path: root/pod/perlfunc.pod

=head1 NAME

perlfunc - Perl builtin functions

=head1 DESCRIPTION

The functions in this section can serve as terms in an expression.
They fall into two major categories: list operators and named unary
operators.  These differ in their precedence relationship with a
following comma.  (See the precedence table in L<perlop>.)  List
operators take more than one argument, while unary operators can never
take more than one argument.  Thus, a comma terminates the argument of
a unary operator, but merely separates the arguments of a list
operator.  A unary operator generally provides a scalar context to its
argument, while a list operator may provide either scalar and list
contexts for its arguments.  If it does both, the scalar arguments will
be first, and the list argument will follow.  (Note that there can only
ever be one list argument.)  For instance, splice() has three scalar
arguments followed by a list.

In the syntax descriptions that follow, list operators that expect a
list (and provide list context for the elements of the list) are shown
with LIST as an argument.  Such a list may consist of any combination
of scalar arguments or list values; the list values will be included
in the list as if each individual element were interpolated at that
point in the list, forming a longer single-dimensional list value.
Elements of the LIST should be separated by commas.

Any function in the list below may be used either with or without
parentheses around its arguments.  (The syntax descriptions omit the
parens.)  If you use the parens, the simple (but occasionally
surprising) rule is this: It I<LOOKS> like a function, therefore it I<IS> a
function, and precedence doesn't matter.  Otherwise it's a list
operator or unary operator, and precedence does matter.  And whitespace
between the function and left parenthesis doesn't count--so you need to
be careful sometimes:

    print 1+2+3;	# Prints 6.
    print(1+2) + 3;	# Prints 3.
    print (1+2)+3;	# Also prints 3!
    print +(1+2)+3;	# Prints 6.
    print ((1+2)+3);	# Prints 6.

If you run Perl with the B<-w> switch it can warn you about this.  For
example, the third line above produces:

    print (...) interpreted as function at - line 1.
    Useless use of integer addition in void context at - line 1.

For functions that can be used in either a scalar or list context,
non-abortive failure is generally indicated in a scalar context by
returning the undefined value, and in a list context by returning the
null list.

Remember the following rule:

=over 8

=item  

I<THERE IS NO GENERAL RULE FOR CONVERTING A LIST INTO A SCALAR!>

=back

Each operator and function decides which sort of value it would be most
appropriate to return in a scalar context.  Some operators return the
length of the list that would have been returned in a list context.  Some
operators return the first value in the list.  Some operators return the
last value in the list.  Some operators return a count of successful
operations.  In general, they do what you want, unless you want
consistency.

=head2 Perl Functions by Category

Here are Perl's functions (including things that look like
functions, like some of the keywords and named operators)
arranged by category.  Some functions appear in more
than one place.

=over

=item Functions for SCALARs or strings

chomp, chop, chr, crypt, hex, index, lc, lcfirst, length,
oct, ord, pack, q/STRING/, qq/STRING/, reverse, rindex,
sprintf, substr, tr///, uc, ucfirst, y///

=item Regular expressions and pattern matching

m//, pos, quotemeta, s///, split, study

=item Numeric functions

abs, atan2, cos, exp, hex, int, log, oct, rand, sin, sqrt,
srand

=item Functions for real @ARRAYs

pop, push, shift, splice, unshift

=item Functions for list data

grep, join, map, qw/STRING/, reverse, sort, unpack

=item Functions for real %HASHes

delete, each, exists, keys, values

=item Input and output functions

binmode, close, closedir, dbmclose, dbmopen, die, eof,
fileno, flock, format, getc, print, printf, read, readdir,
rewinddir, seek, seekdir, select, syscall, sysread,
syswrite, tell, telldir, truncate, warn, write

=item Functions for fixed length data or records

pack, read, syscall, sysread, syswrite, unpack, vec

=item Functions for filehandles, files, or directories

-X, chdir, chmod, chown, chroot, fcntl, glob, ioctl, link,
lstat, mkdir, open, opendir, readlink, rename, rmdir,
stat, symlink, umask, unlink, utime

=item Keywords related to the control flow of your perl program

caller, continue, die, do, dump, eval, exit, goto, last,
next, redo, return, sub, wantarray

=item Keywords related to scoping 

caller, import, local, my, package, use

=item Miscellaneous functions

defined, dump, eval, formline, local, my, reset, scalar,
undef, wantarray

=item Functions for processes and process groups

alarm, exec, fork, getpgrp, getppid, getpriority, kill,
pipe, qx/STRING/, setpgrp, setpriority, sleep, system,
times, wait, waitpid

=item Keywords related to perl modules

do, import, no, package, require, use

=item Keywords related to classes and object-orientedness

bless, dbmclose, dbmopen, package, ref, tie, tied, untie, use

=item Low-level socket functions

accept, bind, connect, getpeername, getsockname,
getsockopt, listen, recv, send, setsockopt, shutdown,
socket, socketpair

=item System V interprocess communication functions

msgctl, msgget, msgrcv, msgsnd, semctl, semget, semop,
shmctl, shmget, shmread, shmwrite

=item Fetching user and group info

endgrent, endhostent, endnetent, endpwent, getgrent,
getgrgid, getgrnam, getlogin, getpwent, getpwnam,
getpwuid, setgrent, setpwent

=item Fetching network info

endprotoent, endservent, gethostbyaddr, gethostbyname,
gethostent, getnetbyaddr, getnetbyname, getnetent,
getprotobyname, getprotobynumber, getprotoent,
getservbyname, getservbyport, getservent, sethostent,
setnetent, setprotoent, setservent

=item Time-related functions

gmtime, localtime, time, times

=back

=head2 Alphabetical Listing of Perl Functions


=over 8

=item -X FILEHANDLE

=item -X EXPR

=item -X

A file test, where X is one of the letters listed below.  This unary
operator takes one argument, either a filename or a filehandle, and
tests the associated file to see if something is true about it.  If the
argument is omitted, tests $_, except for C<-t>, which tests STDIN.
Unless otherwise documented, it returns C<1> for TRUE and C<''> for FALSE, or
the undefined value if the file doesn't exist.  Despite the funny
names, precedence is the same as any other named unary operator, and
the argument may be parenthesized like any other unary operator.  The
operator may be any of:

    -r	File is readable by effective uid/gid.
    -w	File is writable by effective uid/gid.
    -x	File is executable by effective uid/gid.
    -o	File is owned by effective uid.

    -R	File is readable by real uid/gid.
    -W	File is writable by real uid/gid.
    -X	File is executable by real uid/gid.
    -O	File is owned by real uid.

    -e	File exists.
    -z	File has zero size.
    -s	File has non-zero size (returns size).

    -f	File is a plain file.
    -d	File is a directory.
    -l	File is a symbolic link.
    -p	File is a named pipe (FIFO).
    -S	File is a socket.
    -b	File is a block special file.
    -c	File is a character special file.
    -t	Filehandle is opened to a tty.

    -u	File has setuid bit set.
    -g	File has setgid bit set.
    -k	File has sticky bit set.

    -T	File is a text file.
    -B	File is a binary file (opposite of -T).

    -M	Age of file in days when script started.
    -A	Same for access time.
    -C	Same for inode change time.

The interpretation of the file permission operators C<-r>, C<-R>, C<-w>,
C<-W>, C<-x> and C<-X> is based solely on the mode of the file and the
uids and gids of the user.  There may be other reasons you can't actually
read, write or execute the file.  Also note that, for the superuser,
C<-r>, C<-R>, C<-w> and C<-W> always return 1, and C<-x> and C<-X> return
1 if any execute bit is set in the mode.  Scripts run by the superuser may
thus need to do a stat() in order to determine the actual mode of the
file, or temporarily set the uid to something else.

Example:

    while (<>) {
	chop;
	next unless -f $_;	# ignore specials
	...
    }

Note that C<-s/a/b/> does not do a negated substitution.  Saying
C<-exp($foo)> still works as expected, however--only single letters
following a minus are interpreted as file tests.

The C<-T> and C<-B> switches work as follows.  The first block or so of the
file is examined for odd characters such as strange control codes or
characters with the high bit set.  If too many odd characters (>30%)
are found, it's a C<-B> file, otherwise it's a C<-T> file.  Also, any file
containing null in the first block is considered a binary file.  If C<-T>
or C<-B> is used on a filehandle, the current stdio buffer is examined
rather than the first block.  Both C<-T> and C<-B> return TRUE on a null
file, or a file at EOF when testing a filehandle.  Because you have to 
read a file to do the C<-T> test, on most occasions you want to use a C<-f>
against the file first, as in C<next unless -f $file && -T $file>.

If any of the file tests (or either the stat() or lstat() operators) are given the
special filehandle consisting of a solitary underline, then the stat
structure of the previous file test (or stat operator) is used, saving
a system call.  (This doesn't work with C<-t>, and you need to remember
that lstat() and C<-l> will leave values in the stat structure for the
symbolic link, not the real file.)  Example:

    print "Can do.\n" if -r $a || -w _ || -x _;

    stat($filename);
    print "Readable\n" if -r _;
    print "Writable\n" if -w _;
    print "Executable\n" if -x _;
    print "Setuid\n" if -u _;
    print "Setgid\n" if -g _;
    print "Sticky\n" if -k _;
    print "Text\n" if -T _;
    print "Binary\n" if -B _;

=item abs VALUE

Returns the absolute value of its argument.

=item accept NEWSOCKET,GENERICSOCKET

Accepts an incoming socket connect, just as the accept(2) system call
does.  Returns the packed address if it succeeded, FALSE otherwise.
See example in L<perlipc/"Sockets: Client/Server Communication">.

=item alarm SECONDS

Arranges to have a SIGALRM delivered to this process after the
specified number of seconds have elapsed.  (On some machines,
unfortunately, the elapsed time may be up to one second less than you
specified because of how seconds are counted.)  Only one timer may be
counting at once.  Each call disables the previous timer, and an
argument of 0 may be supplied to cancel the previous timer without
starting a new one.  The returned value is the amount of time remaining
on the previous timer.

For delays of finer granularity than one second, you may use Perl's
syscall() interface to access setitimer(2) if your system supports it, 
or else see L</select()> below.  It is not advised to intermix alarm() 
and sleep() calls.

=item atan2 Y,X

Returns the arctangent of Y/X in the range -PI to PI.

=item bind SOCKET,NAME

Binds a network address to a socket, just as the bind system call
does.  Returns TRUE if it succeeded, FALSE otherwise.  NAME should be a
packed address of the appropriate type for the socket.  See the examples in
L<perlipc/"Sockets: Client/Server Communication">.

=item binmode FILEHANDLE

Arranges for the file to be read or written in "binary" mode in operating
systems that distinguish between binary and text files.  Files that are
not in binary mode have CR LF sequences translated to LF on input and LF
translated to CR LF on output.  Binmode has no effect under Unix; in DOS
and similarly archaic systems, it may be imperative--otherwise your
DOS-damaged C library may mangle your file.  The key distinction between
systems that need binmode and those that don't is their text file
formats.  Systems like Unix and Plan9 that delimit lines with a single
character, and that encode that character in C as '\n', do not need
C<binmode>.  The rest need it.  If FILEHANDLE is an expression, the value
is taken as the name of the filehandle.

=item bless REF,CLASSNAME

=item bless REF

This function tells the referenced object (passed as REF) that it is now
an object in the CLASSNAME package--or the current package if no CLASSNAME
is specified, which is often the case.  It returns the reference for
convenience, since a bless() is often the last thing in a constructor.
Always use the two-argument version if the function doing the blessing
might be inherited by a derived class.  See L<perlobj> for more about the
blessing (and blessings) of objects.

=item caller EXPR

=item caller

Returns the context of the current subroutine call.  In a scalar context,
returns TRUE if there is a caller, that is, if we're in a subroutine or
eval() or require(), and FALSE otherwise.  In a list context, returns

    ($package, $filename, $line) = caller;

With EXPR, it returns some extra information that the debugger uses to
print a stack trace.  The value of EXPR indicates how many call frames
to go back before the current one.

    ($package, $filename, $line,
     $subroutine, $hasargs, $wantargs) = caller($i);

Furthermore, when called from within the DB package, caller returns more
detailed information: it sets the list variable @DB::args to be the
arguments with which that subroutine was invoked.

=item chdir EXPR

Changes the working directory to EXPR, if possible.  If EXPR is
omitted, changes to home directory.  Returns TRUE upon success, FALSE
otherwise.  See example under die().

=item chmod LIST

Changes the permissions of a list of files.  The first element of the
list must be the numerical mode, which should probably be an octal
number.  Returns the number of files successfully changed.

    $cnt = chmod 0755, 'foo', 'bar';
    chmod 0755, @executables;

=item chomp VARIABLE

=item chomp LIST

=item chomp

This is a slightly safer version of chop (see below).  It removes any
line ending that corresponds to the current value of C<$/> (also known as
$INPUT_RECORD_SEPARATOR in the C<English> module).  It returns the number
of characters removed.  It's often used to remove the newline from the
end of an input record when you're worried that the final record may be
missing its newline.  When in paragraph mode (C<$/ = "">), it removes all
trailing newlines from the string.  If VARIABLE is omitted, it chomps
$_.  Example:

    while (<>) {
	chomp;	# avoid \n on last field
	@array = split(/:/);
	...
    }

You can actually chomp anything that's an lvalue, including an assignment:

    chomp($cwd = `pwd`);
    chomp($answer = <STDIN>);

If you chomp a list, each element is chomped, and the total number of
characters removed is returned.

=item chop VARIABLE

=item chop LIST

=item chop

Chops off the last character of a string and returns the character
chopped.  It's used primarily to remove the newline from the end of an
input record, but is much more efficient than C<s/\n//> because it neither
scans nor copies the string.  If VARIABLE is omitted, chops $_.
Example:

    while (<>) {
	chop;	# avoid \n on last field
	@array = split(/:/);
	...
    }

You can actually chop anything that's an lvalue, including an assignment:

    chop($cwd = `pwd`);
    chop($answer = <STDIN>);

If you chop a list, each element is chopped.  Only the value of the
last chop is returned.

Note that chop returns the last character.  To return all but the last
character, use C<substr($string, 0, -1)>.

=item chown LIST

Changes the owner (and group) of a list of files.  The first two
elements of the list must be the I<NUMERICAL> uid and gid, in that order.
Returns the number of files successfully changed.

    $cnt = chown $uid, $gid, 'foo', 'bar';
    chown $uid, $gid, @filenames;

Here's an example that looks up non-numeric uids in the passwd file:

    print "User: ";
    chop($user = <STDIN>);
    print "Files: "
    chop($pattern = <STDIN>);

    ($login,$pass,$uid,$gid) = getpwnam($user)
	or die "$user not in passwd file";

    @ary = <${pattern}>;	# expand filenames
    chown $uid, $gid, @ary;

On most systems, you are not allowed to change the ownership of the 
file unless you're the superuser, although you should be able to change
the group to any of your secondary groups.  On insecure systems, these
restrictions may be relaxed, but this is not a portable assumption.

=item chr NUMBER

Returns the character represented by that NUMBER in the character set.
For example, C<chr(65)> is "A" in ASCII.

=item chroot FILENAME

This function works as the system call by the same name: it makes the
named directory the new root directory for all further pathnames that
begin with a "/" by your process and all of its children.  (It doesn't
change your current working directory is unaffected.)  For security
reasons, this call is restricted to the superuser.  If FILENAME is
omitted, does chroot to $_.

=item close FILEHANDLE

Closes the file or pipe associated with the file handle, returning TRUE
only if stdio successfully flushes buffers and closes the system file
descriptor.  You don't have to close FILEHANDLE if you are immediately
going to do another open() on it, since open() will close it for you.  (See
open().)  However, an explicit close on an input file resets the line
counter ($.), while the implicit close done by open() does not.  Also,
closing a pipe will wait for the process executing on the pipe to
complete, in case you want to look at the output of the pipe
afterwards.  Closing a pipe explicitly also puts the status value of
the command into C<$?>.  Example:

    open(OUTPUT, '|sort >foo');	# pipe to sort
    ...				# print stuff to output
    close OUTPUT;		# wait for sort to finish
    open(INPUT, 'foo');		# get sort's results

FILEHANDLE may be an expression whose value gives the real filehandle name.

=item closedir DIRHANDLE

Closes a directory opened by opendir().

=item connect SOCKET,NAME

Attempts to connect to a remote socket, just as the connect system call
does.  Returns TRUE if it succeeded, FALSE otherwise.  NAME should be a
packed address of the appropriate type for the socket.  See the examples in
L<perlipc/"Sockets: Client/Server Communication">.

=item continue BLOCK

Actually a flow control statement rather than a function.  If there is a
C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
C<foreach>), it is always executed just before the conditional is about to
be evaluated again, just like the third part of a C<for> loop in C.  Thus
it can be used to increment a loop variable, even when the loop has been
continued via the C<next> statement (which is similar to the C C<continue>
statement).

=item cos EXPR

Returns the cosine of EXPR (expressed in radians).  If EXPR is omitted
takes cosine of $_.

=item crypt PLAINTEXT,SALT

Encrypts a string exactly like the crypt(3) function in the C library
(assuming that you actually have a version there that has not been
extirpated as a potential munition).  This can prove useful for checking
the password file for lousy passwords, amongst other things.  Only the
guys wearing white hats should do this.

Here's an example that makes sure that whoever runs this program knows
their own password:

    $pwd = (getpwuid($<))[1];
    $salt = substr($pwd, 0, 2);

    system "stty -echo";
    print "Password: ";
    chop($word = <STDIN>);
    print "\n";
    system "stty echo";

    if (crypt($word, $salt) ne $pwd) {
	die "Sorry...\n";
    } else {
	print "ok\n";
    } 

Of course, typing in your own password to whoever asks you 
for it is unwise.

=item dbmclose ASSOC_ARRAY

[This function has been superseded by the untie() function.]

Breaks the binding between a DBM file and an associative array.

=item dbmopen ASSOC,DBNAME,MODE

[This function has been superseded by the tie() function.]

This binds a dbm(3), ndbm(3), sdbm(3), gdbm(), or Berkeley DB file to an
associative array.  ASSOC is the name of the associative array.  (Unlike
normal open, the first argument is I<NOT> a filehandle, even though it
looks like one).  DBNAME is the name of the database (without the F<.dir>
or F<.pag> extension if any).  If the database does not exist, it is
created with protection specified by MODE (as modified by the umask()).
If your system only supports the older DBM functions, you may perform only
one dbmopen() in your program.  In older versions of Perl, if your system
had neither DBM nor ndbm, calling dbmopen() produced a fatal error; it now
falls back to sdbm(3).

If you don't have write access to the DBM file, you can only read
associative array variables, not set them.  If you want to test whether
you can write, either use file tests or try setting a dummy array entry
inside an eval(), which will trap the error.

Note that functions such as keys() and values() may return huge array
values when used on large DBM files.  You may prefer to use the each()
function to iterate over large DBM files.  Example:

    # print out history file offsets
    dbmopen(%HIST,'/usr/lib/news/history',0666);
    while (($key,$val) = each %HIST) {
	print $key, ' = ', unpack('L',$val), "\n";
    }
    dbmclose(%HIST);

See also L<AnyDBM_File> for a more general description of the pros and
cons of the various dbm apparoches, as well as L<DB_File> for a particularly
rich implementation.

=item defined EXPR

Returns a boolean value saying whether EXPR has a real value
or not.  Many operations return the undefined value under exceptional
conditions, such as end of file, uninitialized variable, system error
and such.  This function allows you to distinguish between an undefined
null scalar and a defined null scalar with operations that might return
a real null string, such as referencing elements of an array.  You may
also check to see if arrays or subroutines exist.  Use of defined on
predefined variables is not guaranteed to produce intuitive results.

When used on a hash array element, it tells you whether the value
is defined, not whether the key exists in the hash.  Use exists() for that.

Examples:

    print if defined $switch{'D'};
    print "$val\n" while defined($val = pop(@ary));
    die "Can't readlink $sym: $!"
	unless defined($value = readlink $sym);
    eval '@foo = ()' if defined(@foo);
    die "No XYZ package defined" unless defined %_XYZ;
    sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }

See also undef().

=item delete EXPR

Deletes the specified value from its hash array.  Returns the deleted
value, or the undefined value if nothing was deleted.  Deleting from
C<$ENV{}> modifies the environment.  Deleting from an array tied to a DBM
file deletes the entry from the DBM file.  (But deleting from a tie()d
hash doesn't necessarily return anything.)

The following deletes all the values of an associative array:

    foreach $key (keys %ARRAY) {
	delete $ARRAY{$key};
    }

(But it would be faster to use the undef() command.)  Note that the
EXPR can be arbitrarily complicated as long as the final operation is
a hash key lookup:

    delete $ref->[$x][$y]{$key};

=item die LIST

Outside of an eval(), prints the value of LIST to C<STDERR> and exits with
the current value of $! (errno).  If $! is 0, exits with the value of
C<($? E<gt>E<gt> 8)> (backtick `command` status).  If C<($? E<gt>E<gt> 8)> is 0,
exits with 255.  Inside an eval(), the error message is stuffed into C<$@>,
and the eval() is terminated with the undefined value; this makes die()
the way to raise an exception.

Equivalent examples:

    die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
    chdir '/usr/spool/news' or die "Can't cd to spool: $!\n" 

If the value of EXPR does not end in a newline, the current script line
number and input line number (if any) are also printed, and a newline
is supplied.  Hint: sometimes appending ", stopped" to your message
will cause it to make better sense when the string "at foo line 123" is
appended.  Suppose you are running script "canasta".

    die "/etc/games is no good";
    die "/etc/games is no good, stopped";

produce, respectively

    /etc/games is no good at canasta line 123.
    /etc/games is no good, stopped at canasta line 123.

See also exit() and warn().

=item do BLOCK

Not really a function.  Returns the value of the last command in the
sequence of commands indicated by BLOCK.  When modified by a loop
modifier, executes the BLOCK once before testing the loop condition.
(On other statements the loop modifiers test the conditional first.)

=item do SUBROUTINE(LIST)

A deprecated form of subroutine call.  See L<perlsub>.

=item do EXPR

Uses the value of EXPR as a filename and executes the contents of the
file as a Perl script.  Its primary use is to include subroutines
from a Perl subroutine library.

    do 'stat.pl';

is just like

    eval `cat stat.pl`;

except that it's more efficient, more concise, keeps track of the
current filename for error messages, and searches all the B<-I>
libraries if the file isn't in the current directory (see also the @INC
array in L<perlvar/Predefined Names>).  It's the same, however, in that it does
reparse the file every time you call it, so you probably don't want to
do this inside a loop.

Note that inclusion of library modules is better done with the
use() and require() operators, which also do error checking
and raise an exception if there's a problem.

=item dump LABEL

This causes an immediate core dump.  Primarily this is so that you can
use the B<undump> program to turn your core dump into an executable binary
after having initialized all your variables at the beginning of the
program.  When the new binary is executed it will begin by executing a
C<goto LABEL> (with all the restrictions that C<goto> suffers).  Think of
it as a goto with an intervening core dump and reincarnation.  If LABEL
is omitted, restarts the program from the top.  WARNING: any files
opened at the time of the dump will NOT be open any more when the
program is reincarnated, with possible resulting confusion on the part
of Perl.  See also B<-u> option in L<perlrun>.

Example:

    #!/usr/bin/perl
    require 'getopt.pl';
    require 'stat.pl';
    %days = (
	'Sun' => 1,
	'Mon' => 2,
	'Tue' => 3,
	'Wed' => 4,
	'Thu' => 5,
	'Fri' => 6,
	'Sat' => 7,
    );

    dump QUICKSTART if $ARGV[0] eq '-d';

    QUICKSTART:
    Getopt('f');

=item each ASSOC_ARRAY

Returns a 2-element array consisting of the key and value for the next
value of an associative array, so that you can iterate over it.
Entries are returned in an apparently random order.  When the array is
entirely read, a null array is returned (which when assigned produces a
FALSE (0) value).  The next call to each() after that will start
iterating again.  The iterator can be reset only by reading all the
elements from the array.  You should not add elements to an array while
you're iterating over it.  There is a single iterator for each
associative array, shared by all each(), keys() and values() function
calls in the program.  The following prints out your environment like
the printenv(1) program, only in a different order:

    while (($key,$value) = each %ENV) {
	print "$key=$value\n";
    }

See also keys() and values().

=item eof FILEHANDLE

=item eof ()

=item eof

Returns 1 if the next read on FILEHANDLE will return end of file, or if
FILEHANDLE is not open.  FILEHANDLE may be an expression whose value
gives the real filehandle name.  (Note that this function actually
reads a character and then ungetc()s it, so it is not very useful in an
interactive context.)  Do not read from a terminal file (or call
C<eof(FILEHANDLE)> on it) after end-of-file is reached.  Filetypes such
as terminals may lose the end-of-file condition if you do.

An C<eof> without an argument uses the last file read as argument.
Empty parentheses () may be used to indicate
the pseudofile formed of the files listed on the command line, i.e.
C<eof()> is reasonable to use inside a while (<>) loop to detect the end
of only the last file.  Use C<eof(ARGV)> or eof without the parentheses to
test I<EACH> file in a while (<>) loop.  Examples:

    # reset line numbering on each input file
    while (<>) {
	print "$.\t$_";
	close(ARGV) if (eof);	# Not eof().
    }

    # insert dashes just before last line of last file
    while (<>) {
	if (eof()) {
	    print "--------------\n";
	    close(ARGV);	# close or break; is needed if we
				# are reading from the terminal
	}
	print;
    }

Practical hint: you almost never need to use C<eof> in Perl, because the
input operators return undef when they run out of data.  Testing C<eof>

=item eval EXPR

=item eval BLOCK

EXPR is parsed and executed as if it were a little Perl program.  It
is executed in the context of the current Perl program, so that any
variable settings, subroutine or format definitions remain afterwards.
The value returned is the value of the last expression evaluated, or a
return statement may be used, just as with subroutines.

If there is a syntax error or runtime error, or a die() statement is
executed, an undefined value is returned by eval(), and C<$@> is set to the
error message.  If there was no error, C<$@> is guaranteed to be a null
string.  If EXPR is omitted, evaluates $_.  The final semicolon, if
any, may be omitted from the expression.

Note that, since eval() traps otherwise-fatal errors, it is useful for
determining whether a particular feature (such as socket() or symlink())
is implemented.  It is also Perl's exception trapping mechanism, where
the die operator is used to raise exceptions.

If the code to be executed doesn't vary, you may use the eval-BLOCK
form to trap run-time errors without incurring the penalty of
recompiling each time.  The error, if any, is still returned in C<$@>.
Examples:

    # make divide-by-zero non-fatal
    eval { $answer = $a / $b; }; warn $@ if $@;

    # same thing, but less efficient
    eval '$answer = $a / $b'; warn $@ if $@;

    # a compile-time error
    eval { $answer = };

    # a run-time error
    eval '$answer =';	# sets $@

With an eval(), you should be especially careful to remember what's 
being looked at when:

    eval $x;		# CASE 1
    eval "$x";		# CASE 2

    eval '$x';		# CASE 3
    eval { $x };	# CASE 4

    eval "\$$x++"	# CASE 5
    $$x++;		# CASE 6

Cases 1 and 2 above behave identically: they run the code contained in the
variable $x.  (Although case 2 has misleading double quotes making the
reader wonder what else might be happening (nothing is).) Cases 3 and 4
likewise behave in the same way: they run the code <$x>, which does
nothing at all.  (Case 4 is preferred for purely visual reasons.) Case 5
is a place where normally you I<WOULD> like to use double quotes, except
that in that particular situation, you can just use symbolic references
instead, as in case 6.

=item exec LIST

The exec() function executes a system command I<AND NEVER RETURNS>.  Use
the system() function if you want it to return.

If there is more than one argument in LIST, or if LIST is an array with
more than one value, calls execvp(3) with the arguments in LIST.  If
there is only one scalar argument, the argument is checked for shell
metacharacters.  If there are any, the entire argument is passed to
C</bin/sh -c> for parsing.  If there are none, the argument is split
into words and passed directly to execvp(), which is more efficient.
Note: exec() (and system(0) do not flush your output buffer, so you may
need to set C<$|> to avoid lost output.  Examples:

    exec '/bin/echo', 'Your arguments are: ', @ARGV;
    exec "sort $outfile | uniq";

If you don't really want to execute the first argument, but want to lie
to the program you are executing about its own name, you can specify
the program you actually want to run as an "indirect object" (without a
comma) in front of the LIST.  (This always forces interpretation of the
LIST as a multi-valued list, even if there is only a single scalar in
the list.)  Example:

    $shell = '/bin/csh';
    exec $shell '-sh';		# pretend it's a login shell

or, more directly,

    exec {'/bin/csh'} '-sh';	# pretend it's a login shell

=item exists EXPR

Returns TRUE if the specified hash key exists in its hash array, even
if the corresponding value is undefined.

    print "Exists\n" if exists $array{$key};
    print "Defined\n" if defined $array{$key};
    print "True\n" if $array{$key};

A hash element can only be TRUE if it's defined, and defined if
it exists, but the reverse doesn't necessarily hold true.

Note that the EXPR can be arbitrarily complicated as long as the final
operation is a hash key lookup:

    if (exists $ref->[$x][$y]{$key}) { ... }

=item exit EXPR

Evaluates EXPR and exits immediately with that value.  (Actually, it
calls any defined C<END> routines first, but the C<END> routines may not
abort the exit.  Likewise any object destructors that need to be called
are called before exit.)  Example:

    $ans = <STDIN>;
    exit 0 if $ans =~ /^[Xx]/;

See also die().  If EXPR is omitted, exits with 0 status.

=item exp EXPR

Returns I<e> (the natural logarithm base) to the power of EXPR.  
If EXPR is omitted, gives C<exp($_)>.

=item fcntl FILEHANDLE,FUNCTION,SCALAR

Implements the fcntl(2) function.  You'll probably have to say

    use Fcntl;

first to get the correct function definitions.  Argument processing and
value return works just like ioctl() below.  Note that fcntl() will produce
a fatal error if used on a machine that doesn't implement fcntl(2).
For example:

    use Fcntl;
    fcntl($filehandle, F_GETLK, $packed_return_buffer);

=item fileno FILEHANDLE

Returns the file descriptor for a filehandle.  This is useful for
constructing bitmaps for select().  If FILEHANDLE is an expression, the
value is taken as the name of the filehandle.

=item flock FILEHANDLE,OPERATION

Calls flock(2) on FILEHANDLE.  See L<flock(2)> for definition of
OPERATION.  Returns TRUE for success, FALSE on failure.  Will produce a
fatal error if used on a machine that doesn't implement either flock(2) or
fcntl(2). The fcntl(2) system call will be automatically used if flock(2)
is missing from your system.  This makes flock() the portable file locking
strategy, although it will only lock entire files, not records.  Note also
that some versions of flock() cannot lock things over the network; you
would need to use the more system-specific fcntl() for that.

Here's a mailbox appender for BSD systems.

    $LOCK_SH = 1;
    $LOCK_EX = 2;
    $LOCK_NB = 4;
    $LOCK_UN = 8;

    sub lock {
	flock(MBOX,$LOCK_EX);
	# and, in case someone appended
	# while we were waiting...
	seek(MBOX, 0, 2);
    }

    sub unlock {
	flock(MBOX,$LOCK_UN);
    }

    open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
	    or die "Can't open mailbox: $!";

    lock();
    print MBOX $msg,"\n\n";
    unlock();

See also L<DB_File> for other flock() examples.

=item fork

Does a fork(2) system call.  Returns the child pid to the parent process
and 0 to the child process, or C<undef> if the fork is unsuccessful.
Note: unflushed buffers remain unflushed in both processes, which means
you may need to set C<$|> ($AUTOFLUSH in English) or call the 
autoflush() FileHandle method to avoid duplicate output.

If you fork() without ever waiting on your children, you will accumulate
zombies:

    $SIG{CHLD} = sub { wait };

There's also the double-fork trick (error checking on 
fork() returns omitted);

    unless ($pid = fork) {
	unless (fork) {
	    exec "what you really wanna do";
	    die "no exec";
	    # ... or ...
	    ## (some_perl_code_here)
	    exit 0;
	}
	exit 0;
    }
    waitpid($pid,0);

See also L<perlipc> for more examples of forking and reaping
moribund children.

=item format

Declare a picture format with use by the write() function.  For
example:

    format Something = 
	Test: @<<<<<<<< @||||| @>>>>>
	      $str,     $%,    '$' . int($num)
    .

    $str = "widget";
    $num = $cost/$quantiy;
    $~ = 'Something';
    write;

See L<perlform> for many details and examples.


=item formline PICTURE, LIST

This is an internal function used by C<format>s, though you may call it
too.  It formats (see L<perlform>) a list of values according to the
contents of PICTURE, placing the output into the format output
accumulator, C<$^A> (or $ACCUMULATOR in English).
Eventually, when a write() is done, the contents of
C<$^A> are written to some filehandle, but you could also read C<$^A>
yourself and then set C<$^A> back to "".  Note that a format typically
does one formline() per line of form, but the formline() function itself
doesn't care how many newlines are embedded in the PICTURE.  This means
that the C<~> and C<~~> tokens will treat the entire PICTURE as a single line.
You may therefore need to use multiple formlines to implement a single
record format, just like the format compiler.

Be careful if you put double quotes around the picture, since an "C<@>"
character may be taken to mean the beginning of an array name.
formline() always returns TRUE.  See L<perlform> for other examples.

=item getc FILEHANDLE

=item getc

Returns the next character from the input file attached to FILEHANDLE,
or a null string at end of file.  If FILEHANDLE is omitted, reads from STDIN.
This is not particularly efficient.  It cannot be used to get unbuffered
single-characters, however.  For that, try something more like:

    if ($BSD_STYLE) {
	system "stty cbreak </dev/tty >/dev/tty 2>&1";
    }
    else {
	system "stty", '-icanon', 'eol', "\001"; 
    }

    $key = getc(STDIN);

    if ($BSD_STYLE) {
	system "stty -cbreak </dev/tty >/dev/tty 2>&1";
    }
    else {
	system "stty", 'icanon', 'eol', '^@'; # ascii null
    }
    print "\n";

Determination of whether to whether $BSD_STYLE should be set 
is left as an exercise to the reader.  

See also the C<Term::ReadKey> module from your nearest CPAN site;
details on CPAN can be found on L<perlmod/CPAN> 

=item getlogin

Returns the current login from F</etc/utmp>, if any.  If null, use
getpwuid().  

    $login = getlogin || (getpwuid($<))[0] || "Kilroy";

Do not consider getlogin() for authorentication: it is not as
secure as getpwuid().

=item getpeername SOCKET

Returns the packed sockaddr address of other end of the SOCKET connection.

    use Socket;
    $hersockaddr    = getpeername(SOCK);
    ($port, $iaddr) = unpack_sockaddr_in($hersockaddr);
    $herhostname    = gethostbyaddr($iaddr, AF_INET);
    $herstraddr     = inet_ntoa($iaddr);

=item getpgrp PID

Returns the current process group for the specified PID, 0 for the
current process.  Will raise an exception if used on a machine that
doesn't implement getpgrp(2).  If PID is omitted, returns process
group of current process.

=item getppid

Returns the process id of the parent process.

=item getpriority WHICH,WHO

Returns the current priority for a process, a process group, or a user.
(See L<getpriority(2)>.)  Will raise a fatal exception if used on a
machine that doesn't implement getpriority(2).

=item getpwnam NAME

=item getgrnam NAME

=item gethostbyname NAME

=item getnetbyname NAME

=item getprotobyname NAME

=item getpwuid UID

=item getgrgid GID

=item getservbyname NAME,PROTO

=item gethostbyaddr ADDR,ADDRTYPE

=item getnetbyaddr ADDR,ADDRTYPE

=item getprotobynumber NUMBER

=item getservbyport PORT,PROTO

=item getpwent

=item getgrent

=item gethostent

=item getnetent

=item getprotoent

=item getservent

=item setpwent

=item setgrent

=item sethostent STAYOPEN

=item setnetent STAYOPEN

=item setprotoent STAYOPEN

=item setservent STAYOPEN

=item endpwent

=item endgrent

=item endhostent

=item endnetent

=item endprotoent

=item endservent

These routines perform the same functions as their counterparts in the
system library.  Within a list context, the return values from the
various get routines are as follows:

    ($name,$passwd,$uid,$gid,
       $quota,$comment,$gcos,$dir,$shell) = getpw*
    ($name,$passwd,$gid,$members) = getgr*
    ($name,$aliases,$addrtype,$length,@addrs) = gethost*
    ($name,$aliases,$addrtype,$net) = getnet*
    ($name,$aliases,$proto) = getproto*
    ($name,$aliases,$port,$proto) = getserv*

(If the entry doesn't exist you get a null list.)

Within a scalar context, you get the name, unless the function was a
lookup by name, in which case you get the other thing, whatever it is.
(If the entry doesn't exist you get the undefined value.)  For example:

    $uid = getpwnam
    $name = getpwuid
    $name = getpwent
    $gid = getgrnam
    $name = getgrgid
    $name = getgrent
    etc.

The $members value returned by I<getgr*()> is a space separated list of
the login names of the members of the group.

For the I<gethost*()> functions, if the C<h_errno> variable is supported in
C, it will be returned to you via C<$?> if the function call fails.  The
@addrs value returned by a successful call is a list of the raw
addresses returned by the corresponding system library call.  In the
Internet domain, each address is four bytes long and you can unpack it
by saying something like:

    ($a,$b,$c,$d) = unpack('C4',$addr[0]);

=item getsockname SOCKET

Returns the packed sockaddr address of this end of the SOCKET connection.

    use Socket;
    $mysockaddr = getsockname(SOCK);
    ($port, $myaddr) = unpack_sockaddr_in($mysockaddr);

=item getsockopt SOCKET,LEVEL,OPTNAME

Returns the socket option requested, or undefined if there is an error.

=item glob EXPR

Returns the value of EXPR with filename expansions such as a shell
would do.  This is the internal function implementing the <*.*>
operator, except it's easier to use.

=item gmtime EXPR

Converts a time as returned by the time function to a 9-element array
with the time localized for the standard Greenwich timezone.  
Typically used as follows:


    ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
					    gmtime(time);

All array elements are numeric, and come straight out of a struct tm.
In particular this means that $mon has the range 0..11 and $wday has
the range 0..6.  If EXPR is omitted, does C<gmtime(time())>.

=item goto LABEL

=item goto EXPR

=item goto &NAME

The goto-LABEL form finds the statement labeled with LABEL and resumes
execution there.  It may not be used to go into any construct that
requires initialization, such as a subroutine or a foreach loop.  It
also can't be used to go into a construct that is optimized away.  It
can be used to go almost anywhere else within the dynamic scope,
including out of subroutines, but it's usually better to use some other
construct such as last or die.  The author of Perl has never felt the
need to use this form of goto (in Perl, that is--C is another matter).

The goto-EXPR form expects a label name, whose scope will be resolved
dynamically.  This allows for computed gotos per FORTRAN, but isn't
necessarily recommended if you're optimizing for maintainability:

    goto ("FOO", "BAR", "GLARCH")[$i];

The goto-&NAME form is highly magical, and substitutes a call to the
named subroutine for the currently running subroutine.  This is used by
AUTOLOAD subroutines that wish to load another subroutine and then
pretend that the other subroutine had been called in the first place
(except that any modifications to @_ in the current subroutine are
propagated to the other subroutine.)  After the goto, not even caller()
will be able to tell that this routine was called first.

=item grep BLOCK LIST

=item grep EXPR,LIST

Evaluates the BLOCK or EXPR for each element of LIST (locally setting
$_ to each element) and returns the list value consisting of those
elements for which the expression evaluated to TRUE.  In a scalar
context, returns the number of times the expression was TRUE.

    @foo = grep(!/^#/, @bar);    # weed out comments

or equivalently,

    @foo = grep {!/^#/} @bar;    # weed out comments

Note that, since $_ is a reference into the list value, it can be used
to modify the elements of the array.  While this is useful and
supported, it can cause bizarre results if the LIST is not a named
array.

=item hex EXPR

Interprets EXPR as a hex string and returns the corresponding decimal
value.  (To convert strings that might start with 0 or 0x see
oct().)  If EXPR is omitted, uses $_.

=item import

There is no built-in import() function.  It is merely an ordinary
method (subroutine) defined (or inherited) by modules that wish to export
names to another module.  The use() function calls the import() method
for the package used.  See also L</use>, L<perlmod>, and L<Exporter>.

=item index STR,SUBSTR,POSITION

=item index STR,SUBSTR

Returns the position of the first occurrence of SUBSTR in STR at or after
POSITION.  If POSITION is omitted, starts searching from the beginning of
the string.  The return value is based at 0 (or whatever you've set the $[
variable to--but don't do that).  If the substring is not found, returns
one less than the base, ordinarily -1.

=item int EXPR

Returns the integer portion of EXPR.  If EXPR is omitted, uses $_.

=item ioctl FILEHANDLE,FUNCTION,SCALAR

Implements the ioctl(2) function.  You'll probably have to say

    require "ioctl.ph";	# probably in /usr/local/lib/perl/ioctl.ph

first to get the correct function definitions.  If F<ioctl.ph> doesn't
exist or doesn't have the correct definitions you'll have to roll your
own, based on your C header files such as F<E<lt>sys/ioctl.hE<gt>>.
(There is a Perl script called B<h2ph> that comes with the Perl kit which
may help you in this, but it's non-trivial.)  SCALAR will be read and/or
written depending on the FUNCTION--a pointer to the string value of SCALAR
will be passed as the third argument of the actual ioctl call.  (If SCALAR
has no string value but does have a numeric value, that value will be
passed rather than a pointer to the string value.  To guarantee this to be
TRUE, add a 0 to the scalar before using it.)  The pack() and unpack()
functions are useful for manipulating the values of structures used by
ioctl().  The following example sets the erase character to DEL.

    require 'ioctl.ph';
    $getp = &TIOCGETP;
    die "NO TIOCGETP" if $@ || !$getp;
    $sgttyb_t = "ccccs";		# 4 chars and a short
    if (ioctl(STDIN,$getp,$sgttyb)) {
	@ary = unpack($sgttyb_t,$sgttyb);
	$ary[2] = 127;
	$sgttyb = pack($sgttyb_t,@ary);
	ioctl(STDIN,&TIOCSETP,$sgttyb)
	    || die "Can't ioctl: $!";
    }

The return value of ioctl (and fcntl) is as follows:

	if OS returns:		then Perl returns:
	    -1	  		  undefined value
	     0	 		string "0 but true"
	anything else		    that number

Thus Perl returns TRUE on success and FALSE on failure, yet you can
still easily determine the actual value returned by the operating
system:

    ($retval = ioctl(...)) || ($retval = -1);
    printf "System returned %d\n", $retval;

=item join EXPR,LIST

Joins the separate strings of LIST or ARRAY into a single string with
fields separated by the value of EXPR, and returns the string.
Example:

    $_ = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);

See L<perlfunc/split>.

=item keys ASSOC_ARRAY

Returns a normal array consisting of all the keys of the named
associative array.  (In a scalar context, returns the number of keys.)
The keys are returned in an apparently random order, but it is the same
order as either the values() or each() function produces (given that
the associative array has not been modified).  Here is yet another way
to print your environment:

    @keys = keys %ENV;
    @values = values %ENV;
    while ($#keys >= 0) {
	print pop(@keys), '=', pop(@values), "\n";
    }

or how about sorted by key:

    foreach $key (sort(keys %ENV)) {
	print $key, '=', $ENV{$key}, "\n";
    }

To sort an array by value, you'll need to use a C<sort{}>
function.  Here's a descending numeric sort of a hash by its values:

    foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash)) {
	printf "%4d %s\n", $hash{$key}, $key;
    }

=item kill LIST

Sends a signal to a list of processes.  The first element of 
the list must be the signal to send.  Returns the number of 
processes successfully signaled.

    $cnt = kill 1, $child1, $child2;
    kill 9, @goners;

Unlike in the shell, in Perl if the I<SIGNAL> is negative, it kills
process groups instead of processes.  (On System V, a negative I<PROCESS>
number will also kill process groups, but that's not portable.)  That
means you usually want to use positive not negative signals.  You may also
use a signal name in quotes.  See the L<perlipc/"Signals"> man page for details.

=item last LABEL

=item last

The C<last> command is like the C<break> statement in C (as used in
loops); it immediately exits the loop in question.  If the LABEL is
omitted, the command refers to the innermost enclosing loop.  The
C<continue> block, if any, is not executed:

    LINE: while (<STDIN>) {
	last LINE if /^$/;	# exit when done with header
	...
    }

=item lc EXPR

Returns an lowercased version of EXPR.  This is the internal function
implementing the \L escape in double-quoted strings.  
Should respect any POSIX setlocale() settings.

=item lcfirst EXPR

Returns the value of EXPR with the first character lowercased.  This is
the internal function implementing the \l escape in double-quoted strings.
Should respect any POSIX setlocale() settings.

=item length EXPR

Returns the length in characters of the value of EXPR.  If EXPR is
omitted, returns length of $_.

=item link OLDFILE,NEWFILE

Creates a new filename linked to the old filename.  Returns 1 for
success, 0 otherwise.

=item listen SOCKET,QUEUESIZE

Does the same thing that the listen system call does.  Returns TRUE if
it succeeded, FALSE otherwise.  See example in L<perlipc/"Sockets: Client/Server Communication">.

=item local EXPR

A local modifies the listed variables to be local to the enclosing block,
subroutine, C<eval{}> or C<do>.  If more than one value is listed, the
list must be placed in parens.  See L<perlsub/"Temporary Values via
local()"> for details.

But you really probably want to be using my() instead, because local() isn't
what most people think of as "local").  See L<perlsub/"Private Variables
via my()"> for details.

=item localtime EXPR

Converts a time as returned by the time function to a 9-element array
with the time analyzed for the local timezone.  Typically used as
follows:

    ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
						localtime(time);

All array elements are numeric, and come straight out of a struct tm.
In particular this means that $mon has the range 0..11 and $wday has
the range 0..6.  If EXPR is omitted, does localtime(time).

In a scalar context, prints out the ctime(3) value:

    $now_string = localtime;  # e.g. "Thu Oct 13 04:54:34 1994"

See also L<perlmod/timelocal> and the strftime(3) function available
via the POSIX modulie.

=item log EXPR

Returns logarithm (base I<e>) of EXPR.  If EXPR is omitted, returns log
of $_.

=item lstat FILEHANDLE

=item lstat EXPR

Does the same thing as the stat() function, but stats a symbolic link
instead of the file the symbolic link points to.  If symbolic links are
unimplemented on your system, a normal stat() is done.

=item m//

The match operator.  See L<perlop>.

=item map BLOCK LIST

=item map EXPR,LIST

Evaluates the BLOCK or EXPR for each element of LIST (locally setting $_ to each
element) and returns the list value composed of the results of each such
evaluation.  Evaluates BLOCK or EXPR in a list context, so each element of LIST
may produce zero, one, or more elements in the returned value.

    @chars = map(chr, @nums);

translates a list of numbers to the corresponding characters.  And

    %hash = map { getkey($_) => $_ } @array;

is just a funny way to write

    %hash = ();
    foreach $_ (@array) {
	$hash{getkey($_)} = $_;
    }

=item mkdir FILENAME,MODE

Creates the directory specified by FILENAME, with permissions specified
by MODE (as modified by umask).  If it succeeds it returns 1, otherwise
it returns 0 and sets $! (errno).

=item msgctl ID,CMD,ARG

Calls the System V IPC function msgctl(2).  If CMD is &IPC_STAT, then ARG
must be a variable which will hold the returned msqid_ds structure.
Returns like ioctl: the undefined value for error, "0 but true" for
zero, or the actual return value otherwise.

=item msgget KEY,FLAGS

Calls the System V IPC function msgget(2).  Returns the message queue id,
or the undefined value if there is an error.

=item msgsnd ID,MSG,FLAGS

Calls the System V IPC function msgsnd to send the message MSG to the
message queue ID.  MSG must begin with the long integer message type,
which may be created with C<pack("l", $type)>.  Returns TRUE if
successful, or FALSE if there is an error.

=item msgrcv ID,VAR,SIZE,TYPE,FLAGS

Calls the System V IPC function msgrcv to receive a message from
message queue ID into variable VAR with a maximum message size of
SIZE.  Note that if a message is received, the message type will be the
first thing in VAR, and the maximum length of VAR is SIZE plus the size
of the message type.  Returns TRUE if successful, or FALSE if there is
an error.

=item my EXPR

A "my" declares the listed variables to be local (lexically) to the
enclosing block, subroutine, C<eval>, or C<do/require/use>'d file.  If
more than one value is listed, the list must be placed in parens.  See
L<perlsub/"Private Variables via my()"> for details.

=item next LABEL

=item next

The C<next> command is like the C<continue> statement in C; it starts
the next iteration of the loop:

    LINE: while (<STDIN>) {
	next LINE if /^#/;	# discard comments
	...
    }

Note that if there were a C<continue> block on the above, it would get
executed even on discarded lines.  If the LABEL is omitted, the command
refers to the innermost enclosing loop.

=item no Module LIST

See the "use" function, which "no" is the opposite of.

=item oct EXPR

Interprets EXPR as an octal string and returns the corresponding
decimal value.  (If EXPR happens to start off with 0x, interprets it as
a hex string instead.)  The following will handle decimal, octal, and
hex in the standard Perl or C notation:

    $val = oct($val) if $val =~ /^0/;

If EXPR is omitted, uses $_.

=item open FILEHANDLE,EXPR

=item open FILEHANDLE

Opens the file whose filename is given by EXPR, and associates it with
FILEHANDLE.  If FILEHANDLE is an expression, its value is used as the name
of the real filehandle wanted.  If EXPR is omitted, the scalar variable of
the same name as the FILEHANDLE contains the filename.  If the filename
begins with "<" or nothing, the file is opened for input.  If the filename
begins with ">", the file is opened for output.  If the filename begins
with ">>", the file is opened for appending.  You can put a '+' in front
of the '>' or '<' to indicate that you want both read and write access to
the file; thus '+<' is usually preferred for read/write updates--the '+>'
mode would clobber the file first.  These correspond to the fopen(3) modes
of 'r', 'r+', 'w', 'w+', 'a', and 'a+'.

If the filename begins with "|", the filename is interpreted
as a command to which output is to be piped, and if the filename ends with
a "|", the filename is interpreted See L<perlipc/"Using open() for IPC">
for more examples of this.  as command which pipes input to us.  (You may
not have a raw open() to a command that pipes both in I<and> out, but see See L<open2>,
L<open3>, and L<perlipc/"Bidirectional Communication"> for alternatives.)

Opening '-' opens STDIN and opening '>-' opens STDOUT.  Open returns
non-zero upon success, the undefined value otherwise.  If the open
involved a pipe, the return value happens to be the pid of the
subprocess.  

If you're unfortunate enough to be running Perl on a system that
distinguishes between text files and binary files (modern operating
systems don't care), then you should check out L</binmode> for tips for
dealing with this.  The key distinction between systems that need binmode
and those that don't is their text file formats.  Systems like Unix and
Plan9 that delimit lines with a single character, and that encode that
character in C as '\n', do not need C<binmode>.  The rest need it.

Examples:

    $ARTICLE = 100;
    open ARTICLE or die "Can't find article $ARTICLE: $!\n";
    while (<ARTICLE>) {...

    open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)

    open(DBASE, '+<dbase.mine');	    # open for update

    open(ARTICLE, "caesar <$article |");    # decrypt article

    open(EXTRACT, "|sort >/tmp/Tmp$$");     # $$ is our process id

    # process argument list of files along with any includes

    foreach $file (@ARGV) {
	process($file, 'fh00');
    }

    sub process {
	local($filename, $input) = @_;
	$input++;		# this is a string increment
	unless (open($input, $filename)) {
	    print STDERR "Can't open $filename: $!\n";
	    return;
	}

	while (<$input>) {		# note use of indirection
	    if (/^#include "(.*)"/) {
		process($1, $input);
		next;
	    }
	    ...		# whatever
	}
    }

You may also, in the Bourne shell tradition, specify an EXPR beginning
with ">&", in which case the rest of the string is interpreted as the
name of a filehandle (or file descriptor, if numeric) which is to be
duped and opened.  You may use & after >, >>, <, +>, +>> and +<.  The
mode you specify should match the mode of the original filehandle.
(Duping a filehandle does not take into acount any existing contents of
stdio buffers.)
Here is a script that saves, redirects, and restores STDOUT and
STDERR:

    #!/usr/bin/perl
    open(SAVEOUT, ">&STDOUT");
    open(SAVEERR, ">&STDERR");

    open(STDOUT, ">foo.out") || die "Can't redirect stdout";
    open(STDERR, ">&STDOUT") || die "Can't dup stdout";

    select(STDERR); $| = 1;	# make unbuffered
    select(STDOUT); $| = 1;	# make unbuffered

    print STDOUT "stdout 1\n";	# this works for
    print STDERR "stderr 1\n"; 	# subprocesses too

    close(STDOUT);
    close(STDERR);

    open(STDOUT, ">&SAVEOUT");
    open(STDERR, ">&SAVEERR");

    print STDOUT "stdout 2\n";
    print STDERR "stderr 2\n";


If you specify "<&=N", where N is a number, then Perl will do an
equivalent of C's fdopen() of that file descriptor; this is more
parsimonious of file descriptors.  For example:

    open(FILEHANDLE, "<&=$fd")

If you open a pipe on the command "-", i.e. either "|-" or "-|", then
there is an implicit fork done, and the return value of open is the pid
of the child within the parent process, and 0 within the child
process.  (Use defined($pid) to determine whether the open was successful.)
The filehandle behaves normally for the parent, but i/o to that
filehandle is piped from/to the STDOUT/STDIN of the child process.
In the child process the filehandle isn't opened--i/o happens from/to
the new STDOUT or STDIN.  Typically this is used like the normal
piped open when you want to exercise more control over just how the
pipe command gets executed, such as when you are running setuid, and
don't want to have to scan shell commands for metacharacters.  
The following pairs are more or less equivalent:

    open(FOO, "|tr '[a-z]' '[A-Z]'");
    open(FOO, "|-") || exec 'tr', '[a-z]', '[A-Z]';

    open(FOO, "cat -n '$file'|");
    open(FOO, "-|") || exec 'cat', '-n', $file;

See L<perlipc/"Safe Pipe Opens"> for more examples of this.

Explicitly closing any piped filehandle causes the parent process to
wait for the child to finish, and returns the status value in $?.
Note: on any operation which may do a fork, unflushed buffers remain
unflushed in both processes, which means you may need to set $| to
avoid duplicate output.

Using the FileHandle constructor from the FileHandle package,
you can generate anonymous filehandles which have the scope of whatever
variables hold references to them, and automatically close whenever
and however you leave that scope:

    use FileHandle;
    ...
    sub read_myfile_munged {
	my $ALL = shift;
	my $handle = new FileHandle;
	open($handle, "myfile") or die "myfile: $!";
	$first = <$handle>
	    or return ();     # Automatically closed here.
	mung $first or die "mung failed";	# Or here.
	return $first, <$handle> if $ALL;	# Or here.
	$first;					# Or here.
    }

The filename that is passed to open will have leading and trailing
whitespace deleted.  In order to open a file with arbitrary weird
characters in it, it's necessary to protect any leading and trailing
whitespace thusly:

    $file =~ s#^(\s)#./$1#;
    open(FOO, "< $file\0");

If you want a "real" C open() (see L<open(2)> on your system), then
you should use the sysopen() function.  This is another way to
protect your filenames from interpretation.  For example:

    use FileHandle;
    sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL, 0700)
	or die "sysopen $path: $!";
    HANDLE->autoflush(1);
    HANDLE->print("stuff $$\n");
    seek(HANDLE, 0, 0);
    print "File contains: ", <HANDLE>;

See L</seek()> for some details about mixing reading and writing.

=item opendir DIRHANDLE,EXPR

Opens a directory named EXPR for processing by readdir(), telldir(),
seekdir(), rewinddir() and closedir().  Returns TRUE if successful.
DIRHANDLEs have their own namespace separate from FILEHANDLEs.

=item ord EXPR

Returns the numeric ascii value of the first character of EXPR.  If
EXPR is omitted, uses $_.

=item pack TEMPLATE,LIST

Takes an array or list of values and packs it into a binary structure,
returning the string containing the structure.  The TEMPLATE is a
sequence of characters that give the order and type of values, as
follows:

    A	An ascii string, will be space padded.
    a	An ascii string, will be null padded.
    b	A bit string (ascending bit order, like vec()).
    B	A bit string (descending bit order).
    h	A hex string (low nybble first).
    H	A hex string (high nybble first).

    c	A signed char value.
    C	An unsigned char value.
    s	A signed short value.
    S	An unsigned short value.
    i	A signed integer value.
    I	An unsigned integer value.
    l	A signed long value.
    L	An unsigned long value.

    n	A short in "network" order.
    N	A long in "network" order.
    v	A short in "VAX" (little-endian) order.
    V	A long in "VAX" (little-endian) order.

    f	A single-precision float in the native format.
    d	A double-precision float in the native format.

    p	A pointer to a null-terminated string.
    P	A pointer to a structure (fixed-length string).

    u	A uuencoded string.

    x	A null byte.
    X	Back up a byte.
    @	Null fill to absolute position.

Each letter may optionally be followed by a number which gives a repeat
count.  With all types except "a", "A", "b", "B", "h" and "H", and "P" the
pack function will gobble up that many values from the LIST.  A * for the
repeat count means to use however many items are left.  The "a" and "A"
types gobble just one value, but pack it as a string of length count,
padding with nulls or spaces as necessary.  (When unpacking, "A" strips
trailing spaces and nulls, but "a" does not.)  Likewise, the "b" and "B"
fields pack a string that many bits long.  The "h" and "H" fields pack a
string that many nybbles long.  The "P" packs a pointer to a structure of
the size indicated by the length.  Real numbers (floats and doubles) are
in the native machine format only; due to the multiplicity of floating
formats around, and the lack of a standard "network" representation, no
facility for interchange has been made.  This means that packed floating
point data written on one machine may not be readable on another - even if
both use IEEE floating point arithmetic (as the endian-ness of the memory
representation is not part of the IEEE spec).  Note that Perl uses doubles
internally for all numeric calculation, and converting from double into
float and thence back to double again will lose precision (i.e.
C<unpack("f", pack("f", $foo)>) will not in general equal $foo).

Examples:

    $foo = pack("cccc",65,66,67,68);
    # foo eq "ABCD"
    $foo = pack("c4",65,66,67,68);
    # same thing

    $foo = pack("ccxxcc",65,66,67,68);
    # foo eq "AB\0\0CD"

    $foo = pack("s2",1,2);
    # "\1\0\2\0" on little-endian
    # "\0\1\0\2" on big-endian

    $foo = pack("a4","abcd","x","y","z");
    # "abcd"

    $foo = pack("aaaa","abcd","x","y","z");
    # "axyz"

    $foo = pack("a14","abcdefg");
    # "abcdefg\0\0\0\0\0\0\0"

    $foo = pack("i9pl", gmtime);
    # a real struct tm (on my system anyway)

    sub bintodec {
	unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
    }

The same template may generally also be used in the unpack function.

=item package NAMESPACE

Declares the compilation unit as being in the given namespace.  The scope
of the package declaration is from the declaration itself through the end of
the enclosing block (the same scope as the local() operator).  All further
unqualified dynamic identifiers will be in this namespace.  A package
statement only affects dynamic variables--including those you've used
local() on--but I<not> lexical variables created with my().  Typically it
would be the first declaration in a file to be included by the C<require>
or C<use> operator.  You can switch into a package in more than one place;
it merely influences which symbol table is used by the compiler for the
rest of that block.  You can refer to variables and filehandles in other
packages by prefixing the identifier with the package name and a double
colon:  C<$Package::Variable>.  If the package name is null, the C<main>
package as assumed.  That is, C<$::sail> is equivalent to C<$main::sail>.

See L<perlmod/"Packages"> for more information about packages, modules,
and classes.  See L<perlsub> for other scoping issues.

=item pipe READHANDLE,WRITEHANDLE

Opens a pair of connected pipes like the corresponding system call.
Note that if you set up a loop of piped processes, deadlock can occur
unless you are very careful.  In addition, note that Perl's pipes use
stdio buffering, so you may need to set $| to flush your WRITEHANDLE
after each command, depending on the application.

See L<open2>, L<open3>, and L<perlipc/"Bidirectional Communication">
for examples of such things.

=item pop ARRAY

Pops and returns the last value of the array, shortening the array by
1.  Has a similar effect to

    $tmp = $ARRAY[$#ARRAY--];

If there are no elements in the array, returns the undefined value.
If ARRAY is omitted, pops the
@ARGV array in the main program, and the @_ array in subroutines, just
like shift().

=item pos SCALAR

Returns the offset of where the last C<m//g> search left off for the variable
in question.  May be modified to change that offset.

=item print FILEHANDLE LIST

=item print LIST

=item print

Prints a string or a comma-separated list of strings.  Returns TRUE
if successful.  FILEHANDLE may be a scalar variable name, in which case
the variable contains the name of or a reference to the filehandle, thus introducing one
level of indirection.  (NOTE: If FILEHANDLE is a variable and the next
token is a term, it may be misinterpreted as an operator unless you
interpose a + or put parens around the arguments.)  If FILEHANDLE is
omitted, prints by default to standard output (or to the last selected
output channel--see select()).  If LIST is also omitted, prints $_ to
STDOUT.  To set the default output channel to something other than
STDOUT use the select operation.  Note that, because print takes a
LIST, anything in the LIST is evaluated in a list context, and any
subroutine that you call will have one or more of its expressions
evaluated in a list context.  Also be careful not to follow the print
keyword with a left parenthesis unless you want the corresponding right
parenthesis to terminate the arguments to the print--interpose a + or
put parens around all the arguments.

Note that if you're storing FILEHANDLES in an array or other expression,
you will have to use a block returning its value instead

    print { $files[$i] } "stuff\n";
    print { $OK ? STDOUT : STDERR } "stuff\n";

=item printf FILEHANDLE LIST

=item printf LIST

Equivalent to a "print FILEHANDLE sprintf(LIST)".  The first argument
of the list will be interpreted as the printf format.

=item push ARRAY,LIST

Treats ARRAY as a stack, and pushes the values of LIST
onto the end of ARRAY.  The length of ARRAY increases by the length of
LIST.  Has the same effect as

    for $value (LIST) {
	$ARRAY[++$#ARRAY] = $value;
    }

but is more efficient.  Returns the new number of elements in the array.

=item q/STRING/

=item qq/STRING/

=item qx/STRING/

=item qw/STRING/

Generalized quotes.  See L<perlop>.

=item quotemeta EXPR

Returns the value of EXPR with with all regular expression
metacharacters backslashed.  This is the internal function implementing
the \Q escape in double-quoted strings.

=item rand EXPR

=item rand

Returns a random fractional number between 0 and the value of EXPR.
(EXPR should be positive.)  If EXPR is omitted, returns a value between 
0 and 1.  This function produces repeatable sequences unless srand() 
is invoked.  See also srand().

(Note: if your rand function consistently returns numbers that are too
large or too small, then your version of Perl was probably compiled
with the wrong number of RANDBITS.  As a workaround, you can usually
multiply EXPR by the correct power of 2 to get the range you want.
This will make your script unportable, however.  It's better to recompile
if you can.)

=item read FILEHANDLE,SCALAR,LENGTH,OFFSET

=item read FILEHANDLE,SCALAR,LENGTH

Attempts to read LENGTH bytes of data into variable SCALAR from the
specified FILEHANDLE.  Returns the number of bytes actually read, or
undef if there was an error.  SCALAR will be grown or shrunk to the
length actually read.  An OFFSET may be specified to place the read
data at some other place than the beginning of the string.  This call
is actually implemented in terms of stdio's fread call.  To get a true
read system call, see sysread().

=item readdir DIRHANDLE

Returns the next directory entry for a directory opened by opendir().
If used in a list context, returns all the rest of the entries in the
directory.  If there are no more entries, returns an undefined value in
a scalar context or a null list in a list context.

If you're planning to filetest the return values out of a readdir(), you'd
better prepend the directory in question.  Otherwise, since we didn't
chdir() there, it would have been testing the wrong file.

    opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!";
    @dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR);
    closedir DIR;

=item readlink EXPR

Returns the value of a symbolic link, if symbolic links are
implemented.  If not, gives a fatal error.  If there is some system
error, returns the undefined value and sets $! (errno).  If EXPR is
omitted, uses $_.

=item recv SOCKET,SCALAR,LEN,FLAGS

Receives a message on a socket.  Attempts to receive LENGTH bytes of
data into variable SCALAR from the specified SOCKET filehandle.
Actually does a C recvfrom(), so that it can returns the address of the
sender.  Returns the undefined value if there's an error.  SCALAR will
be grown or shrunk to the length actually read.  Takes the same flags
as the system call of the same name.  
See L<perlipc/"UDP: Message Passing"> for examples.

=item redo LABEL

=item redo

The C<redo> command restarts the loop block without evaluating the
conditional again.  The C<continue> block, if any, is not executed.  If
the LABEL is omitted, the command refers to the innermost enclosing
loop.  This command is normally used by programs that want to lie to
themselves about what was just input:

    # a simpleminded Pascal comment stripper
    # (warning: assumes no { or } in strings)
    LINE: while (<STDIN>) {
	while (s|({.*}.*){.*}|$1 |) {}
	s|{.*}| |;
	if (s|{.*| |) {
	    $front = $_;
	    while (<STDIN>) {
		if (/}/) {	# end of comment?
		    s|^|$front{|;
		    redo LINE;
		}
	    }
	}
	print;
    }

=item ref EXPR

Returns a TRUE value if EXPR is a reference, FALSE otherwise.  The value
returned depends on the type of thing the reference is a reference to.
Builtin types include:

    REF
    SCALAR
    ARRAY
    HASH
    CODE
    GLOB

If the referenced object has been blessed into a package, then that package 
name is returned instead.  You can think of ref() as a typeof() operator.

    if (ref($r) eq "HASH") {
	print "r is a reference to an associative array.\n";
    } 
    if (!ref ($r) {
	print "r is not a reference at all.\n";
    } 

See also L<perlref>.

=item rename OLDNAME,NEWNAME

Changes the name of a file.  Returns 1 for success, 0 otherwise.  Will
not work across filesystem boundaries.

=item require EXPR

=item require

Demands some semantics specified by EXPR, or by $_ if EXPR is not
supplied.  If EXPR is numeric, demands that the current version of Perl
($] or $PERL_VERSION) be equal or greater than EXPR.

Otherwise, demands that a library file be included if it hasn't already
been included.  The file is included via the do-FILE mechanism, which is
essentially just a variety of eval().  Has semantics similar to the following
subroutine:

    sub require {
	local($filename) = @_;
	return 1 if $INC{$filename};
	local($realfilename,$result);
	ITER: {
	    foreach $prefix (@INC) {
		$realfilename = "$prefix/$filename";
		if (-f $realfilename) {
		    $result = do $realfilename;
		    last ITER;
		}
	    }
	    die "Can't find $filename in \@INC";
	}
	die $@ if $@;
	die "$filename did not return true value" unless $result;
	$INC{$filename} = $realfilename;
	$result;
    }

Note that the file will not be included twice under the same specified
name.  The file must return TRUE as the last statement to indicate
successful execution of any initialization code, so it's customary to
end such a file with "1;" unless you're sure it'll return TRUE
otherwise.  But it's better just to put the "C<1;>", in case you add more
statements.

If EXPR is a bare word, the require assumes a "F<.pm>" extension for you,
to make it easy to load standard modules.  This form of loading of 
modules does not risk altering your namespace.

For a yet-more-powerful import facility, see the L</use()> and 
L<perlmod>.

=item reset EXPR

=item reset

Generally used in a C<continue> block at the end of a loop to clear
variables and reset ?? searches so that they work again.  The
expression is interpreted as a list of single characters (hyphens
allowed for ranges).  All variables and arrays beginning with one of
those letters are reset to their pristine state.  If the expression is
omitted, one-match searches (?pattern?) are reset to match again.  Only
resets variables or searches in the current package.  Always returns
1.  Examples:

    reset 'X';		# reset all X variables
    reset 'a-z';	# reset lower case variables
    reset;		# just reset ?? searches

Resetting "A-Z" is not recommended since you'll wipe out your
ARGV and ENV arrays.  Only resets package variables--lexical variables
are unaffected, but they clean themselves up on scope exit anyway,
so anymore you probably want to use them instead.  See L</my>.

=item return LIST

Returns from a subroutine or eval with the value specified.  (Note that
in the absence of a return a subroutine or eval() will automatically
return the value of the last expression evaluated.)

=item reverse LIST

In a list context, returns a list value consisting of the elements
of LIST in the opposite order.  In a scalar context, returns a string
value consisting of the bytes of the first element of LIST in the
opposite order.   

    print reverse <>;			# line tac 

    undef $/;
    print scalar reverse scalar <>;	# byte tac

=item rewinddir DIRHANDLE

Sets the current position to the beginning of the directory for the
readdir() routine on DIRHANDLE.

=item rindex STR,SUBSTR,POSITION

=item rindex STR,SUBSTR

Works just like index except that it returns the position of the LAST
occurrence of SUBSTR in STR.  If POSITION is specified, returns the
last occurrence at or before that position.

=item rmdir FILENAME

Deletes the directory specified by FILENAME if it is empty.  If it
succeeds it returns 1, otherwise it returns 0 and sets $! (errno).  If
FILENAME is omitted, uses $_.

=item s///

The substitution operator.  See L<perlop>.

=item scalar EXPR

Forces EXPR to be interpreted in a scalar context and returns the value
of EXPR.  

    @counts = ( scalar @a, scalar @b, scalar @c );

There is no equivalent operator to force an expression to 
be interpolated in a list context because it's in practice never
needed.  If you really wanted to do so, however, you could use
the construction C<@{[ (some expression) ]}>, but usually a simple
C<(some expression)> suffices.

=item seek FILEHANDLE,POSITION,WHENCE

Randomly positions the file pointer for FILEHANDLE, just like the fseek()
call of stdio.  FILEHANDLE may be an expression whose value gives the name
of the filehandle.  The values for WHENCE are 0 to set the file pointer to
POSITION, 1 to set the it to current plus POSITION, and 2 to set it to EOF
plus offset.  You may use the values SEEK_SET, SEEK_CUR, and SEEK_END for
this from POSIX module.  Returns 1 upon success, 0 otherwise.

On some systems you have to do a seek whenever you switch between reading
and writing.  Amongst other things, this may have the effect of calling
stdio's clearerr(3).  A "whence" of 1 (SEEK_CUR) is useful for not moving
the file pointer:

    seek(TEST,0,1);

This is also useful for applications emulating C<tail -f>.  Once you hit
EOF on your read, and then sleep for a while, you might have to stick in a
seek() to reset things.  First the simple trick listed above to clear the
filepointer.  The seek() doesn't change the current position, but it
I<does> clear the end-of-file condition on the handle, so that the next
C<E<lt>FILE<E<gt>> makes Perl try again to read something.  Hopefully.

If that doesn't work (some stdios are particularly cantankerous), then
you may need something more like this:

    for (;;) {
	for ($curpos = tell(FILE); $_ = <FILE>; $curpos = tell(FILE)) {
	    # search for some stuff and put it into files
	}
	sleep($for_a_while);
	seek(FILE, $curpos, 0);
    }

=item seekdir DIRHANDLE,POS

Sets the current position for the readdir() routine on DIRHANDLE.  POS
must be a value returned by telldir().  Has the same caveats about
possible directory compaction as the corresponding system library
routine.

=item select FILEHANDLE

=item select

Returns the currently selected filehandle.  Sets the current default
filehandle for output, if FILEHANDLE is supplied.  This has two
effects: first, a C<write> or a C<print> without a filehandle will
default to this FILEHANDLE.  Second, references to variables related to
output will refer to this output channel.  For example, if you have to
set the top of form format for more than one output channel, you might
do the following:

    select(REPORT1);
    $^ = 'report1_top';
    select(REPORT2);
    $^ = 'report2_top';

FILEHANDLE may be an expression whose value gives the name of the
actual filehandle.  Thus:

    $oldfh = select(STDERR); $| = 1; select($oldfh);

Some programmers may prefer to think of filehandles as objects with
methods, preferring to write the last example as:

    use FileHandle;
    STDERR->autoflush(1);

=item select RBITS,WBITS,EBITS,TIMEOUT

This calls the select(2) system call with the bitmasks specified, which
can be constructed using fileno() and vec(), along these lines:

    $rin = $win = $ein = '';
    vec($rin,fileno(STDIN),1) = 1;
    vec($win,fileno(STDOUT),1) = 1;
    $ein = $rin | $win;

If you want to select on many filehandles you might wish to write a
subroutine:

    sub fhbits {
	local(@fhlist) = split(' ',$_[0]);
	local($bits);
	for (@fhlist) {
	    vec($bits,fileno($_),1) = 1;
	}
	$bits;
    }
    $rin = fhbits('STDIN TTY SOCK');

The usual idiom is:

    ($nfound,$timeleft) =
      select($rout=$rin, $wout=$win, $eout=$ein, $timeout);

or to block until something becomes ready just do this 

    $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);

Most systems do not both to return anything useful in $timeleft, so
calling select() in a scalar context just returns $nfound.

Any of the bitmasks can also be undef.  The timeout, if specified, is
in seconds, which may be fractional.  Note: not all implementations are
capable of returning the $timeleft.  If not, they always return
$timeleft equal to the supplied $timeout.

You can effect a 250-microsecond sleep this way:

    select(undef, undef, undef, 0.25);

B<WARNING>: Do not attempt to mix buffered I/O (like read() or <FH>)
with select().  You have to use sysread() instead.

=item semctl ID,SEMNUM,CMD,ARG

Calls the System V IPC function semctl.  If CMD is &IPC_STAT or
&GETALL, then ARG must be a variable which will hold the returned
semid_ds structure or semaphore value array.  Returns like ioctl: the
undefined value for error, "0 but true" for zero, or the actual return
value otherwise.

=item semget KEY,NSEMS,FLAGS

Calls the System V IPC function semget.  Returns the semaphore id, or
the undefined value if there is an error.

=item semop KEY,OPSTRING

Calls the System V IPC function semop to perform semaphore operations
such as signaling and waiting.  OPSTRING must be a packed array of
semop structures.  Each semop structure can be generated with
C<pack("sss", $semnum, $semop, $semflag)>.  The number of semaphore
operations is implied by the length of OPSTRING.  Returns TRUE if
successful, or FALSE if there is an error.  As an example, the
following code waits on semaphore $semnum of semaphore id $semid:

    $semop = pack("sss", $semnum, -1, 0);
    die "Semaphore trouble: $!\n" unless semop($semid, $semop);

To signal the semaphore, replace "-1" with "1".

=item send SOCKET,MSG,FLAGS,TO

=item send SOCKET,MSG,FLAGS

Sends a message on a socket.  Takes the same flags as the system call
of the same name.  On unconnected sockets you must specify a
destination to send TO, in which case it does a C sendto().  Returns
the number of characters sent, or the undefined value if there is an
error.
See L<perlipc/"UDP: Message Passing"> for examples.

=item setpgrp PID,PGRP

Sets the current process group for the specified PID, 0 for the current
process.  Will produce a fatal error if used on a machine that doesn't
implement setpgrp(2).

=item setpriority WHICH,WHO,PRIORITY

Sets the current priority for a process, a process group, or a user.
(See setpriority(2).)  Will produce a fatal error if used on a machine
that doesn't implement setpriority(2).

=item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL

Sets the socket option requested.  Returns undefined if there is an
error.  OPTVAL may be specified as undef if you don't want to pass an
argument.

=item shift ARRAY

=item shift

Shifts the first value of the array off and returns it, shortening the
array by 1 and moving everything down.  If there are no elements in the
array, returns the undefined value.  If ARRAY is omitted, shifts the
@ARGV array in the main program, and the @_ array in subroutines.
(This is determined lexically.)  See also unshift(), push(), and pop().
Shift() and unshift() do the same thing to the left end of an array
that push() and pop() do to the right end.

=item shmctl ID,CMD,ARG

Calls the System V IPC function shmctl.  If CMD is &IPC_STAT, then ARG
must be a variable which will hold the returned shmid_ds structure.
Returns like ioctl: the undefined value for error, "0 but true" for
zero, or the actual return value otherwise.

=item shmget KEY,SIZE,FLAGS

Calls the System V IPC function shmget.  Returns the shared memory
segment id, or the undefined value if there is an error.

=item shmread ID,VAR,POS,SIZE

=item shmwrite ID,STRING,POS,SIZE

Reads or writes the System V shared memory segment ID starting at
position POS for size SIZE by attaching to it, copying in/out, and
detaching from it.  When reading, VAR must be a variable which will
hold the data read.  When writing, if STRING is too long, only SIZE
bytes are used; if STRING is too short, nulls are written to fill out
SIZE bytes.  Return TRUE if successful, or FALSE if there is an error.

=item shutdown SOCKET,HOW

Shuts down a socket connection in the manner indicated by HOW, which
has the same interpretation as in the system call of the same name.

=item sin EXPR

Returns the sine of EXPR (expressed in radians).  If EXPR is omitted,
returns sine of $_.

=item sleep EXPR

=item sleep

Causes the script to sleep for EXPR seconds, or forever if no EXPR.
May be interrupted by sending the process a SIGALRM.  Returns the
number of seconds actually slept.  You probably cannot mix alarm() and
sleep() calls, since sleep() is often implemented using alarm().

On some older systems, it may sleep up to a full second less than what
you requested, depending on how it counts seconds.  Most modern systems
always sleep the full amount.

For delays of finer granularity than one second, you may use Perl's
syscall() interface to access setitimer(2) if your system supports it, 
or else see L</select()> below.  

=item socket SOCKET,DOMAIN,TYPE,PROTOCOL

Opens a socket of the specified kind and attaches it to filehandle
SOCKET.  DOMAIN, TYPE and PROTOCOL are specified the same as for the
system call of the same name.  You should "use Socket;" first to get
the proper definitions imported.  See the example in L<perlipc/"Sockets: Client/Server Communication">.

=item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL

Creates an unnamed pair of sockets in the specified domain, of the
specified type.  DOMAIN, TYPE and PROTOCOL are specified the same as
for the system call of the same name.  If unimplemented, yields a fatal
error.  Returns TRUE if successful.

=item sort SUBNAME LIST

=item sort BLOCK LIST

=item sort LIST

Sorts the LIST and returns the sorted list value.  Nonexistent values
of arrays are stripped out.  If SUBNAME or BLOCK is omitted, sorts
in standard string comparison order.  If SUBNAME is specified, it
gives the name of a subroutine that returns an integer less than, equal
to, or greater than 0, depending on how the elements of the array are
to be ordered.  (The <=> and cmp operators are extremely useful in such
routines.)  SUBNAME may be a scalar variable name, in which case the
value provides the name of the subroutine to use.  In place of a
SUBNAME, you can provide a BLOCK as an anonymous, in-line sort
subroutine.

In the interests of efficiency the normal calling code for subroutines is
bypassed, with the following effects: the subroutine may not be a
recursive subroutine, and the two elements to be compared are passed into
the subroutine not via @_ but as the package global variables $a and
$b (see example below).  They are passed by reference, so don't
modify $a and $b.  And don't try to declare them as lexicals either.

Examples:

    # sort lexically
    @articles = sort @files;

    # same thing, but with explicit sort routine
    @articles = sort {$a cmp $b} @files;

    # now case-insensitively
    @articles = sort { uc($a) cmp uc($b)} @files;

    # same thing in reversed order
    @articles = sort {$b cmp $a} @files;

    # sort numerically ascending
    @articles = sort {$a <=> $b} @files;

    # sort numerically descending
    @articles = sort {$b <=> $a} @files;

    # sort using explicit subroutine name
    sub byage {
	$age{$a} <=> $age{$b};	# presuming integers
    }
    @sortedclass = sort byage @class;

    # this sorts the %age associative arrays by value 
    # instead of key using an inline function
    @eldest = sort { $age{$b} <=> $age{$a} } keys %age;

    sub backwards { $b cmp $a; }
    @harry = ('dog','cat','x','Cain','Abel');
    @george = ('gone','chased','yz','Punished','Axed');
    print sort @harry;
	    # prints AbelCaincatdogx
    print sort backwards @harry;
	    # prints xdogcatCainAbel
    print sort @george, 'to', @harry;
	    # prints AbelAxedCainPunishedcatchaseddoggonetoxyz

    # inefficiently sort by descending numeric compare using 
    # the first integer after the first = sign, or the 
    # whole record case-insensitively otherwise

    @new = sort {
	($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
			    ||
	            uc($a)  cmp  uc($b)
    } @old;

    # same thing, but much more efficiently;
    # we'll build auxiliary indices instead
    # for speed
    @nums = @caps = ();
    for (@old) { 
	push @nums, /=(\d+)/;
	push @caps, uc($_);
    } 

    @new = @old[ sort {
			$nums[$b] <=> $nums[$a]
				 ||
			$caps[$a] cmp $caps[$b]
		       } 0..$#old
	       ];

    # same thing using a Schwartzian Transform (no temps)
    @new = map { $_->[0] }
        sort { $b->[1] <=> $a->[1]
                        ||
               $a->[2] cmp $b->[2]
        } map { [$_, /=(\d+)/, uc($_)] } @old;

If you're and using strict, you I<MUST NOT> declare $a
and $b as lexicals.  They are package globals.  That means
if you're in the C<main> package, it's

    @articles = sort {$main::b <=> $main::a} @files;

or just

    @articles = sort {$::b <=> $::a} @files;

but if you're in the C<FooPack> package, it's

    @articles = sort {$FooPack::b <=> $FooPack::a} @files;

=item splice ARRAY,OFFSET,LENGTH,LIST

=item splice ARRAY,OFFSET,LENGTH

=item splice ARRAY,OFFSET

Removes the elements designated by OFFSET and LENGTH from an array, and
replaces them with the elements of LIST, if any.  Returns the elements
removed from the array.  The array grows or shrinks as necessary.  If
LENGTH is omitted, removes everything from OFFSET onward.  The
following equivalencies hold (assuming $[ == 0):

    push(@a,$x,$y)	splice(@a,$#a+1,0,$x,$y)
    pop(@a)		splice(@a,-1)
    shift(@a)		splice(@a,0,1)
    unshift(@a,$x,$y)	splice(@a,0,0,$x,$y)
    $a[$x] = $y		splice(@a,$x,1,$y);

Example, assuming array lengths are passed before arrays:

    sub aeq {	# compare two list values
	local(@a) = splice(@_,0,shift);
	local(@b) = splice(@_,0,shift);
	return 0 unless @a == @b;	# same len?
	while (@a) {
	    return 0 if pop(@a) ne pop(@b);
	}
	return 1;
    }
    if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }

=item split /PATTERN/,EXPR,LIMIT

=item split /PATTERN/,EXPR

=item split /PATTERN/

=item split

Splits a string into an array of strings, and returns it.

If not in a list context, returns the number of fields found and splits into
the @_ array.  (In a list context, you can force the split into @_ by
using C<??> as the pattern delimiters, but it still returns the array
value.)  The use of implicit split to @_ is deprecated, however.

If EXPR is omitted, splits the $_ string.  If PATTERN is also omitted,
splits on whitespace (after skipping any leading whitespace).  Anything
matching PATTERN is taken to be a delimiter separating the fields.  (Note
that the delimiter may be longer than one character.)  If LIMIT is
specified and is not negative, splits into no more than that many fields
(though it may split into fewer).  If LIMIT is unspecified, trailing null
fields are stripped (which potential users of pop() would do well to
remember).  If LIMIT is negative, it is treated as if an arbitrarily large
LIMIT had been specified.

A pattern matching the null string (not to be confused with
a null pattern C<//>, which is just one member of the set of patterns
matching a null string) will split the value of EXPR into separate
characters at each point it matches that way.  For example:

    print join(':', split(/ */, 'hi there'));

produces the output 'h:i:t:h:e:r:e'.

The LIMIT parameter can be used to partially split a line

    ($login, $passwd, $remainder) = split(/:/, $_, 3);

When assigning to a list, if LIMIT is omitted, Perl supplies a LIMIT
one larger than the number of variables in the list, to avoid
unnecessary work.  For the list above LIMIT would have been 4 by
default.  In time critical applications it behooves you not to split
into more fields than you really need.

If the PATTERN contains parentheses, additional array elements are
created from each matching substring in the delimiter.

    split(/([,-])/, "1-10,20");

produces the list value

    (1, '-', 10, ',', 20)

If you had the entire header of a normal Unix email message in $header, 
you could split it up into fields and their values this way:

    $header =~ s/\n\s+/ /g;  # fix continuation lines
    %hdrs   =  (UNIX_FROM => split /^(.*?):\s*/m, $header);

The pattern C</PATTERN/> may be replaced with an expression to specify
patterns that vary at runtime.  (To do runtime compilation only once,
use C</$variable/o>.)

As a special case, specifying a PATTERN of space (C<' '>) will split on
white space just as split with no arguments does.  Thus, split(' ') can
be used to emulate B<awk>'s default behavior, whereas C<split(/ /)>
will give you as many null initial fields as there are leading spaces.
A split on /\s+/ is like a split(' ') except that any leading
whitespace produces a null first field.  A split with no arguments
really does a C<split(' ', $_)> internally.

Example:

    open(passwd, '/etc/passwd');
    while (<passwd>) {
	($login, $passwd, $uid, $gid, $gcos, 
	    $home, $shell) = split(/:/);
	...
    }

(Note that $shell above will still have a newline on it.  See L</chop>, 
L</chomp>, and L</join>.)

=item sprintf FORMAT,LIST

Returns a string formatted by the usual printf conventions of the C
language.  See L<sprintf(3)> or L<printf(3)> on your system for details.
(The * character for an indirectly specified length is not
supported, but you can get the same effect by interpolating a variable
into the pattern.)  Some C libraries' implementations of sprintf() can
dump core when fed ludicrous arguments.

=item sqrt EXPR

Return the square root of EXPR.  If EXPR is omitted, returns square
root of $_.

=item srand EXPR

Sets the random number seed for the C<rand> operator.  If EXPR is omitted,
does C<srand(time)>.  Many folks use an explicit C<srand(time ^ $$)>
instead.  Of course, you'd need something much more random than that for
cryptographic purposes, since it's easy to guess the current time.
Checksumming the compressed output of rapidly changing operating system
status programs is the usual method.  Examples are posted regularly to
the comp.security.unix newsgroup.

=item stat FILEHANDLE

=item stat EXPR

Returns a 13-element array giving the status info for a file, either the
file opened via FILEHANDLE, or named by EXPR.  Returns a null list if
the stat fails.  Typically used as follows:

    ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
       $atime,$mtime,$ctime,$blksize,$blocks)
           = stat($filename);

Not all fields are supported on all filesystem types.  Here are the 
meaning of the fields:

  dev	    device number of filesystem 
  ino	    inode number 
  mode	    file mode  (type and permissions)
  nlink	    number of (hard) links to the file 
  uid	    numeric user ID of file's owner 
  gid	    numer group ID of file's owner 
  rdev	    the device identifier (special files only)
  size	    total size of file, in bytes 
  atime	    last access time since the epoch
  mtime	    last modify time since the epoch
  ctime	    inode change time (NOT creation type!) since the epoch
  blksize   preferred blocksize for file system I/O
  blocks    actual number of blocks allocated

(The epoch was at 00:00 January 1, 1970 GMT.)

If stat is passed the special filehandle consisting of an underline, no
stat is done, but the current contents of the stat structure from the
last stat or filetest are returned.  Example:

    if (-x $file && (($d) = stat(_)) && $d < 0) {
	print "$file is executable NFS file\n";
    }

(This only works on machines for which the device number is negative under NFS.)

=item study SCALAR

=item study

Takes extra time to study SCALAR ($_ if unspecified) in anticipation of
doing many pattern matches on the string before it is next modified.
This may or may not save time, depending on the nature and number of
patterns you are searching on, and on the distribution of character
frequencies in the string to be searched--you probably want to compare
runtimes with and without it to see which runs faster.  Those loops
which scan for many short constant strings (including the constant
parts of more complex patterns) will benefit most.  You may have only
one study active at a time--if you study a different scalar the first
is "unstudied".  (The way study works is this: a linked list of every
character in the string to be searched is made, so we know, for
example, where all the 'k' characters are.  From each search string,
the rarest character is selected, based on some static frequency tables
constructed from some C programs and English text.  Only those places
that contain this "rarest" character are examined.)

For example, here is a loop which inserts index producing entries
before any line containing a certain pattern:

    while (<>) {
	study;
	print ".IX foo\n" if /\bfoo\b/;
	print ".IX bar\n" if /\bbar\b/;
	print ".IX blurfl\n" if /\bblurfl\b/;
	...
	print;
    }

In searching for /\bfoo\b/, only those locations in $_ that contain "f"
will be looked at, because "f" is rarer than "o".  In general, this is
a big win except in pathological cases.  The only question is whether
it saves you more time than it took to build the linked list in the
first place.

Note that if you have to look for strings that you don't know till
runtime, you can build an entire loop as a string and eval that to
avoid recompiling all your patterns all the time.  Together with
undefining $/ to input entire files as one record, this can be very
fast, often faster than specialized programs like fgrep(1).  The following
scans a list of files (@files) for a list of words (@words), and prints
out the names of those files that contain a match:

    $search = 'while (<>) { study;';
    foreach $word (@words) {
	$search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
    }
    $search .= "}";
    @ARGV = @files;
    undef $/;
    eval $search;		# this screams
    $/ = "\n";		# put back to normal input delim
    foreach $file (sort keys(%seen)) {
	print $file, "\n";
    }

=item sub BLOCK

=item sub NAME

=item sub NAME BLOCK

This is subroutine definition, not a real function I<per se>.  With just a
NAME (and possibly prototypes), it's just a forward declaration.  Without
a NAME, it's an anonymous function declaration, and does actually return a
value: the CODE ref of the closure you just created. See L<perlsub> and
L<perlref> for details.

=item substr EXPR,OFFSET,LEN

=item substr EXPR,OFFSET

Extracts a substring out of EXPR and returns it.  First character is at
offset 0, or whatever you've set $[ to.  If OFFSET is negative, starts
that far from the end of the string.  If LEN is omitted, returns
everything to the end of the string.  If LEN is negative, leaves that
many characters off the end of the string.

You can use the substr() function
as an lvalue, in which case EXPR must be an lvalue.  If you assign
something shorter than LEN, the string will shrink, and if you assign
something longer than LEN, the string will grow to accommodate it.  To
keep the string the same length you may need to pad or chop your value
using sprintf().

=item symlink OLDFILE,NEWFILE

Creates a new filename symbolically linked to the old filename.
Returns 1 for success, 0 otherwise.  On systems that don't support
symbolic links, produces a fatal error at run time.  To check for that,
use eval:

    $symlink_exists = (eval 'symlink("","");', $@ eq '');

=item syscall LIST

Calls the system call specified as the first element of the list,
passing the remaining elements as arguments to the system call.  If
unimplemented, produces a fatal error.  The arguments are interpreted
as follows: if a given argument is numeric, the argument is passed as
an int.  If not, the pointer to the string value is passed.  You are
responsible to make sure a string is pre-extended long enough to
receive any result that might be written into a string.  If your
integer arguments are not literals and have never been interpreted in a
numeric context, you may need to add 0 to them to force them to look
like numbers.

    require 'syscall.ph';		# may need to run h2ph
    syscall(&SYS_write, fileno(STDOUT), "hi there\n", 9);

Note that Perl only supports passing of up to 14 arguments to your system call,
which in practice should usually suffice.

=item sysopen FILEHANDLE,FILENAME,MODE

=item sysopen FILEHANDLE,FILENAME,MODE,PERMS

Opens the file whose filename is given by FILENAME, and associates it
with FILEHANDLE.  If FILEHANDLE is an expression, its value is used as
the name of the real filehandle wanted.  This function calls the
underlying operating system's C<open> function with the parameters
FILENAME, MODE, PERMS.

The possible values and flag bits of the MODE parameter are
system-dependent; they are available via the standard module C<Fcntl>.
However, for historical reasons, some values are universal: zero means
read-only, one means write-only, and two means read/write.

If the file named by FILENAME does not exist and the C<open> call
creates it (typically because MODE includes the O_CREAT flag), then
the value of PERMS specifies the permissions of the newly created
file.  If PERMS is omitted, the default value is 0666, which allows
read and write for all.  This default is reasonable: see C<umask>.

=item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET

=item sysread FILEHANDLE,SCALAR,LENGTH

Attempts to read LENGTH bytes of data into variable SCALAR from the
specified FILEHANDLE, using the system call read(2).  It bypasses
stdio, so mixing this with other kinds of reads may cause confusion.
Returns the number of bytes actually read, or undef if there was an
error.  SCALAR will be grown or shrunk to the length actually read.  An
OFFSET may be specified to place the read data at some other place than
the beginning of the string.

=item system LIST

Does exactly the same thing as "exec LIST" except that a fork is done
first, and the parent process waits for the child process to complete.
Note that argument processing varies depending on the number of
arguments.  The return value is the exit status of the program as
returned by the wait() call.  To get the actual exit value divide by
256.  See also L</exec>.  This is I<NOT> what you want to use to capture 
the output from a command, for that you should merely use backticks, as
described in L<perlop/"`STRING`">.

=item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET

=item syswrite FILEHANDLE,SCALAR,LENGTH

Attempts to write LENGTH bytes of data from variable SCALAR to the
specified FILEHANDLE, using the system call write(2).  It bypasses
stdio, so mixing this with prints may cause confusion.  Returns the
number of bytes actually written, or undef if there was an error.  An
OFFSET may be specified to get the write data from some other place than
the beginning of the string.

=item tell FILEHANDLE

=item tell

Returns the current file position for FILEHANDLE.  FILEHANDLE may be an
expression whose value gives the name of the actual filehandle.  If
FILEHANDLE is omitted, assumes the file last read.

=item telldir DIRHANDLE

Returns the current position of the readdir() routines on DIRHANDLE.
Value may be given to seekdir() to access a particular location in a
directory.  Has the same caveats about possible directory compaction as
the corresponding system library routine.

=item tie VARIABLE,CLASSNAME,LIST

This function binds a variable to a package class that will provide the
implementation for the variable.  VARIABLE is the name of the variable
to be enchanted.  CLASSNAME is the name of a class implementing objects
of correct type.  Any additional arguments are passed to the "new"
method of the class (meaning TIESCALAR, TIEARRAY, or TIEHASH).
Typically these are arguments such as might be passed to the dbm_open()
function of C.  The object returned by the "new" method is also
returned by the tie() function, which would be useful if you want to
access other methods in CLASSNAME.

Note that functions such as keys() and values() may return huge array
values when used on large objects, like DBM files.  You may prefer to
use the each() function to iterate over such.  Example:

    # print out history file offsets
    use NDBM_File;
    tie(%HIST, NDBM_File, '/usr/lib/news/history', 1, 0);
    while (($key,$val) = each %HIST) {
	print $key, ' = ', unpack('L',$val), "\n";
    }
    untie(%HIST);

A class implementing an associative array should have the following
methods:

    TIEHASH classname, LIST
    DESTROY this
    FETCH this, key
    STORE this, key, value
    DELETE this, key
    EXISTS this, key
    FIRSTKEY this
    NEXTKEY this, lastkey

A class implementing an ordinary array should have the following methods:

    TIEARRAY classname, LIST
    DESTROY this
    FETCH this, key
    STORE this, key, value
    [others TBD]

A class implementing a scalar should have the following methods:

    TIESCALAR classname, LIST
    DESTROY this
    FETCH this, 
    STORE this, value

Unlike dbmopen(), the tie() function will not use or require a module
for you--you need to do that explicitly yourself.  See L<DB_File>
or the F<Config> module for interesting tie() implementations.

=item tied VARIABLE

Returns a reference to the object underlying VARIABLE (the same value
that was originally returned by the tie() call which bound the variable
to a package.)  Returns the undefined value if VARIABLE isn't tied to a
package.

=item time

Returns the number of non-leap seconds since 00:00:00 UTC, January 1,
1970.  Suitable for feeding to gmtime() and localtime().

=item times

Returns a four-element array giving the user and system times, in
seconds, for this process and the children of this process.

    ($user,$system,$cuser,$csystem) = times;

=item tr///

The translation operator.  See L<perlop>.

=item truncate FILEHANDLE,LENGTH

=item truncate EXPR,LENGTH

Truncates the file opened on FILEHANDLE, or named by EXPR, to the
specified length.  Produces a fatal error if truncate isn't implemented
on your system.

=item uc EXPR

Returns an uppercased version of EXPR.  This is the internal function
implementing the \U escape in double-quoted strings.
Should respect any POSIX setlocale() settings.

=item ucfirst EXPR

Returns the value of EXPR with the first character uppercased.  This is
the internal function implementing the \u escape in double-quoted strings.
Should respect any POSIX setlocale() settings.

=item umask EXPR

=item umask

Sets the umask for the process and returns the old one.  If EXPR is
omitted, merely returns current umask.

=item undef EXPR

=item undef

Undefines the value of EXPR, which must be an lvalue.  Use only on a
scalar value, an entire array, or a subroutine name (using "&").  (Using undef()
will probably not do what you expect on most predefined variables or
DBM list values, so don't do that.)  Always returns the undefined value.  You can omit
the EXPR, in which case nothing is undefined, but you still get an
undefined value that you could, for instance, return from a
subroutine.  Examples:

    undef $foo;
    undef $bar{'blurfl'};
    undef @ary;
    undef %assoc;
    undef &mysub;
    return (wantarray ? () : undef) if $they_blew_it;

=item unlink LIST

Deletes a list of files.  Returns the number of files successfully
deleted.

    $cnt = unlink 'a', 'b', 'c';
    unlink @goners;
    unlink <*.bak>;

Note: unlink will not delete directories unless you are superuser and
the B<-U> flag is supplied to Perl.  Even if these conditions are
met, be warned that unlinking a directory can inflict damage on your
filesystem.  Use rmdir instead.

=item unpack TEMPLATE,EXPR

Unpack does the reverse of pack: it takes a string representing a
structure and expands it out into a list value, returning the array
value.  (In a scalar context, it merely returns the first value
produced.)  The TEMPLATE has the same format as in the pack function.
Here's a subroutine that does substring:

    sub substr {
	local($what,$where,$howmuch) = @_;
	unpack("x$where a$howmuch", $what);
    }

and then there's

    sub ordinal { unpack("c",$_[0]); } # same as ord()

In addition, you may prefix a field with a %<number> to indicate that
you want a <number>-bit checksum of the items instead of the items
themselves.  Default is a 16-bit checksum.  For example, the following
computes the same number as the System V sum program:

    while (<>) {
	$checksum += unpack("%16C*", $_);
    }
    $checksum %= 65536;

The following efficiently counts the number of set bits in a bit vector:

    $setbits = unpack("%32b*", $selectmask);

=item untie VARIABLE

Breaks the binding between a variable and a package.  (See tie().)

=item unshift ARRAY,LIST

Does the opposite of a C<shift>.  Or the opposite of a C<push>,
depending on how you look at it.  Prepends list to the front of the
array, and returns the new number of elements in the array.

    unshift(ARGV, '-e') unless $ARGV[0] =~ /^-/;

Note the LIST is prepended whole, not one element at a time, so the
prepended elements stay in the same order.  Use reverse to do the
reverse.

=item use Module LIST

=item use Module

Imports some semantics into the current package from the named module,
generally by aliasing certain subroutine or variable names into your
package.  It is exactly equivalent to

    BEGIN { require Module; import Module LIST; }

The BEGIN forces the require and import to happen at compile time.  The
require makes sure the module is loaded into memory if it hasn't been
yet.  The import is not a builtin--it's just an ordinary static method
call into the "Module" package to tell the module to import the list of
features back into the current package.  The module can implement its
import method any way it likes, though most modules just choose to
derive their import method via inheritance from the Exporter class that
is defined in the Exporter module. See L<Exporter>.

If you don't want your namespace altered, explicitly supply an empty list:

    use Module ();

That is exactly equivalent to

    BEGIN { require Module; }

Because this is a wide-open interface, pragmas (compiler directives)
are also implemented this way.  Currently implemented pragmas are:

    use integer;
    use diagnostics;
    use sigtrap qw(SEGV BUS);
    use strict  qw(subs vars refs);
    use subs    qw(afunc blurfl);

These pseudomodules import semantics into the current block scope, unlike
ordinary modules, which import symbols into the current package (which are
effective through the end of the file).

There's a corresponding "no" command that unimports meanings imported
by use.

    no integer;
    no strict 'refs';

See L<perlmod> for a list of standard modules and pragmas.

=item utime LIST

Changes the access and modification times on each file of a list of
files.  The first two elements of the list must be the NUMERICAL access
and modification times, in that order.  Returns the number of files
successfully changed.  The inode modification time of each file is set
to the current time.  Example of a "touch" command:

    #!/usr/bin/perl
    $now = time;
    utime $now, $now, @ARGV;

=item values ASSOC_ARRAY

Returns a normal array consisting of all the values of the named
associative array.  (In a scalar context, returns the number of
values.)  The values are returned in an apparently random order, but it
is the same order as either the keys() or each() function would produce
on the same array.  See also keys(), each(), and sort().

=item vec EXPR,OFFSET,BITS

Treats the string in EXPR as a vector of unsigned integers, and
returns the value of the bitfield specified by OFFSET.  BITS specifies
the number of bits that are reserved for each entry in the bit
vector. This must be a power of two from 1 to 32. vec() may also be
assigned to, in which case parens are needed to give the expression
the correct precedence as in

    vec($image, $max_x * $x + $y, 8) = 3;

Vectors created with vec() can also be manipulated with the logical
operators |, & and ^, which will assume a bit vector operation is
desired when both operands are strings.

To transform a bit vector into a string or array of 0's and 1's, use these:

    $bits = unpack("b*", $vector);
    @bits = split(//, unpack("b*", $vector));

If you know the exact length in bits, it can be used in place of the *.

=item wait

Waits for a child process to terminate and returns the pid of the
deceased process, or -1 if there are no child processes.  The status is
returned in $?.

=item waitpid PID,FLAGS

Waits for a particular child process to terminate and returns the pid
of the deceased process, or -1 if there is no such child process.  The
status is returned in $?.  If you say

    use POSIX "wait_h";
    ...
    waitpid(-1,&WNOHANG);

then you can do a non-blocking wait for any process.  Non-blocking wait
is only available on machines supporting either the waitpid(2) or
wait4(2) system calls.  However, waiting for a particular pid with
FLAGS of 0 is implemented everywhere.  (Perl emulates the system call
by remembering the status values of processes that have exited but have
not been harvested by the Perl script yet.)

=item wantarray

Returns TRUE if the context of the currently executing subroutine is
looking for a list value.  Returns FALSE if the context is looking
for a scalar.

    return wantarray ? () : undef;

=item warn LIST

Produces a message on STDERR just like die(), but doesn't exit or
on an exception.

=item write FILEHANDLE

=item write EXPR

=item write

Writes a formatted record (possibly multi-line) to the specified file,
using the format associated with that file.  By default the format for
a file is the one having the same name is the filehandle, but the
format for the current output channel (see the select() function) may be set
explicitly by assigning the name of the format to the $~ variable.

Top of form processing is handled automatically:  if there is
insufficient room on the current page for the formatted record, the
page is advanced by writing a form feed, a special top-of-page format
is used to format the new page header, and then the record is written.
By default the top-of-page format is the name of the filehandle with
"_TOP" appended, but it may be dynamically set to the format of your
choice by assigning the name to the $^ variable while the filehandle is
selected.  The number of lines remaining on the current page is in
variable $-, which can be set to 0 to force a new page.

If FILEHANDLE is unspecified, output goes to the current default output
channel, which starts out as STDOUT but may be changed by the
C<select> operator.  If the FILEHANDLE is an EXPR, then the expression
is evaluated and the resulting string is used to look up the name of
the FILEHANDLE at run time.  For more on formats, see L<perlform>.

Note that write is I<NOT> the opposite of read.  Unfortunately.

=item y///

The translation operator.  See L<perlop/tr///>.

=back