while we were waiting...
1441 flock(MBOX,LOCK_UN);
1444 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1445 or die "Can't open mailbox: $!";
1448 print MBOX $msg,"\n\n";
1451 See also L<DB_File> for other flock() examples.
1455 Does a fork(2) system call. Returns the child pid to the parent process,
1456 C<0> to the child process, or C<undef> if the fork is unsuccessful.
1458 Note: unflushed buffers remain unflushed in both processes, which means
1459 you may need to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()>
1460 method of C<IO::Handle> to avoid duplicate output.
1462 If you C<fork()> without ever waiting on your children, you will accumulate
1465 $SIG{CHLD} = sub { wait };
1467 There's also the double-fork trick (error checking on
1468 C<fork()> returns omitted);
1470 unless ($pid = fork) {
1472 exec "what you really wanna do";
1475 ## (some_perl_code_here)
1482 See also L<perlipc> for more examples of forking and reaping
1485 Note that if your forked child inherits system file descriptors like
1486 STDIN and STDOUT that are actually connected by a pipe or socket, even
1487 if you exit, then the remote server (such as, say, httpd or rsh) won't think
1488 you're done. You should reopen those to F</dev/null> if it's any issue.
1492 Declare a picture format for use by the C<write()> function. For
1496 Test: @<<<<<<<< @||||| @>>>>>
1497 $str, $%, '$' . int($num)
1501 $num = $cost/$quantity;
1505 See L<perlform> for many details and examples.
1507 =item formline PICTURE,LIST
1509 This is an internal function used by C<format>s, though you may call it,
1510 too. It formats (see L<perlform>) a list of values according to the
1511 contents of PICTURE, placing the output into the format output
1512 accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
1513 Eventually, when a C<write()> is done, the contents of
1514 C<$^A> are written to some filehandle, but you could also read C<$^A>
1515 yourself and then set C<$^A> back to C<"">. Note that a format typically
1516 does one C<formline()> per line of form, but the C<formline()> function itself
1517 doesn't care how many newlines are embedded in the PICTURE. This means
1518 that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
1519 You may therefore need to use multiple formlines to implement a single
1520 record format, just like the format compiler.
1522 Be careful if you put double quotes around the picture, because an "C<@>"
1523 character may be taken to mean the beginning of an array name.
1524 C<formline()> always returns TRUE. See L<perlform> for other examples.
1526 =item getc FILEHANDLE
1530 Returns the next character from the input file attached to FILEHANDLE,
1531 or the undefined value at end of file, or if there was an error. If
1532 FILEHANDLE is omitted, reads from STDIN. This is not particularly
1533 efficient. It cannot be used to get unbuffered single-characters,
1534 however. For that, try something more like:
1537 system "stty cbreak </dev/tty >/dev/tty 2>&1";
1540 system "stty", '-icanon', 'eol', "\001";
1546 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
1549 system "stty", 'icanon', 'eol', '^@'; # ASCII null
1553 Determination of whether $BSD_STYLE should be set
1554 is left as an exercise to the reader.
1556 The C<POSIX::getattr()> function can do this more portably on systems
1557 purporting POSIX compliance.
1558 See also the C<Term::ReadKey> module from your nearest CPAN site;
1559 details on CPAN can be found on L<perlmodlib/CPAN>.
1563 Implements the C library function of the same name, which on most
1564 systems returns the current login from F</etc/utmp>, if any. If null,
1567 $login = getlogin || getpwuid($<) || "Kilroy";
1569 Do not consider C<getlogin()> for authentication: it is not as
1570 secure as C<getpwuid()>.
1572 =item getpeername SOCKET
1574 Returns the packed sockaddr address of other end of the SOCKET connection.
1577 $hersockaddr = getpeername(SOCK);
1578 ($port, $iaddr) = unpack_sockaddr_in($hersockaddr);
1579 $herhostname = gethostbyaddr($iaddr, AF_INET);
1580 $herstraddr = inet_ntoa($iaddr);
1584 Returns the current process group for the specified PID. Use
1585 a PID of C<0> to get the current process group for the
1586 current process. Will raise an exception if used on a machine that
1587 doesn't implement getpgrp(2). If PID is omitted, returns process
1588 group of current process. Note that the POSIX version of C<getpgrp()>
1589 does not accept a PID argument, so only C<PID==0> is truly portable.
1593 Returns the process id of the parent process.
1595 =item getpriority WHICH,WHO
1597 Returns the current priority for a process, a process group, or a user.
1598 (See L<getpriority(2)>.) Will raise a fatal exception if used on a
1599 machine that doesn't implement getpriority(2).
1605 =item gethostbyname NAME
1607 =item getnetbyname NAME
1609 =item getprotobyname NAME
1615 =item getservbyname NAME,PROTO
1617 =item gethostbyaddr ADDR,ADDRTYPE
1619 =item getnetbyaddr ADDR,ADDRTYPE
1621 =item getprotobynumber NUMBER
1623 =item getservbyport PORT,PROTO
1641 =item sethostent STAYOPEN
1643 =item setnetent STAYOPEN
1645 =item setprotoent STAYOPEN
1647 =item setservent STAYOPEN
1661 These routines perform the same functions as their counterparts in the
1662 system library. In list context, the return values from the
1663 various get routines are as follows:
1665 ($name,$passwd,$uid,$gid,
1666 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
1667 ($name,$passwd,$gid,$members) = getgr*
1668 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
1669 ($name,$aliases,$addrtype,$net) = getnet*
1670 ($name,$aliases,$proto) = getproto*
1671 ($name,$aliases,$port,$proto) = getserv*
1673 (If the entry doesn't exist you get a null list.)
1675 In scalar context, you get the name, unless the function was a
1676 lookup by name, in which case you get the other thing, whatever it is.
1677 (If the entry doesn't exist you get the undefined value.) For example:
1679 $uid = getpwnam($name);
1680 $name = getpwuid($num);
1682 $gid = getgrnam($name);
1683 $name = getgrgid($num;
1687 In I<getpw*()> the fields C<$quota>, C<$comment>, and C<$expire> are special
1688 cases in the sense that in many systems they are unsupported. If the
1689 C<$quota> is unsupported, it is an empty scalar. If it is supported, it
1690 usually encodes the disk quota. If the C<$comment> field is unsupported,
1691 it is an empty scalar. If it is supported it usually encodes some
1692 administrative comment about the user. In some systems the $quota
1693 field may be C<$change> or C<$age>, fields that have to do with password
1694 aging. In some systems the C<$comment> field may be C<$class>. The C<$expire>
1695 field, if present, encodes the expiration period of the account or the
1696 password. For the availability and the exact meaning of these fields
1697 in your system, please consult your getpwnam(3) documentation and your
1698 F<pwd.h> file. You can also find out from within Perl which meaning
1699 your C<$quota> and C<$comment> fields have and whether you have the C<$expire>
1700 field by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
1701 C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>.
1703 The C<$members> value returned by I<getgr*()> is a space separated list of
1704 the login names of the members of the group.
1706 For the I<gethost*()> functions, if the C<h_errno> variable is supported in
1707 C, it will be returned to you via C<$?> if the function call fails. The
1708 C<@addrs> value returned by a successful call is a list of the raw
1709 addresses returned by the corresponding system library call. In the
1710 Internet domain, each address is four bytes long and you can unpack it
1711 by saying something like:
1713 ($a,$b,$c,$d) = unpack('C4',$addr[0]);
1715 If you get tired of remembering which element of the return list contains
1716 which return value, by-name interfaces are also provided in modules:
1717 C<File::stat>, C<Net::hostent>, C<Net::netent>, C<Net::protoent>, C<Net::servent>,
1718 C<Time::gmtime>, C<Time::localtime>, and C<User::grent>. These override the
1719 normal built-in, replacing them with versions that return objects with
1720 the appropriate names for each field. For example:
1724 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
1726 Even though it looks like they're the same method calls (uid),
1727 they aren't, because a C<File::stat> object is different from a C<User::pwent> object.
1729 =item getsockname SOCKET
1731 Returns the packed sockaddr address of this end of the SOCKET connection.
1734 $mysockaddr = getsockname(SOCK);
1735 ($port, $myaddr) = unpack_sockaddr_in($mysockaddr);
1737 =item getsockopt SOCKET,LEVEL,OPTNAME
1739 Returns the socket option requested, or undef if there is an error.
1745 Returns the value of EXPR with filename expansions such as the standard Unix shell F</bin/sh> would
1746 do. This is the internal function implementing the C<E<lt>*.cE<gt>>
1747 operator, but you can use it directly. If EXPR is omitted, C<$_> is used.
1748 The C<E<lt>*.cE<gt>> operator is discussed in more detail in
1749 L<perlop/"I/O Operators">.
1753 Converts a time as returned by the time function to a 9-element array
1754 with the time localized for the standard Greenwich time zone.
1755 Typically used as follows:
1758 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
1761 All array elements are numeric, and come straight out of a struct tm.
1762 In particular this means that C<$mon> has the range C<0..11> and C<$wday> has
1763 the range C<0..6> with sunday as day C<0>. Also, C<$year> is the number of
1764 years since 1900, that is, C<$year> is C<123> in year 2023, I<not> simply the last two digits of the year.
1766 If EXPR is omitted, does C<gmtime(time())>.
1768 In scalar context, returns the ctime(3) value:
1770 $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
1772 Also see the C<timegm()> function provided by the C<Time::Local> module,
1773 and the strftime(3) function available via the POSIX module.
1775 This scalar value is B<not> locale dependent, see L<perllocale>, but
1776 instead a Perl builtin. Also see the C<Time::Local> module, and the
1777 strftime(3) and mktime(3) function available via the POSIX module. To
1778 get somewhat similar but locale dependent date strings, set up your
1779 locale environment variables appropriately (please see L<perllocale>)
1780 and try for example:
1782 use POSIX qw(strftime);
1783 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
1785 Note that the C<%a> and C<%b>, the short forms of the day of the week
1786 and the month of the year, may not necessarily be three characters wide.
1794 The C<goto-LABEL> form finds the statement labeled with LABEL and resumes
1795 execution there. It may not be used to go into any construct that
1796 requires initialization, such as a subroutine or a C<foreach> loop. It
1797 also can't be used to go into a construct that is optimized away,
1798 or to get out of a block or subroutine given to C<sort()>.
1799 It can be used to go almost anywhere else within the dynamic scope,
1800 including out of subroutines, but it's usually better to use some other
1801 construct such as C<last> or C<die()>. The author of Perl has never felt the
1802 need to use this form of C<goto> (in Perl, that is--C is another matter).
1804 The C<goto-EXPR> form expects a label name, whose scope will be resolved
1805 dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
1806 necessarily recommended if you're optimizing for maintainability:
1808 goto ("FOO", "BAR", "GLARCH")[$i];
1810 The C<goto-&NAME> form is highly magical, and substitutes a call to the
1811 named subroutine for the currently running subroutine. This is used by
1812 C<AUTOLOAD> subroutines that wish to load another subroutine and then
1813 pretend that the other subroutine had been called in the first place
1814 (except that any modifications to C<@_> in the current subroutine are
1815 propagated to the other subroutine.) After the C<goto>, not even C<caller()>
1816 will be able to tell that this routine was called first.
1818 =item grep BLOCK LIST
1820 =item grep EXPR,LIST
1822 This is similar in spirit to, but not the same as, grep(1)
1823 and its relatives. In particular, it is not limited to using
1824 regular expressions.
1826 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
1827 C<$_> to each element) and returns the list value consisting of those
1828 elements for which the expression evaluated to TRUE. In a scalar
1829 context, returns the number of times the expression was TRUE.
1831 @foo = grep(!/^#/, @bar); # weed out comments
1835 @foo = grep {!/^#/} @bar; # weed out comments
1837 Note that, because C<$_> is a reference into the list value, it can be used
1838 to modify the elements of the array. While this is useful and
1839 supported, it can cause bizarre results if the LIST is not a named
1840 array. Similarly, grep returns aliases into the original list,
1841 much like the way that a for loop's index variable aliases the list
1842 elements. That is, modifying an element of a list returned by grep
1843 (for example, in a C<foreach>, C<map()> or another C<grep()>)
1844 actually modifies the element in the original list.
1846 See also L</map> for an array composed of the results of the BLOCK or EXPR.
1852 Interprets EXPR as a hex string and returns the corresponding
1853 value. (To convert strings that might start with either 0 or 0x
1854 see L</oct>.) If EXPR is omitted, uses C<$_>.
1856 print hex '0xAf'; # prints '175'
1857 print hex 'aF'; # same
1861 There is no builtin C<import()> function. It is just an ordinary
1862 method (subroutine) defined (or inherited) by modules that wish to export
1863 names to another module. The C<use()> function calls the C<import()> method
1864 for the package used. See also L</use()>, L<perlmod>, and L<Exporter>.
1866 =item index STR,SUBSTR,POSITION
1868 =item index STR,SUBSTR
1870 Returns the position of the first occurrence of SUBSTR in STR at or after
1871 POSITION. If POSITION is omitted, starts searching from the beginning of
1872 the string. The return value is based at C<0> (or whatever you've set the C<$[>
1873 variable to--but don't do that). If the substring is not found, returns
1874 one less than the base, ordinarily C<-1>.
1880 Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
1881 You should not use this for rounding, because it truncates
1882 towards C<0>, and because machine representations of floating point
1883 numbers can sometimes produce counterintuitive results. Usually C<sprintf()> or C<printf()>,
1884 or the C<POSIX::floor> or C<POSIX::ceil> functions, would serve you better.
1886 =item ioctl FILEHANDLE,FUNCTION,SCALAR
1888 Implements the ioctl(2) function. You'll probably have to say
1890 require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph
1892 first to get the correct function definitions. If F<ioctl.ph> doesn't
1893 exist or doesn't have the correct definitions you'll have to roll your
1894 own, based on your C header files such as F<E<lt>sys/ioctl.hE<gt>>.
1895 (There is a Perl script called B<h2ph> that comes with the Perl kit that
1896 may help you in this, but it's nontrivial.) SCALAR will be read and/or
1897 written depending on the FUNCTION--a pointer to the string value of SCALAR
1898 will be passed as the third argument of the actual C<ioctl()> call. (If SCALAR
1899 has no string value but does have a numeric value, that value will be
1900 passed rather than a pointer to the string value. To guarantee this to be
1901 TRUE, add a C<0> to the scalar before using it.) The C<pack()> and C<unpack()>
1902 functions are useful for manipulating the values of structures used by
1903 C<ioctl()>. The following example sets the erase character to DEL.
1907 die "NO TIOCGETP" if $@ || !$getp;
1908 $sgttyb_t = "ccccs"; # 4 chars and a short
1909 if (ioctl(STDIN,$getp,$sgttyb)) {
1910 @ary = unpack($sgttyb_t,$sgttyb);
1912 $sgttyb = pack($sgttyb_t,@ary);
1913 ioctl(STDIN,&TIOCSETP,$sgttyb)
1914 || die "Can't ioctl: $!";
1917 The return value of C<ioctl()> (and C<fcntl()>) is as follows:
1919 if OS returns: then Perl returns:
1921 0 string "0 but true"
1922 anything else that number
1924 Thus Perl returns TRUE on success and FALSE on failure, yet you can
1925 still easily determine the actual value returned by the operating
1928 ($retval = ioctl(...)) || ($retval = -1);
1929 printf "System returned %d\n", $retval;
1931 The special string "C<0> but true" is exempt from B<-w> complaints
1932 about improper numeric conversions.
1934 =item join EXPR,LIST
1936 Joins the separate strings of LIST into a single string with
1937 fields separated by the value of EXPR, and returns the string.
1940 $_ = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
1946 Returns a list consisting of all the keys of the named hash. (In a
1947 scalar context, returns the number of keys.) The keys are returned in
1948 an apparently random order. The actual random order is subject to
1949 change in future versions of perl, but it is guaranteed to be the same
1950 order as either the C<values()> or C<each()> function produces (given
1951 that the hash has not been modified). As a side effect, it resets
1954 Here is yet another way to print your environment:
1957 @values = values %ENV;
1958 while ($#keys >= 0) {
1959 print pop(@keys), '=', pop(@values), "\n";
1962 or how about sorted by key:
1964 foreach $key (sort(keys %ENV)) {
1965 print $key, '=', $ENV{$key}, "\n";
1968 To sort a hash by value, you'll need to use a C<sort()> function.
1969 Here's a descending numeric sort of a hash by its values:
1971 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
1972 printf "%4d %s\n", $hash{$key}, $key;
1975 As an lvalue C<keys()> allows you to increase the number of hash buckets
1976 allocated for the given hash. This can gain you a measure of efficiency if
1977 you know the hash is going to get big. (This is similar to pre-extending
1978 an array by assigning a larger number to $#array.) If you say
1982 then C<%hash> will have at least 200 buckets allocated for it--256 of them,
1983 in fact, since it rounds up to the next power of two. These
1984 buckets will be retained even if you do C<%hash = ()>, use C<undef
1985 %hash> if you want to free the storage while C<%hash> is still in scope.
1986 You can't shrink the number of buckets allocated for the hash using
1987 C<keys()> in this way (but you needn't worry about doing this by accident,
1988 as trying has no effect).
1990 See also C<each()>, C<values()> and C<sort()>.
1994 Sends a signal to a list of processes. The first element of
1995 the list must be the signal to send. Returns the number of
1996 processes successfully signaled.
1998 $cnt = kill 1, $child1, $child2;
2001 Unlike in the shell, in Perl if the I<SIGNAL> is negative, it kills
2002 process groups instead of processes. (On System V, a negative I<PROCESS>
2003 number will also kill process groups, but that's not portable.) That
2004 means you usually want to use positive not negative signals. You may also
2005 use a signal name in quotes. See L<perlipc/"Signals"> for details.
2011 The C<last> command is like the C<break> statement in C (as used in
2012 loops); it immediately exits the loop in question. If the LABEL is
2013 omitted, the command refers to the innermost enclosing loop. The
2014 C<continue> block, if any, is not executed:
2016 LINE: while (<STDIN>) {
2017 last LINE if /^$/; # exit when done with header
2021 C<last> cannot be used to exit a block which returns a value such as
2022 C<eval {}>, C<sub {}> or C<do {}>.
2024 See also L</continue> for an illustration of how C<last>, C<next>, and
2031 Returns an lowercased version of EXPR. This is the internal function
2032 implementing the C<\L> escape in double-quoted strings.
2033 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
2035 If EXPR is omitted, uses C<$_>.
2041 Returns the value of EXPR with the first character lowercased. This is
2042 the internal function implementing the C<\l> escape in double-quoted strings.
2043 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
2045 If EXPR is omitted, uses C<$_>.
2051 Returns the length in characters of the value of EXPR. If EXPR is
2052 omitted, returns length of C<$_>.
2054 =item link OLDFILE,NEWFILE
2056 Creates a new filename linked to the old filename. Returns TRUE for
2057 success, FALSE otherwise.
2059 =item listen SOCKET,QUEUESIZE
2061 Does the same thing that the listen system call does. Returns TRUE if
2062 it succeeded, FALSE otherwise. See example in L<perlipc/"Sockets: Client/Server Communication">.
2066 A local modifies the listed variables to be local to the enclosing
2067 block, file, or eval. If more than one value is listed, the list must
2068 be placed in parentheses. See L<perlsub/"Temporary Values via local()">
2069 for details, including issues with tied arrays and hashes.
2071 You really probably want to be using C<my()> instead, because C<local()> isn't
2072 what most people think of as "local". See L<perlsub/"Private Variables
2073 via my()"> for details.
2075 =item localtime EXPR
2077 Converts a time as returned by the time function to a 9-element array
2078 with the time analyzed for the local time zone. Typically used as
2082 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
2085 All array elements are numeric, and come straight out of a struct tm.
2086 In particular this means that C<$mon> has the range C<0..11> and C<$wday> has
2087 the range C<0..6> with sunday as day C<0>. Also, C<$year> is the number of
2088 years since 1900, that is, C<$year> is C<123> in year 2023, and I<not> simply the last two digits of the year.
2090 If EXPR is omitted, uses the current time (C<localtime(time)>).
2092 In scalar context, returns the ctime(3) value:
2094 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
2096 This scalar value is B<not> locale dependent, see L<perllocale>, but
2097 instead a Perl builtin. Also see the C<Time::Local> module, and the
2098 strftime(3) and mktime(3) function available via the POSIX module. To
2099 get somewhat similar but locale dependent date strings, set up your
2100 locale environment variables appropriately (please see L<perllocale>)
2101 and try for example:
2103 use POSIX qw(strftime);
2104 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
2106 Note that the C<%a> and C<%b>, the short forms of the day of the week
2107 and the month of the year, may not necessarily be three characters wide.
2113 Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted, returns log
2116 =item lstat FILEHANDLE
2122 Does the same thing as the C<stat()> function (including setting the
2123 special C<_> filehandle) but stats a symbolic link instead of the file
2124 the symbolic link points to. If symbolic links are unimplemented on
2125 your system, a normal C<stat()> is done.
2127 If EXPR is omitted, stats C<$_>.
2131 The match operator. See L<perlop>.
2133 =item map BLOCK LIST
2137 Evaluates the BLOCK or EXPR for each element of LIST (locally setting C<$_> to each
2138 element) and returns the list value composed of the results of each such
2139 evaluation. Evaluates BLOCK or EXPR in a list context, so each element of LIST
2140 may produce zero, one, or more elements in the returned value.
2142 @chars = map(chr, @nums);
2144 translates a list of numbers to the corresponding characters. And
2146 %hash = map { getkey($_) => $_ } @array;
2148 is just a funny way to write
2151 foreach $_ (@array) {
2152 $hash{getkey($_)} = $_;
2155 Note that, because C<$_> is a reference into the list value, it can be used
2156 to modify the elements of the array. While this is useful and
2157 supported, it can cause bizarre results if the LIST is not a named
2158 array. See also L</grep> for an array composed of those items of the
2159 original list for which the BLOCK or EXPR evaluates to true.
2161 =item mkdir FILENAME,MODE
2163 Creates the directory specified by FILENAME, with permissions
2164 specified by MODE (as modified by C<umask>). If it succeeds it
2165 returns TRUE, otherwise it returns FALSE and sets C<$!> (errno).
2167 In general, it is better to create directories with permissive MODEs,
2168 and let the user modify that with their C<umask>, than it is to supply
2169 a restrictive MODE and give the user no way to be more permissive.
2170 The exceptions to this rule are when the file or directory should be
2171 kept private (mail files, for instance). The perlfunc(1) entry on
2172 C<umask> discusses the choice of MODE in more detail.
2174 =item msgctl ID,CMD,ARG
2176 Calls the System V IPC function msgctl(2). You'll probably have to say
2180 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
2181 then ARG must be a variable which will hold the returned C<msqid_ds>
2182 structure. Returns like C<ioctl()>: the undefined value for error, "C<0> but
2183 true" for zero, or the actual return value otherwise. See also
2184 C<IPC::SysV> and C<IPC::Semaphore::Msg> documentation.
2186 =item msgget KEY,FLAGS
2188 Calls the System V IPC function msgget(2). Returns the message queue
2189 id, or the undefined value if there is an error. See also C<IPC::SysV>
2190 and C<IPC::SysV::Msg> documentation.
2192 =item msgsnd ID,MSG,FLAGS
2194 Calls the System V IPC function msgsnd to send the message MSG to the
2195 message queue ID. MSG must begin with the long integer message type,
2196 which may be created with C<pack("l", $type)>. Returns TRUE if
2197 successful, or FALSE if there is an error. See also C<IPC::SysV>
2198 and C<IPC::SysV::Msg> documentation.
2200 =item msgrcv ID,VAR,SIZE,TYPE,FLAGS
2202 Calls the System V IPC function msgrcv to receive a message from
2203 message queue ID into variable VAR with a maximum message size of
2204 SIZE. Note that if a message is received, the message type will be
2205 the first thing in VAR, and the maximum length of VAR is SIZE plus the
2206 size of the message type. Returns TRUE if successful, or FALSE if
2207 there is an error. See also C<IPC::SysV> and C<IPC::SysV::Msg> documentation.
2211 A C<my()> declares the listed variables to be local (lexically) to the
2212 enclosing block, file, or C<eval()>. If
2213 more than one value is listed, the list must be placed in parentheses. See
2214 L<perlsub/"Private Variables via my()"> for details.
2220 The C<next> command is like the C<continue> statement in C; it starts
2221 the next iteration of the loop:
2223 LINE: while (<STDIN>) {
2224 next LINE if /^#/; # discard comments
2228 Note that if there were a C<continue> block on the above, it would get
2229 executed even on discarded lines. If the LABEL is omitted, the command
2230 refers to the innermost enclosing loop.
2232 C<next> cannot be used to exit a block which returns a value such as
2233 C<eval {}>, C<sub {}> or C<do {}>.
2235 See also L</continue> for an illustration of how C<last>, C<next>, and
2238 =item no Module LIST
2240 See the L</use> function, which C<no> is the opposite of.
2246 Interprets EXPR as an octal string and returns the corresponding
2247 value. (If EXPR happens to start off with C<0x>, interprets it as a
2248 hex string. If EXPR starts off with C<0b>, it is interpreted as a
2249 binary string.) The following will handle decimal, binary, octal, and
2250 hex in the standard Perl or C notation:
2252 $val = oct($val) if $val =~ /^0/;
2254 If EXPR is omitted, uses C<$_>. This function is commonly used when
2255 a string such as C<644> needs to be converted into a file mode, for
2256 example. (Although perl will automatically convert strings into
2257 numbers as needed, this automatic conversion assumes base 10.)
2259 =item open FILEHANDLE,EXPR
2261 =item open FILEHANDLE
2263 Opens the file whose filename is given by EXPR, and associates it with
2264 FILEHANDLE. If FILEHANDLE is an expression, its value is used as the
2265 name of the real filehandle wanted. If EXPR is omitted, the scalar
2266 variable of the same name as the FILEHANDLE contains the filename.
2267 (Note that lexical variables--those declared with C<my()>--will not work
2268 for this purpose; so if you're using C<my()>, specify EXPR in your call
2271 If the filename begins with C<'E<lt>'> or nothing, the file is opened for input.
2272 If the filename begins with C<'E<gt>'>, the file is truncated and opened for
2273 output, being created if necessary. If the filename begins with C<'E<gt>E<gt>'>,
2274 the file is opened for appending, again being created if necessary.
2275 You can put a C<'+'> in front of the C<'E<gt>'> or C<'E<lt>'> to indicate that
2276 you want both read and write access to the file; thus C<'+E<lt>'> is almost
2277 always preferred for read/write updates--the C<'+E<gt>'> mode would clobber the
2278 file first. You can't usually use either read-write mode for updating
2279 textfiles, since they have variable length records. See the B<-i>
2280 switch in L<perlrun> for a better approach. The file is created with
2281 permissions of C<0666> modified by the process' C<umask> value.
2283 The prefix and the filename may be separated with spaces.
2284 These various prefixes correspond to the fopen(3) modes of C<'r'>, C<'r+'>, C<'w'>,
2285 C<'w+'>, C<'a'>, and C<'a+'>.
2287 If the filename begins with C<'|'>, the filename is interpreted as a
2288 command to which output is to be piped, and if the filename ends with a
2289 C<'|'>, the filename is interpreted as a command which pipes output to
2290 us. See L<perlipc/"Using open() for IPC">
2291 for more examples of this. (You are not allowed to C<open()> to a command
2292 that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
2293 and L<perlipc/"Bidirectional Communication"> for alternatives.)
2295 Opening C<'-'> opens STDIN and opening C<'E<gt>-'> opens STDOUT. Open returns
2296 nonzero upon success, the undefined value otherwise. If the C<open()>
2297 involved a pipe, the return value happens to be the pid of the
2300 If you're unfortunate enough to be running Perl on a system that
2301 distinguishes between text files and binary files (modern operating
2302 systems don't care), then you should check out L</binmode> for tips for
2303 dealing with this. The key distinction between systems that need C<binmode()>
2304 and those that don't is their text file formats. Systems like Unix, MacOS, and
2305 Plan9, which delimit lines with a single character, and which encode that
2306 character in C as C<"\n">, do not need C<binmode()>. The rest need it.
2308 When opening a file, it's usually a bad idea to continue normal execution
2309 if the request failed, so C<open()> is frequently used in connection with
2310 C<die()>. Even if C<die()> won't do what you want (say, in a CGI script,
2311 where you want to make a nicely formatted error message (but there are
2312 modules that can help with that problem)) you should always check
2313 the return value from opening a file. The infrequent exception is when
2314 working with an unopened filehandle is actually what you want to do.
2319 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
2320 while (<ARTICLE>) {...
2322 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
2323 # if the open fails, output is discarded
2325 open(DBASE, '+<dbase.mine') # open for update
2326 or die "Can't open 'dbase.mine' for update: $!";
2328 open(ARTICLE, "caesar <$article |") # decrypt article
2329 or die "Can't start caesar: $!";
2331 open(EXTRACT, "|sort >/tmp/Tmp$$") # $$ is our process id
2332 or die "Can't start sort: $!";
2334 # process argument list of files along with any includes
2336 foreach $file (@ARGV) {
2337 process($file, 'fh00');
2341 my($filename, $input) = @_;
2342 $input++; # this is a string increment
2343 unless (open($input, $filename)) {
2344 print STDERR "Can't open $filename: $!\n";
2349 while (<$input>) { # note use of indirection
2350 if (/^#include "(.*)"/) {
2351 process($1, $input);
2358 You may also, in the Bourne shell tradition, specify an EXPR beginning
2359 with C<'E<gt>&'>, in which case the rest of the string is interpreted as the
2360 name of a filehandle (or file descriptor, if numeric) to be
2361 duped and opened. You may use C<&> after C<E<gt>>, C<E<gt>E<gt>>, C<E<lt>>, C<+E<gt>>,
2362 C<+E<gt>E<gt>>, and C<+E<lt>>. The
2363 mode you specify should match the mode of the original filehandle.
2364 (Duping a filehandle does not take into account any existing contents of
2366 Here is a script that saves, redirects, and restores STDOUT and
2370 open(OLDOUT, ">&STDOUT");
2371 open(OLDERR, ">&STDERR");
2373 open(STDOUT, ">foo.out") || die "Can't redirect stdout";
2374 open(STDERR, ">&STDOUT") || die "Can't dup stdout";
2376 select(STDERR); $| = 1; # make unbuffered
2377 select(STDOUT); $| = 1; # make unbuffered
2379 print STDOUT "stdout 1\n"; # this works for
2380 print STDERR "stderr 1\n"; # subprocesses too
2385 open(STDOUT, ">&OLDOUT");
2386 open(STDERR, ">&OLDERR");
2388 print STDOUT "stdout 2\n";
2389 print STDERR "stderr 2\n";
2392 If you specify C<'E<lt>&=N'>, where C<N> is a number, then Perl will do an
2393 equivalent of C's C<fdopen()> of that file descriptor; this is more
2394 parsimonious of file descriptors. For example:
2396 open(FILEHANDLE, "<&=$fd")
2398 If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'>, then
2399 there is an implicit fork done, and the return value of open is the pid
2400 of the child within the parent process, and C<0> within the child
2401 process. (Use C<defined($pid)> to determine whether the open was successful.)
2402 The filehandle behaves normally for the parent, but i/o to that
2403 filehandle is piped from/to the STDOUT/STDIN of the child process.
2404 In the child process the filehandle isn't opened--i/o happens from/to
2405 the new STDOUT or STDIN. Typically this is used like the normal
2406 piped open when you want to exercise more control over just how the
2407 pipe command gets executed, such as when you are running setuid, and
2408 don't want to have to scan shell commands for metacharacters.
2409 The following pairs are more or less equivalent:
2411 open(FOO, "|tr '[a-z]' '[A-Z]'");
2412 open(FOO, "|-") || exec 'tr', '[a-z]', '[A-Z]';
2414 open(FOO, "cat -n '$file'|");
2415 open(FOO, "-|") || exec 'cat', '-n', $file;
2417 See L<perlipc/"Safe Pipe Opens"> for more examples of this.
2419 NOTE: On any operation that may do a fork, any unflushed buffers remain
2420 unflushed in both processes, which means you may need to set C<$|> to
2421 avoid duplicate output. On systems that support a close-on-exec flag on
2422 files, the flag will be set for the newly opened file descriptor as
2423 determined by the value of $^F. See L<perlvar/$^F>.
2425 Closing any piped filehandle causes the parent process to wait for the
2426 child to finish, and returns the status value in C<$?>.
2428 The filename passed to open will have leading and trailing
2429 whitespace deleted, and the normal redirection characters
2430 honored. This property, known as "magic open",
2431 can often be used to good effect. A user could specify a filename of
2432 F<"rsh cat file |">, or you could change certain filenames as needed:
2434 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
2435 open(FH, $filename) or die "Can't open $filename: $!";
2437 However, to open a file with arbitrary weird characters in it, it's
2438 necessary to protect any leading and trailing whitespace:
2440 $file =~ s#^(\s)#./$1#;
2441 open(FOO, "< $file\0");
2443 If you want a "real" C C<open()> (see L<open(2)> on your system), then you
2444 should use the C<sysopen()> function, which involves no such magic. This is
2445 another way to protect your filenames from interpretation. For example:
2448 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
2449 or die "sysopen $path: $!";
2450 $oldfh = select(HANDLE); $| = 1; select($oldfh);
2451 print HANDLE "stuff $$\n");
2453 print "File contains: ", <HANDLE>;
2455 Using the constructor from the C<IO::Handle> package (or one of its
2456 subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous
2457 filehandles that have the scope of whatever variables hold references to
2458 them, and automatically close whenever and however you leave that scope:
2462 sub read_myfile_munged {
2464 my $handle = new IO::File;
2465 open($handle, "myfile") or die "myfile: $!";
2467 or return (); # Automatically closed here.
2468 mung $first or die "mung failed"; # Or here.
2469 return $first, <$handle> if $ALL; # Or here.
2473 See L</seek> for some details about mixing reading and writing.
2475 =item opendir DIRHANDLE,EXPR
2477 Opens a directory named EXPR for processing by C<readdir()>, C<telldir()>,
2478 C<seekdir()>, C<rewinddir()>, and C<closedir()>. Returns TRUE if successful.
2479 DIRHANDLEs have their own namespace separate from FILEHANDLEs.
2485 Returns the numeric (ASCII or Unicode) value of the first character of EXPR. If
2486 EXPR is omitted, uses C<$_>. For the reverse, see L</chr>.
2488 =item pack TEMPLATE,LIST
2490 Takes an array or list of values and packs it into a binary structure,
2491 returning the string containing the structure. The TEMPLATE is a
2492 sequence of characters that give the order and type of values, as
2495 a A string with arbitrary binary data, will be null padded.
2496 A An ascii string, will be space padded.
2497 Z A null terminated (asciz) string, will be null padded.
2499 b A bit string (ascending bit order, like vec()).
2500 B A bit string (descending bit order).
2501 h A hex string (low nybble first).
2502 H A hex string (high nybble first).
2504 c A signed char value.
2505 C An unsigned char value. Only does bytes. See U for Unicode.
2507 s A signed short value.
2508 S An unsigned short value.
2509 (This 'short' is _exactly_ 16 bits, which may differ from
2510 what a local C compiler calls 'short'.)
2512 i A signed integer value.
2513 I An unsigned integer value.
2514 (This 'integer' is _at_least_ 32 bits wide. Its exact
2515 size depends on what a local C compiler calls 'int',
2516 and may even be larger than the 'long' described in
2519 l A signed long value.
2520 L An unsigned long value.
2521 (This 'long' is _exactly_ 32 bits, which may differ from
2522 what a local C compiler calls 'long'.)
2524 n A short in "network" (big-endian) order.
2525 N A long in "network" (big-endian) order.
2526 v A short in "VAX" (little-endian) order.
2527 V A long in "VAX" (little-endian) order.
2528 (These 'shorts' and 'longs' are _exactly_ 16 bits and
2529 _exactly_ 32 bits, respectively.)
2531 q A signed quad (64-bit) value.
2532 Q An unsigned quad value.
2533 (Available only if your system supports 64-bit integer values
2534 _and_ if Perl has been compiled to support those.
2535 Causes a fatal error otherwise.)
2537 f A single-precision float in the native format.
2538 d A double-precision float in the native format.
2540 p A pointer to a null-terminated string.
2541 P A pointer to a structure (fixed-length string).
2543 u A uuencoded string.
2544 U A Unicode character number. Encodes to UTF-8 internally.
2545 Works even if C<use utf8> is not in effect.
2547 w A BER compressed integer. Its bytes represent an unsigned
2548 integer in base 128, most significant digit first, with as
2549 few digits as possible. Bit eight (the high bit) is set
2550 on each byte except the last.
2554 @ Null fill to absolute position.
2556 The following rules apply:
2562 Each letter may optionally be followed by a number giving a repeat
2563 count. With all types except C<"a">, C<"A">, C<"Z">, C<"b">, C<"B">, C<"h">,
2564 C<"H">, and C<"P"> the pack function will gobble up that many values from
2565 the LIST. A C<*> for the repeat count means to use however many items are
2570 The C<"a">, C<"A"> and C<"Z"> types gobble just one value, but pack it as a
2571 string of length count, padding with nulls or spaces as necessary. When
2572 unpacking, C<"A"> strips trailing spaces and nulls, C<"Z"> strips everything
2573 after the first null, and C<"a"> returns data verbatim.
2577 Likewise, the C<"b"> and C<"B"> fields pack a string that many bits long.
2581 The C<"h"> and C<"H"> fields pack a string that many nybbles long.
2585 The C<"p"> type packs a pointer to a null-terminated string. You are
2586 responsible for ensuring the string is not a temporary value (which can
2587 potentially get deallocated before you get around to using the packed result).
2588 The C<"P"> type packs a pointer to a structure of the size indicated by the
2589 length. A NULL pointer is created if the corresponding value for C<"p"> or
2594 Real numbers (floats and doubles) are in the native machine format only;
2595 due to the multiplicity of floating formats around, and the lack of a
2596 standard "network" representation, no facility for interchange has been
2597 made. This means that packed floating point data written on one machine
2598 may not be readable on another - even if both use IEEE floating point
2599 arithmetic (as the endian-ness of the memory representation is not part
2602 Note that Perl uses doubles internally for all numeric calculation, and
2603 converting from double into float and thence back to double again will
2604 lose precision (i.e., C<unpack("f", pack("f", $foo)>) will not in general
2611 $foo = pack("CCCC",65,66,67,68);
2613 $foo = pack("C4",65,66,67,68);
2615 $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);
2616 # same thing with Unicode circled letters
2618 $foo = pack("ccxxcc",65,66,67,68);
2621 $foo = pack("s2",1,2);
2622 # "\1\0\2\0" on little-endian
2623 # "\0\1\0\2" on big-endian
2625 $foo = pack("a4","abcd","x","y","z");
2628 $foo = pack("aaaa","abcd","x","y","z");
2631 $foo = pack("a14","abcdefg");
2632 # "abcdefg\0\0\0\0\0\0\0"
2634 $foo = pack("i9pl", gmtime);
2635 # a real struct tm (on my system anyway)
2637 $utmp_template = "Z8 Z8 Z16 L";
2638 $utmp = pack($utmp_template, @utmp1);
2639 # a struct utmp (BSDish)
2641 @utmp2 = unpack($utmp_template, $utmp);
2642 # "@utmp1" eq "@utmp2"
2645 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
2648 The same template may generally also be used in unpack().
2652 =item package NAMESPACE
2654 Declares the compilation unit as being in the given namespace. The scope
2655 of the package declaration is from the declaration itself through the end of
2656 the enclosing block (the same scope as the C<local()> operator). All further
2657 unqualified dynamic identifiers will be in this namespace. A package
2658 statement affects only dynamic variables--including those you've used
2659 C<local()> on--but I<not> lexical variables created with C<my()>. Typically it
2660 would be the first declaration in a file to be included by the C<require>
2661 or C<use> operator. You can switch into a package in more than one place;
2662 it merely influences which symbol table is used by the compiler for the
2663 rest of that block. You can refer to variables and filehandles in other
2664 packages by prefixing the identifier with the package name and a double
2665 colon: C<$Package::Variable>. If the package name is null, the C<main>
2666 package as assumed. That is, C<$::sail> is equivalent to C<$main::sail>.
2668 If NAMESPACE is omitted, then there is no current package, and all
2669 identifiers must be fully qualified or lexicals. This is stricter
2670 than C<use strict>, since it also extends to function names.
2672 See L<perlmod/"Packages"> for more information about packages, modules,
2673 and classes. See L<perlsub> for other scoping issues.
2675 =item pipe READHANDLE,WRITEHANDLE
2677 Opens a pair of connected pipes like the corresponding system call.
2678 Note that if you set up a loop of piped processes, deadlock can occur
2679 unless you are very careful. In addition, note that Perl's pipes use
2680 stdio buffering, so you may need to set C<$|> to flush your WRITEHANDLE
2681 after each command, depending on the application.
2683 See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
2684 for examples of such things.
2686 On systems that support a close-on-exec flag on files, the flag will be set
2687 for the newly opened file descriptors as determined by the value of $^F.
2694 Pops and returns the last value of the array, shortening the array by
2695 1. Has a similar effect to
2697 $tmp = $ARRAY[$#ARRAY--];
2699 If there are no elements in the array, returns the undefined value.
2700 If ARRAY is omitted, pops the
2701 C<@ARGV> array in the main program, and the C<@_> array in subroutines, just
2708 Returns the offset of where the last C<m//g> search left off for the variable
2709 is in question (C<$_> is used when the variable is not specified). May be
2710 modified to change that offset. Such modification will also influence
2711 the C<\G> zero-width assertion in regular expressions. See L<perlre> and
2714 =item print FILEHANDLE LIST
2720 Prints a string or a comma-separated list of strings. Returns TRUE
2721 if successful. FILEHANDLE may be a scalar variable name, in which case
2722 the variable contains the name of or a reference to the filehandle, thus introducing one
2723 level of indirection. (NOTE: If FILEHANDLE is a variable and the next
2724 token is a term, it may be misinterpreted as an operator unless you
2725 interpose a C<+> or put parentheses around the arguments.) If FILEHANDLE is
2726 omitted, prints by default to standard output (or to the last selected
2727 output channel--see L</select>). If LIST is also omitted, prints C<$_> to
2728 the currently selected output channel. To set the default output channel to something other than
2729 STDOUT use the select operation. Note that, because print takes a
2730 LIST, anything in the LIST is evaluated in list context, and any
2731 subroutine that you call will have one or more of its expressions
2732 evaluated in list context. Also be careful not to follow the print
2733 keyword with a left parenthesis unless you want the corresponding right
2734 parenthesis to terminate the arguments to the print--interpose a C<+> or
2735 put parentheses around all the arguments.
2737 Note that if you're storing FILEHANDLES in an array or other expression,
2738 you will have to use a block returning its value instead:
2740 print { $files[$i] } "stuff\n";
2741 print { $OK ? STDOUT : STDERR } "stuff\n";
2743 =item printf FILEHANDLE FORMAT, LIST
2745 =item printf FORMAT, LIST
2747 Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that C<$\>
2748 (the output record separator) is not appended. The first argument
2749 of the list will be interpreted as the C<printf()> format. If C<use locale> is
2750 in effect, the character used for the decimal point in formatted real numbers
2751 is affected by the LC_NUMERIC locale. See L<perllocale>.
2753 Don't fall into the trap of using a C<printf()> when a simple
2754 C<print()> would do. The C<print()> is more efficient and less
2757 =item prototype FUNCTION
2759 Returns the prototype of a function as a string (or C<undef> if the
2760 function has no prototype). FUNCTION is a reference to, or the name of,
2761 the function whose prototype you want to retrieve.
2763 If FUNCTION is a string starting with C<CORE::>, the rest is taken as
2764 a name for Perl builtin. If builtin is not I<overridable> (such as
2765 C<qw//>) or its arguments cannot be expressed by a prototype (such as
2766 C<system()>) - in other words, the builtin does not behave like a Perl
2767 function - returns C<undef>. Otherwise, the string describing the
2768 equivalent prototype is returned.
2770 =item push ARRAY,LIST
2772 Treats ARRAY as a stack, and pushes the values of LIST
2773 onto the end of ARRAY. The length of ARRAY increases by the length of
2774 LIST. Has the same effect as
2777 $ARRAY[++$#ARRAY] = $value;
2780 but is more efficient. Returns the new number of elements in the array.
2792 Generalized quotes. See L<perlop/"Regexp Quote-Like Operators">.
2794 =item quotemeta EXPR
2798 Returns the value of EXPR with all non-alphanumeric
2799 characters backslashed. (That is, all characters not matching
2800 C</[A-Za-z_0-9]/> will be preceded by a backslash in the
2801 returned string, regardless of any locale settings.)
2802 This is the internal function implementing
2803 the C<\Q> escape in double-quoted strings.
2805 If EXPR is omitted, uses C<$_>.
2811 Returns a random fractional number greater than or equal to C<0> and less
2812 than the value of EXPR. (EXPR should be positive.) If EXPR is
2813 omitted, the value C<1> is used. Automatically calls C<srand()> unless
2814 C<srand()> has already been called. See also C<srand()>.
2816 (Note: If your rand function consistently returns numbers that are too
2817 large or too small, then your version of Perl was probably compiled
2818 with the wrong number of RANDBITS.)
2820 =item read FILEHANDLE,SCALAR,LENGTH,OFFSET
2822 =item read FILEHANDLE,SCALAR,LENGTH
2824 Attempts to read LENGTH bytes of data into variable SCALAR from the
2825 specified FILEHANDLE. Returns the number of bytes actually read,
2826 C<0> at end of file, or undef if there was an error. SCALAR will be grown
2827 or shrunk to the length actually read. An OFFSET may be specified to
2828 place the read data at some other place than the beginning of the
2829 string. This call is actually implemented in terms of stdio's fread(3)
2830 call. To get a true read(2) system call, see C<sysread()>.
2832 =item readdir DIRHANDLE
2834 Returns the next directory entry for a directory opened by C<opendir()>.
2835 If used in list context, returns all the rest of the entries in the
2836 directory. If there are no more entries, returns an undefined value in
2837 scalar context or a null list in list context.
2839 If you're planning to filetest the return values out of a C<readdir()>, you'd
2840 better prepend the directory in question. Otherwise, because we didn't
2841 C<chdir()> there, it would have been testing the wrong file.
2843 opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!";
2844 @dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR);
2849 Reads from the filehandle whose typeglob is contained in EXPR. In scalar
2850 context, each call reads and returns the next line, until end-of-file is
2851 reached, whereupon the subsequent call returns undef. In list context,
2852 reads until end-of-file is reached and returns a list of lines. Note that
2853 the notion of "line" used here is however you may have defined it
2854 with C<$/> or C<$INPUT_RECORD_SEPARATOR>). See L<perlvar/"$/">.
2856 When C<$/> is set to C<undef> and when readline() is in a scalar
2857 context (i.e. file slurp mode), it returns C<''> the first time,
2858 followed by C<undef> subsequently.
2860 This is the internal function implementing the C<E<lt>EXPRE<gt>>
2861 operator, but you can use it directly. The C<E<lt>EXPRE<gt>>
2862 operator is discussed in more detail in L<perlop/"I/O Operators">.
2865 $line = readline(*STDIN); # same thing
2871 Returns the value of a symbolic link, if symbolic links are
2872 implemented. If not, gives a fatal error. If there is some system
2873 error, returns the undefined value and sets C<$!> (errno). If EXPR is
2874 omitted, uses C<$_>.
2878 EXPR is executed as a system command.
2879 The collected standard output of the command is returned.
2880 In scalar context, it comes back as a single (potentially
2881 multi-line) string. In list context, returns a list of lines
2882 (however you've defined lines with C<$/> or C<$INPUT_RECORD_SEPARATOR>).
2883 This is the internal function implementing the C<qx/EXPR/>
2884 operator, but you can use it directly. The C<qx/EXPR/>
2885 operator is discussed in more detail in L<perlop/"I/O Operators">.
2887 =item recv SOCKET,SCALAR,LEN,FLAGS
2889 Receives a message on a socket. Attempts to receive LENGTH bytes of
2890 data into variable SCALAR from the specified SOCKET filehandle.
2891 Actually does a C C<recvfrom()>, so that it can return the address of the
2892 sender. Returns the undefined value if there's an error. SCALAR will
2893 be grown or shrunk to the length actually read. Takes the same flags
2894 as the system call of the same name.
2895 See L<perlipc/"UDP: Message Passing"> for examples.
2901 The C<redo> command restarts the loop block without evaluating the
2902 conditional again. The C<continue> block, if any, is not executed. If
2903 the LABEL is omitted, the command refers to the innermost enclosing
2904 loop. This command is normally used by programs that want to lie to
2905 themselves about what was just input:
2907 # a simpleminded Pascal comment stripper
2908 # (warning: assumes no { or } in strings)
2909 LINE: while (<STDIN>) {
2910 while (s|({.*}.*){.*}|$1 |) {}
2915 if (/}/) { # end of comment?
2924 C<redo> cannot be used to retry a block which returns a value such as
2925 C<eval {}>, C<sub {}> or C<do {}>.
2927 See also L</continue> for an illustration of how C<last>, C<next>, and
2934 Returns a TRUE value if EXPR is a reference, FALSE otherwise. If EXPR
2935 is not specified, C<$_> will be used. The value returned depends on the
2936 type of thing the reference is a reference to.
2937 Builtin types include:
2946 If the referenced object has been blessed into a package, then that package
2947 name is returned instead. You can think of C<ref()> as a C<typeof()> operator.
2949 if (ref($r) eq "HASH") {
2950 print "r is a reference to a hash.\n";
2953 print "r is not a reference at all.\n";
2956 See also L<perlref>.
2958 =item rename OLDNAME,NEWNAME
2960 Changes the name of a file. Returns C<1> for success, C<0> otherwise. Will
2961 not work across file system boundaries.
2967 Demands some semantics specified by EXPR, or by C<$_> if EXPR is not
2968 supplied. If EXPR is numeric, demands that the current version of Perl
2969 (C<$]> or $PERL_VERSION) be equal or greater than EXPR.
2971 Otherwise, demands that a library file be included if it hasn't already
2972 been included. The file is included via the do-FILE mechanism, which is
2973 essentially just a variety of C<eval()>. Has semantics similar to the following
2978 return 1 if $INC{$filename};
2979 my($realfilename,$result);
2981 foreach $prefix (@INC) {
2982 $realfilename = "$prefix/$filename";
2983 if (-f $realfilename) {
2984 $result = do $realfilename;
2988 die "Can't find $filename in \@INC";
2991 die "$filename did not return true value" unless $result;
2992 $INC{$filename} = $realfilename;
2996 Note that the file will not be included twice under the same specified
2997 name. The file must return TRUE as the last statement to indicate
2998 successful execution of any initialization code, so it's customary to
2999 end such a file with "C<1;>" unless you're sure it'll return TRUE
3000 otherwise. But it's better just to put the "C<1;>", in case you add more
3003 If EXPR is a bareword, the require assumes a "F<.pm>" extension and
3004 replaces "F<::>" with "F</>" in the filename for you,
3005 to make it easy to load standard modules. This form of loading of
3006 modules does not risk altering your namespace.
3008 In other words, if you try this:
3010 require Foo::Bar; # a splendid bareword
3012 The require function will actually look for the "F<Foo/Bar.pm>" file in the
3013 directories specified in the C<@INC> array.
3015 But if you try this:
3017 $class = 'Foo::Bar';
3018 require $class; # $class is not a bareword
3020 require "Foo::Bar"; # not a bareword because of the ""
3022 The require function will look for the "F<Foo::Bar>" file in the @INC array and
3023 will complain about not finding "F<Foo::Bar>" there. In this case you can do:
3025 eval "require $class";
3027 For a yet-more-powerful import facility, see L</use> and L<perlmod>.
3033 Generally used in a C<continue> block at the end of a loop to clear
3034 variables and reset C<??> searches so that they work again. The
3035 expression is interpreted as a list of single characters (hyphens
3036 allowed for ranges). All variables and arrays beginning with one of
3037 those letters are reset to their pristine state. If the expression is
3038 omitted, one-match searches (C<?pattern?>) are reset to match again. Resets
3039 only variables or searches in the current package. Always returns
3042 reset 'X'; # reset all X variables
3043 reset 'a-z'; # reset lower case variables
3044 reset; # just reset ?? searches
3046 Resetting C<"A-Z"> is not recommended because you'll wipe out your
3047 C<@ARGV> and C<@INC> arrays and your C<%ENV> hash. Resets only package variables--lexical variables
3048 are unaffected, but they clean themselves up on scope exit anyway,
3049 so you'll probably want to use them instead. See L</my>.
3055 Returns from a subroutine, C<eval()>, or C<do FILE> with the value
3056 given in EXPR. Evaluation of EXPR may be in list, scalar, or void
3057 context, depending on how the return value will be used, and the context
3058 may vary from one execution to the next (see C<wantarray()>). If no EXPR
3059 is given, returns an empty list in list context, an undefined value in
3060 scalar context, or nothing in a void context.
3062 (Note that in the absence of a return, a subroutine, eval, or do FILE
3063 will automatically return the value of the last expression evaluated.)
3067 In list context, returns a list value consisting of the elements
3068 of LIST in the opposite order. In scalar context, concatenates the
3069 elements of LIST, and returns a string value with all the characters
3070 in the opposite order.
3072 print reverse <>; # line tac, last line first
3074 undef $/; # for efficiency of <>
3075 print scalar reverse <>; # character tac, last line tsrif
3077 This operator is also handy for inverting a hash, although there are some
3078 caveats. If a value is duplicated in the original hash, only one of those
3079 can be represented as a key in the inverted hash. Also, this has to
3080 unwind one hash and build a whole new one, which may take some time
3083 %by_name = reverse %by_address; # Invert the hash
3085 =item rewinddir DIRHANDLE
3087 Sets the current position to the beginning of the directory for the
3088 C<readdir()> routine on DIRHANDLE.
3090 =item rindex STR,SUBSTR,POSITION
3092 =item rindex STR,SUBSTR
3094 Works just like index except that it returns the position of the LAST
3095 occurrence of SUBSTR in STR. If POSITION is specified, returns the
3096 last occurrence at or before that position.
3098 =item rmdir FILENAME
3102 Deletes the directory specified by FILENAME if that directory is empty. If it
3103 succeeds it returns TRUE, otherwise it returns FALSE and sets C<$!> (errno). If
3104 FILENAME is omitted, uses C<$_>.
3108 The substitution operator. See L<perlop>.
3112 Forces EXPR to be interpreted in scalar context and returns the value
3115 @counts = ( scalar @a, scalar @b, scalar @c );
3117 There is no equivalent operator to force an expression to
3118 be interpolated in list context because it's in practice never
3119 needed. If you really wanted to do so, however, you could use
3120 the construction C<@{[ (some expression) ]}>, but usually a simple
3121 C<(some expression)> suffices.
3123 Though C<scalar> can be considered in general to be a unary operator,
3124 EXPR is also allowed to be a parenthesized list. The list in fact
3125 behaves as a scalar comma expression, evaluating all but the last
3126 element in void context and returning the final element evaluated in
3129 The following single statement:
3131 print uc(scalar(&foo,$bar)),$baz;
3133 is the moral equivalent of these two:
3136 print(uc($bar),$baz);
3138 See L<perlop> for more details on unary operators and the comma operator.
3140 =item seek FILEHANDLE,POSITION,WHENCE
3142 Sets FILEHANDLE's position, just like the C<fseek()> call of C<stdio()>.
3143 FILEHANDLE may be an expression whose value gives the name of the
3144 filehandle. The values for WHENCE are C<0> to set the new position to
3145 POSITION, C<1> to set it to the current position plus POSITION, and C<2> to
3146 set it to EOF plus POSITION (typically negative). For WHENCE you may
3147 use the constants C<SEEK_SET>, C<SEEK_CUR>, and C<SEEK_END> from either the
3148 C<IO::Seekable> or the POSIX module. Returns C<1> upon success, C<0> otherwise.
3150 If you want to position file for C<sysread()> or C<syswrite()>, don't use
3151 C<seek()> -- buffering makes its effect on the file's system position
3152 unpredictable and non-portable. Use C<sysseek()> instead.
3154 On some systems you have to do a seek whenever you switch between reading
3155 and writing. Amongst other things, this may have the effect of calling
3156 stdio's clearerr(3). A WHENCE of C<1> (C<SEEK_CUR>) is useful for not moving
3161 This is also useful for applications emulating C<tail -f>. Once you hit
3162 EOF on your read, and then sleep for a while, you might have to stick in a
3163 seek() to reset things. The C<seek()> doesn't change the current position,
3164 but it I<does> clear the end-of-file condition on the handle, so that the
3165 next C<E<lt>FILEE<gt>> makes Perl try again to read something. We hope.
3167 If that doesn't work (some stdios are particularly cantankerous), then
3168 you may need something more like this:
3171 for ($curpos = tell(FILE); $_ = <FILE>;
3172 $curpos = tell(FILE)) {
3173 # search for some stuff and put it into files
3175 sleep($for_a_while);
3176 seek(FILE, $curpos, 0);
3179 =item seekdir DIRHANDLE,POS
3181 Sets the current position for the C<readdir()> routine on DIRHANDLE. POS
3182 must be a value returned by C<telldir()>. Has the same caveats about
3183 possible directory compaction as the corresponding system library
3186 =item select FILEHANDLE
3190 Returns the currently selected filehandle. Sets the current default
3191 filehandle for output, if FILEHANDLE is supplied. This has two
3192 effects: first, a C<write()> or a C<print()> without a filehandle will
3193 default to this FILEHANDLE. Second, references to variables related to
3194 output will refer to this output channel. For example, if you have to
3195 set the top of form format for more than one output channel, you might
3203 FILEHANDLE may be an expression whose value gives the name of the
3204 actual filehandle. Thus:
3206 $oldfh = select(STDERR); $| = 1; select($oldfh);
3208 Some programmers may prefer to think of filehandles as objects with
3209 methods, preferring to write the last example as:
3212 STDERR->autoflush(1);
3214 =item select RBITS,WBITS,EBITS,TIMEOUT
3216 This calls the select(2) system call with the bit masks specified, which
3217 can be constructed using C<fileno()> and C<vec()>, along these lines:
3219 $rin = $win = $ein = '';
3220 vec($rin,fileno(STDIN),1) = 1;
3221 vec($win,fileno(STDOUT),1) = 1;
3224 If you want to select on many filehandles you might wish to write a
3228 my(@fhlist) = split(' ',$_[0]);
3231 vec($bits,fileno($_),1) = 1;
3235 $rin = fhbits('STDIN TTY SOCK');
3239 ($nfound,$timeleft) =
3240 select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
3242 or to block until something becomes ready just do this
3244 $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
3246 Most systems do not bother to return anything useful in C<$timeleft>, so
3247 calling select() in scalar context just returns C<$nfound>.
3249 Any of the bit masks can also be undef. The timeout, if specified, is
3250 in seconds, which may be fractional. Note: not all implementations are
3251 capable of returning theC<$timeleft>. If not, they always return
3252 C<$timeleft> equal to the supplied C<$timeout>.
3254 You can effect a sleep of 250 milliseconds this way:
3256 select(undef, undef, undef, 0.25);
3258 B<WARNING>: One should not attempt to mix buffered I/O (like C<read()>
3259 or E<lt>FHE<gt>) with C<select()>, except as permitted by POSIX, and even
3260 then only on POSIX systems. You have to use C<sysread()> instead.
3262 =item semctl ID,SEMNUM,CMD,ARG
3264 Calls the System V IPC function C<semctl()>. You'll probably have to say
3268 first to get the correct constant definitions. If CMD is IPC_STAT or
3269 GETALL, then ARG must be a variable which will hold the returned
3270 semid_ds structure or semaphore value array. Returns like C<ioctl()>: the
3271 undefined value for error, "C<0> but true" for zero, or the actual return
3272 value otherwise. See also C<IPC::SysV> and C<IPC::Semaphore> documentation.
3274 =item semget KEY,NSEMS,FLAGS
3276 Calls the System V IPC function semget. Returns the semaphore id, or
3277 the undefined value if there is an error. See also C<IPC::SysV> and
3278 C<IPC::SysV::Semaphore> documentation.
3280 =item semop KEY,OPSTRING
3282 Calls the System V IPC function semop to perform semaphore operations
3283 such as signaling and waiting. OPSTRING must be a packed array of
3284 semop structures. Each semop structure can be generated with
3285 C<pack("sss", $semnum, $semop, $semflag)>. The number of semaphore
3286 operations is implied by the length of OPSTRING. Returns TRUE if
3287 successful, or FALSE if there is an error. As an example, the
3288 following code waits on semaphore C<$semnum> of semaphore id C<$semid>:
3290 $semop = pack("sss", $semnum, -1, 0);
3291 die "Semaphore trouble: $!\n" unless semop($semid, $semop);
3293 To signal the semaphore, replace C<-1> with C<1>. See also C<IPC::SysV>
3294 and C<IPC::SysV::Semaphore> documentation.
3296 =item send SOCKET,MSG,FLAGS,TO
3298 =item send SOCKET,MSG,FLAGS
3300 Sends a message on a socket. Takes the same flags as the system call
3301 of the same name. On unconnected sockets you must specify a
3302 destination to send TO, in which case it does a C C<sendto()>. Returns
3303 the number of characters sent, or the undefined value if there is an
3305 See L<perlipc/"UDP: Message Passing"> for examples.
3307 =item setpgrp PID,PGRP
3309 Sets the current process group for the specified PID, C<0> for the current
3310 process. Will produce a fatal error if used on a machine that doesn't
3311 implement setpgrp(2). If the arguments are omitted, it defaults to
3312 C<0,0>. Note that the POSIX version of C<setpgrp()> does not accept any
3313 arguments, so only setpgrp C<0,0> is portable.
3315 =item setpriority WHICH,WHO,PRIORITY
3317 Sets the current priority for a process, a process group, or a user.
3318 (See setpriority(2).) Will produce a fatal error if used on a machine
3319 that doesn't implement setpriority(2).
3321 =item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
3323 Sets the socket option requested. Returns undefined if there is an
3324 error. OPTVAL may be specified as C<undef> if you don't want to pass an
3331 Shifts the first value of the array off and returns it, shortening the
3332 array by 1 and moving everything down. If there are no elements in the
3333 array, returns the undefined value. If ARRAY is omitted, shifts the
3334 C<@_> array within the lexical scope of subroutines and formats, and the
3335 C<@ARGV> array at file scopes or within the lexical scopes established by
3336 the C<eval ''>, C<BEGIN {}>, C<END {}>, and C<INIT {}> constructs.
3337 See also C<unshift()>, C<push()>, and C<pop()>. C<Shift()> and C<unshift()> do the
3338 same thing to the left end of an array that C<pop()> and C<push()> do to the
3341 =item shmctl ID,CMD,ARG
3343 Calls the System V IPC function shmctl. You'll probably have to say
3347 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
3348 then ARG must be a variable which will hold the returned C<shmid_ds>
3349 structure. Returns like ioctl: the undefined value for error, "C<0> but
3350 true" for zero, or the actual return value otherwise.
3351 See also C<IPC::SysV> documentation.
3353 =item shmget KEY,SIZE,FLAGS
3355 Calls the System V IPC function shmget. Returns the shared memory
3356 segment id, or the undefined value if there is an error.
3357 See also C<IPC::SysV> documentation.
3359 =item shmread ID,VAR,POS,SIZE
3361 =item shmwrite ID,STRING,POS,SIZE
3363 Reads or writes the System V shared memory segment ID starting at
3364 position POS for size SIZE by attaching to it, copying in/out, and
3365 detaching from it. When reading, VAR must be a variable that will
3366 hold the data read. When writing, if STRING is too long, only SIZE
3367 bytes are used; if STRING is too short, nulls are written to fill out
3368 SIZE bytes. Return TRUE if successful, or FALSE if there is an error.
3369 See also C<IPC::SysV> documentation.
3371 =item shutdown SOCKET,HOW
3373 Shuts down a socket connection in the manner indicated by HOW, which
3374 has the same interpretation as in the system call of the same name.
3376 shutdown(SOCKET, 0); # I/we have stopped reading data
3377 shutdown(SOCKET, 1); # I/we have stopped writing data
3378 shutdown(SOCKET, 2); # I/we have stopped using this socket
3380 This is useful with sockets when you want to tell the other
3381 side you're done writing but not done reading, or vice versa.
3382 It's also a more insistent form of close because it also
3383 disables the filedescriptor in any forked copies in other
3390 Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
3391 returns sine of C<$_>.
3393 For the inverse sine operation, you may use the C<POSIX::asin()>
3394 function, or use this relation:
3396 sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
3402 Causes the script to sleep for EXPR seconds, or forever if no EXPR.
3403 May be interrupted if the process receives a signal such as C<SIGALRM>.
3404 Returns the number of seconds actually slept. You probably cannot
3405 mix C<alarm()> and C<sleep()> calls, because C<sleep()> is often implemented
3408 On some older systems, it may sleep up to a full second less than what
3409 you requested, depending on how it counts seconds. Most modern systems
3410 always sleep the full amount. They may appear to sleep longer than that,
3411 however, because your process might not be scheduled right away in a
3412 busy multitasking system.
3414 For delays of finer granularity than one second, you may use Perl's
3415 C<syscall()> interface to access setitimer(2) if your system supports it,
3416 or else see L</select> above.
3418 See also the POSIX module's C<sigpause()> function.
3420 =item socket SOCKET,DOMAIN,TYPE,PROTOCOL
3422 Opens a socket of the specified kind and attaches it to filehandle
3423 SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for the
3424 system call of the same name. You should "C<use Socket;>" first to get
3425 the proper definitions imported. See the example in L<perlipc/"Sockets: Client/Server Communication">.
3427 =item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
3429 Creates an unnamed pair of sockets in the specified domain, of the
3430 specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as
3431 for the system call of the same name. If unimplemented, yields a fatal
3432 error. Returns TRUE if successful.
3434 Some systems defined C<pipe()> in terms of C<socketpair()>, in which a call
3435 to C<pipe(Rdr, Wtr)> is essentially:
3438 socketpair(Rdr, Wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC);
3439 shutdown(Rdr, 1); # no more writing for reader
3440 shutdown(Wtr, 0); # no more reading for writer
3442 See L<perlipc> for an example of socketpair use.
3444 =item sort SUBNAME LIST
3446 =item sort BLOCK LIST
3450 Sorts the LIST and returns the sorted list value. If SUBNAME or BLOCK
3451 is omitted, C<sort()>s in standard string comparison order. If SUBNAME is
3452 specified, it gives the name of a subroutine that returns an integer
3453 less than, equal to, or greater than C<0>, depending on how the elements
3454 of the array are to be ordered. (The C<E<lt>=E<gt>> and C<cmp>
3455 operators are extremely useful in such routines.) SUBNAME may be a
3456 scalar variable name (unsubscripted), in which case the value provides
3457 the name of (or a reference to) the actual subroutine to use. In place
3458 of a SUBNAME, you can provide a BLOCK as an anonymous, in-line sort
3461 In the interests of efficiency the normal calling code for subroutines is
3462 bypassed, with the following effects: the subroutine may not be a
3463 recursive subroutine, and the two elements to be compared are passed into
3464 the subroutine not via C<@_> but as the package global variables C<$a> and
3465 C<$b> (see example below). They are passed by reference, so don't
3466 modify C<$a> and C<$b>. And don't try to declare them as lexicals either.
3468 You also cannot exit out of the sort block or subroutine using any of the
3469 loop control operators described in L<perlsyn> or with C<goto()>.
3471 When C<use locale> is in effect, C<sort LIST> sorts LIST according to the
3472 current collation locale. See L<perllocale>.
3477 @articles = sort @files;
3479 # same thing, but with explicit sort routine
3480 @articles = sort {$a cmp $b} @files;
3482 # now case-insensitively
3483 @articles = sort {uc($a) cmp uc($b)} @files;
3485 # same thing in reversed order
3486 @articles = sort {$b cmp $a} @files;
3488 # sort numerically ascending
3489 @articles = sort {$a <=> $b} @files;
3491 # sort numerically descending
3492 @articles = sort {$b <=> $a} @files;
3494 # sort using explicit subroutine name
3496 $age{$a} <=> $age{$b}; # presuming numeric
3498 @sortedclass = sort byage @class;
3500 # this sorts the %age hash by value instead of key
3501 # using an in-line function
3502 @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
3504 sub backwards { $b cmp $a; }
3505 @harry = ('dog','cat','x','Cain','Abel');
3506 @george = ('gone','chased','yz','Punished','Axed');
3508 # prints AbelCaincatdogx
3509 print sort backwards @harry;
3510 # prints xdogcatCainAbel
3511 print sort @george, 'to', @harry;
3512 # prints AbelAxedCainPunishedcatchaseddoggonetoxyz
3514 # inefficiently sort by descending numeric compare using
3515 # the first integer after the first = sign, or the
3516 # whole record case-insensitively otherwise
3519 ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
3524 # same thing, but much more efficiently;
3525 # we'll build auxiliary indices instead
3529 push @nums, /=(\d+)/;
3534 $nums[$b] <=> $nums[$a]
3536 $caps[$a] cmp $caps[$b]
3540 # same thing using a Schwartzian Transform (no temps)
3541 @new = map { $_->[0] }
3542 sort { $b->[1] <=> $a->[1]
3545 } map { [$_, /=(\d+)/, uc($_)] } @old;
3547 If you're using strict, you I<MUST NOT> declare C<$a>
3548 and C<$b> as lexicals. They are package globals. That means
3549 if you're in the C<main> package, it's
3551 @articles = sort {$main::b <=> $main::a} @files;
3555 @articles = sort {$::b <=> $::a} @files;
3557 but if you're in the C<FooPack> package, it's
3559 @articles = sort {$FooPack::b <=> $FooPack::a} @files;
3561 The comparison function is required to behave. If it returns
3562 inconsistent results (sometimes saying C<$x[1]> is less than C<$x[2]> and
3563 sometimes saying the opposite, for example) the results are not
3566 =item splice ARRAY,OFFSET,LENGTH,LIST
3568 =item splice ARRAY,OFFSET,LENGTH
3570 =item splice ARRAY,OFFSET
3572 Removes the elements designated by OFFSET and LENGTH from an array, and
3573 replaces them with the elements of LIST, if any. In list context,
3574 returns the elements removed from the array. In scalar context,
3575 returns the last element removed, or C<undef> if no elements are
3576 removed. The array grows or shrinks as necessary.
3577 If OFFSET is negative then it start that far from the end of the array.
3578 If LENGTH is omitted, removes everything from OFFSET onward.
3579 If LENGTH is negative, leave that many elements off the end of the array.
3580 The following equivalences hold (assuming C<$[ == 0>):
3582 push(@a,$x,$y) splice(@a,@a,0,$x,$y)
3583 pop(@a) splice(@a,-1)
3584 shift(@a) splice(@a,0,1)
3585 unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
3586 $a[$x] = $y splice(@a,$x,1,$y)
3588 Example, assuming array lengths are passed before arrays:
3590 sub aeq { # compare two list values
3591 my(@a) = splice(@_,0,shift);
3592 my(@b) = splice(@_,0,shift);
3593 return 0 unless @a == @b; # same len?
3595 return 0 if pop(@a) ne pop(@b);
3599 if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
3601 =item split /PATTERN/,EXPR,LIMIT
3603 =item split /PATTERN/,EXPR
3605 =item split /PATTERN/
3609 Splits a string into an array of strings, and returns it. By default,
3610 empty leading fields are preserved, and empty trailing ones are deleted.
3612 If not in list context, returns the number of fields found and splits into
3613 the C<@_> array. (In list context, you can force the split into C<@_> by
3614 using C<??> as the pattern delimiters, but it still returns the list
3615 value.) The use of implicit split to C<@_> is deprecated, however, because
3616 it clobbers your subroutine arguments.
3618 If EXPR is omitted, splits the C<$_> string. If PATTERN is also omitted,
3619 splits on whitespace (after skipping any leading whitespace). Anything
3620 matching PATTERN is taken to be a delimiter separating the fields. (Note
3621 that the delimiter may be longer than one character.)
3623 If LIMIT is specified and positive, splits into no more than that
3624 many fields (though it may split into fewer). If LIMIT is unspecified
3625 or zero, trailing null fields are stripped (which potential users
3626 of C<pop()> would do well to remember). If LIMIT is negative, it is
3627 treated as if an arbitrarily large LIMIT had been specified.
3629 A pattern matching the null string (not to be confused with
3630 a null pattern C<//>, which is just one member of the set of patterns
3631 matching a null string) will split the value of EXPR into separate
3632 characters at each point it matches that way. For example:
3634 print join(':', split(/ */, 'hi there'));
3636 produces the output 'h:i:t:h:e:r:e'.
3638 The LIMIT parameter can be used to split a line partially
3640 ($login, $passwd, $remainder) = split(/:/, $_, 3);
3642 When assigning to a list, if LIMIT is omitted, Perl supplies a LIMIT
3643 one larger than the number of variables in the list, to avoid
3644 unnecessary work. For the list above LIMIT would have been 4 by
3645 default. In time critical applications it behooves you not to split
3646 into more fields than you really need.
3648 If the PATTERN contains parentheses, additional array elements are
3649 created from each matching substring in the delimiter.
3651 split(/([,-])/, "1-10,20", 3);
3653 produces the list value
3655 (1, '-', 10, ',', 20)
3657 If you had the entire header of a normal Unix email message in C<$header>,
3658 you could split it up into fields and their values this way:
3660 $header =~ s/\n\s+/ /g; # fix continuation lines
3661 %hdrs = (UNIX_FROM => split /^(\S*?):\s*/m, $header);
3663 The pattern C</PATTERN/> may be replaced with an expression to specify
3664 patterns that vary at runtime. (To do runtime compilation only once,
3665 use C</$variable/o>.)
3667 As a special case, specifying a PATTERN of space (C<' '>) will split on
3668 white space just as C<split()> with no arguments does. Thus, C<split(' ')> can
3669 be used to emulate B<awk>'s default behavior, whereas C<split(/ /)>
3670 will give you as many null initial fields as there are leading spaces.
3671 A C<split()> on C</\s+/> is like a C<split(' ')> except that any leading
3672 whitespace produces a null first field. A C<split()> with no arguments
3673 really does a C<split(' ', $_)> internally.
3677 open(PASSWD, '/etc/passwd');
3679 ($login, $passwd, $uid, $gid,
3680 $gcos, $home, $shell) = split(/:/);
3684 (Note that C<$shell> above will still have a newline on it. See L</chop>,
3685 L</chomp>, and L</join>.)
3687 =item sprintf FORMAT, LIST
3689 Returns a string formatted by the usual C<printf()> conventions of the
3690 C library function C<sprintf()>. See L<sprintf(3)> or L<printf(3)>
3691 on your system for an explanation of the general principles.
3693 Perl does its own C<sprintf()> formatting -- it emulates the C
3694 function C<sprintf()>, but it doesn't use it (except for floating-point
3695 numbers, and even then only the standard modifiers are allowed). As a
3696 result, any non-standard extensions in your local C<sprintf()> are not
3697 available from Perl.
3699 Perl's C<sprintf()> permits the following universally-known conversions:
3702 %c a character with the given number
3704 %d a signed integer, in decimal
3705 %u an unsigned integer, in decimal
3706 %o an unsigned integer, in octal
3707 %x an unsigned integer, in hexadecimal
3708 %e a floating-point number, in scientific notation
3709 %f a floating-point number, in fixed decimal notation
3710 %g a floating-point number, in %e or %f notation
3712 In addition, Perl permits the following widely-supported conversions:
3714 %X like %x, but using upper-case letters
3715 %E like %e, but using an upper-case "E"
3716 %G like %g, but with an upper-case "E" (if applicable)
3717 %b an unsigned integer, in binary
3718 %p a pointer (outputs the Perl value's address in hexadecimal)
3719 %n special: *stores* the number of characters output so far
3720 into the next variable in the parameter list
3722 Finally, for backward (and we do mean "backward") compatibility, Perl
3723 permits these unnecessary but widely-supported conversions:
3726 %D a synonym for %ld
3727 %U a synonym for %lu
3728 %O a synonym for %lo
3731 Perl permits the following universally-known flags between the C<%>
3732 and the conversion letter:
3734 space prefix positive number with a space
3735 + prefix positive number with a plus sign
3736 - left-justify within the field
3737 0 use zeros, not spaces, to right-justify
3738 # prefix non-zero octal with "0", non-zero hex with "0x"
3739 number minimum field width
3740 .number "precision": digits after decimal point for
3741 floating-point, max length for string, minimum length
3743 l interpret integer as C type "long" or "unsigned long"
3744 h interpret integer as C type "short" or "unsigned short"
3746 There is also one Perl-specific flag:
3748 V interpret integer as Perl's standard integer type
3750 Where a number would appear in the flags, an asterisk ("C<*>") may be
3751 used instead, in which case Perl uses the next item in the parameter
3752 list as the given number (that is, as the field width or precision).
3753 If a field width obtained through "C<*>" is negative, it has the same
3754 effect as the "C<->" flag: left-justification.
3756 If C<use locale> is in effect, the character used for the decimal
3757 point in formatted real numbers is affected by the LC_NUMERIC locale.
3764 Return the square root of EXPR. If EXPR is omitted, returns square
3771 Sets the random number seed for the C<rand()> operator. If EXPR is
3772 omitted, uses a semi-random value supplied by the kernel (if it supports
3773 the F</dev/urandom> device) or based on the current time and process
3774 ID, among other things. In versions of Perl prior to 5.004 the default
3775 seed was just the current C<time()>. This isn't a particularly good seed,
3776 so many old programs supply their own seed value (often C<time ^ $$> or
3777 C<time ^ ($$ + ($$ E<lt>E<lt> 15))>), but that isn't necessary any more.
3779 In fact, it's usually not necessary to call C<srand()> at all, because if
3780 it is not called explicitly, it is called implicitly at the first use of
3781 the C<rand()> operator. However, this was not the case in version of Perl
3782 before 5.004, so if your script will run under older Perl versions, it
3783 should call C<srand()>.
3785 Note that you need something much more random than the default seed for
3786 cryptographic purposes. Checksumming the compressed output of one or more
3787 rapidly changing operating system status programs is the usual method. For
3790 srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip`);
3792 If you're particularly concerned with this, see the C<Math::TrulyRandom>
3795 Do I<not> call C<srand()> multiple times in your program unless you know
3796 exactly what you're doing and why you're doing it. The point of the
3797 function is to "seed" the C<rand()> function so that C<rand()> can produce
3798 a different sequence each time you run your program. Just do it once at the
3799 top of your program, or you I<won't> get random numbers out of C<rand()>!
3801 Frequently called programs (like CGI scripts) that simply use
3805 for a seed can fall prey to the mathematical property that
3809 one-third of the time. So don't do that.
3811 =item stat FILEHANDLE
3817 Returns a 13-element list giving the status info for a file, either
3818 the file opened via FILEHANDLE, or named by EXPR. If EXPR is omitted,
3819 it stats C<$_>. Returns a null list if the stat fails. Typically used
3822 ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
3823 $atime,$mtime,$ctime,$blksize,$blocks)
3826 Not all fields are supported on all filesystem types. Here are the
3827 meaning of the fields:
3829 0 dev device number of filesystem
3831 2 mode file mode (type and permissions)
3832 3 nlink number of (hard) links to the file
3833 4 uid numeric user ID of file's owner
3834 5 gid numeric group ID of file's owner
3835 6 rdev the device identifier (special files only)
3836 7 size total size of file, in bytes
3837 8 atime last access time since the epoch
3838 9 mtime last modify time since the epoch
3839 10 ctime inode change time (NOT creation time!) since the epoch
3840 11 blksize preferred block size for file system I/O
3841 12 blocks actual number of blocks allocated
3843 (The epoch was at 00:00 January 1, 1970 GMT.)
3845 If stat is passed the special filehandle consisting of an underline, no
3846 stat is done, but the current contents of the stat structure from the
3847 last stat or filetest are returned. Example:
3849 if (-x $file && (($d) = stat(_)) && $d < 0) {
3850 print "$file is executable NFS file\n";
3853 (This works on machines only for which the device number is negative under NFS.)
3855 In scalar context, C<stat()> returns a boolean value indicating success
3856 or failure, and, if successful, sets the information associated with
3857 the special filehandle C<_>.
3863 Takes extra time to study SCALAR (C<$_> if unspecified) in anticipation of
3864 doing many pattern matches on the string before it is next modified.
3865 This may or may not save time, depending on the nature and number of
3866 patterns you are searching on, and on the distribution of character
3867 frequencies in the string to be searched -- you probably want to compare
3868 run times with and without it to see which runs faster. Those loops
3869 which scan for many short constant strings (including the constant
3870 parts of more complex patterns) will benefit most. You may have only
3871 one C<study()> active at a time -- if you study a different scalar the first
3872 is "unstudied". (The way C<study()> works is this: a linked list of every
3873 character in the string to be searched is made, so we know, for
3874 example, where all the C<'k'> characters are. From each search string,
3875 the rarest character is selected, based on some static frequency tables
3876 constructed from some C programs and English text. Only those places
3877 that contain this "rarest" character are examined.)
3879 For example, here is a loop that inserts index producing entries
3880 before any line containing a certain pattern:
3884 print ".IX foo\n" if /\bfoo\b/;
3885 print ".IX bar\n" if /\bbar\b/;
3886 print ".IX blurfl\n" if /\bblurfl\b/;
3891 In searching for C</\bfoo\b/>, only those locations in C<$_> that contain C<"f">
3892 will be looked at, because C<"f"> is rarer than C<"o">. In general, this is
3893 a big win except in pathological cases. The only question is whether
3894 it saves you more time than it took to build the linked list in the
3897 Note that if you have to look for strings that you don't know till
3898 runtime, you can build an entire loop as a string and C<eval()> that to
3899 avoid recompiling all your patterns all the time. Together with
3900 undefining C<$/> to input entire files as one record, this can be very
3901 fast, often faster than specialized programs like fgrep(1). The following
3902 scans a list of files (C<@files>) for a list of words (C<@words>), and prints
3903 out the names of those files that contain a match:
3905 $search = 'while (<>) { study;';
3906 foreach $word (@words) {
3907 $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
3912 eval $search; # this screams
3913 $/ = "\n"; # put back to normal input delimiter
3914 foreach $file (sort keys(%seen)) {
3922 =item sub NAME BLOCK
3924 This is subroutine definition, not a real function I<per se>. With just a
3925 NAME (and possibly prototypes), it's just a forward declaration. Without
3926 a NAME, it's an anonymous function declaration, and does actually return a
3927 value: the CODE ref of the closure you just created. See L<perlsub> and
3928 L<perlref> for details.
3930 =item substr EXPR,OFFSET,LEN,REPLACEMENT
3932 =item substr EXPR,OFFSET,LEN
3934 =item substr EXPR,OFFSET
3936 Extracts a substring out of EXPR and returns it. First character is at
3937 offset C<0>, or whatever you've set C<$[> to (but don't do that).
3938 If OFFSET is negative (or more precisely, less than C<$[>), starts
3939 that far from the end of the string. If LEN is omitted, returns
3940 everything to the end of the string. If LEN is negative, leaves that
3941 many characters off the end of the string.
3943 If you specify a substring that is partly outside the string, the part
3944 within the string is returned. If the substring is totally outside
3945 the string a warning is produced.
3947 You can use the C<substr()> function
3948 as an lvalue, in which case EXPR must be an lvalue. If you assign
3949 something shorter than LEN, the string will shrink, and if you assign
3950 something longer than LEN, the string will grow to accommodate it. To
3951 keep the string the same length you may need to pad or chop your value
3954 An alternative to using C<substr()> as an lvalue is to specify the
3955 replacement string as the 4th argument. This allows you to replace
3956 parts of the EXPR and return what was there before in one operation.
3958 =item symlink OLDFILE,NEWFILE
3960 Creates a new filename symbolically linked to the old filename.
3961 Returns C<1> for success, C<0> otherwise. On systems that don't support
3962 symbolic links, produces a fatal error at run time. To check for that,
3965 $symlink_exists = eval { symlink("",""); 1 };
3969 Calls the system call specified as the first element of the list,
3970 passing the remaining elements as arguments to the system call. If
3971 unimplemented, produces a fatal error. The arguments are interpreted
3972 as follows: if a given argument is numeric, the argument is passed as
3973 an int. If not, the pointer to the string value is passed. You are
3974 responsible to make sure a string is pre-extended long enough to
3975 receive any result that might be written into a string. You can't use a
3976 string literal (or other read-only string) as an argument to C<syscall()>
3977 because Perl has to assume that any string pointer might be written
3979 integer arguments are not literals and have never been interpreted in a
3980 numeric context, you may need to add C<0> to them to force them to look
3981 like numbers. This emulates the C<syswrite()> function (or vice versa):
3983 require 'syscall.ph'; # may need to run h2ph
3985 syscall(&SYS_write, fileno(STDOUT), $s, length $s);
3987 Note that Perl supports passing of up to only 14 arguments to your system call,
3988 which in practice should usually suffice.
3990 Syscall returns whatever value returned by the system call it calls.
3991 If the system call fails, C<syscall()> returns C<-1> and sets C<$!> (errno).
3992 Note that some system calls can legitimately return C<-1>. The proper
3993 way to handle such calls is to assign C<$!=0;> before the call and
3994 check the value of C<$!> if syscall returns C<-1>.
3996 There's a problem with C<syscall(&SYS_pipe)>: it returns the file
3997 number of the read end of the pipe it creates. There is no way
3998 to retrieve the file number of the other end. You can avoid this
3999 problem by using C<pipe()> instead.
4001 =item sysopen FILEHANDLE,FILENAME,MODE
4003 =item sysopen FILEHANDLE,FILENAME,MODE,PERMS
4005 Opens the file whose filename is given by FILENAME, and associates it
4006 with FILEHANDLE. If FILEHANDLE is an expression, its value is used as
4007 the name of the real filehandle wanted. This function calls the
4008 underlying operating system's C<open()> function with the parameters
4009 FILENAME, MODE, PERMS.
4011 The possible values and flag bits of the MODE parameter are
4012 system-dependent; they are available via the standard module C<Fcntl>.
4013 For historical reasons, some values work on almost every system
4014 supported by perl: zero means read-only, one means write-only, and two
4015 means read/write. We know that these values do I<not> work under
4016 OS/390 & VM/ESA Unix and on the Macintosh; you probably don't want to
4017 use them in new code.
4019 If the file named by FILENAME does not exist and the C<open()> call creates
4020 it (typically because MODE includes the C<O_CREAT> flag), then the value of
4021 PERMS specifies the permissions of the newly created file. If you omit
4022 the PERMS argument to C<sysopen()>, Perl uses the octal value C<0666>.
4023 These permission values need to be in octal, and are modified by your
4024 process's current C<umask>.
4026 Seldom if ever use C<0644> as argument to C<sysopen()> because that
4027 takes away the user's option to have a more permissive umask. Better
4028 to omit it. See the perlfunc(1) entry on C<umask> for more on this.
4030 The C<IO::File> module provides a more object-oriented approach, if you're
4031 into that kind of thing.
4033 =item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
4035 =item sysread FILEHANDLE,SCALAR,LENGTH
4037 Attempts to read LENGTH bytes of data into variable SCALAR from the
4038 specified FILEHANDLE, using the system call read(2). It bypasses stdio,
4039 so mixing this with other kinds of reads, C<print()>, C<write()>,
4040 C<seek()>, C<tell()>, or C<eof()> can cause confusion because stdio
4041 usually buffers data. Returns the number of bytes actually read, C<0>
4042 at end of file, or undef if there was an error. SCALAR will be grown or
4043 shrunk so that the last byte actually read is the last byte of the
4044 scalar after the read.
4046 An OFFSET may be specified to place the read data at some place in the
4047 string other than the beginning. A negative OFFSET specifies
4048 placement at that many bytes counting backwards from the end of the
4049 string. A positive OFFSET greater than the length of SCALAR results
4050 in the string being padded to the required size with C<"\0"> bytes before
4051 the result of the read is appended.
4053 =item sysseek FILEHANDLE,POSITION,WHENCE
4055 Sets FILEHANDLE's system position using the system call lseek(2). It
4056 bypasses stdio, so mixing this with reads (other than C<sysread()>),
4057 C<print()>, C<write()>, C<seek()>, C<tell()>, or C<eof()> may cause
4058 confusion. FILEHANDLE may be an expression whose value gives the name
4059 of the filehandle. The values for WHENCE are C<0> to set the new
4060 position to POSITION, C<1> to set the it to the current position plus
4061 POSITION, and C<2> to set it to EOF plus POSITION (typically negative).
4062 For WHENCE, you may use the constants C<SEEK_SET>, C<SEEK_CUR>, and
4063 C<SEEK_END> from either the C<IO::Seekable> or the POSIX module.
4065 Returns the new position, or the undefined value on failure. A position
4066 of zero is returned as the string "C<0> but true"; thus C<sysseek()> returns
4067 TRUE on success and FALSE on failure, yet you can still easily determine
4072 =item system PROGRAM LIST
4074 Does exactly the same thing as "C<exec LIST>" except that a fork is done
4075 first, and the parent process waits for the child process to complete.
4076 Note that argument processing varies depending on the number of
4077 arguments. If there is more than one argument in LIST, or if LIST is
4078 an array with more than one value, starts the program given by the
4079 first element of the list with arguments given by the rest of the list.
4080 If there is only one scalar argument, the argument is
4081 checked for shell metacharacters, and if there are any, the entire
4082 argument is passed to the system's command shell for parsing (this is
4083 C</bin/sh -c> on Unix platforms, but varies on other platforms). If
4084 there are no shell metacharacters in the argument, it is split into
4085 words and passed directly to C<execvp()>, which is more efficient.
4087 The return value is the exit status of the program as
4088 returned by the C<wait()> call. To get the actual exit value divide by
4089 256. See also L</exec>. This is I<NOT> what you want to use to capture
4090 the output from a command, for that you should use merely backticks or
4091 C<qx//>, as described in L<perlop/"`STRING`">.
4093 Like C<exec()>, C<system()> allows you to lie to a program about its name if
4094 you use the "C<system PROGRAM LIST>" syntax. Again, see L</exec>.
4096 Because C<system()> and backticks block C<SIGINT> and C<SIGQUIT>, killing the
4097 program they're running doesn't actually interrupt your program.
4099 @args = ("command", "arg1", "arg2");
4101 or die "system @args failed: $?"
4103 You can check all the failure possibilities by inspecting
4106 $exit_value = $? >> 8;
4107 $signal_num = $? & 127;
4108 $dumped_core = $? & 128;
4110 When the arguments get executed via the system shell, results
4111 and return codes will be subject to its quirks and capabilities.
4112 See L<perlop/"`STRING`"> and L</exec> for details.
4114 =item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
4116 =item syswrite FILEHANDLE,SCALAR,LENGTH
4118 =item syswrite FILEHANDLE,SCALAR
4120 Attempts to write LENGTH bytes of data from variable SCALAR to the
4121 specified FILEHANDLE, using the system call write(2). If LENGTH is
4122 not specified, writes whole SCALAR. It bypasses
4123 stdio, so mixing this with reads (other than C<sysread())>, C<print()>,
4124 C<write()>, C<seek()>, C<tell()>, or C<eof()> may cause confusion
4125 because stdio usually buffers data. Returns the number of bytes
4126 actually written, or C<undef> if there was an error. If the LENGTH is
4127 greater than the available data in the SCALAR after the OFFSET, only as
4128 much data as is available will be written.
4130 An OFFSET may be specified to write the data from some part of the
4131 string other than the beginning. A negative OFFSET specifies writing
4132 that many bytes counting backwards from the end of the string. In the
4133 case the SCALAR is empty you can use OFFSET but only zero offset.
4135 =item tell FILEHANDLE
4139 Returns the current position for FILEHANDLE. FILEHANDLE may be an
4140 expression whose value gives the name of the actual filehandle. If
4141 FILEHANDLE is omitted, assumes the file last read.
4143 =item telldir DIRHANDLE
4145 Returns the current position of the C<readdir()> routines on DIRHANDLE.
4146 Value may be given to C<seekdir()> to access a particular location in a
4147 directory. Has the same caveats about possible directory compaction as
4148 the corresponding system library routine.
4150 =item tie VARIABLE,CLASSNAME,LIST
4152 This function binds a variable to a package class that will provide the
4153 implementation for the variable. VARIABLE is the name of the variable
4154 to be enchanted. CLASSNAME is the name of a class implementing objects
4155 of correct type. Any additional arguments are passed to the "C<new()>"
4156 method of the class (meaning C<TIESCALAR>, C<TIEHANDLE>, C<TIEARRAY>,
4157 or C<TIEHASH>). Typically these are arguments such as might be passed
4158 to the C<dbm_open()> function of C. The object returned by the "C<new()>"
4159 method is also returned by the C<tie()> function, which would be useful
4160 if you want to access other methods in CLASSNAME.
4162 Note that functions such as C<keys()> and C<values()> may return huge lists
4163 when used on large objects, like DBM files. You may prefer to use the
4164 C<each()> function to iterate over such. Example:
4166 # print out history file offsets
4168 tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
4169 while (($key,$val) = each %HIST) {
4170 print $key, ' = ', unpack('L',$val), "\n";
4174 A class implementing a hash should have the following methods:
4176 TIEHASH classname, LIST
4178 STORE this, key, value
4183 NEXTKEY this, lastkey
4186 A class implementing an ordinary array should have the following methods:
4188 TIEARRAY classname, LIST
4190 STORE this, key, value
4192 STORESIZE this, count
4198 SPLICE this, offset, length, LIST
4202 A class implementing a file handle should have the following methods:
4204 TIEHANDLE classname, LIST
4205 READ this, scalar, length, offset
4208 WRITE this, scalar, length, offset
4210 PRINTF this, format, LIST
4214 A class implementing a scalar should have the following methods:
4216 TIESCALAR classname, LIST
4221 Not all methods indicated above need be implemented. See L<perltie>,
4222 L<Tie::Hash>, L<Tie::Array>, L<Tie::Scalar> and L<Tie::Handle>.
4224 Unlike C<dbmopen()>, the C<tie()> function will not use or require a module
4225 for you--you need to do that explicitly yourself. See L<DB_File>
4226 or the F<Config> module for interesting C<tie()> implementations.
4228 For further details see L<perltie>, L<"tied VARIABLE">.
4232 Returns a reference to the object underlying VARIABLE (the same value
4233 that was originally returned by the C<tie()> call that bound the variable
4234 to a package.) Returns the undefined value if VARIABLE isn't tied to a
4239 Returns the number of non-leap seconds since whatever time the system
4240 considers to be the epoch (that's 00:00:00, January 1, 1904 for MacOS,
4241 and 00:00:00 UTC, January 1, 1970 for most other systems).
4242 Suitable for feeding to C<gmtime()> and C<localtime()>.
4246 Returns a four-element list giving the user and system times, in
4247 seconds, for this process and the children of this process.
4249 ($user,$system,$cuser,$csystem) = times;
4253 The transliteration operator. Same as C<y///>. See L<perlop>.
4255 =item truncate FILEHANDLE,LENGTH
4257 =item truncate EXPR,LENGTH
4259 Truncates the file opened on FILEHANDLE, or named by EXPR, to the
4260 specified length. Produces a fatal error if truncate isn't implemented
4261 on your system. Returns TRUE if successful, the undefined value
4268 Returns an uppercased version of EXPR. This is the internal function
4269 implementing the C<\U> escape in double-quoted strings.
4270 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
4271 Under Unicode (C<use utf8>) it uses the standard Unicode uppercase mappings. (It
4272 does not attempt to do titlecase mapping on initial letters. See C<ucfirst()> for that.)
4274 If EXPR is omitted, uses C<$_>.
4280 Returns the value of EXPR with the first character
4281 in uppercase (titlecase in Unicode). This is
4282 the internal function implementing the C<\u> escape in double-quoted strings.
4283 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
4285 If EXPR is omitted, uses C<$_>.
4291 Sets the umask for the process to EXPR and returns the previous value.
4292 If EXPR is omitted, merely returns the current umask.
4294 The Unix permission C<rwxr-x---> is represented as three sets of three
4295 bits, or three octal digits: C<0750> (the leading 0 indicates octal
4296 and isn't one of the digits). The C<umask> value is such a number
4297 representing disabled permissions bits. The permission (or "mode")
4298 values you pass C<mkdir> or C<sysopen> are modified by your umask, so
4299 even if you tell C<sysopen> to create a file with permissions C<0777>,
4300 if your umask is C<0022> then the file will actually be created with
4301 permissions C<0755>. If your C<umask> were C<0027> (group can't
4302 write; others can't read, write, or execute), then passing
4303 C<sysopen()> C<0666> would create a file with mode C<0640> (C<0666 &~
4306 Here's some advice: supply a creation mode of C<0666> for regular
4307 files (in C<sysopen()>) and one of C<0777> for directories (in
4308 C<mkdir()>) and executable files. This gives users the freedom of
4309 choice: if they want protected files, they might choose process umasks
4310 of C<022>, C<027>, or even the particularly antisocial mask of C<077>.
4311 Programs should rarely if ever make policy decisions better left to
4312 the user. The exception to this is when writing files that should be
4313 kept private: mail files, web browser cookies, I<.rhosts> files, and
4316 If umask(2) is not implemented on your system and you are trying to
4317 restrict access for I<yourself> (i.e., (EXPR & 0700) > 0), produces a
4318 fatal error at run time. If umask(2) is not implemented and you are
4319 not trying to restrict access for yourself, returns C<undef>.
4321 Remember that a umask is a number, usually given in octal; it is I<not> a
4322 string of octal digits. See also L</oct>, if all you have is a string.
4330 Undefines the value of EXPR, which must be an lvalue. Use only on a
4331 scalar value, an array (using "C<@>"), a hash (using "C<%>"), a subroutine
4332 (using "C<&>"), or a typeglob (using "<*>"). (Saying C<undef $hash{$key}>
4333 will probably not do what you expect on most predefined variables or
4334 DBM list values, so don't do that; see L<delete>.) Always returns the
4335 undefined value. You can omit the EXPR, in which case nothing is
4336 undefined, but you still get an undefined value that you could, for
4337 instance, return from a subroutine, assign to a variable or pass as a
4338 parameter. Examples:
4341 undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'};
4345 undef *xyz; # destroys $xyz, @xyz, %xyz, &xyz, etc.
4346 return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it;
4347 select undef, undef, undef, 0.25;
4348 ($a, $b, undef, $c) = &foo; # Ignore third value returned
4350 Note that this is a unary operator, not a list operator.
4356 Deletes a list of files. Returns the number of files successfully
4359 $cnt = unlink 'a', 'b', 'c';
4363 Note: C<unlink()> will not delete directories unless you are superuser and
4364 the B<-U> flag is supplied to Perl. Even if these conditions are
4365 met, be warned that unlinking a directory can inflict damage on your
4366 filesystem. Use C<rmdir()> instead.
4368 If LIST is omitted, uses C<$_>.
4370 =item unpack TEMPLATE,EXPR
4372 C<Unpack()> does the reverse of C<pack()>: it takes a string representing a
4373 structure and expands it out into a list value, returning the array
4374 value. (In scalar context, it returns merely the first value
4375 produced.) The TEMPLATE has the same format as in the C<pack()> function.
4376 Here's a subroutine that does substring:
4379 my($what,$where,$howmuch) = @_;
4380 unpack("x$where a$howmuch", $what);
4385 sub ordinal { unpack("c",$_[0]); } # same as ord()
4387 In addition, you may prefix a field with a %E<lt>numberE<gt> to indicate that
4388 you want a E<lt>numberE<gt>-bit checksum of the items instead of the items
4389 themselves. Default is a 16-bit checksum. For example, the following
4390 computes the same number as the System V sum program:
4393 $checksum += unpack("%32C*", $_);
4397 The following efficiently counts the number of set bits in a bit vector:
4399 $setbits = unpack("%32b*", $selectmask);
4401 See L</pack> for more examples.
4403 =item untie VARIABLE
4405 Breaks the binding between a variable and a package. (See C<tie()>.)
4407 =item unshift ARRAY,LIST
4409 Does the opposite of a C<shift()>. Or the opposite of a C<push()>,
4410 depending on how you look at it. Prepends list to the front of the
4411 array, and returns the new number of elements in the array.
4413 unshift(ARGV, '-e') unless $ARGV[0] =~ /^-/;
4415 Note the LIST is prepended whole, not one element at a time, so the
4416 prepended elements stay in the same order. Use C<reverse()> to do the
4419 =item use Module LIST
4423 =item use Module VERSION LIST
4427 Imports some semantics into the current package from the named module,
4428 generally by aliasing certain subroutine or variable names into your
4429 package. It is exactly equivalent to
4431 BEGIN { require Module; import Module LIST; }
4433 except that Module I<must> be a bareword.
4435 If the first argument to C<use> is a number, it is treated as a version
4436 number instead of a module name. If the version of the Perl interpreter
4437 is less than VERSION, then an error message is printed and Perl exits
4438 immediately. This is often useful if you need to check the current
4439 Perl version before C<use>ing library modules that have changed in
4440 incompatible ways from older versions of Perl. (We try not to do
4441 this more than we have to.)
4443 The C<BEGIN> forces the C<require> and C<import()> to happen at compile time. The
4444 C<require> makes sure the module is loaded into memory if it hasn't been
4445 yet. The C<import()> is not a builtin--it's just an ordinary static method
4446 call into the "C<Module>" package to tell the module to import the list of
4447 features back into the current package. The module can implement its
4448 C<import()> method any way it likes, though most modules just choose to
4449 derive their C<import()> method via inheritance from the C<Exporter> class that
4450 is defined in the C<Exporter> module. See L<Exporter>. If no C<import()>
4451 method can be found then the error is currently silently ignored. This
4452 may change to a fatal error in a future version.
4454 If you don't want your namespace altered, explicitly supply an empty list:
4458 That is exactly equivalent to
4460 BEGIN { require Module }
4462 If the VERSION argument is present between Module and LIST, then the
4463 C<use> will call the VERSION method in class Module with the given
4464 version as an argument. The default VERSION method, inherited from
4465 the Universal class, croaks if the given version is larger than the
4466 value of the variable C<$Module::VERSION>. (Note that there is not a
4467 comma after VERSION!)
4469 Because this is a wide-open interface, pragmas (compiler directives)
4470 are also implemented this way. Currently implemented pragmas are:
4474 use sigtrap qw(SEGV BUS);
4475 use strict qw(subs vars refs);
4476 use subs qw(afunc blurfl);
4478 Some of these these pseudo-modules import semantics into the current
4479 block scope (like C<strict> or C<integer>, unlike ordinary modules,
4480 which import symbols into the current package (which are effective
4481 through the end of the file).
4483 There's a corresponding "C<no>" command that unimports meanings imported
4484 by C<use>, i.e., it calls C<unimport Module LIST> instead of C<import()>.
4489 If no C<unimport()> method can be found the call fails with a fatal error.
4491 See L<perlmod> for a list of standard modules and pragmas.
4495 Changes the access and modification times on each file of a list of
4496 files. The first two elements of the list must be the NUMERICAL access
4497 and modification times, in that order. Returns the number of files
4498 successfully changed. The inode modification time of each file is set
4499 to the current time. This code has the same effect as the "C<touch>"
4500 command if the files already exist:
4504 utime $now, $now, @ARGV;
4508 Returns a list consisting of all the values of the named hash. (In a
4509 scalar context, returns the number of values.) The values are
4510 returned in an apparently random order. The actual random order is
4511 subject to change in future versions of perl, but it is guaranteed to
4512 be the same order as either the C<keys()> or C<each()> function would
4513 produce on the same (unmodified) hash.
4515 As a side effect, it resets HASH's iterator. See also C<keys()>, C<each()>,
4518 =item vec EXPR,OFFSET,BITS
4520 Treats the string in EXPR as a vector of unsigned integers, and
4521 returns the value of the bit field specified by OFFSET. BITS specifies
4522 the number of bits that are reserved for each entry in the bit
4523 vector. This must be a power of two from 1 to 32. C<vec()> may also be
4524 assigned to, in which case parentheses are needed to give the expression
4525 the correct precedence as in
4527 vec($image, $max_x * $x + $y, 8) = 3;
4529 Vectors created with C<vec()> can also be manipulated with the logical
4530 operators C<|>, C<&>, and C<^>, which will assume a bit vector operation is
4531 desired when both operands are strings.
4533 The following code will build up an ASCII string saying C<'PerlPerlPerl'>.
4534 The comments show the string after each step. Note that this code works
4535 in the same way on big-endian or little-endian machines.
4538 vec($foo, 0, 32) = 0x5065726C; # 'Perl'
4539 vec($foo, 2, 16) = 0x5065; # 'PerlPe'
4540 vec($foo, 3, 16) = 0x726C; # 'PerlPerl'
4541 vec($foo, 8, 8) = 0x50; # 'PerlPerlP'
4542 vec($foo, 9, 8) = 0x65; # 'PerlPerlPe'
4543 vec($foo, 20, 4) = 2; # 'PerlPerlPe' . "\x02"
4544 vec($foo, 21, 4) = 7; # 'PerlPerlPer'
4546 vec($foo, 45, 2) = 3; # 'PerlPerlPer' . "\x0c"
4547 vec($foo, 93, 1) = 1; # 'PerlPerlPer' . "\x2c"
4548 vec($foo, 94, 1) = 1; # 'PerlPerlPerl'
4551 To transform a bit vector into a string or array of 0's and 1's, use these:
4553 $bits = unpack("b*", $vector);
4554 @bits = split(//, unpack("b*", $vector));
4556 If you know the exact length in bits, it can be used in place of the C<*>.
4560 Waits for a child process to terminate and returns the pid of the
4561 deceased process, or C<-1> if there are no child processes. The status is
4562 returned in C<$?>. Note that a return value of C<-1> could mean that
4563 child processes are being automatically reaped, as described in L<perlipc>.
4565 =item waitpid PID,FLAGS
4567 Waits for a particular child process to terminate and returns the pid
4568 of the deceased process, or C<-1> if there is no such child process. The
4569 status is returned in C<$?>. If you say
4571 use POSIX ":sys_wait_h";
4573 waitpid(-1,&WNOHANG);
4575 then you can do a non-blocking wait for any process. Non-blocking wait
4576 is available on machines supporting either the waitpid(2) or
4577 wait4(2) system calls. However, waiting for a particular pid with
4578 FLAGS of C<0> is implemented everywhere. (Perl emulates the system call
4579 by remembering the status values of processes that have exited but have
4580 not been harvested by the Perl script yet.)
4582 Note that a return value of C<-1> could mean that child processes are being
4583 automatically reaped. See L<perlipc> for details, and for other examples.
4587 Returns TRUE if the context of the currently executing subroutine is
4588 looking for a list value. Returns FALSE if the context is looking
4589 for a scalar. Returns the undefined value if the context is looking
4590 for no value (void context).
4592 return unless defined wantarray; # don't bother doing more
4593 my @a = complex_calculation();
4594 return wantarray ? @a : "@a";
4598 Produces a message on STDERR just like C<die()>, but doesn't exit or throw
4601 If LIST is empty and C<$@> already contains a value (typically from a
4602 previous eval) that value is used after appending C<"\t...caught">
4603 to C<$@>. This is useful for staying almost, but not entirely similar to
4606 If C<$@> is empty then the string C<"Warning: Something's wrong"> is used.
4608 No message is printed if there is a C<$SIG{__WARN__}> handler
4609 installed. It is the handler's responsibility to deal with the message
4610 as it sees fit (like, for instance, converting it into a C<die()>). Most
4611 handlers must therefore make arrangements to actually display the
4612 warnings that they are not prepared to deal with, by calling C<warn()>
4613 again in the handler. Note that this is quite safe and will not
4614 produce an endless loop, since C<__WARN__> hooks are not called from
4617 You will find this behavior is slightly different from that of
4618 C<$SIG{__DIE__}> handlers (which don't suppress the error text, but can
4619 instead call C<die()> again to change it).
4621 Using a C<__WARN__> handler provides a powerful way to silence all
4622 warnings (even the so-called mandatory ones). An example:
4624 # wipe out *all* compile-time warnings
4625 BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } }
4627 my $foo = 20; # no warning about duplicate my $foo,
4628 # but hey, you asked for it!
4629 # no compile-time or run-time warnings before here
4632 # run-time warnings enabled after here
4633 warn "\$foo is alive and $foo!"; # does show up
4635 See L<perlvar> for details on setting C<%SIG> entries, and for more
4638 =item write FILEHANDLE
4644 Writes a formatted record (possibly multi-line) to the specified FILEHANDLE,
4645 using the format associated with that file. By default the format for
4646 a file is the one having the same name as the filehandle, but the
4647 format for the current output channel (see the C<select()> function) may be set
4648 explicitly by assigning the name of the format to the C<$~> variable.
4650 Top of form processing is handled automatically: if there is
4651 insufficient room on the current page for the formatted record, the
4652 page is advanced by writing a form feed, a special top-of-page format
4653 is used to format the new page header, and then the record is written.
4654 By default the top-of-page format is the name of the filehandle with
4655 "_TOP" appended, but it may be dynamically set to the format of your
4656 choice by assigning the name to the C<$^> variable while the filehandle is
4657 selected. The number of lines remaining on the current page is in
4658 variable C<$->, which can be set to C<0> to force a new page.
4660 If FILEHANDLE is unspecified, output goes to the current default output
4661 channel, which starts out as STDOUT but may be changed by the
4662 C<select()> operator. If the FILEHANDLE is an EXPR, then the expression
4663 is evaluated and the resulting string is used to look up the name of
4664 the FILEHANDLE at run time. For more on formats, see L<perlform>.
4666 Note that write is I<NOT> the opposite of C<read()>. Unfortunately.
4670 The transliteration operator. Same as C<tr///>. See L<perlop>.