3 perlfunc - Perl builtin functions
7 The functions in this section can serve as terms in an expression.
8 They fall into two major categories: list operators and named unary
9 operators. These differ in their precedence relationship with a
10 following comma. (See the precedence table in L<perlop>.) List
11 operators take more than one argument, while unary operators can never
12 take more than one argument. Thus, a comma terminates the argument of
13 a unary operator, but merely separates the arguments of a list
14 operator. A unary operator generally provides a scalar context to its
15 argument, while a list operator may provide either scalar and list
16 contexts for its arguments. If it does both, the scalar arguments will
17 be first, and the list argument will follow. (Note that there can ever
18 be only one such list argument.) For instance, splice() has three scalar
19 arguments followed by a list.
21 In the syntax descriptions that follow, list operators that expect a
22 list (and provide list context for the elements of the list) are shown
23 with LIST as an argument. Such a list may consist of any combination
24 of scalar arguments or list values; the list values will be included
25 in the list as if each individual element were interpolated at that
26 point in the list, forming a longer single-dimensional list value.
27 Elements of the LIST should be separated by commas.
29 Any function in the list below may be used either with or without
30 parentheses around its arguments. (The syntax descriptions omit the
31 parentheses.) If you use the parentheses, the simple (but occasionally
32 surprising) rule is this: It I<LOOKS> like a function, therefore it I<IS> a
33 function, and precedence doesn't matter. Otherwise it's a list
34 operator or unary operator, and precedence does matter. And whitespace
35 between the function and left parenthesis doesn't count--so you need to
38 print 1+2+4; # Prints 7.
39 print(1+2) + 4; # Prints 3.
40 print (1+2)+4; # Also prints 3!
41 print +(1+2)+4; # Prints 7.
42 print ((1+2)+4); # Prints 7.
44 If you run Perl with the B<-w> switch it can warn you about this. For
45 example, the third line above produces:
47 print (...) interpreted as function at - line 1.
48 Useless use of integer addition in void context at - line 1.
50 For functions that can be used in either a scalar or list context,
51 nonabortive failure is generally indicated in a scalar context by
52 returning the undefined value, and in a list context by returning the
55 Remember the following important rule: There is B<no rule> that relates
56 the behavior of an expression in list context to its behavior in scalar
57 context, or vice versa. It might do two totally different things.
58 Each operator and function decides which sort of value it would be most
59 appropriate to return in a scalar context. Some operators return the
60 length of the list that would have been returned in list context. Some
61 operators return the first value in the list. Some operators return the
62 last value in the list. Some operators return a count of successful
63 operations. In general, they do what you want, unless you want
66 An named array in scalar context is quite different from what would at
67 first glance appear to be a list in scalar context. You can't get a list
68 like C<(1,2,3)> into being in scalar context, because the compiler knows
69 the context at compile time. It would generate the scalar comma operator
70 there, not the list construction version of the comma. That means it
71 was never a list to start with.
73 In general, functions in Perl that serve as wrappers for system calls
74 of the same name (like chown(2), fork(2), closedir(2), etc.) all return
75 true when they succeed and C<undef> otherwise, as is usually mentioned
76 in the descriptions below. This is different from the C interfaces,
77 which return C<-1> on failure. Exceptions to this rule are C<wait()>,
78 C<waitpid()>, and C<syscall()>. System calls also set the special C<$!>
79 variable on failure. Other functions do not, except accidentally.
81 =head2 Perl Functions by Category
83 Here are Perl's functions (including things that look like
84 functions, like some keywords and named operators)
85 arranged by category. Some functions appear in more
90 =item Functions for SCALARs or strings
92 C<chomp>, C<chop>, C<chr>, C<crypt>, C<hex>, C<index>, C<lc>, C<lcfirst>,
93 C<length>, C<oct>, C<ord>, C<pack>, C<q/STRING/>, C<qq/STRING/>, C<reverse>,
94 C<rindex>, C<sprintf>, C<substr>, C<tr///>, C<uc>, C<ucfirst>, C<y///>
96 =item Regular expressions and pattern matching
98 C<m//>, C<pos>, C<quotemeta>, C<s///>, C<split>, C<study>, C<qr//>
100 =item Numeric functions
102 C<abs>, C<atan2>, C<cos>, C<exp>, C<hex>, C<int>, C<log>, C<oct>, C<rand>,
103 C<sin>, C<sqrt>, C<srand>
105 =item Functions for real @ARRAYs
107 C<pop>, C<push>, C<shift>, C<splice>, C<unshift>
109 =item Functions for list data
111 C<grep>, C<join>, C<map>, C<qw/STRING/>, C<reverse>, C<sort>, C<unpack>
113 =item Functions for real %HASHes
115 C<delete>, C<each>, C<exists>, C<keys>, C<values>
117 =item Input and output functions
119 C<binmode>, C<close>, C<closedir>, C<dbmclose>, C<dbmopen>, C<die>, C<eof>,
120 C<fileno>, C<flock>, C<format>, C<getc>, C<print>, C<printf>, C<read>,
121 C<readdir>, C<rewinddir>, C<seek>, C<seekdir>, C<select>, C<syscall>,
122 C<sysread>, C<sysseek>, C<syswrite>, C<tell>, C<telldir>, C<truncate>,
125 =item Functions for fixed length data or records
127 C<pack>, C<read>, C<syscall>, C<sysread>, C<syswrite>, C<unpack>, C<vec>
129 =item Functions for filehandles, files, or directories
131 C<-I<X>>, C<chdir>, C<chmod>, C<chown>, C<chroot>, C<fcntl>, C<glob>,
132 C<ioctl>, C<link>, C<lstat>, C<mkdir>, C<open>, C<opendir>,
133 C<readlink>, C<rename>, C<rmdir>, C<stat>, C<symlink>, C<umask>,
136 =item Keywords related to the control flow of your perl program
138 C<caller>, C<continue>, C<die>, C<do>, C<dump>, C<eval>, C<exit>,
139 C<goto>, C<last>, C<next>, C<redo>, C<return>, C<sub>, C<wantarray>
141 =item Keywords related to scoping
143 C<caller>, C<import>, C<local>, C<my>, C<package>, C<use>
145 =item Miscellaneous functions
147 C<defined>, C<dump>, C<eval>, C<formline>, C<local>, C<my>, C<reset>,
148 C<scalar>, C<undef>, C<wantarray>
150 =item Functions for processes and process groups
152 C<alarm>, C<exec>, C<fork>, C<getpgrp>, C<getppid>, C<getpriority>, C<kill>,
153 C<pipe>, C<qx/STRING/>, C<setpgrp>, C<setpriority>, C<sleep>, C<system>,
154 C<times>, C<wait>, C<waitpid>
156 =item Keywords related to perl modules
158 C<do>, C<import>, C<no>, C<package>, C<require>, C<use>
160 =item Keywords related to classes and object-orientedness
162 C<bless>, C<dbmclose>, C<dbmopen>, C<package>, C<ref>, C<tie>, C<tied>,
165 =item Low-level socket functions
167 C<accept>, C<bind>, C<connect>, C<getpeername>, C<getsockname>,
168 C<getsockopt>, C<listen>, C<recv>, C<send>, C<setsockopt>, C<shutdown>,
169 C<socket>, C<socketpair>
171 =item System V interprocess communication functions
173 C<msgctl>, C<msgget>, C<msgrcv>, C<msgsnd>, C<semctl>, C<semget>, C<semop>,
174 C<shmctl>, C<shmget>, C<shmread>, C<shmwrite>
176 =item Fetching user and group info
178 C<endgrent>, C<endhostent>, C<endnetent>, C<endpwent>, C<getgrent>,
179 C<getgrgid>, C<getgrnam>, C<getlogin>, C<getpwent>, C<getpwnam>,
180 C<getpwuid>, C<setgrent>, C<setpwent>
182 =item Fetching network info
184 C<endprotoent>, C<endservent>, C<gethostbyaddr>, C<gethostbyname>,
185 C<gethostent>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
186 C<getprotobyname>, C<getprotobynumber>, C<getprotoent>,
187 C<getservbyname>, C<getservbyport>, C<getservent>, C<sethostent>,
188 C<setnetent>, C<setprotoent>, C<setservent>
190 =item Time-related functions
192 C<gmtime>, C<localtime>, C<time>, C<times>
194 =item Functions new in perl5
196 C<abs>, C<bless>, C<chomp>, C<chr>, C<exists>, C<formline>, C<glob>,
197 C<import>, C<lc>, C<lcfirst>, C<map>, C<my>, C<no>, C<prototype>, C<qx>,
198 C<qw>, C<readline>, C<readpipe>, C<ref>, C<sub*>, C<sysopen>, C<tie>,
199 C<tied>, C<uc>, C<ucfirst>, C<untie>, C<use>
201 * - C<sub> was a keyword in perl4, but in perl5 it is an
202 operator, which can be used in expressions.
204 =item Functions obsoleted in perl5
206 C<dbmclose>, C<dbmopen>
212 Perl was born in UNIX and therefore it can access all the common UNIX
213 system calls. In non-UNIX environments the functionality of many
214 UNIX system calls may not be available or the details of the available
215 functionality may be slightly different. The Perl functions affected
218 C<-X>, C<binmode>, C<chmod>, C<chown>, C<chroot>, C<crypt>,
219 C<dbmclose>, C<dbmopen>, C<dump>, C<endgrent>, C<endhostent>,
220 C<endnetent>, C<endprotoent>, C<endpwent>, C<endservent>, C<exec>,
221 C<fcntl>, C<flock>, C<fork>, C<getgrent>, C<getgrgid>, C<gethostent>,
222 C<getlogin>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
223 C<getppid>, C<getprgp>, C<getpriority>, C<getprotobynumber>,
224 C<getprotoent>, C<getpwent>, C<getpwnam>, C<getpwuid>,
225 C<getservbyport>, C<getservent>, C<getsockopt>, C<glob>, C<ioctl>,
226 C<kill>, C<link>, C<lstat>, C<msgctl>, C<msgget>, C<msgrcv>,
227 C<msgsnd>, C<open>, C<pipe>, C<readlink>, C<select>, C<semctl>,
228 C<semget>, C<semop>, C<setgrent>, C<sethostent>, C<setnetent>,
229 C<setpgrp>, C<setpriority>, C<setprotoent>, C<setpwent>,
230 C<setservent>, C<setsockopt>, C<shmctl>, C<shmget>, C<shmread>,
231 C<shmwrite>, C<socketpair>, C<stat>, C<symlink>, C<syscall>,
232 C<sysopen>, C<system>, C<times>, C<truncate>, C<umask>, C<utime>,
235 For more information about the portability of these functions, see
236 L<perlport> and other available platform-specific documentation.
238 =head2 Alphabetical Listing of Perl Functions
242 =item I<-X> FILEHANDLE
248 A file test, where X is one of the letters listed below. This unary
249 operator takes one argument, either a filename or a filehandle, and
250 tests the associated file to see if something is true about it. If the
251 argument is omitted, tests C<$_>, except for C<-t>, which tests STDIN.
252 Unless otherwise documented, it returns C<1> for TRUE and C<''> for FALSE, or
253 the undefined value if the file doesn't exist. Despite the funny
254 names, precedence is the same as any other named unary operator, and
255 the argument may be parenthesized like any other unary operator. The
256 operator may be any of:
257 X<-r>X<-w>X<-x>X<-o>X<-R>X<-W>X<-X>X<-O>X<-e>X<-z>X<-s>X<-f>X<-d>X<-l>X<-p>
258 X<-S>X<-b>X<-c>X<-t>X<-u>X<-g>X<-k>X<-T>X<-B>X<-M>X<-A>X<-C>
260 -r File is readable by effective uid/gid.
261 -w File is writable by effective uid/gid.
262 -x File is executable by effective uid/gid.
263 -o File is owned by effective uid.
265 -R File is readable by real uid/gid.
266 -W File is writable by real uid/gid.
267 -X File is executable by real uid/gid.
268 -O File is owned by real uid.
271 -z File has zero size.
272 -s File has nonzero size (returns size).
274 -f File is a plain file.
275 -d File is a directory.
276 -l File is a symbolic link.
277 -p File is a named pipe (FIFO), or Filehandle is a pipe.
279 -b File is a block special file.
280 -c File is a character special file.
281 -t Filehandle is opened to a tty.
283 -u File has setuid bit set.
284 -g File has setgid bit set.
285 -k File has sticky bit set.
287 -T File is a text file.
288 -B File is a binary file (opposite of -T).
290 -M Age of file in days when script started.
291 -A Same for access time.
292 -C Same for inode change time.
298 next unless -f $_; # ignore specials
302 The interpretation of the file permission operators C<-r>, C<-R>,
303 C<-w>, C<-W>, C<-x>, and C<-X> is by default based solely on the mode
304 of the file and the uids and gids of the user. There may be other
305 reasons you can't actually read, write, or execute the file. Such
306 reasons may be for example network filesystem access controls, ACLs
307 (access control lists), read-only filesystems, and unrecognized
310 Also note that, for the superuser on the local filesystems, C<-r>,
311 C<-R>, C<-w>, and C<-W> always return 1, and C<-x> and C<-X> return 1
312 if any execute bit is set in the mode. Scripts run by the superuser
313 may thus need to do a stat() to determine the actual mode of the file,
314 or temporarily set the uid to something else.
316 If you are using ACLs, there is a pragma called C<filetest> that may
317 produce more accurate results than the bare stat() mode bits.
319 When under the C<use filetest 'access'> the above-mentioned filetests
320 will test whether the permission can (not) be granted using the
321 access() family of system calls. Also note that the C<-x> and C<-X> may
322 under this pragma return true even if there are no execute permission
323 bits set (nor any extra execute permission ACLs). This strangeness is
324 due to the underlying system calls' definitions. Read the
325 documentation for the C<filetest> pragma for more information.
327 Note that C<-s/a/b/> does not do a negated substitution. Saying
328 C<-exp($foo)> still works as expected, however--only single letters
329 following a minus are interpreted as file tests.
331 The C<-T> and C<-B> switches work as follows. The first block or so of the
332 file is examined for odd characters such as strange control codes or
333 characters with the high bit set. If too many strange characters (E<gt>30%)
334 are found, it's a C<-B> file, otherwise it's a C<-T> file. Also, any file
335 containing null in the first block is considered a binary file. If C<-T>
336 or C<-B> is used on a filehandle, the current stdio buffer is examined
337 rather than the first block. Both C<-T> and C<-B> return TRUE on a null
338 file, or a file at EOF when testing a filehandle. Because you have to
339 read a file to do the C<-T> test, on most occasions you want to use a C<-f>
340 against the file first, as in C<next unless -f $file && -T $file>.
342 If any of the file tests (or either the C<stat()> or C<lstat()> operators) are given
343 the special filehandle consisting of a solitary underline, then the stat
344 structure of the previous file test (or stat operator) is used, saving
345 a system call. (This doesn't work with C<-t>, and you need to remember
346 that lstat() and C<-l> will leave values in the stat structure for the
347 symbolic link, not the real file.) Example:
349 print "Can do.\n" if -r $a || -w _ || -x _;
352 print "Readable\n" if -r _;
353 print "Writable\n" if -w _;
354 print "Executable\n" if -x _;
355 print "Setuid\n" if -u _;
356 print "Setgid\n" if -g _;
357 print "Sticky\n" if -k _;
358 print "Text\n" if -T _;
359 print "Binary\n" if -B _;
365 Returns the absolute value of its argument.
366 If VALUE is omitted, uses C<$_>.
368 =item accept NEWSOCKET,GENERICSOCKET
370 Accepts an incoming socket connect, just as the accept(2) system call
371 does. Returns the packed address if it succeeded, FALSE otherwise.
372 See example in L<perlipc/"Sockets: Client/Server Communication">.
378 Arranges to have a SIGALRM delivered to this process after the
379 specified number of seconds have elapsed. If SECONDS is not specified,
380 the value stored in C<$_> is used. (On some machines,
381 unfortunately, the elapsed time may be up to one second less than you
382 specified because of how seconds are counted.) Only one timer may be
383 counting at once. Each call disables the previous timer, and an
384 argument of C<0> may be supplied to cancel the previous timer without
385 starting a new one. The returned value is the amount of time remaining
386 on the previous timer.
388 For delays of finer granularity than one second, you may use Perl's
389 C<syscall()> interface to access setitimer(2) if your system supports it,
390 or else see L</select()>. It is usually a mistake to intermix C<alarm()>
391 and C<sleep()> calls.
393 If you want to use C<alarm()> to time out a system call you need to use an
394 C<eval()>/C<die()> pair. You can't rely on the alarm causing the system call to
395 fail with C<$!> set to C<EINTR> because Perl sets up signal handlers to
396 restart system calls on some systems. Using C<eval()>/C<die()> always works,
397 modulo the caveats given in L<perlipc/"Signals">.
400 local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
402 $nread = sysread SOCKET, $buffer, $size;
406 die unless $@ eq "alarm\n"; # propagate unexpected errors
415 Returns the arctangent of Y/X in the range -PI to PI.
417 For the tangent operation, you may use the C<POSIX::tan()>
418 function, or use the familiar relation:
420 sub tan { sin($_[0]) / cos($_[0]) }
422 =item bind SOCKET,NAME
424 Binds a network address to a socket, just as the bind system call
425 does. Returns TRUE if it succeeded, FALSE otherwise. NAME should be a
426 packed address of the appropriate type for the socket. See the examples in
427 L<perlipc/"Sockets: Client/Server Communication">.
429 =item binmode FILEHANDLE
431 Arranges for the file to be read or written in "binary" mode in operating
432 systems that distinguish between binary and text files. Files that are
433 not in binary mode have CR LF sequences translated to LF on input and LF
434 translated to CR LF on output. Binmode has no effect under Unix; in MS-DOS
435 and similarly archaic systems, it may be imperative--otherwise your
436 MS-DOS-damaged C library may mangle your file. The key distinction between
437 systems that need C<binmode()> and those that don't is their text file
438 formats. Systems like Unix, MacOS, and Plan9 that delimit lines with a single
439 character, and that encode that character in C as C<"\n">, do not need
440 C<binmode()>. The rest need it. If FILEHANDLE is an expression, the value
441 is taken as the name of the filehandle.
443 =item bless REF,CLASSNAME
447 This function tells the thingy referenced by REF that it is now
448 an object in the CLASSNAME package--or the current package if no CLASSNAME
449 is specified, which is often the case. It returns the reference for
450 convenience, because a C<bless()> is often the last thing in a constructor.
451 Always use the two-argument version if the function doing the blessing
452 might be inherited by a derived class. See L<perltoot> and L<perlobj>
453 for more about the blessing (and blessings) of objects.
455 Consider always blessing objects in CLASSNAMEs that are mixed case.
456 Namespaces with all lowercase names are considered reserved for Perl
457 pragmata. Builtin types have all uppercase names, so to prevent confusion,
458 it is best to avoid such package names as well.
460 See L<perlmod/"Perl Modules">.
466 Returns the context of the current subroutine call. In scalar context,
467 returns the caller's package name if there is a caller, that is, if
468 we're in a subroutine or C<eval()> or C<require()>, and the undefined value
469 otherwise. In list context, returns
471 ($package, $filename, $line) = caller;
473 With EXPR, it returns some extra information that the debugger uses to
474 print a stack trace. The value of EXPR indicates how many call frames
475 to go back before the current one.
477 ($package, $filename, $line, $subroutine,
478 $hasargs, $wantarray, $evaltext, $is_require) = caller($i);
480 Here C<$subroutine> may be C<"(eval)"> if the frame is not a subroutine
481 call, but an C<eval()>. In such a case additional elements C<$evaltext> and
482 C<$is_require> are set: C<$is_require> is true if the frame is created by a
483 C<require> or C<use> statement, C<$evaltext> contains the text of the
484 C<eval EXPR> statement. In particular, for a C<eval BLOCK> statement,
485 C<$filename> is C<"(eval)">, but C<$evaltext> is undefined. (Note also that
486 each C<use> statement creates a C<require> frame inside an C<eval EXPR>)
489 Furthermore, when called from within the DB package, caller returns more
490 detailed information: it sets the list variable C<@DB::args> to be the
491 arguments with which the subroutine was invoked.
493 Be aware that the optimizer might have optimized call frames away before
494 C<caller()> had a chance to get the information. That means that C<caller(N)>
495 might not return information about the call frame you expect it do, for
496 C<N E<gt> 1>. In particular, C<@DB::args> might have information from the
497 previous time C<caller()> was called.
501 Changes the working directory to EXPR, if possible. If EXPR is
502 omitted, changes to home directory. Returns TRUE upon success, FALSE
503 otherwise. See example under C<die()>.
507 Changes the permissions of a list of files. The first element of the
508 list must be the numerical mode, which should probably be an octal
509 number, and which definitely should I<not> a string of octal digits:
510 C<0644> is okay, C<'0644'> is not. Returns the number of files
511 successfully changed. See also L</oct>, if all you have is a string.
513 $cnt = chmod 0755, 'foo', 'bar';
514 chmod 0755, @executables;
515 $mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to
517 $mode = '0644'; chmod oct($mode), 'foo'; # this is better
518 $mode = 0644; chmod $mode, 'foo'; # this is best
526 This is a slightly safer version of L</chop>. It removes any
527 line ending that corresponds to the current value of C<$/> (also known as
528 $INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
529 number of characters removed from all its arguments. It's often used to
530 remove the newline from the end of an input record when you're worried
531 that the final record may be missing its newline. When in paragraph mode
532 (C<$/ = "">), it removes all trailing newlines from the string. If
533 VARIABLE is omitted, it chomps C<$_>. Example:
536 chomp; # avoid \n on last field
541 You can actually chomp anything that's an lvalue, including an assignment:
544 chomp($answer = <STDIN>);
546 If you chomp a list, each element is chomped, and the total number of
547 characters removed is returned.
555 Chops off the last character of a string and returns the character
556 chopped. It's used primarily to remove the newline from the end of an
557 input record, but is much more efficient than C<s/\n//> because it neither
558 scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
562 chop; # avoid \n on last field
567 You can actually chop anything that's an lvalue, including an assignment:
570 chop($answer = <STDIN>);
572 If you chop a list, each element is chopped. Only the value of the
573 last C<chop()> is returned.
575 Note that C<chop()> returns the last character. To return all but the last
576 character, use C<substr($string, 0, -1)>.
580 Changes the owner (and group) of a list of files. The first two
581 elements of the list must be the I<NUMERICAL> uid and gid, in that order.
582 Returns the number of files successfully changed.
584 $cnt = chown $uid, $gid, 'foo', 'bar';
585 chown $uid, $gid, @filenames;
587 Here's an example that looks up nonnumeric uids in the passwd file:
590 chop($user = <STDIN>);
592 chop($pattern = <STDIN>);
594 ($login,$pass,$uid,$gid) = getpwnam($user)
595 or die "$user not in passwd file";
597 @ary = glob($pattern); # expand filenames
598 chown $uid, $gid, @ary;
600 On most systems, you are not allowed to change the ownership of the
601 file unless you're the superuser, although you should be able to change
602 the group to any of your secondary groups. On insecure systems, these
603 restrictions may be relaxed, but this is not a portable assumption.
609 Returns the character represented by that NUMBER in the character set.
610 For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
611 chr(0x263a) is a Unicode smiley face (but only within the scope of a
612 C<use utf8>). For the reverse, use L</ord>.
614 If NUMBER is omitted, uses C<$_>.
616 =item chroot FILENAME
620 This function works like the system call by the same name: it makes the
621 named directory the new root directory for all further pathnames that
622 begin with a C<"/"> by your process and all its children. (It doesn't
623 change your current working directory, which is unaffected.) For security
624 reasons, this call is restricted to the superuser. If FILENAME is
625 omitted, does a C<chroot()> to C<$_>.
627 =item close FILEHANDLE
631 Closes the file or pipe associated with the file handle, returning TRUE
632 only if stdio successfully flushes buffers and closes the system file
633 descriptor. Closes the currently selected filehandle if the argument
636 You don't have to close FILEHANDLE if you are immediately going to do
637 another C<open()> on it, because C<open()> will close it for you. (See
638 C<open()>.) However, an explicit C<close()> on an input file resets the line
639 counter (C<$.>), while the implicit close done by C<open()> does not.
641 If the file handle came from a piped open C<close()> will additionally
642 return FALSE if one of the other system calls involved fails or if the
643 program exits with non-zero status. (If the only problem was that the
644 program exited non-zero C<$!> will be set to C<0>.) Also, closing a pipe
645 waits for the process executing on the pipe to complete, in case you
646 want to look at the output of the pipe afterwards. Closing a pipe
647 explicitly also puts the exit status value of the command into C<$?>.
651 open(OUTPUT, '|sort >foo') # pipe to sort
652 or die "Can't start sort: $!";
653 #... # print stuff to output
654 close OUTPUT # wait for sort to finish
655 or warn $! ? "Error closing sort pipe: $!"
656 : "Exit status $? from sort";
657 open(INPUT, 'foo') # get sort's results
658 or die "Can't open 'foo' for input: $!";
660 FILEHANDLE may be an expression whose value can be used as an indirect
661 filehandle, usually the real filehandle name.
663 =item closedir DIRHANDLE
665 Closes a directory opened by C<opendir()> and returns the success of that
668 DIRHANDLE may be an expression whose value can be used as an indirect
669 dirhandle, usually the real dirhandle name.
671 =item connect SOCKET,NAME
673 Attempts to connect to a remote socket, just as the connect system call
674 does. Returns TRUE if it succeeded, FALSE otherwise. NAME should be a
675 packed address of the appropriate type for the socket. See the examples in
676 L<perlipc/"Sockets: Client/Server Communication">.
680 Actually a flow control statement rather than a function. If there is a
681 C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
682 C<foreach>), it is always executed just before the conditional is about to
683 be evaluated again, just like the third part of a C<for> loop in C. Thus
684 it can be used to increment a loop variable, even when the loop has been
685 continued via the C<next> statement (which is similar to the C C<continue>
688 C<last>, C<next>, or C<redo> may appear within a C<continue>
689 block. C<last> and C<redo> will behave as if they had been executed within
690 the main block. So will C<next>, but since it will execute a C<continue>
691 block, it may be more entertaining.
694 ### redo always comes here
697 ### next always comes here
699 # then back the top to re-check EXPR
701 ### last always comes here
703 Omitting the C<continue> section is semantically equivalent to using an
704 empty one, logically enough. In that case, C<next> goes directly back
705 to check the condition at the top of the loop.
709 Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
710 takes cosine of C<$_>.
712 For the inverse cosine operation, you may use the C<POSIX::acos()>
713 function, or use this relation:
715 sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
717 =item crypt PLAINTEXT,SALT
719 Encrypts a string exactly like the crypt(3) function in the C library
720 (assuming that you actually have a version there that has not been
721 extirpated as a potential munition). This can prove useful for checking
722 the password file for lousy passwords, amongst other things. Only the
723 guys wearing white hats should do this.
725 Note that C<crypt()> is intended to be a one-way function, much like breaking
726 eggs to make an omelette. There is no (known) corresponding decrypt
727 function. As a result, this function isn't all that useful for
728 cryptography. (For that, see your nearby CPAN mirror.)
730 When verifying an existing encrypted string you should use the encrypted
731 text as the salt (like C<crypt($plain, $crypted) eq $crypted>). This
732 allows your code to work with the standard C<crypt()> and with more
733 exotic implementations. When choosing a new salt create a random two
734 character string whose characters come from the set C<[./0-9A-Za-z]>
735 (like C<join '', ('.', '/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>).
737 Here's an example that makes sure that whoever runs this program knows
740 $pwd = (getpwuid($<))[1];
744 chomp($word = <STDIN>);
748 if (crypt($word, $pwd) ne $pwd) {
754 Of course, typing in your own password to whoever asks you
759 [This function has been superseded by the C<untie()> function.]
761 Breaks the binding between a DBM file and a hash.
763 =item dbmopen HASH,DBNAME,MODE
765 [This function has been superseded by the C<tie()> function.]
767 This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
768 hash. HASH is the name of the hash. (Unlike normal C<open()>, the first
769 argument is I<NOT> a filehandle, even though it looks like one). DBNAME
770 is the name of the database (without the F<.dir> or F<.pag> extension if
771 any). If the database does not exist, it is created with protection
772 specified by MODE (as modified by the C<umask()>). If your system supports
773 only the older DBM functions, you may perform only one C<dbmopen()> in your
774 program. In older versions of Perl, if your system had neither DBM nor
775 ndbm, calling C<dbmopen()> produced a fatal error; it now falls back to
778 If you don't have write access to the DBM file, you can only read hash
779 variables, not set them. If you want to test whether you can write,
780 either use file tests or try setting a dummy hash entry inside an C<eval()>,
781 which will trap the error.
783 Note that functions such as C<keys()> and C<values()> may return huge lists
784 when used on large DBM files. You may prefer to use the C<each()>
785 function to iterate over large DBM files. Example:
787 # print out history file offsets
788 dbmopen(%HIST,'/usr/lib/news/history',0666);
789 while (($key,$val) = each %HIST) {
790 print $key, ' = ', unpack('L',$val), "\n";
794 See also L<AnyDBM_File> for a more general description of the pros and
795 cons of the various dbm approaches, as well as L<DB_File> for a particularly
802 Returns a Boolean value telling whether EXPR has a value other than
803 the undefined value C<undef>. If EXPR is not present, C<$_> will be
806 Many operations return C<undef> to indicate failure, end of file,
807 system error, uninitialized variable, and other exceptional
808 conditions. This function allows you to distinguish C<undef> from
809 other values. (A simple Boolean test will not distinguish among
810 C<undef>, zero, the empty string, and C<"0">, which are all equally
811 false.) Note that since C<undef> is a valid scalar, its presence
812 doesn't I<necessarily> indicate an exceptional condition: C<pop()>
813 returns C<undef> when its argument is an empty array, I<or> when the
814 element to return happens to be C<undef>.
816 You may also use C<defined()> to check whether a subroutine exists, by
817 saying C<defined &func> without parentheses. On the other hand, use
818 of C<defined()> upon aggregates (hashes and arrays) is not guaranteed to
819 produce intuitive results, and should probably be avoided.
821 When used on a hash element, it tells you whether the value is defined,
822 not whether the key exists in the hash. Use L</exists> for the latter
827 print if defined $switch{'D'};
828 print "$val\n" while defined($val = pop(@ary));
829 die "Can't readlink $sym: $!"
830 unless defined($value = readlink $sym);
831 sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
832 $debugging = 0 unless defined $debugging;
834 Note: Many folks tend to overuse C<defined()>, and then are surprised to
835 discover that the number C<0> and C<""> (the zero-length string) are, in fact,
836 defined values. For example, if you say
840 The pattern match succeeds, and C<$1> is defined, despite the fact that it
841 matched "nothing". But it didn't really match nothing--rather, it
842 matched something that happened to be C<0> characters long. This is all
843 very above-board and honest. When a function returns an undefined value,
844 it's an admission that it couldn't give you an honest answer. So you
845 should use C<defined()> only when you're questioning the integrity of what
846 you're trying to do. At other times, a simple comparison to C<0> or C<""> is
849 Currently, using C<defined()> on an entire array or hash reports whether
850 memory for that aggregate has ever been allocated. So an array you set
851 to the empty list appears undefined initially, and one that once was full
852 and that you then set to the empty list still appears defined. You
853 should instead use a simple test for size:
855 if (@an_array) { print "has array elements\n" }
856 if (%a_hash) { print "has hash members\n" }
858 Using C<undef()> on these, however, does clear their memory and then report
859 them as not defined anymore, but you shouldn't do that unless you don't
860 plan to use them again, because it saves time when you load them up
861 again to have memory already ready to be filled. The normal way to
862 free up space used by an aggregate is to assign the empty list.
864 This counterintuitive behavior of C<defined()> on aggregates may be
865 changed, fixed, or broken in a future release of Perl.
867 See also L</undef>, L</exists>, L</ref>.
871 Deletes the specified key(s) and their associated values from a hash.
872 For each key, returns the deleted value associated with that key, or
873 the undefined value if there was no such key. Deleting from C<$ENV{}>
874 modifies the environment. Deleting from a hash tied to a DBM file
875 deletes the entry from the DBM file. (But deleting from a C<tie()>d hash
876 doesn't necessarily return anything.)
878 The following deletes all the values of a hash:
880 foreach $key (keys %HASH) {
886 delete @HASH{keys %HASH}
888 (But both of these are slower than just assigning the empty list, or
889 using C<undef()>.) Note that the EXPR can be arbitrarily complicated as
890 long as the final operation is a hash element lookup or hash slice:
892 delete $ref->[$x][$y]{$key};
893 delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
897 Outside an C<eval()>, prints the value of LIST to C<STDERR> and exits with
898 the current value of C<$!> (errno). If C<$!> is C<0>, exits with the value of
899 C<($? E<gt>E<gt> 8)> (backtick `command` status). If C<($? E<gt>E<gt> 8)>
900 is C<0>, exits with C<255>. Inside an C<eval(),> the error message is stuffed into
901 C<$@> and the C<eval()> is terminated with the undefined value. This makes
902 C<die()> the way to raise an exception.
906 die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
907 chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
909 If the value of EXPR does not end in a newline, the current script line
910 number and input line number (if any) are also printed, and a newline
911 is supplied. Hint: sometimes appending C<", stopped"> to your message
912 will cause it to make better sense when the string C<"at foo line 123"> is
913 appended. Suppose you are running script "canasta".
915 die "/etc/games is no good";
916 die "/etc/games is no good, stopped";
918 produce, respectively
920 /etc/games is no good at canasta line 123.
921 /etc/games is no good, stopped at canasta line 123.
923 See also C<exit()> and C<warn()>.
925 If LIST is empty and C<$@> already contains a value (typically from a
926 previous eval) that value is reused after appending C<"\t...propagated">.
927 This is useful for propagating exceptions:
930 die unless $@ =~ /Expected exception/;
932 If C<$@> is empty then the string C<"Died"> is used.
934 You can arrange for a callback to be run just before the C<die()> does
935 its deed, by setting the C<$SIG{__DIE__}> hook. The associated handler
936 will be called with the error text and can change the error message, if
937 it sees fit, by calling C<die()> again. See L<perlvar/$SIG{expr}> for details on
938 setting C<%SIG> entries, and L<"eval BLOCK"> for some examples.
940 Note that the C<$SIG{__DIE__}> hook is called even inside eval()ed
941 blocks/strings. If one wants the hook to do nothing in such
946 as the first line of the handler (see L<perlvar/$^S>).
950 Not really a function. Returns the value of the last command in the
951 sequence of commands indicated by BLOCK. When modified by a loop
952 modifier, executes the BLOCK once before testing the loop condition.
953 (On other statements the loop modifiers test the conditional first.)
955 C<do BLOCK> does I<not> count as a loop, so the loop control statements
956 C<next>, C<last> or C<redo> cannot be used to leave or restart the block.
958 =item do SUBROUTINE(LIST)
960 A deprecated form of subroutine call. See L<perlsub>.
964 Uses the value of EXPR as a filename and executes the contents of the
965 file as a Perl script. Its primary use is to include subroutines
966 from a Perl subroutine library.
972 scalar eval `cat stat.pl`;
974 except that it's more efficient and concise, keeps track of the
975 current filename for error messages, and searches all the B<-I>
976 libraries if the file isn't in the current directory (see also the @INC
977 array in L<perlvar/Predefined Names>). It is also different in how
978 code evaluated with C<do FILENAME> doesn't see lexicals in the enclosing
979 scope like C<eval STRING> does. It's the same, however, in that it does
980 reparse the file every time you call it, so you probably don't want to
981 do this inside a loop.
983 If C<do> cannot read the file, it returns undef and sets C<$!> to the
984 error. If C<do> can read the file but cannot compile it, it
985 returns undef and sets an error message in C<$@>. If the file is
986 successfully compiled, C<do> returns the value of the last expression
989 Note that inclusion of library modules is better done with the
990 C<use()> and C<require()> operators, which also do automatic error checking
991 and raise an exception if there's a problem.
993 You might like to use C<do> to read in a program configuration
994 file. Manual error checking can be done this way:
996 # read in config files: system first, then user
997 for $file ("/share/prog/defaults.rc",
998 "$ENV{HOME}/.someprogrc") {
999 unless ($return = do $file) {
1000 warn "couldn't parse $file: $@" if $@;
1001 warn "couldn't do $file: $!" unless defined $return;
1002 warn "couldn't run $file" unless $return;
1010 This causes an immediate core dump. Primarily this is so that you can
1011 use the B<undump> program to turn your core dump into an executable binary
1012 after having initialized all your variables at the beginning of the
1013 program. When the new binary is executed it will begin by executing a
1014 C<goto LABEL> (with all the restrictions that C<goto> suffers). Think of
1015 it as a goto with an intervening core dump and reincarnation. If C<LABEL>
1016 is omitted, restarts the program from the top. WARNING: Any files
1017 opened at the time of the dump will NOT be open any more when the
1018 program is reincarnated, with possible resulting confusion on the part
1019 of Perl. See also B<-u> option in L<perlrun>.
1024 require 'getopt.pl';
1036 dump QUICKSTART if $ARGV[0] eq '-d';
1041 This operator is largely obsolete, partly because it's very hard to
1042 convert a core file into an executable, and because the real perl-to-C
1043 compiler has superseded it.
1047 When called in list context, returns a 2-element list consisting of the
1048 key and value for the next element of a hash, so that you can iterate over
1049 it. When called in scalar context, returns the key for only the "next"
1050 element in the hash. (Note: Keys may be C<"0"> or C<"">, which are logically
1051 false; you may wish to avoid constructs like C<while ($k = each %foo) {}>
1054 Entries are returned in an apparently random order. The actual random
1055 order is subject to change in future versions of perl, but it is guaranteed
1056 to be in the same order as either the C<keys()> or C<values()> function
1057 would produce on the same (unmodified) hash.
1059 When the hash is entirely read, a null array is returned in list context
1060 (which when assigned produces a FALSE (C<0>) value), and C<undef> in
1061 scalar context. The next call to C<each()> after that will start iterating
1062 again. There is a single iterator for each hash, shared by all C<each()>,
1063 C<keys()>, and C<values()> function calls in the program; it can be reset by
1064 reading all the elements from the hash, or by evaluating C<keys HASH> or
1065 C<values HASH>. If you add or delete elements of a hash while you're
1066 iterating over it, you may get entries skipped or duplicated, so don't.
1068 The following prints out your environment like the printenv(1) program,
1069 only in a different order:
1071 while (($key,$value) = each %ENV) {
1072 print "$key=$value\n";
1075 See also C<keys()>, C<values()> and C<sort()>.
1077 =item eof FILEHANDLE
1083 Returns 1 if the next read on FILEHANDLE will return end of file, or if
1084 FILEHANDLE is not open. FILEHANDLE may be an expression whose value
1085 gives the real filehandle. (Note that this function actually
1086 reads a character and then C<ungetc()>s it, so isn't very useful in an
1087 interactive context.) Do not read from a terminal file (or call
1088 C<eof(FILEHANDLE)> on it) after end-of-file is reached. Filetypes such
1089 as terminals may lose the end-of-file condition if you do.
1091 An C<eof> without an argument uses the last file read as argument.
1092 Using C<eof()> with empty parentheses is very different. It indicates the pseudo file formed of
1093 the files listed on the command line, i.e., C<eof()> is reasonable to
1094 use inside a C<while (E<lt>E<gt>)> loop to detect the end of only the
1095 last file. Use C<eof(ARGV)> or eof without the parentheses to test
1096 I<EACH> file in a while (E<lt>E<gt>) loop. Examples:
1098 # reset line numbering on each input file
1100 next if /^\s*#/; # skip comments
1103 close ARGV if eof; # Not eof()!
1106 # insert dashes just before last line of last file
1108 if (eof()) { # check for end of current file
1109 print "--------------\n";
1110 close(ARGV); # close or break; is needed if we
1111 # are reading from the terminal
1116 Practical hint: you almost never need to use C<eof> in Perl, because the
1117 input operators return false values when they run out of data, or if there
1124 In the first form, the return value of EXPR is parsed and executed as if it
1125 were a little Perl program. The value of the expression (which is itself
1126 determined within scalar context) is first parsed, and if there weren't any
1127 errors, executed in the context of the current Perl program, so that any
1128 variable settings or subroutine and format definitions remain afterwards.
1129 Note that the value is parsed every time the eval executes. If EXPR is
1130 omitted, evaluates C<$_>. This form is typically used to delay parsing
1131 and subsequent execution of the text of EXPR until run time.
1133 In the second form, the code within the BLOCK is parsed only once--at the
1134 same time the code surrounding the eval itself was parsed--and executed
1135 within the context of the current Perl program. This form is typically
1136 used to trap exceptions more efficiently than the first (see below), while
1137 also providing the benefit of checking the code within BLOCK at compile
1140 The final semicolon, if any, may be omitted from the value of EXPR or within
1143 In both forms, the value returned is the value of the last expression
1144 evaluated inside the mini-program; a return statement may be also used, just
1145 as with subroutines. The expression providing the return value is evaluated
1146 in void, scalar, or list context, depending on the context of the eval itself.
1147 See L</wantarray> for more on how the evaluation context can be determined.
1149 If there is a syntax error or runtime error, or a C<die()> statement is
1150 executed, an undefined value is returned by C<eval()>, and C<$@> is set to the
1151 error message. If there was no error, C<$@> is guaranteed to be a null
1152 string. Beware that using C<eval()> neither silences perl from printing
1153 warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
1154 To do either of those, you have to use the C<$SIG{__WARN__}> facility. See
1155 L</warn> and L<perlvar>.
1157 Note that, because C<eval()> traps otherwise-fatal errors, it is useful for
1158 determining whether a particular feature (such as C<socket()> or C<symlink()>)
1159 is implemented. It is also Perl's exception trapping mechanism, where
1160 the die operator is used to raise exceptions.
1162 If the code to be executed doesn't vary, you may use the eval-BLOCK
1163 form to trap run-time errors without incurring the penalty of
1164 recompiling each time. The error, if any, is still returned in C<$@>.
1167 # make divide-by-zero nonfatal
1168 eval { $answer = $a / $b; }; warn $@ if $@;
1170 # same thing, but less efficient
1171 eval '$answer = $a / $b'; warn $@ if $@;
1173 # a compile-time error
1174 eval { $answer = }; # WRONG
1177 eval '$answer ='; # sets $@
1179 When using the C<eval{}> form as an exception trap in libraries, you may
1180 wish not to trigger any C<__DIE__> hooks that user code may have
1181 installed. You can use the C<local $SIG{__DIE__}> construct for this
1182 purpose, as shown in this example:
1184 # a very private exception trap for divide-by-zero
1185 eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
1188 This is especially significant, given that C<__DIE__> hooks can call
1189 C<die()> again, which has the effect of changing their error messages:
1191 # __DIE__ hooks may modify error messages
1193 local $SIG{'__DIE__'} =
1194 sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
1195 eval { die "foo lives here" };
1196 print $@ if $@; # prints "bar lives here"
1199 With an C<eval()>, you should be especially careful to remember what's
1200 being looked at when:
1206 eval { $x }; # CASE 4
1208 eval "\$$x++"; # CASE 5
1211 Cases 1 and 2 above behave identically: they run the code contained in
1212 the variable C<$x>. (Although case 2 has misleading double quotes making
1213 the reader wonder what else might be happening (nothing is).) Cases 3
1214 and 4 likewise behave in the same way: they run the code C<'$x'>, which
1215 does nothing but return the value of C<$x>. (Case 4 is preferred for
1216 purely visual reasons, but it also has the advantage of compiling at
1217 compile-time instead of at run-time.) Case 5 is a place where
1218 normally you I<WOULD> like to use double quotes, except that in this
1219 particular situation, you can just use symbolic references instead, as
1222 C<eval BLOCK> does I<not> count as a loop, so the loop control statements
1223 C<next>, C<last> or C<redo> cannot be used to leave or restart the block.
1228 =item exec PROGRAM LIST
1230 The C<exec()> function executes a system command I<AND NEVER RETURNS> -
1231 use C<system()> instead of C<exec()> if you want it to return. It fails and
1232 returns FALSE only if the command does not exist I<and> it is executed
1233 directly instead of via your system's command shell (see below).
1235 Since it's a common mistake to use C<exec()> instead of C<system()>, Perl
1236 warns you if there is a following statement which isn't C<die()>, C<warn()>,
1237 or C<exit()> (if C<-w> is set - but you always do that). If you
1238 I<really> want to follow an C<exec()> with some other statement, you
1239 can use one of these styles to avoid the warning:
1241 exec ('foo') or print STDERR "couldn't exec foo: $!";
1242 { exec ('foo') }; print STDERR "couldn't exec foo: $!";
1244 If there is more than one argument in LIST, or if LIST is an array
1245 with more than one value, calls execvp(3) with the arguments in LIST.
1246 If there is only one scalar argument or an array with one element in it,
1247 the argument is checked for shell metacharacters, and if there are any,
1248 the entire argument is passed to the system's command shell for parsing
1249 (this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
1250 If there are no shell metacharacters in the argument, it is split into
1251 words and passed directly to C<execvp()>, which is more efficient. Note:
1252 C<exec()> and C<system()> do not flush your output buffer, so you may need to
1253 set C<$|> to avoid lost output. Examples:
1255 exec '/bin/echo', 'Your arguments are: ', @ARGV;
1256 exec "sort $outfile | uniq";
1258 If you don't really want to execute the first argument, but want to lie
1259 to the program you are executing about its own name, you can specify
1260 the program you actually want to run as an "indirect object" (without a
1261 comma) in front of the LIST. (This always forces interpretation of the
1262 LIST as a multivalued list, even if there is only a single scalar in
1265 $shell = '/bin/csh';
1266 exec $shell '-sh'; # pretend it's a login shell
1270 exec {'/bin/csh'} '-sh'; # pretend it's a login shell
1272 When the arguments get executed via the system shell, results will
1273 be subject to its quirks and capabilities. See L<perlop/"`STRING`">
1276 Using an indirect object with C<exec()> or C<system()> is also more secure.
1277 This usage forces interpretation of the arguments as a multivalued list,
1278 even if the list had just one argument. That way you're safe from the
1279 shell expanding wildcards or splitting up words with whitespace in them.
1281 @args = ( "echo surprise" );
1283 system @args; # subject to shell escapes
1285 system { $args[0] } @args; # safe even with one-arg list
1287 The first version, the one without the indirect object, ran the I<echo>
1288 program, passing it C<"surprise"> an argument. The second version
1289 didn't--it tried to run a program literally called I<"echo surprise">,
1290 didn't find it, and set C<$?> to a non-zero value indicating failure.
1292 Note that C<exec()> will not call your C<END> blocks, nor will it call
1293 any C<DESTROY> methods in your objects.
1297 Returns TRUE if the specified hash key exists in its hash array, even
1298 if the corresponding value is undefined.
1300 print "Exists\n" if exists $array{$key};
1301 print "Defined\n" if defined $array{$key};
1302 print "True\n" if $array{$key};
1304 A hash element can be TRUE only if it's defined, and defined if
1305 it exists, but the reverse doesn't necessarily hold true.
1307 Note that the EXPR can be arbitrarily complicated as long as the final
1308 operation is a hash key lookup:
1310 if (exists $ref->{"A"}{"B"}{$key}) { ... }
1312 Although the last element will not spring into existence just because its
1313 existence was tested, intervening ones will. Thus C<$ref-E<gt>{"A"}>
1314 C<$ref-E<gt>{"B"}> will spring into existence due to the existence
1315 test for a $key element. This autovivification may be fixed in a later
1320 Evaluates EXPR and exits immediately with that value. (Actually, it
1321 calls any defined C<END> routines first, but the C<END> routines may not
1322 abort the exit. Likewise any object destructors that need to be called
1323 are called before exit.) Example:
1326 exit 0 if $ans =~ /^[Xx]/;
1328 See also C<die()>. If EXPR is omitted, exits with C<0> status. The only
1329 universally portable values for EXPR are C<0> for success and C<1> for error;
1330 all other values are subject to unpredictable interpretation depending
1331 on the environment in which the Perl program is running.
1333 You shouldn't use C<exit()> to abort a subroutine if there's any chance that
1334 someone might want to trap whatever error happened. Use C<die()> instead,
1335 which can be trapped by an C<eval()>.
1337 All C<END{}> blocks are run at exit time. See L<perlsub> for details.
1343 Returns I<e> (the natural logarithm base) to the power of EXPR.
1344 If EXPR is omitted, gives C<exp($_)>.
1346 =item fcntl FILEHANDLE,FUNCTION,SCALAR
1348 Implements the fcntl(2) function. You'll probably have to say
1352 first to get the correct constant definitions. Argument processing and
1353 value return works just like C<ioctl()> below.
1357 fcntl($filehandle, F_GETFL, $packed_return_buffer)
1358 or die "can't fcntl F_GETFL: $!";
1360 You don't have to check for C<defined()> on the return from
1361 C<fnctl()>. Like C<ioctl()>, it maps a C<0> return from the system
1362 call into "C<0> but true" in Perl. This string is true in
1363 boolean context and C<0> in numeric context. It is also
1364 exempt from the normal B<-w> warnings on improper numeric
1367 Note that C<fcntl()> will produce a fatal error if used on a machine that
1368 doesn't implement fcntl(2).
1370 =item fileno FILEHANDLE
1372 Returns the file descriptor for a filehandle. This is useful for
1373 constructing bitmaps for C<select()> and low-level POSIX tty-handling
1374 operations. If FILEHANDLE is an expression, the value is taken as
1375 an indirect filehandle, generally its name.
1377 You can use this to find out whether two handles refer to the
1378 same underlying descriptor:
1380 if (fileno(THIS) == fileno(THAT)) {
1381 print "THIS and THAT are dups\n";
1384 =item flock FILEHANDLE,OPERATION
1386 Calls flock(2), or an emulation of it, on FILEHANDLE. Returns TRUE for
1387 success, FALSE on failure. Produces a fatal error if used on a machine
1388 that doesn't implement flock(2), fcntl(2) locking, or lockf(3). C<flock()>
1389 is Perl's portable file locking interface, although it locks only entire
1392 On many platforms (including most versions or clones of Unix), locks
1393 established by C<flock()> are B<merely advisory>. Such discretionary locks
1394 are more flexible, but offer fewer guarantees. This means that files
1395 locked with C<flock()> may be modified by programs that do not also use
1396 C<flock()>. Windows NT and OS/2 are among the platforms which
1397 enforce mandatory locking. See your local documentation for details.
1399 OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
1400 LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
1401 you can use the symbolic names if import them from the Fcntl module,
1402 either individually, or as a group using the ':flock' tag. LOCK_SH
1403 requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
1404 releases a previously requested lock. If LOCK_NB is added to LOCK_SH or
1405 LOCK_EX then C<flock()> will return immediately rather than blocking
1406 waiting for the lock (check the return status to see if you got it).
1408 To avoid the possibility of mis-coordination, Perl flushes FILEHANDLE
1409 before (un)locking it.
1411 Note that the emulation built with lockf(3) doesn't provide shared
1412 locks, and it requires that FILEHANDLE be open with write intent. These
1413 are the semantics that lockf(3) implements. Most (all?) systems
1414 implement lockf(3) in terms of fcntl(2) locking, though, so the
1415 differing semantics shouldn't bite too many people.
1417 Note also that some versions of C<flock()> cannot lock things over the
1418 network; you would need to use the more system-specific C<fcntl()> for
1419 that. If you like you can force Perl to ignore your system's flock(2)
1420 function, and so provide its own fcntl(2)-based emulation, by passing
1421 the switch C<-Ud_flock> to the F<Configure> program when you configure
1424 Here's a mailbox appender for BSD systems.
1426 use Fcntl ':flock'; # import LOCK_* constants
1429 flock(MBOX,LOCK_EX);
1430 # and, in case someone appended
1431 # while we were waiting...
1436 flock(MBOX,LOCK_UN);
1439 open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
1440 or die "Can't open mailbox: $!";
1443 print MBOX $msg,"\n\n";
1446 See also L<DB_File> for other flock() examples.
1450 Does a fork(2) system call. Returns the child pid to the parent process,
1451 C<0> to the child process, or C<undef> if the fork is unsuccessful.
1453 Note: unflushed buffers remain unflushed in both processes, which means
1454 you may need to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()>
1455 method of C<IO::Handle> to avoid duplicate output.
1457 If you C<fork()> without ever waiting on your children, you will accumulate
1460 $SIG{CHLD} = sub { wait };
1462 There's also the double-fork trick (error checking on
1463 C<fork()> returns omitted);
1465 unless ($pid = fork) {
1467 exec "what you really wanna do";
1470 ## (some_perl_code_here)
1477 See also L<perlipc> for more examples of forking and reaping
1480 Note that if your forked child inherits system file descriptors like
1481 STDIN and STDOUT that are actually connected by a pipe or socket, even
1482 if you exit, then the remote server (such as, say, httpd or rsh) won't think
1483 you're done. You should reopen those to F</dev/null> if it's any issue.
1487 Declare a picture format for use by the C<write()> function. For
1491 Test: @<<<<<<<< @||||| @>>>>>
1492 $str, $%, '$' . int($num)
1496 $num = $cost/$quantity;
1500 See L<perlform> for many details and examples.
1502 =item formline PICTURE,LIST
1504 This is an internal function used by C<format>s, though you may call it,
1505 too. It formats (see L<perlform>) a list of values according to the
1506 contents of PICTURE, placing the output into the format output
1507 accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
1508 Eventually, when a C<write()> is done, the contents of
1509 C<$^A> are written to some filehandle, but you could also read C<$^A>
1510 yourself and then set C<$^A> back to C<"">. Note that a format typically
1511 does one C<formline()> per line of form, but the C<formline()> function itself
1512 doesn't care how many newlines are embedded in the PICTURE. This means
1513 that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
1514 You may therefore need to use multiple formlines to implement a single
1515 record format, just like the format compiler.
1517 Be careful if you put double quotes around the picture, because an "C<@>"
1518 character may be taken to mean the beginning of an array name.
1519 C<formline()> always returns TRUE. See L<perlform> for other examples.
1521 =item getc FILEHANDLE
1525 Returns the next character from the input file attached to FILEHANDLE,
1526 or the undefined value at end of file, or if there was an error. If
1527 FILEHANDLE is omitted, reads from STDIN. This is not particularly
1528 efficient. It cannot be used to get unbuffered single-characters,
1529 however. For that, try something more like:
1532 system "stty cbreak </dev/tty >/dev/tty 2>&1";
1535 system "stty", '-icanon', 'eol', "\001";
1541 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
1544 system "stty", 'icanon', 'eol', '^@'; # ASCII null
1548 Determination of whether $BSD_STYLE should be set
1549 is left as an exercise to the reader.
1551 The C<POSIX::getattr()> function can do this more portably on systems
1552 purporting POSIX compliance.
1553 See also the C<Term::ReadKey> module from your nearest CPAN site;
1554 details on CPAN can be found on L<perlmodlib/CPAN>.
1558 Implements the C library function of the same name, which on most
1559 systems returns the current login from F</etc/utmp>, if any. If null,
1562 $login = getlogin || getpwuid($<) || "Kilroy";
1564 Do not consider C<getlogin()> for authentication: it is not as
1565 secure as C<getpwuid()>.
1567 =item getpeername SOCKET
1569 Returns the packed sockaddr address of other end of the SOCKET connection.
1572 $hersockaddr = getpeername(SOCK);
1573 ($port, $iaddr) = unpack_sockaddr_in($hersockaddr);
1574 $herhostname = gethostbyaddr($iaddr, AF_INET);
1575 $herstraddr = inet_ntoa($iaddr);
1579 Returns the current process group for the specified PID. Use
1580 a PID of C<0> to get the current process group for the
1581 current process. Will raise an exception if used on a machine that
1582 doesn't implement getpgrp(2). If PID is omitted, returns process
1583 group of current process. Note that the POSIX version of C<getpgrp()>
1584 does not accept a PID argument, so only C<PID==0> is truly portable.
1588 Returns the process id of the parent process.
1590 =item getpriority WHICH,WHO
1592 Returns the current priority for a process, a process group, or a user.
1593 (See L<getpriority(2)>.) Will raise a fatal exception if used on a
1594 machine that doesn't implement getpriority(2).
1600 =item gethostbyname NAME
1602 =item getnetbyname NAME
1604 =item getprotobyname NAME
1610 =item getservbyname NAME,PROTO
1612 =item gethostbyaddr ADDR,ADDRTYPE
1614 =item getnetbyaddr ADDR,ADDRTYPE
1616 =item getprotobynumber NUMBER
1618 =item getservbyport PORT,PROTO
1636 =item sethostent STAYOPEN
1638 =item setnetent STAYOPEN
1640 =item setprotoent STAYOPEN
1642 =item setservent STAYOPEN
1656 These routines perform the same functions as their counterparts in the
1657 system library. In list context, the return values from the
1658 various get routines are as follows:
1660 ($name,$passwd,$uid,$gid,
1661 $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
1662 ($name,$passwd,$gid,$members) = getgr*
1663 ($name,$aliases,$addrtype,$length,@addrs) = gethost*
1664 ($name,$aliases,$addrtype,$net) = getnet*
1665 ($name,$aliases,$proto) = getproto*
1666 ($name,$aliases,$port,$proto) = getserv*
1668 (If the entry doesn't exist you get a null list.)
1670 In scalar context, you get the name, unless the function was a
1671 lookup by name, in which case you get the other thing, whatever it is.
1672 (If the entry doesn't exist you get the undefined value.) For example:
1674 $uid = getpwnam($name);
1675 $name = getpwuid($num);
1677 $gid = getgrnam($name);
1678 $name = getgrgid($num;
1682 In I<getpw*()> the fields C<$quota>, C<$comment>, and C<$expire> are special
1683 cases in the sense that in many systems they are unsupported. If the
1684 C<$quota> is unsupported, it is an empty scalar. If it is supported, it
1685 usually encodes the disk quota. If the C<$comment> field is unsupported,
1686 it is an empty scalar. If it is supported it usually encodes some
1687 administrative comment about the user. In some systems the $quota
1688 field may be C<$change> or C<$age>, fields that have to do with password
1689 aging. In some systems the C<$comment> field may be C<$class>. The C<$expire>
1690 field, if present, encodes the expiration period of the account or the
1691 password. For the availability and the exact meaning of these fields
1692 in your system, please consult your getpwnam(3) documentation and your
1693 F<pwd.h> file. You can also find out from within Perl which meaning
1694 your C<$quota> and C<$comment> fields have and whether you have the C<$expire>
1695 field by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
1696 C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>.
1698 The C<$members> value returned by I<getgr*()> is a space separated list of
1699 the login names of the members of the group.
1701 For the I<gethost*()> functions, if the C<h_errno> variable is supported in
1702 C, it will be returned to you via C<$?> if the function call fails. The
1703 C<@addrs> value returned by a successful call is a list of the raw
1704 addresses returned by the corresponding system library call. In the
1705 Internet domain, each address is four bytes long and you can unpack it
1706 by saying something like:
1708 ($a,$b,$c,$d) = unpack('C4',$addr[0]);
1710 If you get tired of remembering which element of the return list contains
1711 which return value, by-name interfaces are also provided in modules:
1712 C<File::stat>, C<Net::hostent>, C<Net::netent>, C<Net::protoent>, C<Net::servent>,
1713 C<Time::gmtime>, C<Time::localtime>, and C<User::grent>. These override the
1714 normal built-in, replacing them with versions that return objects with
1715 the appropriate names for each field. For example:
1719 $is_his = (stat($filename)->uid == pwent($whoever)->uid);
1721 Even though it looks like they're the same method calls (uid),
1722 they aren't, because a C<File::stat> object is different from a C<User::pwent> object.
1724 =item getsockname SOCKET
1726 Returns the packed sockaddr address of this end of the SOCKET connection.
1729 $mysockaddr = getsockname(SOCK);
1730 ($port, $myaddr) = unpack_sockaddr_in($mysockaddr);
1732 =item getsockopt SOCKET,LEVEL,OPTNAME
1734 Returns the socket option requested, or undef if there is an error.
1740 Returns the value of EXPR with filename expansions such as the standard Unix shell F</bin/sh> would
1741 do. This is the internal function implementing the C<E<lt>*.cE<gt>>
1742 operator, but you can use it directly. If EXPR is omitted, C<$_> is used.
1743 The C<E<lt>*.cE<gt>> operator is discussed in more detail in
1744 L<perlop/"I/O Operators">.
1748 Converts a time as returned by the time function to a 9-element array
1749 with the time localized for the standard Greenwich time zone.
1750 Typically used as follows:
1753 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
1756 All array elements are numeric, and come straight out of a struct tm.
1757 In particular this means that C<$mon> has the range C<0..11> and C<$wday> has
1758 the range C<0..6> with sunday as day C<0>. Also, C<$year> is the number of
1759 years since 1900, that is, C<$year> is C<123> in year 2023, I<not> simply the last two digits of the year.
1761 If EXPR is omitted, does C<gmtime(time())>.
1763 In scalar context, returns the ctime(3) value:
1765 $now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
1767 Also see the C<timegm()> function provided by the C<Time::Local> module,
1768 and the strftime(3) function available via the POSIX module.
1770 This scalar value is B<not> locale dependent, see L<perllocale>, but
1771 instead a Perl builtin. Also see the C<Time::Local> module, and the
1772 strftime(3) and mktime(3) function available via the POSIX module. To
1773 get somewhat similar but locale dependent date strings, set up your
1774 locale environment variables appropriately (please see L<perllocale>)
1775 and try for example:
1777 use POSIX qw(strftime);
1778 $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
1780 Note that the C<%a> and C<%b>, the short forms of the day of the week
1781 and the month of the year, may not necessarily be three characters wide.
1789 The C<goto-LABEL> form finds the statement labeled with LABEL and resumes
1790 execution there. It may not be used to go into any construct that
1791 requires initialization, such as a subroutine or a C<foreach> loop. It
1792 also can't be used to go into a construct that is optimized away,
1793 or to get out of a block or subroutine given to C<sort()>.
1794 It can be used to go almost anywhere else within the dynamic scope,
1795 including out of subroutines, but it's usually better to use some other
1796 construct such as C<last> or C<die()>. The author of Perl has never felt the
1797 need to use this form of C<goto> (in Perl, that is--C is another matter).
1799 The C<goto-EXPR> form expects a label name, whose scope will be resolved
1800 dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
1801 necessarily recommended if you're optimizing for maintainability:
1803 goto ("FOO", "BAR", "GLARCH")[$i];
1805 The C<goto-&NAME> form is highly magical, and substitutes a call to the
1806 named subroutine for the currently running subroutine. This is used by
1807 C<AUTOLOAD> subroutines that wish to load another subroutine and then
1808 pretend that the other subroutine had been called in the first place
1809 (except that any modifications to C<@_> in the current subroutine are
1810 propagated to the other subroutine.) After the C<goto>, not even C<caller()>
1811 will be able to tell that this routine was called first.
1813 =item grep BLOCK LIST
1815 =item grep EXPR,LIST
1817 This is similar in spirit to, but not the same as, grep(1)
1818 and its relatives. In particular, it is not limited to using
1819 regular expressions.
1821 Evaluates the BLOCK or EXPR for each element of LIST (locally setting
1822 C<$_> to each element) and returns the list value consisting of those
1823 elements for which the expression evaluated to TRUE. In a scalar
1824 context, returns the number of times the expression was TRUE.
1826 @foo = grep(!/^#/, @bar); # weed out comments
1830 @foo = grep {!/^#/} @bar; # weed out comments
1832 Note that, because C<$_> is a reference into the list value, it can be used
1833 to modify the elements of the array. While this is useful and
1834 supported, it can cause bizarre results if the LIST is not a named
1835 array. Similarly, grep returns aliases into the original list,
1836 much like the way that a for loop's index variable aliases the list
1837 elements. That is, modifying an element of a list returned by grep
1838 (for example, in a C<foreach>, C<map()> or another C<grep()>)
1839 actually modifies the element in the original list.
1841 See also L</map> for an array composed of the results of the BLOCK or EXPR.
1847 Interprets EXPR as a hex string and returns the corresponding
1848 value. (To convert strings that might start with either 0 or 0x
1849 see L</oct>.) If EXPR is omitted, uses C<$_>.
1851 print hex '0xAf'; # prints '175'
1852 print hex 'aF'; # same
1856 There is no builtin C<import()> function. It is just an ordinary
1857 method (subroutine) defined (or inherited) by modules that wish to export
1858 names to another module. The C<use()> function calls the C<import()> method
1859 for the package used. See also L</use()>, L<perlmod>, and L<Exporter>.
1861 =item index STR,SUBSTR,POSITION
1863 =item index STR,SUBSTR
1865 Returns the position of the first occurrence of SUBSTR in STR at or after
1866 POSITION. If POSITION is omitted, starts searching from the beginning of
1867 the string. The return value is based at C<0> (or whatever you've set the C<$[>
1868 variable to--but don't do that). If the substring is not found, returns
1869 one less than the base, ordinarily C<-1>.
1875 Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
1876 You should not use this for rounding, because it truncates
1877 towards C<0>, and because machine representations of floating point
1878 numbers can sometimes produce counterintuitive results. Usually C<sprintf()> or C<printf()>,
1879 or the C<POSIX::floor> or C<POSIX::ceil> functions, would serve you better.
1881 =item ioctl FILEHANDLE,FUNCTION,SCALAR
1883 Implements the ioctl(2) function. You'll probably have to say
1885 require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph
1887 first to get the correct function definitions. If F<ioctl.ph> doesn't
1888 exist or doesn't have the correct definitions you'll have to roll your
1889 own, based on your C header files such as F<E<lt>sys/ioctl.hE<gt>>.
1890 (There is a Perl script called B<h2ph> that comes with the Perl kit that
1891 may help you in this, but it's nontrivial.) SCALAR will be read and/or
1892 written depending on the FUNCTION--a pointer to the string value of SCALAR
1893 will be passed as the third argument of the actual C<ioctl()> call. (If SCALAR
1894 has no string value but does have a numeric value, that value will be
1895 passed rather than a pointer to the string value. To guarantee this to be
1896 TRUE, add a C<0> to the scalar before using it.) The C<pack()> and C<unpack()>
1897 functions are useful for manipulating the values of structures used by
1898 C<ioctl()>. The following example sets the erase character to DEL.
1902 die "NO TIOCGETP" if $@ || !$getp;
1903 $sgttyb_t = "ccccs"; # 4 chars and a short
1904 if (ioctl(STDIN,$getp,$sgttyb)) {
1905 @ary = unpack($sgttyb_t,$sgttyb);
1907 $sgttyb = pack($sgttyb_t,@ary);
1908 ioctl(STDIN,&TIOCSETP,$sgttyb)
1909 || die "Can't ioctl: $!";
1912 The return value of C<ioctl()> (and C<fcntl()>) is as follows:
1914 if OS returns: then Perl returns:
1916 0 string "0 but true"
1917 anything else that number
1919 Thus Perl returns TRUE on success and FALSE on failure, yet you can
1920 still easily determine the actual value returned by the operating
1923 ($retval = ioctl(...)) || ($retval = -1);
1924 printf "System returned %d\n", $retval;
1926 The special string "C<0> but true" is excempt from B<-w> complaints
1927 about improper numeric conversions.
1929 =item join EXPR,LIST
1931 Joins the separate strings of LIST into a single string with
1932 fields separated by the value of EXPR, and returns the string.
1935 $_ = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
1941 Returns a list consisting of all the keys of the named hash. (In a
1942 scalar context, returns the number of keys.) The keys are returned in
1943 an apparently random order. The actual random order is subject to
1944 change in future versions of perl, but it is guaranteed to be the same
1945 order as either the C<values()> or C<each()> function produces (given
1946 that the hash has not been modified). As a side effect, it resets
1949 Here is yet another way to print your environment:
1952 @values = values %ENV;
1953 while ($#keys >= 0) {
1954 print pop(@keys), '=', pop(@values), "\n";
1957 or how about sorted by key:
1959 foreach $key (sort(keys %ENV)) {
1960 print $key, '=', $ENV{$key}, "\n";
1963 To sort a hash by value, you'll need to use a C<sort()> function.
1964 Here's a descending numeric sort of a hash by its values:
1966 foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
1967 printf "%4d %s\n", $hash{$key}, $key;
1970 As an lvalue C<keys()> allows you to increase the number of hash buckets
1971 allocated for the given hash. This can gain you a measure of efficiency if
1972 you know the hash is going to get big. (This is similar to pre-extending
1973 an array by assigning a larger number to $#array.) If you say
1977 then C<%hash> will have at least 200 buckets allocated for it--256 of them,
1978 in fact, since it rounds up to the next power of two. These
1979 buckets will be retained even if you do C<%hash = ()>, use C<undef
1980 %hash> if you want to free the storage while C<%hash> is still in scope.
1981 You can't shrink the number of buckets allocated for the hash using
1982 C<keys()> in this way (but you needn't worry about doing this by accident,
1983 as trying has no effect).
1985 See also C<each()>, C<values()> and C<sort()>.
1989 Sends a signal to a list of processes. The first element of
1990 the list must be the signal to send. Returns the number of
1991 processes successfully signaled.
1993 $cnt = kill 1, $child1, $child2;
1996 Unlike in the shell, in Perl if the I<SIGNAL> is negative, it kills
1997 process groups instead of processes. (On System V, a negative I<PROCESS>
1998 number will also kill process groups, but that's not portable.) That
1999 means you usually want to use positive not negative signals. You may also
2000 use a signal name in quotes. See L<perlipc/"Signals"> for details.
2006 The C<last> command is like the C<break> statement in C (as used in
2007 loops); it immediately exits the loop in question. If the LABEL is
2008 omitted, the command refers to the innermost enclosing loop. The
2009 C<continue> block, if any, is not executed:
2011 LINE: while (<STDIN>) {
2012 last LINE if /^$/; # exit when done with header
2016 C<last> cannot be used to exit a block which returns a value such as
2017 C<eval {}>, C<sub {}> or C<do {}>.
2019 See also L</continue> for an illustration of how C<last>, C<next>, and
2026 Returns an lowercased version of EXPR. This is the internal function
2027 implementing the C<\L> escape in double-quoted strings.
2028 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
2030 If EXPR is omitted, uses C<$_>.
2036 Returns the value of EXPR with the first character lowercased. This is
2037 the internal function implementing the C<\l> escape in double-quoted strings.
2038 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
2040 If EXPR is omitted, uses C<$_>.
2046 Returns the length in characters of the value of EXPR. If EXPR is
2047 omitted, returns length of C<$_>.
2049 =item link OLDFILE,NEWFILE
2051 Creates a new filename linked to the old filename. Returns TRUE for
2052 success, FALSE otherwise.
2054 =item listen SOCKET,QUEUESIZE
2056 Does the same thing that the listen system call does. Returns TRUE if
2057 it succeeded, FALSE otherwise. See example in L<perlipc/"Sockets: Client/Server Communication">.
2061 A local modifies the listed variables to be local to the enclosing
2062 block, file, or eval. If more than one value is listed, the list must
2063 be placed in parentheses. See L<perlsub/"Temporary Values via local()">
2064 for details, including issues with tied arrays and hashes.
2066 You really probably want to be using C<my()> instead, because C<local()> isn't
2067 what most people think of as "local". See L<perlsub/"Private Variables
2068 via my()"> for details.
2070 =item localtime EXPR
2072 Converts a time as returned by the time function to a 9-element array
2073 with the time analyzed for the local time zone. Typically used as
2077 ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
2080 All array elements are numeric, and come straight out of a struct tm.
2081 In particular this means that C<$mon> has the range C<0..11> and C<$wday> has
2082 the range C<0..6> with sunday as day C<0>. Also, C<$year> is the number of
2083 years since 1900, that is, C<$year> is C<123> in year 2023, and I<not> simply the last two digits of the year.
2085 If EXPR is omitted, uses the current time (C<localtime(time)>).
2087 In scalar context, returns the ctime(3) value:
2089 $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
2091 This scalar value is B<not> locale dependent, see L<perllocale>, but
2092 instead a Perl builtin. Also see the C<Time::Local> module, and the
2093 strftime(3) and mktime(3) function available via the POSIX module. To
2094 get somewhat similar but locale dependent date strings, set up your
2095 locale environment variables appropriately (please see L<perllocale>)
2096 and try for example:
2098 use POSIX qw(strftime);
2099 $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
2101 Note that the C<%a> and C<%b>, the short forms of the day of the week
2102 and the month of the year, may not necessarily be three characters wide.
2108 Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted, returns log
2111 =item lstat FILEHANDLE
2117 Does the same thing as the C<stat()> function (including setting the
2118 special C<_> filehandle) but stats a symbolic link instead of the file
2119 the symbolic link points to. If symbolic links are unimplemented on
2120 your system, a normal C<stat()> is done.
2122 If EXPR is omitted, stats C<$_>.
2126 The match operator. See L<perlop>.
2128 =item map BLOCK LIST
2132 Evaluates the BLOCK or EXPR for each element of LIST (locally setting C<$_> to each
2133 element) and returns the list value composed of the results of each such
2134 evaluation. Evaluates BLOCK or EXPR in a list context, so each element of LIST
2135 may produce zero, one, or more elements in the returned value.
2137 @chars = map(chr, @nums);
2139 translates a list of numbers to the corresponding characters. And
2141 %hash = map { getkey($_) => $_ } @array;
2143 is just a funny way to write
2146 foreach $_ (@array) {
2147 $hash{getkey($_)} = $_;
2150 Note that, because C<$_> is a reference into the list value, it can be used
2151 to modify the elements of the array. While this is useful and
2152 supported, it can cause bizarre results if the LIST is not a named
2153 array. See also L</grep> for an array composed of those items of the
2154 original list for which the BLOCK or EXPR evaluates to true.
2156 =item mkdir FILENAME,MODE
2158 Creates the directory specified by FILENAME, with permissions
2159 specified by MODE (as modified by C<umask>). If it succeeds it
2160 returns TRUE, otherwise it returns FALSE and sets C<$!> (errno).
2162 In general, it is better to create directories with permissive MODEs,
2163 and let the user modify that with their C<umask>, than it is to supply
2164 a restrictive MODE and give the user no way to be more permissive.
2165 The exceptions to this rule are when the file or directory should be
2166 kept private (mail files, for instance). The perlfunc(1) entry on
2167 C<umask> discusses the choice of MODE in more detail.
2169 =item msgctl ID,CMD,ARG
2171 Calls the System V IPC function msgctl(2). You'll probably have to say
2175 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
2176 then ARG must be a variable which will hold the returned C<msqid_ds>
2177 structure. Returns like C<ioctl()>: the undefined value for error, "C<0> but
2178 true" for zero, or the actual return value otherwise. See also
2179 C<IPC::SysV> and C<IPC::Semaphore::Msg> documentation.
2181 =item msgget KEY,FLAGS
2183 Calls the System V IPC function msgget(2). Returns the message queue
2184 id, or the undefined value if there is an error. See also C<IPC::SysV>
2185 and C<IPC::SysV::Msg> documentation.
2187 =item msgsnd ID,MSG,FLAGS
2189 Calls the System V IPC function msgsnd to send the message MSG to the
2190 message queue ID. MSG must begin with the long integer message type,
2191 which may be created with C<pack("l", $type)>. Returns TRUE if
2192 successful, or FALSE if there is an error. See also C<IPC::SysV>
2193 and C<IPC::SysV::Msg> documentation.
2195 =item msgrcv ID,VAR,SIZE,TYPE,FLAGS
2197 Calls the System V IPC function msgrcv to receive a message from
2198 message queue ID into variable VAR with a maximum message size of
2199 SIZE. Note that if a message is received, the message type will be
2200 the first thing in VAR, and the maximum length of VAR is SIZE plus the
2201 size of the message type. Returns TRUE if successful, or FALSE if
2202 there is an error. See also C<IPC::SysV> and C<IPC::SysV::Msg> documentation.
2206 A C<my()> declares the listed variables to be local (lexically) to the
2207 enclosing block, file, or C<eval()>. If
2208 more than one value is listed, the list must be placed in parentheses. See
2209 L<perlsub/"Private Variables via my()"> for details.
2215 The C<next> command is like the C<continue> statement in C; it starts
2216 the next iteration of the loop:
2218 LINE: while (<STDIN>) {
2219 next LINE if /^#/; # discard comments
2223 Note that if there were a C<continue> block on the above, it would get
2224 executed even on discarded lines. If the LABEL is omitted, the command
2225 refers to the innermost enclosing loop.
2227 C<next> cannot be used to exit a block which returns a value such as
2228 C<eval {}>, C<sub {}> or C<do {}>.
2230 See also L</continue> for an illustration of how C<last>, C<next>, and
2233 =item no Module LIST
2235 See the L</use> function, which C<no> is the opposite of.
2241 Interprets EXPR as an octal string and returns the corresponding
2242 value. (If EXPR happens to start off with C<0x>, interprets it as a
2243 hex string. If EXPR starts off with C<0b>, it is interpreted as a
2244 binary string.) The following will handle decimal, binary, octal, and
2245 hex in the standard Perl or C notation:
2247 $val = oct($val) if $val =~ /^0/;
2249 If EXPR is omitted, uses C<$_>. This function is commonly used when
2250 a string such as C<644> needs to be converted into a file mode, for
2251 example. (Although perl will automatically convert strings into
2252 numbers as needed, this automatic conversion assumes base 10.)
2254 =item open FILEHANDLE,EXPR
2256 =item open FILEHANDLE
2258 Opens the file whose filename is given by EXPR, and associates it with
2259 FILEHANDLE. If FILEHANDLE is an expression, its value is used as the
2260 name of the real filehandle wanted. If EXPR is omitted, the scalar
2261 variable of the same name as the FILEHANDLE contains the filename.
2262 (Note that lexical variables--those declared with C<my()>--will not work
2263 for this purpose; so if you're using C<my()>, specify EXPR in your call
2266 If the filename begins with C<'E<lt>'> or nothing, the file is opened for input.
2267 If the filename begins with C<'E<gt>'>, the file is truncated and opened for
2268 output, being created if necessary. If the filename begins with C<'E<gt>E<gt>'>,
2269 the file is opened for appending, again being created if necessary.
2270 You can put a C<'+'> in front of the C<'E<gt>'> or C<'E<lt>'> to indicate that
2271 you want both read and write access to the file; thus C<'+E<lt>'> is almost
2272 always preferred for read/write updates--the C<'+E<gt>'> mode would clobber the
2273 file first. You can't usually use either read-write mode for updating
2274 textfiles, since they have variable length records. See the B<-i>
2275 switch in L<perlrun> for a better approach. The file is created with
2276 permissions of C<0666> modified by the process' C<umask> value.
2278 The prefix and the filename may be separated with spaces.
2279 These various prefixes correspond to the fopen(3) modes of C<'r'>, C<'r+'>, C<'w'>,
2280 C<'w+'>, C<'a'>, and C<'a+'>.
2282 If the filename begins with C<'|'>, the filename is interpreted as a
2283 command to which output is to be piped, and if the filename ends with a
2284 C<'|'>, the filename is interpreted as a command which pipes output to
2285 us. See L<perlipc/"Using open() for IPC">
2286 for more examples of this. (You are not allowed to C<open()> to a command
2287 that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
2288 and L<perlipc/"Bidirectional Communication"> for alternatives.)
2290 Opening C<'-'> opens STDIN and opening C<'E<gt>-'> opens STDOUT. Open returns
2291 nonzero upon success, the undefined value otherwise. If the C<open()>
2292 involved a pipe, the return value happens to be the pid of the
2295 If you're unfortunate enough to be running Perl on a system that
2296 distinguishes between text files and binary files (modern operating
2297 systems don't care), then you should check out L</binmode> for tips for
2298 dealing with this. The key distinction between systems that need C<binmode()>
2299 and those that don't is their text file formats. Systems like Unix, MacOS, and
2300 Plan9, which delimit lines with a single character, and which encode that
2301 character in C as C<"\n">, do not need C<binmode()>. The rest need it.
2303 When opening a file, it's usually a bad idea to continue normal execution
2304 if the request failed, so C<open()> is frequently used in connection with
2305 C<die()>. Even if C<die()> won't do what you want (say, in a CGI script,
2306 where you want to make a nicely formatted error message (but there are
2307 modules that can help with that problem)) you should always check
2308 the return value from opening a file. The infrequent exception is when
2309 working with an unopened filehandle is actually what you want to do.
2314 open ARTICLE or die "Can't find article $ARTICLE: $!\n";
2315 while (<ARTICLE>) {...
2317 open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
2318 # if the open fails, output is discarded
2320 open(DBASE, '+<dbase.mine') # open for update
2321 or die "Can't open 'dbase.mine' for update: $!";
2323 open(ARTICLE, "caesar <$article |") # decrypt article
2324 or die "Can't start caesar: $!";
2326 open(EXTRACT, "|sort >/tmp/Tmp$$") # $$ is our process id
2327 or die "Can't start sort: $!";
2329 # process argument list of files along with any includes
2331 foreach $file (@ARGV) {
2332 process($file, 'fh00');
2336 my($filename, $input) = @_;
2337 $input++; # this is a string increment
2338 unless (open($input, $filename)) {
2339 print STDERR "Can't open $filename: $!\n";
2344 while (<$input>) { # note use of indirection
2345 if (/^#include "(.*)"/) {
2346 process($1, $input);
2353 You may also, in the Bourne shell tradition, specify an EXPR beginning
2354 with C<'E<gt>&'>, in which case the rest of the string is interpreted as the
2355 name of a filehandle (or file descriptor, if numeric) to be
2356 duped and opened. You may use C<&> after C<E<gt>>, C<E<gt>E<gt>>, C<E<lt>>, C<+E<gt>>,
2357 C<+E<gt>E<gt>>, and C<+E<lt>>. The
2358 mode you specify should match the mode of the original filehandle.
2359 (Duping a filehandle does not take into account any existing contents of
2361 Here is a script that saves, redirects, and restores STDOUT and
2365 open(OLDOUT, ">&STDOUT");
2366 open(OLDERR, ">&STDERR");
2368 open(STDOUT, ">foo.out") || die "Can't redirect stdout";
2369 open(STDERR, ">&STDOUT") || die "Can't dup stdout";
2371 select(STDERR); $| = 1; # make unbuffered
2372 select(STDOUT); $| = 1; # make unbuffered
2374 print STDOUT "stdout 1\n"; # this works for
2375 print STDERR "stderr 1\n"; # subprocesses too
2380 open(STDOUT, ">&OLDOUT");
2381 open(STDERR, ">&OLDERR");
2383 print STDOUT "stdout 2\n";
2384 print STDERR "stderr 2\n";
2387 If you specify C<'E<lt>&=N'>, where C<N> is a number, then Perl will do an
2388 equivalent of C's C<fdopen()> of that file descriptor; this is more
2389 parsimonious of file descriptors. For example:
2391 open(FILEHANDLE, "<&=$fd")
2393 If you open a pipe on the command C<'-'>, i.e., either C<'|-'> or C<'-|'>, then
2394 there is an implicit fork done, and the return value of open is the pid
2395 of the child within the parent process, and C<0> within the child
2396 process. (Use C<defined($pid)> to determine whether the open was successful.)
2397 The filehandle behaves normally for the parent, but i/o to that
2398 filehandle is piped from/to the STDOUT/STDIN of the child process.
2399 In the child process the filehandle isn't opened--i/o happens from/to
2400 the new STDOUT or STDIN. Typically this is used like the normal
2401 piped open when you want to exercise more control over just how the
2402 pipe command gets executed, such as when you are running setuid, and
2403 don't want to have to scan shell commands for metacharacters.
2404 The following pairs are more or less equivalent:
2406 open(FOO, "|tr '[a-z]' '[A-Z]'");
2407 open(FOO, "|-") || exec 'tr', '[a-z]', '[A-Z]';
2409 open(FOO, "cat -n '$file'|");
2410 open(FOO, "-|") || exec 'cat', '-n', $file;
2412 See L<perlipc/"Safe Pipe Opens"> for more examples of this.
2414 NOTE: On any operation that may do a fork, any unflushed buffers remain
2415 unflushed in both processes, which means you may need to set C<$|> to
2416 avoid duplicate output. On systems that support a close-on-exec flag on
2417 files, the flag will be set for the newly opened file descriptor as
2418 determined by the value of $^F. See L<perlvar/$^F>.
2420 Closing any piped filehandle causes the parent process to wait for the
2421 child to finish, and returns the status value in C<$?>.
2423 The filename passed to open will have leading and trailing
2424 whitespace deleted, and the normal redirection characters
2425 honored. This property, known as "magic open",
2426 can often be used to good effect. A user could specify a filename of
2427 F<"rsh cat file |">, or you could change certain filenames as needed:
2429 $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
2430 open(FH, $filename) or die "Can't open $filename: $!";
2432 However, to open a file with arbitrary weird characters in it, it's
2433 necessary to protect any leading and trailing whitespace:
2435 $file =~ s#^(\s)#./$1#;
2436 open(FOO, "< $file\0");
2438 If you want a "real" C C<open()> (see L<open(2)> on your system), then you
2439 should use the C<sysopen()> function, which involves no such magic. This is
2440 another way to protect your filenames from interpretation. For example:
2443 sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
2444 or die "sysopen $path: $!";
2445 $oldfh = select(HANDLE); $| = 1; select($oldfh);
2446 print HANDLE "stuff $$\n");
2448 print "File contains: ", <HANDLE>;
2450 Using the constructor from the C<IO::Handle> package (or one of its
2451 subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous
2452 filehandles that have the scope of whatever variables hold references to
2453 them, and automatically close whenever and however you leave that scope:
2457 sub read_myfile_munged {
2459 my $handle = new IO::File;
2460 open($handle, "myfile") or die "myfile: $!";
2462 or return (); # Automatically closed here.
2463 mung $first or die "mung failed"; # Or here.
2464 return $first, <$handle> if $ALL; # Or here.
2468 See L</seek> for some details about mixing reading and writing.
2470 =item opendir DIRHANDLE,EXPR
2472 Opens a directory named EXPR for processing by C<readdir()>, C<telldir()>,
2473 C<seekdir()>, C<rewinddir()>, and C<closedir()>. Returns TRUE if successful.
2474 DIRHANDLEs have their own namespace separate from FILEHANDLEs.
2480 Returns the numeric (ASCII or Unicode) value of the first character of EXPR. If
2481 EXPR is omitted, uses C<$_>. For the reverse, see L</chr>.
2483 =item pack TEMPLATE,LIST
2485 Takes an array or list of values and packs it into a binary structure,
2486 returning the string containing the structure. The TEMPLATE is a
2487 sequence of characters that give the order and type of values, as
2490 a A string with arbitrary binary data, will be null padded.
2491 A An ascii string, will be space padded.
2492 Z A null terminated (asciz) string, will be null padded.
2494 b A bit string (ascending bit order, like vec()).
2495 B A bit string (descending bit order).
2496 h A hex string (low nybble first).
2497 H A hex string (high nybble first).
2499 c A signed char value.
2500 C An unsigned char value. Only does bytes. See U for Unicode.
2502 s A signed short value.
2503 S An unsigned short value.
2504 (This 'short' is _exactly_ 16 bits, which may differ from
2505 what a local C compiler calls 'short'.)
2507 i A signed integer value.
2508 I An unsigned integer value.
2509 (This 'integer' is _at_least_ 32 bits wide. Its exact
2510 size depends on what a local C compiler calls 'int',
2511 and may even be larger than the 'long' described in
2514 l A signed long value.
2515 L An unsigned long value.
2516 (This 'long' is _exactly_ 32 bits, which may differ from
2517 what a local C compiler calls 'long'.)
2519 n A short in "network" (big-endian) order.
2520 N A long in "network" (big-endian) order.
2521 v A short in "VAX" (little-endian) order.
2522 V A long in "VAX" (little-endian) order.
2523 (These 'shorts' and 'longs' are _exactly_ 16 bits and
2524 _exactly_ 32 bits, respectively.)
2526 q A signed quad (64-bit) value.
2527 Q An unsigned quad value.
2528 (Available only if your system supports 64-bit integer values
2529 _and_ if Perl has been compiled to support those.
2530 Causes a fatal error otherwise.)
2532 f A single-precision float in the native format.
2533 d A double-precision float in the native format.
2535 p A pointer to a null-terminated string.
2536 P A pointer to a structure (fixed-length string).
2538 u A uuencoded string.
2539 U A Unicode character number. Encodes to UTF-8 internally.
2540 Works even if C<use utf8> is not in effect.
2542 w A BER compressed integer. Its bytes represent an unsigned
2543 integer in base 128, most significant digit first, with as
2544 few digits as possible. Bit eight (the high bit) is set
2545 on each byte except the last.
2549 @ Null fill to absolute position.
2551 The following rules apply:
2557 Each letter may optionally be followed by a number giving a repeat
2558 count. With all types except C<"a">, C<"A">, C<"Z">, C<"b">, C<"B">, C<"h">,
2559 C<"H">, and C<"P"> the pack function will gobble up that many values from
2560 the LIST. A C<*> for the repeat count means to use however many items are
2565 The C<"a">, C<"A"> and C<"Z"> types gobble just one value, but pack it as a
2566 string of length count, padding with nulls or spaces as necessary. When
2567 unpacking, C<"A"> strips trailing spaces and nulls, C<"Z"> strips everything
2568 after the first null, and C<"a"> returns data verbatim.
2572 Likewise, the C<"b"> and C<"B"> fields pack a string that many bits long.
2576 The C<"h"> and C<"H"> fields pack a string that many nybbles long.
2580 The C<"p"> type packs a pointer to a null-terminated string. You are
2581 responsible for ensuring the string is not a temporary value (which can
2582 potentially get deallocated before you get around to using the packed result).
2583 The C<"P"> type packs a pointer to a structure of the size indicated by the
2584 length. A NULL pointer is created if the corresponding value for C<"p"> or
2589 Real numbers (floats and doubles) are in the native machine format only;
2590 due to the multiplicity of floating formats around, and the lack of a
2591 standard "network" representation, no facility for interchange has been
2592 made. This means that packed floating point data written on one machine
2593 may not be readable on another - even if both use IEEE floating point
2594 arithmetic (as the endian-ness of the memory representation is not part
2597 Note that Perl uses doubles internally for all numeric calculation, and
2598 converting from double into float and thence back to double again will
2599 lose precision (i.e., C<unpack("f", pack("f", $foo)>) will not in general
2606 $foo = pack("CCCC",65,66,67,68);
2608 $foo = pack("C4",65,66,67,68);
2610 $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);
2611 # same thing with Unicode circled letters
2613 $foo = pack("ccxxcc",65,66,67,68);
2616 $foo = pack("s2",1,2);
2617 # "\1\0\2\0" on little-endian
2618 # "\0\1\0\2" on big-endian
2620 $foo = pack("a4","abcd","x","y","z");
2623 $foo = pack("aaaa","abcd","x","y","z");
2626 $foo = pack("a14","abcdefg");
2627 # "abcdefg\0\0\0\0\0\0\0"
2629 $foo = pack("i9pl", gmtime);
2630 # a real struct tm (on my system anyway)
2632 $utmp_template = "Z8 Z8 Z16 L";
2633 $utmp = pack($utmp_template, @utmp1);
2634 # a struct utmp (BSDish)
2636 @utmp2 = unpack($utmp_template, $utmp);
2637 # "@utmp1" eq "@utmp2"
2640 unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
2643 The same template may generally also be used in unpack().
2647 =item package NAMESPACE
2649 Declares the compilation unit as being in the given namespace. The scope
2650 of the package declaration is from the declaration itself through the end of
2651 the enclosing block (the same scope as the C<local()> operator). All further
2652 unqualified dynamic identifiers will be in this namespace. A package
2653 statement affects only dynamic variables--including those you've used
2654 C<local()> on--but I<not> lexical variables created with C<my()>. Typically it
2655 would be the first declaration in a file to be included by the C<require>
2656 or C<use> operator. You can switch into a package in more than one place;
2657 it merely influences which symbol table is used by the compiler for the
2658 rest of that block. You can refer to variables and filehandles in other
2659 packages by prefixing the identifier with the package name and a double
2660 colon: C<$Package::Variable>. If the package name is null, the C<main>
2661 package as assumed. That is, C<$::sail> is equivalent to C<$main::sail>.
2663 If NAMESPACE is omitted, then there is no current package, and all
2664 identifiers must be fully qualified or lexicals. This is stricter
2665 than C<use strict>, since it also extends to function names.
2667 See L<perlmod/"Packages"> for more information about packages, modules,
2668 and classes. See L<perlsub> for other scoping issues.
2670 =item pipe READHANDLE,WRITEHANDLE
2672 Opens a pair of connected pipes like the corresponding system call.
2673 Note that if you set up a loop of piped processes, deadlock can occur
2674 unless you are very careful. In addition, note that Perl's pipes use
2675 stdio buffering, so you may need to set C<$|> to flush your WRITEHANDLE
2676 after each command, depending on the application.
2678 See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
2679 for examples of such things.
2681 On systems that support a close-on-exec flag on files, the flag will be set
2682 for the newly opened file descriptors as determined by the value of $^F.
2689 Pops and returns the last value of the array, shortening the array by
2690 1. Has a similar effect to
2692 $tmp = $ARRAY[$#ARRAY--];
2694 If there are no elements in the array, returns the undefined value.
2695 If ARRAY is omitted, pops the
2696 C<@ARGV> array in the main program, and the C<@_> array in subroutines, just
2703 Returns the offset of where the last C<m//g> search left off for the variable
2704 is in question (C<$_> is used when the variable is not specified). May be
2705 modified to change that offset. Such modification will also influence
2706 the C<\G> zero-width assertion in regular expressions. See L<perlre> and
2709 =item print FILEHANDLE LIST
2715 Prints a string or a comma-separated list of strings. Returns TRUE
2716 if successful. FILEHANDLE may be a scalar variable name, in which case
2717 the variable contains the name of or a reference to the filehandle, thus introducing one
2718 level of indirection. (NOTE: If FILEHANDLE is a variable and the next
2719 token is a term, it may be misinterpreted as an operator unless you
2720 interpose a C<+> or put parentheses around the arguments.) If FILEHANDLE is
2721 omitted, prints by default to standard output (or to the last selected
2722 output channel--see L</select>). If LIST is also omitted, prints C<$_> to
2723 the currently selected output channel. To set the default output channel to something other than
2724 STDOUT use the select operation. Note that, because print takes a
2725 LIST, anything in the LIST is evaluated in list context, and any
2726 subroutine that you call will have one or more of its expressions
2727 evaluated in list context. Also be careful not to follow the print
2728 keyword with a left parenthesis unless you want the corresponding right
2729 parenthesis to terminate the arguments to the print--interpose a C<+> or
2730 put parentheses around all the arguments.
2732 Note that if you're storing FILEHANDLES in an array or other expression,
2733 you will have to use a block returning its value instead:
2735 print { $files[$i] } "stuff\n";
2736 print { $OK ? STDOUT : STDERR } "stuff\n";
2738 =item printf FILEHANDLE FORMAT, LIST
2740 =item printf FORMAT, LIST
2742 Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that C<$\>
2743 (the output record separator) is not appended. The first argument
2744 of the list will be interpreted as the C<printf()> format. If C<use locale> is
2745 in effect, the character used for the decimal point in formatted real numbers
2746 is affected by the LC_NUMERIC locale. See L<perllocale>.
2748 Don't fall into the trap of using a C<printf()> when a simple
2749 C<print()> would do. The C<print()> is more efficient and less
2752 =item prototype FUNCTION
2754 Returns the prototype of a function as a string (or C<undef> if the
2755 function has no prototype). FUNCTION is a reference to, or the name of,
2756 the function whose prototype you want to retrieve.
2758 If FUNCTION is a string starting with C<CORE::>, the rest is taken as
2759 a name for Perl builtin. If builtin is not I<overridable> (such as
2760 C<qw//>) or its arguments cannot be expressed by a prototype (such as
2761 C<system()>) - in other words, the builtin does not behave like a Perl
2762 function - returns C<undef>. Otherwise, the string describing the
2763 equivalent prototype is returned.
2765 =item push ARRAY,LIST
2767 Treats ARRAY as a stack, and pushes the values of LIST
2768 onto the end of ARRAY. The length of ARRAY increases by the length of
2769 LIST. Has the same effect as
2772 $ARRAY[++$#ARRAY] = $value;
2775 but is more efficient. Returns the new number of elements in the array.
2787 Generalized quotes. See L<perlop/"Regexp Quote-Like Operators">.
2789 =item quotemeta EXPR
2793 Returns the value of EXPR with all non-alphanumeric
2794 characters backslashed. (That is, all characters not matching
2795 C</[A-Za-z_0-9]/> will be preceded by a backslash in the
2796 returned string, regardless of any locale settings.)
2797 This is the internal function implementing
2798 the C<\Q> escape in double-quoted strings.
2800 If EXPR is omitted, uses C<$_>.
2806 Returns a random fractional number greater than or equal to C<0> and less
2807 than the value of EXPR. (EXPR should be positive.) If EXPR is
2808 omitted, the value C<1> is used. Automatically calls C<srand()> unless
2809 C<srand()> has already been called. See also C<srand()>.
2811 (Note: If your rand function consistently returns numbers that are too
2812 large or too small, then your version of Perl was probably compiled
2813 with the wrong number of RANDBITS.)
2815 =item read FILEHANDLE,SCALAR,LENGTH,OFFSET
2817 =item read FILEHANDLE,SCALAR,LENGTH
2819 Attempts to read LENGTH bytes of data into variable SCALAR from the
2820 specified FILEHANDLE. Returns the number of bytes actually read,
2821 C<0> at end of file, or undef if there was an error. SCALAR will be grown
2822 or shrunk to the length actually read. An OFFSET may be specified to
2823 place the read data at some other place than the beginning of the
2824 string. This call is actually implemented in terms of stdio's fread(3)
2825 call. To get a true read(2) system call, see C<sysread()>.
2827 =item readdir DIRHANDLE
2829 Returns the next directory entry for a directory opened by C<opendir()>.
2830 If used in list context, returns all the rest of the entries in the
2831 directory. If there are no more entries, returns an undefined value in
2832 scalar context or a null list in list context.
2834 If you're planning to filetest the return values out of a C<readdir()>, you'd
2835 better prepend the directory in question. Otherwise, because we didn't
2836 C<chdir()> there, it would have been testing the wrong file.
2838 opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!";
2839 @dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR);
2844 Reads from the filehandle whose typeglob is contained in EXPR. In scalar
2845 context, each call reads and returns the next line, until end-of-file is
2846 reached, whereupon the subsequent call returns undef. In list context,
2847 reads until end-of-file is reached and returns a list of lines. Note that
2848 the notion of "line" used here is however you may have defined it
2849 with C<$/> or C<$INPUT_RECORD_SEPARATOR>). See L<perlvar/"$/">.
2851 When C<$/> is set to C<undef> and when readline() is in a scalar
2852 context (i.e. file slurp mode), it returns C<''> the first time,
2853 followed by C<undef> subsequently.
2855 This is the internal function implementing the C<E<lt>EXPRE<gt>>
2856 operator, but you can use it directly. The C<E<lt>EXPRE<gt>>
2857 operator is discussed in more detail in L<perlop/"I/O Operators">.
2860 $line = readline(*STDIN); # same thing
2866 Returns the value of a symbolic link, if symbolic links are
2867 implemented. If not, gives a fatal error. If there is some system
2868 error, returns the undefined value and sets C<$!> (errno). If EXPR is
2869 omitted, uses C<$_>.
2873 EXPR is executed as a system command.
2874 The collected standard output of the command is returned.
2875 In scalar context, it comes back as a single (potentially
2876 multi-line) string. In list context, returns a list of lines
2877 (however you've defined lines with C<$/> or C<$INPUT_RECORD_SEPARATOR>).
2878 This is the internal function implementing the C<qx/EXPR/>
2879 operator, but you can use it directly. The C<qx/EXPR/>
2880 operator is discussed in more detail in L<perlop/"I/O Operators">.
2882 =item recv SOCKET,SCALAR,LEN,FLAGS
2884 Receives a message on a socket. Attempts to receive LENGTH bytes of
2885 data into variable SCALAR from the specified SOCKET filehandle.
2886 Actually does a C C<recvfrom()>, so that it can return the address of the
2887 sender. Returns the undefined value if there's an error. SCALAR will
2888 be grown or shrunk to the length actually read. Takes the same flags
2889 as the system call of the same name.
2890 See L<perlipc/"UDP: Message Passing"> for examples.
2896 The C<redo> command restarts the loop block without evaluating the
2897 conditional again. The C<continue> block, if any, is not executed. If
2898 the LABEL is omitted, the command refers to the innermost enclosing
2899 loop. This command is normally used by programs that want to lie to
2900 themselves about what was just input:
2902 # a simpleminded Pascal comment stripper
2903 # (warning: assumes no { or } in strings)
2904 LINE: while (<STDIN>) {
2905 while (s|({.*}.*){.*}|$1 |) {}
2910 if (/}/) { # end of comment?
2919 C<redo> cannot be used to retry a block which returns a value such as
2920 C<eval {}>, C<sub {}> or C<do {}>.
2922 See also L</continue> for an illustration of how C<last>, C<next>, and
2929 Returns a TRUE value if EXPR is a reference, FALSE otherwise. If EXPR
2930 is not specified, C<$_> will be used. The value returned depends on the
2931 type of thing the reference is a reference to.
2932 Builtin types include:
2941 If the referenced object has been blessed into a package, then that package
2942 name is returned instead. You can think of C<ref()> as a C<typeof()> operator.
2944 if (ref($r) eq "HASH") {
2945 print "r is a reference to a hash.\n";
2948 print "r is not a reference at all.\n";
2951 See also L<perlref>.
2953 =item rename OLDNAME,NEWNAME
2955 Changes the name of a file. Returns C<1> for success, C<0> otherwise. Will
2956 not work across file system boundaries.
2962 Demands some semantics specified by EXPR, or by C<$_> if EXPR is not
2963 supplied. If EXPR is numeric, demands that the current version of Perl
2964 (C<$]> or $PERL_VERSION) be equal or greater than EXPR.
2966 Otherwise, demands that a library file be included if it hasn't already
2967 been included. The file is included via the do-FILE mechanism, which is
2968 essentially just a variety of C<eval()>. Has semantics similar to the following
2973 return 1 if $INC{$filename};
2974 my($realfilename,$result);
2976 foreach $prefix (@INC) {
2977 $realfilename = "$prefix/$filename";
2978 if (-f $realfilename) {
2979 $result = do $realfilename;
2983 die "Can't find $filename in \@INC";
2986 die "$filename did not return true value" unless $result;
2987 $INC{$filename} = $realfilename;
2991 Note that the file will not be included twice under the same specified
2992 name. The file must return TRUE as the last statement to indicate
2993 successful execution of any initialization code, so it's customary to
2994 end such a file with "C<1;>" unless you're sure it'll return TRUE
2995 otherwise. But it's better just to put the "C<1;>", in case you add more
2998 If EXPR is a bareword, the require assumes a "F<.pm>" extension and
2999 replaces "F<::>" with "F</>" in the filename for you,
3000 to make it easy to load standard modules. This form of loading of
3001 modules does not risk altering your namespace.
3003 In other words, if you try this:
3005 require Foo::Bar; # a splendid bareword
3007 The require function will actually look for the "F<Foo/Bar.pm>" file in the
3008 directories specified in the C<@INC> array.
3010 But if you try this:
3012 $class = 'Foo::Bar';
3013 require $class; # $class is not a bareword
3015 require "Foo::Bar"; # not a bareword because of the ""
3017 The require function will look for the "F<Foo::Bar>" file in the @INC array and
3018 will complain about not finding "F<Foo::Bar>" there. In this case you can do:
3020 eval "require $class";
3022 For a yet-more-powerful import facility, see L</use> and L<perlmod>.
3028 Generally used in a C<continue> block at the end of a loop to clear
3029 variables and reset C<??> searches so that they work again. The
3030 expression is interpreted as a list of single characters (hyphens
3031 allowed for ranges). All variables and arrays beginning with one of
3032 those letters are reset to their pristine state. If the expression is
3033 omitted, one-match searches (C<?pattern?>) are reset to match again. Resets
3034 only variables or searches in the current package. Always returns
3037 reset 'X'; # reset all X variables
3038 reset 'a-z'; # reset lower case variables
3039 reset; # just reset ?? searches
3041 Resetting C<"A-Z"> is not recommended because you'll wipe out your
3042 C<@ARGV> and C<@INC> arrays and your C<%ENV> hash. Resets only package variables--lexical variables
3043 are unaffected, but they clean themselves up on scope exit anyway,
3044 so you'll probably want to use them instead. See L</my>.
3050 Returns from a subroutine, C<eval()>, or C<do FILE> with the value
3051 given in EXPR. Evaluation of EXPR may be in list, scalar, or void
3052 context, depending on how the return value will be used, and the context
3053 may vary from one execution to the next (see C<wantarray()>). If no EXPR
3054 is given, returns an empty list in list context, an undefined value in
3055 scalar context, or nothing in a void context.
3057 (Note that in the absence of a return, a subroutine, eval, or do FILE
3058 will automatically return the value of the last expression evaluated.)
3062 In list context, returns a list value consisting of the elements
3063 of LIST in the opposite order. In scalar context, concatenates the
3064 elements of LIST, and returns a string value with all the characters
3065 in the opposite order.
3067 print reverse <>; # line tac, last line first
3069 undef $/; # for efficiency of <>
3070 print scalar reverse <>; # character tac, last line tsrif
3072 This operator is also handy for inverting a hash, although there are some
3073 caveats. If a value is duplicated in the original hash, only one of those
3074 can be represented as a key in the inverted hash. Also, this has to
3075 unwind one hash and build a whole new one, which may take some time
3078 %by_name = reverse %by_address; # Invert the hash
3080 =item rewinddir DIRHANDLE
3082 Sets the current position to the beginning of the directory for the
3083 C<readdir()> routine on DIRHANDLE.
3085 =item rindex STR,SUBSTR,POSITION
3087 =item rindex STR,SUBSTR
3089 Works just like index except that it returns the position of the LAST
3090 occurrence of SUBSTR in STR. If POSITION is specified, returns the
3091 last occurrence at or before that position.
3093 =item rmdir FILENAME
3097 Deletes the directory specified by FILENAME if that directory is empty. If it
3098 succeeds it returns TRUE, otherwise it returns FALSE and sets C<$!> (errno). If
3099 FILENAME is omitted, uses C<$_>.
3103 The substitution operator. See L<perlop>.
3107 Forces EXPR to be interpreted in scalar context and returns the value
3110 @counts = ( scalar @a, scalar @b, scalar @c );
3112 There is no equivalent operator to force an expression to
3113 be interpolated in list context because it's in practice never
3114 needed. If you really wanted to do so, however, you could use
3115 the construction C<@{[ (some expression) ]}>, but usually a simple
3116 C<(some expression)> suffices.
3118 Though C<scalar> can be considered in general to be a unary operator,
3119 EXPR is also allowed to be a parenthesized list. The list in fact
3120 behaves as a scalar comma expression, evaluating all but the last
3121 element in void context and returning the final element evaluated in
3124 The following single statement:
3126 print uc(scalar(&foo,$bar)),$baz;
3128 is the moral equivalent of these two:
3131 print(uc($bar),$baz);
3133 See L<perlop> for more details on unary operators and the comma operator.
3135 =item seek FILEHANDLE,POSITION,WHENCE
3137 Sets FILEHANDLE's position, just like the C<fseek()> call of C<stdio()>.
3138 FILEHANDLE may be an expression whose value gives the name of the
3139 filehandle. The values for WHENCE are C<0> to set the new position to
3140 POSITION, C<1> to set it to the current position plus POSITION, and C<2> to
3141 set it to EOF plus POSITION (typically negative). For WHENCE you may
3142 use the constants C<SEEK_SET>, C<SEEK_CUR>, and C<SEEK_END> from either the
3143 C<IO::Seekable> or the POSIX module. Returns C<1> upon success, C<0> otherwise.
3145 If you want to position file for C<sysread()> or C<syswrite()>, don't use
3146 C<seek()> -- buffering makes its effect on the file's system position
3147 unpredictable and non-portable. Use C<sysseek()> instead.
3149 On some systems you have to do a seek whenever you switch between reading
3150 and writing. Amongst other things, this may have the effect of calling
3151 stdio's clearerr(3). A WHENCE of C<1> (C<SEEK_CUR>) is useful for not moving
3156 This is also useful for applications emulating C<tail -f>. Once you hit
3157 EOF on your read, and then sleep for a while, you might have to stick in a
3158 seek() to reset things. The C<seek()> doesn't change the current position,
3159 but it I<does> clear the end-of-file condition on the handle, so that the
3160 next C<E<lt>FILEE<gt>> makes Perl try again to read something. We hope.
3162 If that doesn't work (some stdios are particularly cantankerous), then
3163 you may need something more like this:
3166 for ($curpos = tell(FILE); $_ = <FILE>;
3167 $curpos = tell(FILE)) {
3168 # search for some stuff and put it into files
3170 sleep($for_a_while);
3171 seek(FILE, $curpos, 0);
3174 =item seekdir DIRHANDLE,POS
3176 Sets the current position for the C<readdir()> routine on DIRHANDLE. POS
3177 must be a value returned by C<telldir()>. Has the same caveats about
3178 possible directory compaction as the corresponding system library
3181 =item select FILEHANDLE
3185 Returns the currently selected filehandle. Sets the current default
3186 filehandle for output, if FILEHANDLE is supplied. This has two
3187 effects: first, a C<write()> or a C<print()> without a filehandle will
3188 default to this FILEHANDLE. Second, references to variables related to
3189 output will refer to this output channel. For example, if you have to
3190 set the top of form format for more than one output channel, you might
3198 FILEHANDLE may be an expression whose value gives the name of the
3199 actual filehandle. Thus:
3201 $oldfh = select(STDERR); $| = 1; select($oldfh);
3203 Some programmers may prefer to think of filehandles as objects with
3204 methods, preferring to write the last example as:
3207 STDERR->autoflush(1);
3209 =item select RBITS,WBITS,EBITS,TIMEOUT
3211 This calls the select(2) system call with the bit masks specified, which
3212 can be constructed using C<fileno()> and C<vec()>, along these lines:
3214 $rin = $win = $ein = '';
3215 vec($rin,fileno(STDIN),1) = 1;
3216 vec($win,fileno(STDOUT),1) = 1;
3219 If you want to select on many filehandles you might wish to write a
3223 my(@fhlist) = split(' ',$_[0]);
3226 vec($bits,fileno($_),1) = 1;
3230 $rin = fhbits('STDIN TTY SOCK');
3234 ($nfound,$timeleft) =
3235 select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
3237 or to block until something becomes ready just do this
3239 $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
3241 Most systems do not bother to return anything useful in C<$timeleft>, so
3242 calling select() in scalar context just returns C<$nfound>.
3244 Any of the bit masks can also be undef. The timeout, if specified, is
3245 in seconds, which may be fractional. Note: not all implementations are
3246 capable of returning theC<$timeleft>. If not, they always return
3247 C<$timeleft> equal to the supplied C<$timeout>.
3249 You can effect a sleep of 250 milliseconds this way:
3251 select(undef, undef, undef, 0.25);
3253 B<WARNING>: One should not attempt to mix buffered I/O (like C<read()>
3254 or E<lt>FHE<gt>) with C<select()>, except as permitted by POSIX, and even
3255 then only on POSIX systems. You have to use C<sysread()> instead.
3257 =item semctl ID,SEMNUM,CMD,ARG
3259 Calls the System V IPC function C<semctl()>. You'll probably have to say
3263 first to get the correct constant definitions. If CMD is IPC_STAT or
3264 GETALL, then ARG must be a variable which will hold the returned
3265 semid_ds structure or semaphore value array. Returns like C<ioctl()>: the
3266 undefined value for error, "C<0> but true" for zero, or the actual return
3267 value otherwise. See also C<IPC::SysV> and C<IPC::Semaphore> documentation.
3269 =item semget KEY,NSEMS,FLAGS
3271 Calls the System V IPC function semget. Returns the semaphore id, or
3272 the undefined value if there is an error. See also C<IPC::SysV> and
3273 C<IPC::SysV::Semaphore> documentation.
3275 =item semop KEY,OPSTRING
3277 Calls the System V IPC function semop to perform semaphore operations
3278 such as signaling and waiting. OPSTRING must be a packed array of
3279 semop structures. Each semop structure can be generated with
3280 C<pack("sss", $semnum, $semop, $semflag)>. The number of semaphore
3281 operations is implied by the length of OPSTRING. Returns TRUE if
3282 successful, or FALSE if there is an error. As an example, the
3283 following code waits on semaphore C<$semnum> of semaphore id C<$semid>:
3285 $semop = pack("sss", $semnum, -1, 0);
3286 die "Semaphore trouble: $!\n" unless semop($semid, $semop);
3288 To signal the semaphore, replace C<-1> with C<1>. See also C<IPC::SysV>
3289 and C<IPC::SysV::Semaphore> documentation.
3291 =item send SOCKET,MSG,FLAGS,TO
3293 =item send SOCKET,MSG,FLAGS
3295 Sends a message on a socket. Takes the same flags as the system call
3296 of the same name. On unconnected sockets you must specify a
3297 destination to send TO, in which case it does a C C<sendto()>. Returns
3298 the number of characters sent, or the undefined value if there is an
3300 See L<perlipc/"UDP: Message Passing"> for examples.
3302 =item setpgrp PID,PGRP
3304 Sets the current process group for the specified PID, C<0> for the current
3305 process. Will produce a fatal error if used on a machine that doesn't
3306 implement setpgrp(2). If the arguments are omitted, it defaults to
3307 C<0,0>. Note that the POSIX version of C<setpgrp()> does not accept any
3308 arguments, so only setpgrp C<0,0> is portable.
3310 =item setpriority WHICH,WHO,PRIORITY
3312 Sets the current priority for a process, a process group, or a user.
3313 (See setpriority(2).) Will produce a fatal error if used on a machine
3314 that doesn't implement setpriority(2).
3316 =item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
3318 Sets the socket option requested. Returns undefined if there is an
3319 error. OPTVAL may be specified as C<undef> if you don't want to pass an
3326 Shifts the first value of the array off and returns it, shortening the
3327 array by 1 and moving everything down. If there are no elements in the
3328 array, returns the undefined value. If ARRAY is omitted, shifts the
3329 C<@_> array within the lexical scope of subroutines and formats, and the
3330 C<@ARGV> array at file scopes or within the lexical scopes established by
3331 the C<eval ''>, C<BEGIN {}>, C<END {}>, and C<INIT {}> constructs.
3332 See also C<unshift()>, C<push()>, and C<pop()>. C<Shift()> and C<unshift()> do the
3333 same thing to the left end of an array that C<pop()> and C<push()> do to the
3336 =item shmctl ID,CMD,ARG
3338 Calls the System V IPC function shmctl. You'll probably have to say
3342 first to get the correct constant definitions. If CMD is C<IPC_STAT>,
3343 then ARG must be a variable which will hold the returned C<shmid_ds>
3344 structure. Returns like ioctl: the undefined value for error, "C<0> but
3345 true" for zero, or the actual return value otherwise.
3346 See also C<IPC::SysV> documentation.
3348 =item shmget KEY,SIZE,FLAGS
3350 Calls the System V IPC function shmget. Returns the shared memory
3351 segment id, or the undefined value if there is an error.
3352 See also C<IPC::SysV> documentation.
3354 =item shmread ID,VAR,POS,SIZE
3356 =item shmwrite ID,STRING,POS,SIZE
3358 Reads or writes the System V shared memory segment ID starting at
3359 position POS for size SIZE by attaching to it, copying in/out, and
3360 detaching from it. When reading, VAR must be a variable that will
3361 hold the data read. When writing, if STRING is too long, only SIZE
3362 bytes are used; if STRING is too short, nulls are written to fill out
3363 SIZE bytes. Return TRUE if successful, or FALSE if there is an error.
3364 See also C<IPC::SysV> documentation.
3366 =item shutdown SOCKET,HOW
3368 Shuts down a socket connection in the manner indicated by HOW, which
3369 has the same interpretation as in the system call of the same name.
3371 shutdown(SOCKET, 0); # I/we have stopped reading data
3372 shutdown(SOCKET, 1); # I/we have stopped writing data
3373 shutdown(SOCKET, 2); # I/we have stopped using this socket
3375 This is useful with sockets when you want to tell the other
3376 side you're done writing but not done reading, or vice versa.
3377 It's also a more insistent form of close because it also
3378 disables the filedescriptor in any forked copies in other
3385 Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
3386 returns sine of C<$_>.
3388 For the inverse sine operation, you may use the C<POSIX::asin()>
3389 function, or use this relation:
3391 sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
3397 Causes the script to sleep for EXPR seconds, or forever if no EXPR.
3398 May be interrupted if the process receives a signal such as C<SIGALRM>.
3399 Returns the number of seconds actually slept. You probably cannot
3400 mix C<alarm()> and C<sleep()> calls, because C<sleep()> is often implemented
3403 On some older systems, it may sleep up to a full second less than what
3404 you requested, depending on how it counts seconds. Most modern systems
3405 always sleep the full amount. They may appear to sleep longer than that,
3406 however, because your process might not be scheduled right away in a
3407 busy multitasking system.
3409 For delays of finer granularity than one second, you may use Perl's
3410 C<syscall()> interface to access setitimer(2) if your system supports it,
3411 or else see L</select> above.
3413 See also the POSIX module's C<sigpause()> function.
3415 =item socket SOCKET,DOMAIN,TYPE,PROTOCOL
3417 Opens a socket of the specified kind and attaches it to filehandle
3418 SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for the
3419 system call of the same name. You should "C<use Socket;>" first to get
3420 the proper definitions imported. See the example in L<perlipc/"Sockets: Client/Server Communication">.
3422 =item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
3424 Creates an unnamed pair of sockets in the specified domain, of the
3425 specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as
3426 for the system call of the same name. If unimplemented, yields a fatal
3427 error. Returns TRUE if successful.
3429 Some systems defined C<pipe()> in terms of C<socketpair()>, in which a call
3430 to C<pipe(Rdr, Wtr)> is essentially:
3433 socketpair(Rdr, Wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC);
3434 shutdown(Rdr, 1); # no more writing for reader
3435 shutdown(Wtr, 0); # no more reading for writer
3437 See L<perlipc> for an example of socketpair use.
3439 =item sort SUBNAME LIST
3441 =item sort BLOCK LIST
3445 Sorts the LIST and returns the sorted list value. If SUBNAME or BLOCK
3446 is omitted, C<sort()>s in standard string comparison order. If SUBNAME is
3447 specified, it gives the name of a subroutine that returns an integer
3448 less than, equal to, or greater than C<0>, depending on how the elements
3449 of the array are to be ordered. (The C<E<lt>=E<gt>> and C<cmp>
3450 operators are extremely useful in such routines.) SUBNAME may be a
3451 scalar variable name (unsubscripted), in which case the value provides
3452 the name of (or a reference to) the actual subroutine to use. In place
3453 of a SUBNAME, you can provide a BLOCK as an anonymous, in-line sort
3456 In the interests of efficiency the normal calling code for subroutines is
3457 bypassed, with the following effects: the subroutine may not be a
3458 recursive subroutine, and the two elements to be compared are passed into
3459 the subroutine not via C<@_> but as the package global variables C<$a> and
3460 C<$b> (see example below). They are passed by reference, so don't
3461 modify C<$a> and C<$b>. And don't try to declare them as lexicals either.
3463 You also cannot exit out of the sort block or subroutine using any of the
3464 loop control operators described in L<perlsyn> or with C<goto()>.
3466 When C<use locale> is in effect, C<sort LIST> sorts LIST according to the
3467 current collation locale. See L<perllocale>.
3472 @articles = sort @files;
3474 # same thing, but with explicit sort routine
3475 @articles = sort {$a cmp $b} @files;
3477 # now case-insensitively
3478 @articles = sort {uc($a) cmp uc($b)} @files;
3480 # same thing in reversed order
3481 @articles = sort {$b cmp $a} @files;
3483 # sort numerically ascending
3484 @articles = sort {$a <=> $b} @files;
3486 # sort numerically descending
3487 @articles = sort {$b <=> $a} @files;
3489 # sort using explicit subroutine name
3491 $age{$a} <=> $age{$b}; # presuming numeric
3493 @sortedclass = sort byage @class;
3495 # this sorts the %age hash by value instead of key
3496 # using an in-line function
3497 @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
3499 sub backwards { $b cmp $a; }
3500 @harry = ('dog','cat','x','Cain','Abel');
3501 @george = ('gone','chased','yz','Punished','Axed');
3503 # prints AbelCaincatdogx
3504 print sort backwards @harry;
3505 # prints xdogcatCainAbel
3506 print sort @george, 'to', @harry;
3507 # prints AbelAxedCainPunishedcatchaseddoggonetoxyz
3509 # inefficiently sort by descending numeric compare using
3510 # the first integer after the first = sign, or the
3511 # whole record case-insensitively otherwise
3514 ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
3519 # same thing, but much more efficiently;
3520 # we'll build auxiliary indices instead
3524 push @nums, /=(\d+)/;
3529 $nums[$b] <=> $nums[$a]
3531 $caps[$a] cmp $caps[$b]
3535 # same thing using a Schwartzian Transform (no temps)
3536 @new = map { $_->[0] }
3537 sort { $b->[1] <=> $a->[1]
3540 } map { [$_, /=(\d+)/, uc($_)] } @old;
3542 If you're using strict, you I<MUST NOT> declare C<$a>
3543 and C<$b> as lexicals. They are package globals. That means
3544 if you're in the C<main> package, it's
3546 @articles = sort {$main::b <=> $main::a} @files;
3550 @articles = sort {$::b <=> $::a} @files;
3552 but if you're in the C<FooPack> package, it's
3554 @articles = sort {$FooPack::b <=> $FooPack::a} @files;
3556 The comparison function is required to behave. If it returns
3557 inconsistent results (sometimes saying C<$x[1]> is less than C<$x[2]> and
3558 sometimes saying the opposite, for example) the results are not
3561 =item splice ARRAY,OFFSET,LENGTH,LIST
3563 =item splice ARRAY,OFFSET,LENGTH
3565 =item splice ARRAY,OFFSET
3567 Removes the elements designated by OFFSET and LENGTH from an array, and
3568 replaces them with the elements of LIST, if any. In list context,
3569 returns the elements removed from the array. In scalar context,
3570 returns the last element removed, or C<undef> if no elements are
3571 removed. The array grows or shrinks as necessary.
3572 If OFFSET is negative then it start that far from the end of the array.
3573 If LENGTH is omitted, removes everything from OFFSET onward.
3574 If LENGTH is negative, leave that many elements off the end of the array.
3575 The following equivalences hold (assuming C<$[ == 0>):
3577 push(@a,$x,$y) splice(@a,@a,0,$x,$y)
3578 pop(@a) splice(@a,-1)
3579 shift(@a) splice(@a,0,1)
3580 unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
3581 $a[$x] = $y splice(@a,$x,1,$y)
3583 Example, assuming array lengths are passed before arrays:
3585 sub aeq { # compare two list values
3586 my(@a) = splice(@_,0,shift);
3587 my(@b) = splice(@_,0,shift);
3588 return 0 unless @a == @b; # same len?
3590 return 0 if pop(@a) ne pop(@b);
3594 if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
3596 =item split /PATTERN/,EXPR,LIMIT
3598 =item split /PATTERN/,EXPR
3600 =item split /PATTERN/
3604 Splits a string into an array of strings, and returns it. By default,
3605 empty leading fields are preserved, and empty trailing ones are deleted.
3607 If not in list context, returns the number of fields found and splits into
3608 the C<@_> array. (In list context, you can force the split into C<@_> by
3609 using C<??> as the pattern delimiters, but it still returns the list
3610 value.) The use of implicit split to C<@_> is deprecated, however, because
3611 it clobbers your subroutine arguments.
3613 If EXPR is omitted, splits the C<$_> string. If PATTERN is also omitted,
3614 splits on whitespace (after skipping any leading whitespace). Anything
3615 matching PATTERN is taken to be a delimiter separating the fields. (Note
3616 that the delimiter may be longer than one character.)
3618 If LIMIT is specified and positive, splits into no more than that
3619 many fields (though it may split into fewer). If LIMIT is unspecified
3620 or zero, trailing null fields are stripped (which potential users
3621 of C<pop()> would do well to remember). If LIMIT is negative, it is
3622 treated as if an arbitrarily large LIMIT had been specified.
3624 A pattern matching the null string (not to be confused with
3625 a null pattern C<//>, which is just one member of the set of patterns
3626 matching a null string) will split the value of EXPR into separate
3627 characters at each point it matches that way. For example:
3629 print join(':', split(/ */, 'hi there'));
3631 produces the output 'h:i:t:h:e:r:e'.
3633 The LIMIT parameter can be used to split a line partially
3635 ($login, $passwd, $remainder) = split(/:/, $_, 3);
3637 When assigning to a list, if LIMIT is omitted, Perl supplies a LIMIT
3638 one larger than the number of variables in the list, to avoid
3639 unnecessary work. For the list above LIMIT would have been 4 by
3640 default. In time critical applications it behooves you not to split
3641 into more fields than you really need.
3643 If the PATTERN contains parentheses, additional array elements are
3644 created from each matching substring in the delimiter.
3646 split(/([,-])/, "1-10,20", 3);
3648 produces the list value
3650 (1, '-', 10, ',', 20)
3652 If you had the entire header of a normal Unix email message in C<$header>,
3653 you could split it up into fields and their values this way:
3655 $header =~ s/\n\s+/ /g; # fix continuation lines
3656 %hdrs = (UNIX_FROM => split /^(\S*?):\s*/m, $header);
3658 The pattern C</PATTERN/> may be replaced with an expression to specify
3659 patterns that vary at runtime. (To do runtime compilation only once,
3660 use C</$variable/o>.)
3662 As a special case, specifying a PATTERN of space (C<' '>) will split on
3663 white space just as C<split()> with no arguments does. Thus, C<split(' ')> can
3664 be used to emulate B<awk>'s default behavior, whereas C<split(/ /)>
3665 will give you as many null initial fields as there are leading spaces.
3666 A C<split()> on C</\s+/> is like a C<split(' ')> except that any leading
3667 whitespace produces a null first field. A C<split()> with no arguments
3668 really does a C<split(' ', $_)> internally.
3672 open(PASSWD, '/etc/passwd');
3674 ($login, $passwd, $uid, $gid,
3675 $gcos, $home, $shell) = split(/:/);
3679 (Note that C<$shell> above will still have a newline on it. See L</chop>,
3680 L</chomp>, and L</join>.)
3682 =item sprintf FORMAT, LIST
3684 Returns a string formatted by the usual C<printf()> conventions of the
3685 C library function C<sprintf()>. See L<sprintf(3)> or L<printf(3)>
3686 on your system for an explanation of the general principles.
3688 Perl does its own C<sprintf()> formatting -- it emulates the C
3689 function C<sprintf()>, but it doesn't use it (except for floating-point
3690 numbers, and even then only the standard modifiers are allowed). As a
3691 result, any non-standard extensions in your local C<sprintf()> are not
3692 available from Perl.
3694 Perl's C<sprintf()> permits the following universally-known conversions:
3697 %c a character with the given number
3699 %d a signed integer, in decimal
3700 %u an unsigned integer, in decimal
3701 %o an unsigned integer, in octal
3702 %x an unsigned integer, in hexadecimal
3703 %e a floating-point number, in scientific notation
3704 %f a floating-point number, in fixed decimal notation
3705 %g a floating-point number, in %e or %f notation
3707 In addition, Perl permits the following widely-supported conversions:
3709 %X like %x, but using upper-case letters
3710 %E like %e, but using an upper-case "E"
3711 %G like %g, but with an upper-case "E" (if applicable)
3712 %b an unsigned integer, in binary
3713 %p a pointer (outputs the Perl value's address in hexadecimal)
3714 %n special: *stores* the number of characters output so far
3715 into the next variable in the parameter list
3717 Finally, for backward (and we do mean "backward") compatibility, Perl
3718 permits these unnecessary but widely-supported conversions:
3721 %D a synonym for %ld
3722 %U a synonym for %lu
3723 %O a synonym for %lo
3726 Perl permits the following universally-known flags between the C<%>
3727 and the conversion letter:
3729 space prefix positive number with a space
3730 + prefix positive number with a plus sign
3731 - left-justify within the field
3732 0 use zeros, not spaces, to right-justify
3733 # prefix non-zero octal with "0", non-zero hex with "0x"
3734 number minimum field width
3735 .number "precision": digits after decimal point for
3736 floating-point, max length for string, minimum length
3738 l interpret integer as C type "long" or "unsigned long"
3739 h interpret integer as C type "short" or "unsigned short"
3741 There is also one Perl-specific flag:
3743 V interpret integer as Perl's standard integer type
3745 Where a number would appear in the flags, an asterisk ("C<*>") may be
3746 used instead, in which case Perl uses the next item in the parameter
3747 list as the given number (that is, as the field width or precision).
3748 If a field width obtained through "C<*>" is negative, it has the same
3749 effect as the "C<->" flag: left-justification.
3751 If C<use locale> is in effect, the character used for the decimal
3752 point in formatted real numbers is affected by the LC_NUMERIC locale.
3759 Return the square root of EXPR. If EXPR is omitted, returns square
3766 Sets the random number seed for the C<rand()> operator. If EXPR is
3767 omitted, uses a semi-random value supplied by the kernel (if it supports
3768 the F</dev/urandom> device) or based on the current time and process
3769 ID, among other things. In versions of Perl prior to 5.004 the default
3770 seed was just the current C<time()>. This isn't a particularly good seed,
3771 so many old programs supply their own seed value (often C<time ^ $$> or
3772 C<time ^ ($$ + ($$ E<lt>E<lt> 15))>), but that isn't necessary any more.
3774 In fact, it's usually not necessary to call C<srand()> at all, because if
3775 it is not called explicitly, it is called implicitly at the first use of
3776 the C<rand()> operator. However, this was not the case in version of Perl
3777 before 5.004, so if your script will run under older Perl versions, it
3778 should call C<srand()>.
3780 Note that you need something much more random than the default seed for
3781 cryptographic purposes. Checksumming the compressed output of one or more
3782 rapidly changing operating system status programs is the usual method. For
3785 srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip`);
3787 If you're particularly concerned with this, see the C<Math::TrulyRandom>
3790 Do I<not> call C<srand()> multiple times in your program unless you know
3791 exactly what you're doing and why you're doing it. The point of the
3792 function is to "seed" the C<rand()> function so that C<rand()> can produce
3793 a different sequence each time you run your program. Just do it once at the
3794 top of your program, or you I<won't> get random numbers out of C<rand()>!
3796 Frequently called programs (like CGI scripts) that simply use
3800 for a seed can fall prey to the mathematical property that
3804 one-third of the time. So don't do that.
3806 =item stat FILEHANDLE
3812 Returns a 13-element list giving the status info for a file, either
3813 the file opened via FILEHANDLE, or named by EXPR. If EXPR is omitted,
3814 it stats C<$_>. Returns a null list if the stat fails. Typically used
3817 ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
3818 $atime,$mtime,$ctime,$blksize,$blocks)
3821 Not all fields are supported on all filesystem types. Here are the
3822 meaning of the fields:
3824 0 dev device number of filesystem
3826 2 mode file mode (type and permissions)
3827 3 nlink number of (hard) links to the file
3828 4 uid numeric user ID of file's owner
3829 5 gid numeric group ID of file's owner
3830 6 rdev the device identifier (special files only)
3831 7 size total size of file, in bytes
3832 8 atime last access time since the epoch
3833 9 mtime last modify time since the epoch
3834 10 ctime inode change time (NOT creation time!) since the epoch
3835 11 blksize preferred block size for file system I/O
3836 12 blocks actual number of blocks allocated
3838 (The epoch was at 00:00 January 1, 1970 GMT.)
3840 If stat is passed the special filehandle consisting of an underline, no
3841 stat is done, but the current contents of the stat structure from the
3842 last stat or filetest are returned. Example:
3844 if (-x $file && (($d) = stat(_)) && $d < 0) {
3845 print "$file is executable NFS file\n";
3848 (This works on machines only for which the device number is negative under NFS.)
3850 In scalar context, C<stat()> returns a boolean value indicating success
3851 or failure, and, if successful, sets the information associated with
3852 the special filehandle C<_>.
3858 Takes extra time to study SCALAR (C<$_> if unspecified) in anticipation of
3859 doing many pattern matches on the string before it is next modified.
3860 This may or may not save time, depending on the nature and number of
3861 patterns you are searching on, and on the distribution of character
3862 frequencies in the string to be searched -- you probably want to compare
3863 run times with and without it to see which runs faster. Those loops
3864 which scan for many short constant strings (including the constant
3865 parts of more complex patterns) will benefit most. You may have only
3866 one C<study()> active at a time -- if you study a different scalar the first
3867 is "unstudied". (The way C<study()> works is this: a linked list of every
3868 character in the string to be searched is made, so we know, for
3869 example, where all the C<'k'> characters are. From each search string,
3870 the rarest character is selected, based on some static frequency tables
3871 constructed from some C programs and English text. Only those places
3872 that contain this "rarest" character are examined.)
3874 For example, here is a loop that inserts index producing entries
3875 before any line containing a certain pattern:
3879 print ".IX foo\n" if /\bfoo\b/;
3880 print ".IX bar\n" if /\bbar\b/;
3881 print ".IX blurfl\n" if /\bblurfl\b/;
3886 In searching for C</\bfoo\b/>, only those locations in C<$_> that contain C<"f">
3887 will be looked at, because C<"f"> is rarer than C<"o">. In general, this is
3888 a big win except in pathological cases. The only question is whether
3889 it saves you more time than it took to build the linked list in the
3892 Note that if you have to look for strings that you don't know till
3893 runtime, you can build an entire loop as a string and C<eval()> that to
3894 avoid recompiling all your patterns all the time. Together with
3895 undefining C<$/> to input entire files as one record, this can be very
3896 fast, often faster than specialized programs like fgrep(1). The following
3897 scans a list of files (C<@files>) for a list of words (C<@words>), and prints
3898 out the names of those files that contain a match:
3900 $search = 'while (<>) { study;';
3901 foreach $word (@words) {
3902 $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
3907 eval $search; # this screams
3908 $/ = "\n"; # put back to normal input delimiter
3909 foreach $file (sort keys(%seen)) {
3917 =item sub NAME BLOCK
3919 This is subroutine definition, not a real function I<per se>. With just a
3920 NAME (and possibly prototypes), it's just a forward declaration. Without
3921 a NAME, it's an anonymous function declaration, and does actually return a
3922 value: the CODE ref of the closure you just created. See L<perlsub> and
3923 L<perlref> for details.
3925 =item substr EXPR,OFFSET,LEN,REPLACEMENT
3927 =item substr EXPR,OFFSET,LEN
3929 =item substr EXPR,OFFSET
3931 Extracts a substring out of EXPR and returns it. First character is at
3932 offset C<0>, or whatever you've set C<$[> to (but don't do that).
3933 If OFFSET is negative (or more precisely, less than C<$[>), starts
3934 that far from the end of the string. If LEN is omitted, returns
3935 everything to the end of the string. If LEN is negative, leaves that
3936 many characters off the end of the string.
3938 If you specify a substring that is partly outside the string, the part
3939 within the string is returned. If the substring is totally outside
3940 the string a warning is produced.
3942 You can use the C<substr()> function
3943 as an lvalue, in which case EXPR must be an lvalue. If you assign
3944 something shorter than LEN, the string will shrink, and if you assign
3945 something longer than LEN, the string will grow to accommodate it. To
3946 keep the string the same length you may need to pad or chop your value
3949 An alternative to using C<substr()> as an lvalue is to specify the
3950 replacement string as the 4th argument. This allows you to replace
3951 parts of the EXPR and return what was there before in one operation.
3953 =item symlink OLDFILE,NEWFILE
3955 Creates a new filename symbolically linked to the old filename.
3956 Returns C<1> for success, C<0> otherwise. On systems that don't support
3957 symbolic links, produces a fatal error at run time. To check for that,
3960 $symlink_exists = eval { symlink("",""); 1 };
3964 Calls the system call specified as the first element of the list,
3965 passing the remaining elements as arguments to the system call. If
3966 unimplemented, produces a fatal error. The arguments are interpreted
3967 as follows: if a given argument is numeric, the argument is passed as
3968 an int. If not, the pointer to the string value is passed. You are
3969 responsible to make sure a string is pre-extended long enough to
3970 receive any result that might be written into a string. You can't use a
3971 string literal (or other read-only string) as an argument to C<syscall()>
3972 because Perl has to assume that any string pointer might be written
3974 integer arguments are not literals and have never been interpreted in a
3975 numeric context, you may need to add C<0> to them to force them to look
3976 like numbers. This emulates the C<syswrite()> function (or vice versa):
3978 require 'syscall.ph'; # may need to run h2ph
3980 syscall(&SYS_write, fileno(STDOUT), $s, length $s);
3982 Note that Perl supports passing of up to only 14 arguments to your system call,
3983 which in practice should usually suffice.
3985 Syscall returns whatever value returned by the system call it calls.
3986 If the system call fails, C<syscall()> returns C<-1> and sets C<$!> (errno).
3987 Note that some system calls can legitimately return C<-1>. The proper
3988 way to handle such calls is to assign C<$!=0;> before the call and
3989 check the value of C<$!> if syscall returns C<-1>.
3991 There's a problem with C<syscall(&SYS_pipe)>: it returns the file
3992 number of the read end of the pipe it creates. There is no way
3993 to retrieve the file number of the other end. You can avoid this
3994 problem by using C<pipe()> instead.
3996 =item sysopen FILEHANDLE,FILENAME,MODE
3998 =item sysopen FILEHANDLE,FILENAME,MODE,PERMS
4000 Opens the file whose filename is given by FILENAME, and associates it
4001 with FILEHANDLE. If FILEHANDLE is an expression, its value is used as
4002 the name of the real filehandle wanted. This function calls the
4003 underlying operating system's C<open()> function with the parameters
4004 FILENAME, MODE, PERMS.
4006 The possible values and flag bits of the MODE parameter are
4007 system-dependent; they are available via the standard module C<Fcntl>.
4008 For historical reasons, some values work on almost every system
4009 supported by perl: zero means read-only, one means write-only, and two
4010 means read/write. We know that these values do I<not> work under
4011 OS/390 & VM/ESA Unix and on the Macintosh; you probably don't want to
4012 use them in new code.
4014 If the file named by FILENAME does not exist and the C<open()> call creates
4015 it (typically because MODE includes the C<O_CREAT> flag), then the value of
4016 PERMS specifies the permissions of the newly created file. If you omit
4017 the PERMS argument to C<sysopen()>, Perl uses the octal value C<0666>.
4018 These permission values need to be in octal, and are modified by your
4019 process's current C<umask>.
4021 Seldom if ever use C<0644> as argument to C<sysopen()> because that
4022 takes away the user's option to have a more permissive umask. Better
4023 to omit it. See the perlfunc(1) entry on C<umask> for more on this.
4025 The C<IO::File> module provides a more object-oriented approach, if you're
4026 into that kind of thing.
4028 =item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
4030 =item sysread FILEHANDLE,SCALAR,LENGTH
4032 Attempts to read LENGTH bytes of data into variable SCALAR from the
4033 specified FILEHANDLE, using the system call read(2). It bypasses stdio,
4034 so mixing this with other kinds of reads, C<print()>, C<write()>,
4035 C<seek()>, C<tell()>, or C<eof()> can cause confusion because stdio
4036 usually buffers data. Returns the number of bytes actually read, C<0>
4037 at end of file, or undef if there was an error. SCALAR will be grown or
4038 shrunk so that the last byte actually read is the last byte of the
4039 scalar after the read.
4041 An OFFSET may be specified to place the read data at some place in the
4042 string other than the beginning. A negative OFFSET specifies
4043 placement at that many bytes counting backwards from the end of the
4044 string. A positive OFFSET greater than the length of SCALAR results
4045 in the string being padded to the required size with C<"\0"> bytes before
4046 the result of the read is appended.
4048 =item sysseek FILEHANDLE,POSITION,WHENCE
4050 Sets FILEHANDLE's system position using the system call lseek(2). It
4051 bypasses stdio, so mixing this with reads (other than C<sysread()>),
4052 C<print()>, C<write()>, C<seek()>, C<tell()>, or C<eof()> may cause
4053 confusion. FILEHANDLE may be an expression whose value gives the name
4054 of the filehandle. The values for WHENCE are C<0> to set the new
4055 position to POSITION, C<1> to set the it to the current position plus
4056 POSITION, and C<2> to set it to EOF plus POSITION (typically negative).
4057 For WHENCE, you may use the constants C<SEEK_SET>, C<SEEK_CUR>, and
4058 C<SEEK_END> from either the C<IO::Seekable> or the POSIX module.
4060 Returns the new position, or the undefined value on failure. A position
4061 of zero is returned as the string "C<0> but true"; thus C<sysseek()> returns
4062 TRUE on success and FALSE on failure, yet you can still easily determine
4067 =item system PROGRAM LIST
4069 Does exactly the same thing as "C<exec LIST>" except that a fork is done
4070 first, and the parent process waits for the child process to complete.
4071 Note that argument processing varies depending on the number of
4072 arguments. If there is more than one argument in LIST, or if LIST is
4073 an array with more than one value, starts the program given by the
4074 first element of the list with arguments given by the rest of the list.
4075 If there is only one scalar argument, the argument is
4076 checked for shell metacharacters, and if there are any, the entire
4077 argument is passed to the system's command shell for parsing (this is
4078 C</bin/sh -c> on Unix platforms, but varies on other platforms). If
4079 there are no shell metacharacters in the argument, it is split into
4080 words and passed directly to C<execvp()>, which is more efficient.
4082 The return value is the exit status of the program as
4083 returned by the C<wait()> call. To get the actual exit value divide by
4084 256. See also L</exec>. This is I<NOT> what you want to use to capture
4085 the output from a command, for that you should use merely backticks or
4086 C<qx//>, as described in L<perlop/"`STRING`">.
4088 Like C<exec()>, C<system()> allows you to lie to a program about its name if
4089 you use the "C<system PROGRAM LIST>" syntax. Again, see L</exec>.
4091 Because C<system()> and backticks block C<SIGINT> and C<SIGQUIT>, killing the
4092 program they're running doesn't actually interrupt your program.
4094 @args = ("command", "arg1", "arg2");
4096 or die "system @args failed: $?"
4098 You can check all the failure possibilities by inspecting
4101 $exit_value = $? >> 8;
4102 $signal_num = $? & 127;
4103 $dumped_core = $? & 128;
4105 When the arguments get executed via the system shell, results
4106 and return codes will be subject to its quirks and capabilities.
4107 See L<perlop/"`STRING`"> and L</exec> for details.
4109 =item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
4111 =item syswrite FILEHANDLE,SCALAR,LENGTH
4113 =item syswrite FILEHANDLE,SCALAR
4115 Attempts to write LENGTH bytes of data from variable SCALAR to the
4116 specified FILEHANDLE, using the system call write(2). If LENGTH is
4117 not specified, writes whole SCALAR. It bypasses
4118 stdio, so mixing this with reads (other than C<sysread())>, C<print()>,
4119 C<write()>, C<seek()>, C<tell()>, or C<eof()> may cause confusion
4120 because stdio usually buffers data. Returns the number of bytes
4121 actually written, or C<undef> if there was an error. If the LENGTH is
4122 greater than the available data in the SCALAR after the OFFSET, only as
4123 much data as is available will be written.
4125 An OFFSET may be specified to write the data from some part of the
4126 string other than the beginning. A negative OFFSET specifies writing
4127 that many bytes counting backwards from the end of the string. In the
4128 case the SCALAR is empty you can use OFFSET but only zero offset.
4130 =item tell FILEHANDLE
4134 Returns the current position for FILEHANDLE. FILEHANDLE may be an
4135 expression whose value gives the name of the actual filehandle. If
4136 FILEHANDLE is omitted, assumes the file last read.
4138 =item telldir DIRHANDLE
4140 Returns the current position of the C<readdir()> routines on DIRHANDLE.
4141 Value may be given to C<seekdir()> to access a particular location in a
4142 directory. Has the same caveats about possible directory compaction as
4143 the corresponding system library routine.
4145 =item tie VARIABLE,CLASSNAME,LIST
4147 This function binds a variable to a package class that will provide the
4148 implementation for the variable. VARIABLE is the name of the variable
4149 to be enchanted. CLASSNAME is the name of a class implementing objects
4150 of correct type. Any additional arguments are passed to the "C<new()>"
4151 method of the class (meaning C<TIESCALAR>, C<TIEHANDLE>, C<TIEARRAY>,
4152 or C<TIEHASH>). Typically these are arguments such as might be passed
4153 to the C<dbm_open()> function of C. The object returned by the "C<new()>"
4154 method is also returned by the C<tie()> function, which would be useful
4155 if you want to access other methods in CLASSNAME.
4157 Note that functions such as C<keys()> and C<values()> may return huge lists
4158 when used on large objects, like DBM files. You may prefer to use the
4159 C<each()> function to iterate over such. Example:
4161 # print out history file offsets
4163 tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
4164 while (($key,$val) = each %HIST) {
4165 print $key, ' = ', unpack('L',$val), "\n";
4169 A class implementing a hash should have the following methods:
4171 TIEHASH classname, LIST
4173 STORE this, key, value
4178 NEXTKEY this, lastkey
4181 A class implementing an ordinary array should have the following methods:
4183 TIEARRAY classname, LIST
4185 STORE this, key, value
4187 STORESIZE this, count
4193 SPLICE this, offset, length, LIST
4197 A class implementing a file handle should have the following methods:
4199 TIEHANDLE classname, LIST
4200 READ this, scalar, length, offset
4203 WRITE this, scalar, length, offset
4205 PRINTF this, format, LIST
4209 A class implementing a scalar should have the following methods:
4211 TIESCALAR classname, LIST
4216 Not all methods indicated above need be implemented. See L<perltie>,
4217 L<Tie::Hash>, L<Tie::Array>, L<Tie::Scalar> and L<Tie::Handle>.
4219 Unlike C<dbmopen()>, the C<tie()> function will not use or require a module
4220 for you--you need to do that explicitly yourself. See L<DB_File>
4221 or the F<Config> module for interesting C<tie()> implementations.
4223 For further details see L<perltie>, L<"tied VARIABLE">.
4227 Returns a reference to the object underlying VARIABLE (the same value
4228 that was originally returned by the C<tie()> call that bound the variable
4229 to a package.) Returns the undefined value if VARIABLE isn't tied to a
4234 Returns the number of non-leap seconds since whatever time the system
4235 considers to be the epoch (that's 00:00:00, January 1, 1904 for MacOS,
4236 and 00:00:00 UTC, January 1, 1970 for most other systems).
4237 Suitable for feeding to C<gmtime()> and C<localtime()>.
4241 Returns a four-element list giving the user and system times, in
4242 seconds, for this process and the children of this process.
4244 ($user,$system,$cuser,$csystem) = times;
4248 The transliteration operator. Same as C<y///>. See L<perlop>.
4250 =item truncate FILEHANDLE,LENGTH
4252 =item truncate EXPR,LENGTH
4254 Truncates the file opened on FILEHANDLE, or named by EXPR, to the
4255 specified length. Produces a fatal error if truncate isn't implemented
4256 on your system. Returns TRUE if successful, the undefined value
4263 Returns an uppercased version of EXPR. This is the internal function
4264 implementing the C<\U> escape in double-quoted strings.
4265 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
4266 Under Unicode (C<use utf8>) it uses the standard Unicode uppercase mappings. (It
4267 does not attempt to do titlecase mapping on initial letters. See C<ucfirst()> for that.)
4269 If EXPR is omitted, uses C<$_>.
4275 Returns the value of EXPR with the first character
4276 in uppercase (titlecase in Unicode). This is
4277 the internal function implementing the C<\u> escape in double-quoted strings.
4278 Respects current LC_CTYPE locale if C<use locale> in force. See L<perllocale>.
4280 If EXPR is omitted, uses C<$_>.
4286 Sets the umask for the process to EXPR and returns the previous value.
4287 If EXPR is omitted, merely returns the current umask.
4289 The Unix permission C<rwxr-x---> is represented as three sets of three
4290 bits, or three octal digits: C<0750> (the leading 0 indicates octal
4291 and isn't one of the digits). The C<umask> value is such a number
4292 representing disabled permissions bits. The permission (or "mode")
4293 values you pass C<mkdir> or C<sysopen> are modified by your umask, so
4294 even if you tell C<sysopen> to create a file with permissions C<0777>,
4295 if your umask is C<0022> then the file will actually be created with
4296 permissions C<0755>. If your C<umask> were C<0027> (group can't
4297 write; others can't read, write, or execute), then passing
4298 C<sysopen()> C<0666> would create a file with mode C<0640> (C<0666 &~
4301 Here's some advice: supply a creation mode of C<0666> for regular
4302 files (in C<sysopen()>) and one of C<0777> for directories (in
4303 C<mkdir()>) and executable files. This gives users the freedom of
4304 choice: if they want protected files, they might choose process umasks
4305 of C<022>, C<027>, or even the particularly antisocial mask of C<077>.
4306 Programs should rarely if ever make policy decisions better left to
4307 the user. The exception to this is when writing files that should be
4308 kept private: mail files, web browser cookies, I<.rhosts> files, and
4311 If umask(2) is not implemented on your system and you are trying to
4312 restrict access for I<yourself> (i.e., (EXPR & 0700) > 0), produces a
4313 fatal error at run time. If umask(2) is not implemented and you are
4314 not trying to restrict access for yourself, returns C<undef>.
4316 Remember that a umask is a number, usually given in octal; it is I<not> a
4317 string of octal digits. See also L</oct>, if all you have is a string.
4325 Undefines the value of EXPR, which must be an lvalue. Use only on a
4326 scalar value, an array (using "C<@>"), a hash (using "C<%>"), a subroutine
4327 (using "C<&>"), or a typeglob (using "<*>"). (Saying C<undef $hash{$key}>
4328 will probably not do what you expect on most predefined variables or
4329 DBM list values, so don't do that; see L<delete>.) Always returns the
4330 undefined value. You can omit the EXPR, in which case nothing is
4331 undefined, but you still get an undefined value that you could, for
4332 instance, return from a subroutine, assign to a variable or pass as a
4333 parameter. Examples:
4336 undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'};
4340 undef *xyz; # destroys $xyz, @xyz, %xyz, &xyz, etc.
4341 return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it;
4342 select undef, undef, undef, 0.25;
4343 ($a, $b, undef, $c) = &foo; # Ignore third value returned
4345 Note that this is a unary operator, not a list operator.
4351 Deletes a list of files. Returns the number of files successfully
4354 $cnt = unlink 'a', 'b', 'c';
4358 Note: C<unlink()> will not delete directories unless you are superuser and
4359 the B<-U> flag is supplied to Perl. Even if these conditions are
4360 met, be warned that unlinking a directory can inflict damage on your
4361 filesystem. Use C<rmdir()> instead.
4363 If LIST is omitted, uses C<$_>.
4365 =item unpack TEMPLATE,EXPR
4367 C<Unpack()> does the reverse of C<pack()>: it takes a string representing a
4368 structure and expands it out into a list value, returning the array
4369 value. (In scalar context, it returns merely the first value
4370 produced.) The TEMPLATE has the same format as in the C<pack()> function.
4371 Here's a subroutine that does substring:
4374 my($what,$where,$howmuch) = @_;
4375 unpack("x$where a$howmuch", $what);
4380 sub ordinal { unpack("c",$_[0]); } # same as ord()
4382 In addition, you may prefix a field with a %E<lt>numberE<gt> to indicate that
4383 you want a E<lt>numberE<gt>-bit checksum of the items instead of the items
4384 themselves. Default is a 16-bit checksum. For example, the following
4385 computes the same number as the System V sum program:
4388 $checksum += unpack("%32C*", $_);
4392 The following efficiently counts the number of set bits in a bit vector:
4394 $setbits = unpack("%32b*", $selectmask);
4396 See L</pack> for more examples.
4398 =item untie VARIABLE
4400 Breaks the binding between a variable and a package. (See C<tie()>.)
4402 =item unshift ARRAY,LIST
4404 Does the opposite of a C<shift()>. Or the opposite of a C<push()>,
4405 depending on how you look at it. Prepends list to the front of the
4406 array, and returns the new number of elements in the array.
4408 unshift(ARGV, '-e') unless $ARGV[0] =~ /^-/;
4410 Note the LIST is prepended whole, not one element at a time, so the
4411 prepended elements stay in the same order. Use C<reverse()> to do the
4414 =item use Module LIST
4418 =item use Module VERSION LIST
4422 Imports some semantics into the current package from the named module,
4423 generally by aliasing certain subroutine or variable names into your
4424 package. It is exactly equivalent to
4426 BEGIN { require Module; import Module LIST; }
4428 except that Module I<must> be a bareword.
4430 If the first argument to C<use> is a number, it is treated as a version
4431 number instead of a module name. If the version of the Perl interpreter
4432 is less than VERSION, then an error message is printed and Perl exits
4433 immediately. This is often useful if you need to check the current
4434 Perl version before C<use>ing library modules that have changed in
4435 incompatible ways from older versions of Perl. (We try not to do
4436 this more than we have to.)
4438 The C<BEGIN> forces the C<require> and C<import()> to happen at compile time. The
4439 C<require> makes sure the module is loaded into memory if it hasn't been
4440 yet. The C<import()> is not a builtin--it's just an ordinary static method
4441 call into the "C<Module>" package to tell the module to import the list of
4442 features back into the current package. The module can implement its
4443 C<import()> method any way it likes, though most modules just choose to
4444 derive their C<import()> method via inheritance from the C<Exporter> class that
4445 is defined in the C<Exporter> module. See L<Exporter>. If no C<import()>
4446 method can be found then the error is currently silently ignored. This
4447 may change to a fatal error in a future version.
4449 If you don't want your namespace altered, explicitly supply an empty list:
4453 That is exactly equivalent to
4455 BEGIN { require Module }
4457 If the VERSION argument is present between Module and LIST, then the
4458 C<use> will call the VERSION method in class Module with the given
4459 version as an argument. The default VERSION method, inherited from
4460 the Universal class, croaks if the given version is larger than the
4461 value of the variable C<$Module::VERSION>. (Note that there is not a
4462 comma after VERSION!)
4464 Because this is a wide-open interface, pragmas (compiler directives)
4465 are also implemented this way. Currently implemented pragmas are:
4469 use sigtrap qw(SEGV BUS);
4470 use strict qw(subs vars refs);
4471 use subs qw(afunc blurfl);
4473 Some of these these pseudo-modules import semantics into the current
4474 block scope (like C<strict> or C<integer>, unlike ordinary modules,
4475 which import symbols into the current package (which are effective
4476 through the end of the file).
4478 There's a corresponding "C<no>" command that unimports meanings imported
4479 by C<use>, i.e., it calls C<unimport Module LIST> instead of C<import()>.
4484 If no C<unimport()> method can be found the call fails with a fatal error.
4486 See L<perlmod> for a list of standard modules and pragmas.
4490 Changes the access and modification times on each file of a list of
4491 files. The first two elements of the list must be the NUMERICAL access
4492 and modification times, in that order. Returns the number of files
4493 successfully changed. The inode modification time of each file is set
4494 to the current time. This code has the same effect as the "C<touch>"
4495 command if the files already exist:
4499 utime $now, $now, @ARGV;
4503 Returns a list consisting of all the values of the named hash. (In a
4504 scalar context, returns the number of values.) The values are
4505 returned in an apparently random order. The actual random order is
4506 subject to change in future versions of perl, but it is guaranteed to
4507 be the same order as either the C<keys()> or C<each()> function would
4508 produce on the same (unmodified) hash.
4510 As a side effect, it resets HASH's iterator. See also C<keys()>, C<each()>,
4513 =item vec EXPR,OFFSET,BITS
4515 Treats the string in EXPR as a vector of unsigned integers, and
4516 returns the value of the bit field specified by OFFSET. BITS specifies
4517 the number of bits that are reserved for each entry in the bit
4518 vector. This must be a power of two from 1 to 32. C<vec()> may also be
4519 assigned to, in which case parentheses are needed to give the expression
4520 the correct precedence as in
4522 vec($image, $max_x * $x + $y, 8) = 3;
4524 Vectors created with C<vec()> can also be manipulated with the logical
4525 operators C<|>, C<&>, and C<^>, which will assume a bit vector operation is
4526 desired when both operands are strings.
4528 The following code will build up an ASCII string saying C<'PerlPerlPerl'>.
4529 The comments show the string after each step. Note that this code works
4530 in the same way on big-endian or little-endian machines.
4533 vec($foo, 0, 32) = 0x5065726C; # 'Perl'
4534 vec($foo, 2, 16) = 0x5065; # 'PerlPe'
4535 vec($foo, 3, 16) = 0x726C; # 'PerlPerl'
4536 vec($foo, 8, 8) = 0x50; # 'PerlPerlP'
4537 vec($foo, 9, 8) = 0x65; # 'PerlPerlPe'
4538 vec($foo, 20, 4) = 2; # 'PerlPerlPe' . "\x02"
4539 vec($foo, 21, 4) = 7; # 'PerlPerlPer'
4541 vec($foo, 45, 2) = 3; # 'PerlPerlPer' . "\x0c"
4542 vec($foo, 93, 1) = 1; # 'PerlPerlPer' . "\x2c"
4543 vec($foo, 94, 1) = 1; # 'PerlPerlPerl'
4546 To transform a bit vector into a string or array of 0's and 1's, use these:
4548 $bits = unpack("b*", $vector);
4549 @bits = split(//, unpack("b*", $vector));
4551 If you know the exact length in bits, it can be used in place of the C<*>.
4555 Waits for a child process to terminate and returns the pid of the
4556 deceased process, or C<-1> if there are no child processes. The status is
4557 returned in C<$?>. Note that a return value of C<-1> could mean that
4558 child processes are being automatically reaped, as described in L<perlipc>.
4560 =item waitpid PID,FLAGS
4562 Waits for a particular child process to terminate and returns the pid
4563 of the deceased process, or C<-1> if there is no such child process. The
4564 status is returned in C<$?>. If you say
4566 use POSIX ":sys_wait_h";
4568 waitpid(-1,&WNOHANG);
4570 then you can do a non-blocking wait for any process. Non-blocking wait
4571 is available on machines supporting either the waitpid(2) or
4572 wait4(2) system calls. However, waiting for a particular pid with
4573 FLAGS of C<0> is implemented everywhere. (Perl emulates the system call
4574 by remembering the status values of processes that have exited but have
4575 not been harvested by the Perl script yet.)
4577 Note that a return value of C<-1> could mean that child processes are being
4578 automatically reaped. See L<perlipc> for details, and for other examples.
4582 Returns TRUE if the context of the currently executing subroutine is
4583 looking for a list value. Returns FALSE if the context is looking
4584 for a scalar. Returns the undefined value if the context is looking
4585 for no value (void context).
4587 return unless defined wantarray; # don't bother doing more
4588 my @a = complex_calculation();
4589 return wantarray ? @a : "@a";
4593 Produces a message on STDERR just like C<die()>, but doesn't exit or throw
4596 If LIST is empty and C<$@> already contains a value (typically from a
4597 previous eval) that value is used after appending C<"\t...caught">
4598 to C<$@>. This is useful for staying almost, but not entirely similar to
4601 If C<$@> is empty then the string C<"Warning: Something's wrong"> is used.
4603 No message is printed if there is a C<$SIG{__WARN__}> handler
4604 installed. It is the handler's responsibility to deal with the message
4605 as it sees fit (like, for instance, converting it into a C<die()>). Most
4606 handlers must therefore make arrangements to actually display the
4607 warnings that they are not prepared to deal with, by calling C<warn()>
4608 again in the handler. Note that this is quite safe and will not
4609 produce an endless loop, since C<__WARN__> hooks are not called from
4612 You will find this behavior is slightly different from that of
4613 C<$SIG{__DIE__}> handlers (which don't suppress the error text, but can
4614 instead call C<die()> again to change it).
4616 Using a C<__WARN__> handler provides a powerful way to silence all
4617 warnings (even the so-called mandatory ones). An example:
4619 # wipe out *all* compile-time warnings
4620 BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } }
4622 my $foo = 20; # no warning about duplicate my $foo,
4623 # but hey, you asked for it!
4624 # no compile-time or run-time warnings before here
4627 # run-time warnings enabled after here
4628 warn "\$foo is alive and $foo!"; # does show up
4630 See L<perlvar> for details on setting C<%SIG> entries, and for more
4633 =item write FILEHANDLE
4639 Writes a formatted record (possibly multi-line) to the specified FILEHANDLE,
4640 using the format associated with that file. By default the format for
4641 a file is the one having the same name as the filehandle, but the
4642 format for the current output channel (see the C<select()> function) may be set
4643 explicitly by assigning the name of the format to the C<$~> variable.
4645 Top of form processing is handled automatically: if there is
4646 insufficient room on the current page for the formatted record, the
4647 page is advanced by writing a form feed, a special top-of-page format
4648 is used to format the new page header, and then the record is written.
4649 By default the top-of-page format is the name of the filehandle with
4650 "_TOP" appended, but it may be dynamically set to the format of your
4651 choice by assigning the name to the C<$^> variable while the filehandle is
4652 selected. The number of lines remaining on the current page is in
4653 variable C<$->, which can be set to C<0> to force a new page.
4655 If FILEHANDLE is unspecified, output goes to the current default output
4656 channel, which starts out as STDOUT but may be changed by the
4657 C<select()> operator. If the FILEHANDLE is an EXPR, then the expression
4658 is evaluated and the resulting string is used to look up the name of
4659 the FILEHANDLE at run time. For more on formats, see L<perlform>.
4661 Note that write is I<NOT> the opposite of C<read()>. Unfortunately.
4665 The transliteration operator. Same as C<tr///>. See L<perlop>.