perlfaq update from Tom Christiansen

[perl5.git] / 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 or 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 ever
be only one such list argument.)  For instance, splice() has three scalar
arguments followed by a list, whereas gethostbyname() has four scalar
arguments.

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
parentheses.)  If you use the parentheses, 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+4;	# Prints 7.
    print(1+2) + 4;	# Prints 3.
    print (1+2)+4;	# Also prints 3!
    print +(1+2)+4;	# Prints 7.
    print ((1+2)+4);	# Prints 7.

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.

A few functions take no arguments at all, and therefore work as neither
unary nor list operators.  These include such functions as C<time>
and C<endpwent>.  For example, C<time+86_400> always means
C<time() + 86_400>.

For functions that can be used in either a scalar or list context,
nonabortive 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 important rule: There is B<no rule> that relates
the behavior of an expression in list context to its behavior in scalar
context, or vice versa.  It might do two totally different things.
Each operator and function decides which sort of value it would be most
appropriate to return in scalar context.  Some operators return the
length of the list that would have been returned in 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.

An named array in scalar context is quite different from what would at
first glance appear to be a list in scalar context.  You can't get a list
like C<(1,2,3)> into being in scalar context, because the compiler knows
the context at compile time.  It would generate the scalar comma operator
there, not the list construction version of the comma.  That means it
was never a list to start with.

In general, functions in Perl that serve as wrappers for system calls
of the same name (like chown(2), fork(2), closedir(2), etc.) all return
true when they succeed and C<undef> otherwise, as is usually mentioned
in the descriptions below.  This is different from the C interfaces,
which return C<-1> on failure.  Exceptions to this rule are C<wait()>,
C<waitpid()>, and C<syscall()>.  System calls also set the special C<$!>
variable on failure.  Other functions do not, except accidentally.

=head2 Perl Functions by Category

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

=over

=item Functions for SCALARs or strings

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

=item Regular expressions and pattern matching

C<m//>, C<pos>, C<quotemeta>, C<s///>, C<split>, C<study>, C<qr//>

=item Numeric functions

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

=item Functions for real @ARRAYs

C<pop>, C<push>, C<shift>, C<splice>, C<unshift>

=item Functions for list data

C<grep>, C<join>, C<map>, C<qw/STRING/>, C<reverse>, C<sort>, C<unpack>

=item Functions for real %HASHes

C<delete>, C<each>, C<exists>, C<keys>, C<values>

=item Input and output functions

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

=item Functions for fixed length data or records

C<pack>, C<read>, C<syscall>, C<sysread>, C<syswrite>, C<unpack>, C<vec>

=item Functions for filehandles, files, or directories

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

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

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

=item Keywords related to scoping

C<caller>, C<import>, C<local>, C<my>, C<package>, C<use>

=item Miscellaneous functions

C<defined>, C<dump>, C<eval>, C<formline>, C<local>, C<my>, C<reset>,
C<scalar>, C<undef>, C<wantarray>

=item Functions for processes and process groups

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

=item Keywords related to perl modules

C<do>, C<import>, C<no>, C<package>, C<require>, C<use>

=item Keywords related to classes and object-orientedness

C<bless>, C<dbmclose>, C<dbmopen>, C<package>, C<ref>, C<tie>, C<tied>,
C<untie>, C<use>

=item Low-level socket functions

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

=item System V interprocess communication functions

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

=item Fetching user and group info

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

=item Fetching network info

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

=item Time-related functions

C<gmtime>, C<localtime>, C<time>, C<times>

=item Functions new in perl5

C<abs>, C<bless>, C<chomp>, C<chr>, C<exists>, C<formline>, C<glob>,
C<import>, C<lc>, C<lcfirst>, C<map>, C<my>, C<no>, C<prototype>, C<qx>,
C<qw>, C<readline>, C<readpipe>, C<ref>, C<sub*>, C<sysopen>, C<tie>,
C<tied>, C<uc>, C<ucfirst>, C<untie>, C<use>

* - C<sub> was a keyword in perl4, but in perl5 it is an
operator, which can be used in expressions.

=item Functions obsoleted in perl5

C<dbmclose>, C<dbmopen>

=back

=head2 Portability

Perl was born in Unix and can therefore access all common Unix
system calls.  In non-Unix environments, the functionality of some
Unix system calls may not be available, or details of the available
functionality may differ slightly.  The Perl functions affected
by this are:

C<-X>, C<binmode>, C<chmod>, C<chown>, C<chroot>, C<crypt>,
C<dbmclose>, C<dbmopen>, C<dump>, C<endgrent>, C<endhostent>,
C<endnetent>, C<endprotoent>, C<endpwent>, C<endservent>, C<exec>,
C<fcntl>, C<flock>, C<fork>, C<getgrent>, C<getgrgid>, C<gethostent>,
C<getlogin>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
C<getppid>, C<getprgp>, C<getpriority>, C<getprotobynumber>,
C<getprotoent>, C<getpwent>, C<getpwnam>, C<getpwuid>,
C<getservbyport>, C<getservent>, C<getsockopt>, C<glob>, C<ioctl>,
C<kill>, C<link>, C<lstat>, C<msgctl>, C<msgget>, C<msgrcv>,
C<msgsnd>, C<open>, C<pipe>, C<readlink>, C<rename>, C<select>, C<semctl>,
C<semget>, C<semop>, C<setgrent>, C<sethostent>, C<setnetent>,
C<setpgrp>, C<setpriority>, C<setprotoent>, C<setpwent>,
C<setservent>, C<setsockopt>, C<shmctl>, C<shmget>, C<shmread>,
C<shmwrite>, C<socket>, C<socketpair>, C<stat>, C<symlink>, C<syscall>,
C<sysopen>, C<system>, C<times>, C<truncate>, C<umask>, C<unlink>,
C<utime>, C<wait>, C<waitpid>

For more information about the portability of these functions, see
L<perlport> and other available platform-specific documentation.

=head2 Alphabetical Listing of Perl Functions

=over 8

=item I<-X> FILEHANDLE

=item I<-X> EXPR

=item I<-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 C<$_>, 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:
X<-r>X<-w>X<-x>X<-o>X<-R>X<-W>X<-X>X<-O>X<-e>X<-z>X<-s>X<-f>X<-d>X<-l>X<-p>
X<-S>X<-b>X<-c>X<-t>X<-u>X<-g>X<-k>X<-T>X<-B>X<-M>X<-A>X<-C>

    -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 nonzero 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), or Filehandle is a pipe.
    -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 an ASCII 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.

Example:

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

The interpretation of the file permission operators C<-r>, C<-R>,
C<-w>, C<-W>, C<-x>, and C<-X> is by default 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.  Such
reasons may be for example network filesystem access controls, ACLs
(access control lists), read-only filesystems, and unrecognized
executable formats.

Also note that, for the superuser on the local filesystems, the C<-r>,
C<-R>, C<-w>, and C<-W> tests 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() to determine the actual mode of the file,
or temporarily set their effective uid to something else.

If you are using ACLs, there is a pragma called C<filetest> that may
produce more accurate results than the bare stat() mode bits.
When under the C<use filetest 'access'> the above-mentioned filetests
will test whether the permission can (not) be granted using the
access() family of system calls.  Also note that the C<-x> and C<-X> may
under this pragma return true even if there are no execute permission
bits set (nor any extra execute permission ACLs).  This strangeness is
due to the underlying system calls' definitions.  Read the
documentation for the C<filetest> pragma for more information.

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 strange characters (E<gt>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 C<stat()> or C<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

=item abs

Returns the absolute value of its argument.
If VALUE is omitted, uses C<$_>.

=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 the example in L<perlipc/"Sockets: Client/Server Communication">.

=item alarm SECONDS

=item alarm

Arranges to have a SIGALRM delivered to this process after the
specified number of seconds have elapsed.  If SECONDS is not specified,
the value stored in C<$_> is used. (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 C<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
four-arugment version of select() leaving the first three arguments
undefined, or you might be able to use the C<syscall()> interface to
access setitimer(2) if your system supports it.  The Time::HiRes module
from CPAN may also prove useful.

It is usually a mistake to intermix C<alarm()>
and C<sleep()> calls.

If you want to use C<alarm()> to time out a system call you need to use an
C<eval()>/C<die()> pair.  You can't rely on the alarm causing the system call to
fail with C<$!> set to C<EINTR> because Perl sets up signal handlers to
restart system calls on some systems.  Using C<eval()>/C<die()> always works,
modulo the caveats given in L<perlipc/"Signals">.

    eval {
	local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
	alarm $timeout;
	$nread = sysread SOCKET, $buffer, $size;
	alarm 0;
    };
    if ($@) {
	die unless $@ eq "alarm\n";   # propagate unexpected errors
    	# timed out
    }
    else {
    	# didn't
    }

=item atan2 Y,X

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

For the tangent operation, you may use the C<POSIX::tan()>
function, or use the familiar relation:

    sub tan { sin($_[0]) / cos($_[0])  }

=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
many sytems, but in MS-DOS and similarly archaic systems, it may be
imperative--otherwise your MS-DOS-damaged C library may mangle your file.
The key distinction between systems that need C<binmode()> and those
that don't is their text file formats.  Systems like Unix, MacOS, and
Plan9 that delimit lines with a single character, and that encode that
character in C as C<"\n">, do not need C<binmode()>.  The rest may need it.
If FILEHANDLE is an expression, the value is taken as the name of the
filehandle.

If the system does care about it, using it when you shouldn't is just as
perilous as failing to use it when you should.  Fortunately for most of
us, you can't go wrong using binmode() on systems that don't care about
it, though.

=item bless REF,CLASSNAME

=item bless REF

This function tells the thingy referenced by REF that it is now an object
in the CLASSNAME package.  If CLASSNAME is omitted, the current package
is used.  Because a C<bless()> is often the last thing in a constructor.
it returns the reference for convenience.  Always use the two-argument
version if the function doing the blessing might be inherited by a
derived class.  See L<perltoot> and L<perlobj> for more about the blessing
(and blessings) of objects.

Consider always blessing objects in CLASSNAMEs that are mixed case.
Namespaces with all lowercase names are considered reserved for
Perl pragmata.  Builtin types have all uppercase names, so to prevent
confusion, you may wish to avoid such package names as well.  Make sure
that CLASSNAME is a true value.

See L<perlmod/"Perl Modules">.

=item caller EXPR

=item caller

Returns the context of the current subroutine call.  In scalar context,
returns the caller's package name if there is a caller, that is, if
we're in a subroutine or C<eval()> or C<require()>, and the undefined value
otherwise.  In 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, $wantarray, $evaltext, $is_require) = caller($i);

Here C<$subroutine> may be C<"(eval)"> if the frame is not a subroutine
call, but an C<eval()>.  In such a case additional elements C<$evaltext> and
C<$is_require> are set: C<$is_require> is true if the frame is created by a
C<require> or C<use> statement, C<$evaltext> contains the text of the
C<eval EXPR> statement.  In particular, for a C<eval BLOCK> statement,
C<$filename> is C<"(eval)">, but C<$evaltext> is undefined.  (Note also that
each C<use> statement creates a C<require> frame inside an C<eval EXPR>)
frame.

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

Be aware that the optimizer might have optimized call frames away before
C<caller()> had a chance to get the information. That means that C<caller(N)>
might not return information about the call frame you expect it do, for
C<N E<gt> 1>. In particular, C<@DB::args> might have information from the 
previous time C<caller()> was called.

=item chdir EXPR

Changes the working directory to EXPR, if possible.  If EXPR is omitted,
changes to the user's home directory.  Returns TRUE upon success,
FALSE otherwise.  See the example under C<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, and which definitely should I<not> a string of octal digits:
C<0644> is okay, C<'0644'> is not.  Returns the number of files
successfully changed.  See also L</oct>, if all you have is a string.

    $cnt = chmod 0755, 'foo', 'bar';
    chmod 0755, @executables;
    $mode = '0644'; chmod $mode, 'foo';      # !!! sets mode to
                                             # --w----r-T
    $mode = '0644'; chmod oct($mode), 'foo'; # this is better
    $mode = 0644;   chmod $mode, 'foo';      # this is best

=item chomp VARIABLE

=item chomp LIST

=item chomp

This safer version of L</chop> removes any trailing string
that corresponds to the current value of C<$/> (also known as
$INPUT_RECORD_SEPARATOR in the C<English> module).  It returns the total
number of characters removed from all its arguments.  It's often used to
remove the newline from the end of an input record when you're worried
that the final record may be missing its newline.  When in paragraph
mode (C<$/ = "">), it removes all trailing newlines from the string.
When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is
a reference to an integer or the like, see L<perlvar>) chomp() won't
remove anything.  If VARIABLE is omitted, it chomps C<$_>.  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 C<$_>.
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 C<chop()> is returned.

Note that C<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 nonnumeric 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 = glob($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

=item chr

Returns the character represented by that NUMBER in the character set.
For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
chr(0x263a) is a Unicode smiley face (but only within the scope of
a C<use utf8>).  For the reverse, use L</ord>.  
See L<utf8> for more about Unicode.

If NUMBER is omitted, uses C<$_>.

=item chroot FILENAME

=item chroot

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

=item close FILEHANDLE

=item close

Closes the file or pipe associated with the file handle, returning TRUE
only if stdio successfully flushes buffers and closes the system file
descriptor. Closes the currently selected filehandle if the argument
is omitted.

You don't have to close FILEHANDLE if you are immediately going to do
another C<open()> on it, because C<open()> will close it for you.  (See
C<open()>.)  However, an explicit C<close()> on an input file resets the line
counter (C<$.>), while the implicit close done by C<open()> does not.

If the file handle came from a piped open C<close()> will additionally
return FALSE if one of the other system calls involved fails or if the
program exits with non-zero status.  (If the only problem was that the
program exited non-zero C<$!> will be set to C<0>.)  Closing a pipe 
also waits for the process executing on the pipe to complete, in case you
want to look at the output of the pipe afterwards, and 
implicitly puts the exit status value of that command into C<$?>.

Example:

    open(OUTPUT, '|sort >foo')  # pipe to sort
        or die "Can't start sort: $!";
    #...			# print stuff to output
    close OUTPUT		# wait for sort to finish
        or warn $! ? "Error closing sort pipe: $!"
                   : "Exit status $? from sort";
    open(INPUT, 'foo')		# get sort's results
        or die "Can't open 'foo' for input: $!";

FILEHANDLE may be an expression whose value can be used as an indirect
filehandle, usually the real filehandle name.

=item closedir DIRHANDLE

Closes a directory opened by C<opendir()> and returns the success of that
system call.

DIRHANDLE may be an expression whose value can be used as an indirect
dirhandle, usually the real dirhandle name.

=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).

C<last>, C<next>, or C<redo> may appear within a C<continue>
block. C<last> and C<redo> will behave as if they had been executed within
the main block. So will C<next>, but since it will execute a C<continue>
block, it may be more entertaining.

    while (EXPR) {
	### redo always comes here
	do_something;
    } continue {
	### next always comes here
	do_something_else;
	# then back the top to re-check EXPR
    }
    ### last always comes here

Omitting the C<continue> section is semantically equivalent to using an
empty one, logically enough. In that case, C<next> goes directly back
to check the condition at the top of the loop.

=item cos EXPR

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

For the inverse cosine operation, you may use the C<POSIX::acos()>
function, or use this relation:

    sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }

=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.

Note that C<crypt()> is intended to be a one-way function, much like breaking
eggs to make an omelette.  There is no (known) corresponding decrypt
function.  As a result, this function isn't all that useful for
cryptography.  (For that, see your nearby CPAN mirror.)

When verifying an existing encrypted string you should use the encrypted
text as the salt (like C<crypt($plain, $crypted) eq $crypted>).  This
allows your code to work with the standard C<crypt()> and with more
exotic implementations.  When choosing a new salt create a random two
character string whose characters come from the set C<[./0-9A-Za-z]>
(like C<join '', ('.', '/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>).

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

    $pwd = (getpwuid($<))[1];

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

    if (crypt($word, $pwd) 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 HASH

[This function has been largely superseded by the C<untie()> function.]

Breaks the binding between a DBM file and a hash.

=item dbmopen HASH,DBNAME,MODE

[This function has been largely superseded by the C<tie()> function.]

This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
hash.  HASH is the name of the hash.  (Unlike normal C<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 C<umask()>).  If your system supports
only the older DBM functions, you may perform only one C<dbmopen()> in your
program.  In older versions of Perl, if your system had neither DBM nor
ndbm, calling C<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 hash
variables, not set them.  If you want to test whether you can write,
either use file tests or try setting a dummy hash entry inside an C<eval()>,
which will trap the error.

Note that functions such as C<keys()> and C<values()> may return huge lists
when used on large DBM files.  You may prefer to use the C<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 approaches, as well as L<DB_File> for a particularly
rich implementation.

You can control which DBM library you use by loading that library
before you call dbmopen():

    use DB_File;
    dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
	or die "Can't open netscape history file: $!";

=item defined EXPR

=item defined

Returns a Boolean value telling whether EXPR has a value other than
the undefined value C<undef>.  If EXPR is not present, C<$_> will be
checked.

Many operations return C<undef> to indicate failure, end of file,
system error, uninitialized variable, and other exceptional
conditions.  This function allows you to distinguish C<undef> from
other values.  (A simple Boolean test will not distinguish among
C<undef>, zero, the empty string, and C<"0">, which are all equally
false.)  Note that since C<undef> is a valid scalar, its presence
doesn't I<necessarily> indicate an exceptional condition: C<pop()>
returns C<undef> when its argument is an empty array, I<or> when the
element to return happens to be C<undef>.

You may also use C<defined()> to check whether a subroutine exists, by
saying C<defined &func> without parentheses.  On the other hand, use
of C<defined()> upon aggregates (hashes and arrays) is not guaranteed to
produce intuitive results, and should probably be avoided.

When used on a hash element, it tells you whether the value is defined,
not whether the key exists in the hash.  Use L</exists> for the latter
purpose.

Examples:

    print if defined $switch{'D'};
    print "$val\n" while defined($val = pop(@ary));
    die "Can't readlink $sym: $!"
	unless defined($value = readlink $sym);
    sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
    $debugging = 0 unless defined $debugging;

Note:  Many folks tend to overuse C<defined()>, and then are surprised to
discover that the number C<0> and C<""> (the zero-length string) are, in fact,
defined values.  For example, if you say

    "ab" =~ /a(.*)b/;

The pattern match succeeds, and C<$1> is defined, despite the fact that it
matched "nothing".  But it didn't really match nothing--rather, it
matched something that happened to be zero characters long.  This is all
very above-board and honest.  When a function returns an undefined value,
it's an admission that it couldn't give you an honest answer.  So you
should use C<defined()> only when you're questioning the integrity of what
you're trying to do.  At other times, a simple comparison to C<0> or C<""> is
what you want.

Currently, using C<defined()> on an entire array or hash reports whether
memory for that aggregate has ever been allocated.  So an array you set
to the empty list appears undefined initially, and one that once was full
and that you then set to the empty list still appears defined.  You
should instead use a simple test for size:

    if (@an_array) { print "has array elements\n" }
    if (%a_hash)   { print "has hash members\n"   }

Using C<undef()> on these, however, does clear their memory and then report
them as not defined anymore, but you shouldn't do that unless you don't
plan to use them again, because it saves time when you load them up
again to have memory already ready to be filled.  The normal way to 
free up space used by an aggregate is to assign the empty list.

This counterintuitive behavior of C<defined()> on aggregates may be
changed, fixed, or broken in a future release of Perl.

See also L</undef>, L</exists>, L</ref>.

=item delete EXPR

Deletes the specified key(s) and their associated values from a hash.
For each key, returns the deleted value associated with that key, or
the undefined value if there was no such key.  Deleting from C<$ENV{}>
modifies the environment.  Deleting from a hash tied to a DBM file
deletes the entry from the DBM file.  (But deleting from a C<tie()>d hash
doesn't necessarily return anything.)

The following deletes all the values of a hash:

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

And so does this:

    delete @HASH{keys %HASH}

But both of these are slower than just assigning the empty list
or undefining it:

    %hash = ();		# completely empty %hash
    undef %hash;	# forget %hash every existed

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

    delete $ref->[$x][$y]{$key};
    delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};

=item die LIST

Outside an C<eval()>, prints the value of LIST to C<STDERR> and exits with
the current value of C<$!> (errno).  If C<$!> is C<0>, exits with the value of
C<($? E<gt>E<gt> 8)> (backtick `command` status).  If C<($? E<gt>E<gt> 8)>
is C<0>, exits with C<255>.  Inside an C<eval(),> the error message is stuffed into
C<$@> and the C<eval()> is terminated with the undefined value.  This makes
C<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.  Note that the "input line number" (also known as "chunk")
is subject to whatever notion of "line" happens to be currently in
effect, and is also available as the special variable C<$.>.
See L<perlvar/"$/"> and L<perlvar/"$.">.

Hint: sometimes appending C<", stopped"> to your message
will cause it to make better sense when the string C<"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(), warn(), and the Carp module.

If LIST is empty and C<$@> already contains a value (typically from a
previous eval) that value is reused after appending C<"\t...propagated">.
This is useful for propagating exceptions:

    eval { ... };
    die unless $@ =~ /Expected exception/;

If C<$@> is empty then the string C<"Died"> is used.

die() can also be called with a reference argument.  If this happens to be
trapped within an eval(), $@ contains the reference.  This behavior permits
a more elaborate exception handling implementation using objects that
maintain arbitary state about the nature of the exception.  Such a scheme
is sometimes preferable to matching particular string values of $@ using
regular expressions.  Here's an example:

    eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
    if ($@) {
        if (ref($@) && UNIVERSAL::isa($@,"Some::Module::Exception")) {
            # handle Some::Module::Exception
        }
        else {
            # handle all other possible exceptions
        }
    }

Since perl will stringify uncaught exception messages before displaying
them, you may want to overload stringification operations on such custom
exception objects.  See L<overload> for details about that.

You can arrange for a callback to be run just before the C<die()> does
its deed, by setting the C<$SIG{__DIE__}> hook.  The associated handler
will be called with the error text and can change the error message, if
it sees fit, by calling C<die()> again.  See L<perlvar/$SIG{expr}> for details on
setting C<%SIG> entries, and L<"eval BLOCK"> for some examples.

Note that the C<$SIG{__DIE__}> hook is currently called even inside
eval()ed blocks/strings!  If one wants the hook to do nothing in such
situations, put

	die @_ if $^S;

as the first line of the handler (see L<perlvar/$^S>).  Because this
promotes action at a distance, this counterintuitive behavior may be fixed
in a future release.

=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.)

C<do BLOCK> does I<not> count as a loop, so the loop control statements
C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
See L<perlsyn> for alternative strategies.

=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

    scalar eval `cat stat.pl`;

except that it's more efficient and concise, keeps track of the current
filename for error messages, searches the @INC libraries, and updates
C<%INC> if the file is found.  See L<perlvar/Predefined Names> for these
variables.  It also differs in that code evaluated with C<do FILENAME>
cannot see lexicals in the enclosing scope; C<eval STRING> does.  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.

If C<do> cannot read the file, it returns undef and sets C<$!> to the
error.  If C<do> can read the file but cannot compile it, it
returns undef and sets an error message in C<$@>.   If the file is
successfully compiled, C<do> returns the value of the last expression
evaluated.

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

You might like to use C<do> to read in a program configuration
file.  Manual error checking can be done this way:

    # read in config files: system first, then user 
    for $file ("/share/prog/defaults.rc",
               "$ENV{HOME}/.someprogrc") 
   {
	unless ($return = do $file) {
	    warn "couldn't parse $file: $@" if $@;
	    warn "couldn't do $file: $!"    unless defined $return;
	    warn "couldn't run $file"       unless $return;
	}
    }

=item dump LABEL

=item dump

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 C<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');

This operator is largely obsolete, partly because it's very hard to 
convert a core file into an executable, and because the real perl-to-C
compiler has superseded it.

=item each HASH

When called in list context, returns a 2-element list consisting of the
key and value for the next element of a hash, so that you can iterate over
it.  When called in scalar context, returns the key for only the "next"
element in the hash.  (Note: Keys may be C<"0"> or C<"">, which are logically
false; you may wish to avoid constructs like C<while ($k = each %foo) {}>
for this reason.)

Entries are returned in an apparently random order.  The actual random
order is subject to change in future versions of perl, but it is guaranteed
to be in the same order as either the C<keys()> or C<values()> function
would produce on the same (unmodified) hash.

When the hash is entirely read, a null array is returned in list context
(which when assigned produces a FALSE (C<0>) value), and C<undef> in
scalar context.  The next call to C<each()> after that will start iterating
again.  There is a single iterator for each hash, shared by all C<each()>,
C<keys()>, and C<values()> function calls in the program; it can be reset by
reading all the elements from the hash, or by evaluating C<keys HASH> or
C<values HASH>.  If you add or delete elements of a hash while you're
iterating over it, you may get entries skipped or duplicated, so don't.

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 C<keys()>, C<values()> and C<sort()>.

=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.  (Note that this function actually
reads a character and then C<ungetc()>s it, so isn't 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.
Using C<eof()> with empty parentheses is very different.  It indicates
the pseudo file formed of the files listed on the command line, i.e.,
C<eof()> is reasonable to use inside a C<while (E<lt>E<gt>)> 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 (E<lt>E<gt>) loop.  Examples:

    # reset line numbering on each input file
    while (<>) {
	next if /^\s*#/;	# skip comments 
	print "$.\t$_";
    } continue {
	close ARGV  if eof;	# Not eof()!
    }

    # insert dashes just before last line of last file
    while (<>) {
	if (eof()) {		# check for end of current file
	    print "--------------\n";
	    close(ARGV);	# close or last; 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 false values when they run out of data, or if there
was an error.

=item eval EXPR

=item eval BLOCK

In the first form, the return value of EXPR is parsed and executed as if it
were a little Perl program.  The value of the expression (which is itself
determined within scalar context) is first parsed, and if there weren't any
errors, executed in the context of the current Perl program, so that any
variable settings or subroutine and format definitions remain afterwards.
Note that the value is parsed every time the eval executes.  If EXPR is
omitted, evaluates C<$_>.  This form is typically used to delay parsing
and subsequent execution of the text of EXPR until run time.

In the second form, the code within the BLOCK is parsed only once--at the
same time the code surrounding the eval itself was parsed--and executed
within the context of the current Perl program.  This form is typically
used to trap exceptions more efficiently than the first (see below), while
also providing the benefit of checking the code within BLOCK at compile
time.

The final semicolon, if any, may be omitted from the value of EXPR or within
the BLOCK.

In both forms, the value returned is the value of the last expression
evaluated inside the mini-program; a return statement may be also used, just
as with subroutines.  The expression providing the return value is evaluated
in void, scalar, or list context, depending on the context of the eval itself.
See L</wantarray> for more on how the evaluation context can be determined.

If there is a syntax error or runtime error, or a C<die()> statement is
executed, an undefined value is returned by C<eval()>, and C<$@> is set to the
error message.  If there was no error, C<$@> is guaranteed to be a null
string.  Beware that using C<eval()> neither silences perl from printing
warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
To do either of those, you have to use the C<$SIG{__WARN__}> facility.  See
L</warn> and L<perlvar>.

Note that, because C<eval()> traps otherwise-fatal errors, it is useful for
determining whether a particular feature (such as C<socket()> or C<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 nonfatal
    eval { $answer = $a / $b; }; warn $@ if $@;

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

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

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

Due to the current arguably broken state of C<__DIE__> hooks, when using
the C<eval{}> form as an exception trap in libraries, you may wish not
to trigger any C<__DIE__> hooks that user code may have installed.
You can use the C<local $SIG{__DIE__}> construct for this purpose,
as shown in this example:

    # a very private exception trap for divide-by-zero
    eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
    warn $@ if $@;

This is especially significant, given that C<__DIE__> hooks can call
C<die()> again, which has the effect of changing their error messages:

    # __DIE__ hooks may modify error messages
    {
       local $SIG{'__DIE__'} =
              sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
       eval { die "foo lives here" };
       print $@ if $@;                # prints "bar lives here"
    }

Because this promotes action at a distance, this counterintuive behavior
may be fixed in a future release.

With an C<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 C<$x>.  (Although case 2 has misleading double quotes making
the reader wonder what else might be happening (nothing is).)  Cases 3
and 4 likewise behave in the same way: they run the code C<'$x'>, which
does nothing but return the value of C<$x>.  (Case 4 is preferred for
purely visual reasons, but it also has the advantage of compiling at
compile-time instead of at run-time.)  Case 5 is a place where
normally you I<WOULD> like to use double quotes, except that in this
particular situation, you can just use symbolic references instead, as
in case 6.

C<eval BLOCK> does I<not> count as a loop, so the loop control statements
C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.

=item exec LIST

=item exec PROGRAM LIST

The C<exec()> function executes a system command I<AND NEVER RETURNS> -
use C<system()> instead of C<exec()> if you want it to return. It fails and
returns FALSE only if the command does not exist I<and> it is executed
directly instead of via your system's command shell (see below).

Since it's a common mistake to use C<exec()> instead of C<system()>, Perl
warns you if there is a following statement which isn't C<die()>, C<warn()>,
or C<exit()> (if C<-w> is set  -  but you always do that).   If you
I<really> want to follow an C<exec()> with some other statement, you
can use one of these styles to avoid the warning:

    exec ('foo')   or print STDERR "couldn't exec foo: $!";
    { exec ('foo') }; print STDERR "couldn't exec foo: $!";

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 or an array with one element in it,
the argument is checked for shell metacharacters, and if there are any,
the entire argument is passed to the system's command shell for parsing
(this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
If there are no shell metacharacters in the argument, it is split into
words and passed directly to C<execvp()>, which is more efficient.

All files opened for output are flushed before attempting the exec().

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 multivalued 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

When the arguments get executed via the system shell, results will
be subject to its quirks and capabilities.  See L<perlop/"`STRING`">
for details.

Using an indirect object with C<exec()> or C<system()> is also more secure.
This usage forces interpretation of the arguments as a multivalued list,
even if the list had just one argument.  That way you're safe from the
shell expanding wildcards or splitting up words with whitespace in them.

    @args = ( "echo surprise" );

    exec @args;               # subject to shell escapes
                                # if @args == 1
    exec { $args[0] } @args;  # safe even with one-arg list

The first version, the one without the indirect object, ran the I<echo>
program, passing it C<"surprise"> an argument.  The second version
didn't--it tried to run a program literally called I<"echo surprise">,
didn't find it, and set C<$?> to a non-zero value indicating failure.

Note that C<exec()> will not call your C<END> blocks, nor will it call
any C<DESTROY> methods in your objects.

=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 be TRUE only 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 $ref->{A}->{B}->{$key}) 	{ }
    if (exists $hash{A}{B}{$key}) 	{ }

Although the last element will not spring into existence just because
its existence was tested, intervening ones will.  Thus C<$ref-E<gt>{"A"}>
and C<$ref-E<gt>{"A"}-E<gt>{"B"}> will spring into existence due to the
existence test for a $key element.  This happens anywhere the arrow
operator is used, including even 

    undef $ref;
    if (exists $ref->{"Some key"})	{ }
    print $ref; 	    # prints HASH(0x80d3d5c)

This surprising autovivification in what does not at first--or even
second--glance appear to be an lvalue context may be fixed in a future
release.

=item exit EXPR

Evaluates EXPR and exits immediately with that value.    Example:

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

See also C<die()>.  If EXPR is omitted, exits with C<0> status.  The only
universally recognized values for EXPR are C<0> for success and C<1>
for error; other values are subject to interpretation depending on the
environment in which the Perl program is running.  For example, exiting
69 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
the mailer to return the item undelivered, but that's not true everywhere.

Don't use C<exit()> to abort a subroutine if there's any chance that
someone might want to trap whatever error happened.  Use C<die()> instead,
which can be trapped by an C<eval()>.

The exit() function does not always exit immediately. It calls any
defined C<END> routines first, but these C<END> routines may not
themselves abort the exit. Likewise any object destructors that need to
be called are called before the real exit.  If this is a problem, you
can call C<POSIX:_exit($status)> to avoid END and destructor processing.
See L<perlsub> for details.

=item exp EXPR

=item exp

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 constant definitions.  Argument processing and
value return works just like C<ioctl()> below.  
For example:

    use Fcntl;
    fcntl($filehandle, F_GETFL, $packed_return_buffer)
	or die "can't fcntl F_GETFL: $!";

You don't have to check for C<defined()> on the return from C<fnctl()>.
Like C<ioctl()>, it maps a C<0> return from the system call into "C<0>
but true" in Perl.  This string is true in boolean context and C<0>
in numeric context.  It is also exempt from the normal B<-w> warnings
on improper numeric conversions.

Note that C<fcntl()> will produce a fatal error if used on a machine that
doesn't implement fcntl(2).  See the Fcntl module or your fcntl(2)
manpage to learn what functions are available on your system.

=item fileno FILEHANDLE

Returns the file descriptor for a filehandle, or undefined if the
filehandle is not open.  This is mainly useful for constructing
bitmaps for C<select()> and low-level POSIX tty-handling operations.
If FILEHANDLE is an expression, the value is taken as an indirect
filehandle, generally its name.

You can use this to find out whether two handles refer to the 
same underlying descriptor:

    if (fileno(THIS) == fileno(THAT)) {
	print "THIS and THAT are dups\n";
    } 

=item flock FILEHANDLE,OPERATION

Calls flock(2), or an emulation of it, on FILEHANDLE.  Returns TRUE
for success, FALSE on failure.  Produces a fatal error if used on a
machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
C<flock()> is Perl's portable file locking interface, although it locks
only entire files, not records.

Two potentially non-obvious but traditional C<flock> semantics are
that it waits indefinitely until the lock is granted, and that its locks
B<merely advisory>.  Such discretionary locks are more flexible, but offer
fewer guarantees.  This means that files locked with C<flock()> may be
modified by programs that do not also use C<flock()>.  See L<perlport>,
your port's specific documentation, or your system-specific local manpages
for details.  It's best to assume traditional behavior if you're writing
portable programs.  (But if you're not, you should as always feel perfectly
free to write for your own system's idiosyncrasies (sometimes called
"features").  Slavish adherence to portability concerns shouldn't get
in the way of your getting your job done.)

OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
LOCK_NB.  These constants are traditionally valued 1, 2, 8 and 4, but
you can use the symbolic names if import them from the Fcntl module,
either individually, or as a group using the ':flock' tag.  LOCK_SH
requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
releases a previously requested lock.  If LOCK_NB is added to LOCK_SH or
LOCK_EX then C<flock()> will return immediately rather than blocking
waiting for the lock (check the return status to see if you got it).

To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
before locking or unlocking it.

Note that the emulation built with lockf(3) doesn't provide shared
locks, and it requires that FILEHANDLE be open with write intent.  These
are the semantics that lockf(3) implements.  Most if not all systems
implement lockf(3) in terms of fcntl(2) locking, though, so the
differing semantics shouldn't bite too many people.

Note also that some versions of C<flock()> cannot lock things over the
network; you would need to use the more system-specific C<fcntl()> for
that.  If you like you can force Perl to ignore your system's flock(2)
function, and so provide its own fcntl(2)-based emulation, by passing
the switch C<-Ud_flock> to the F<Configure> program when you configure
perl.

Here's a mailbox appender for BSD systems.

    use Fcntl ':flock'; # import LOCK_* constants

    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();

On systems that support a real flock(), locks are inherited across fork()
calls, whereas those that must resort to the more capricious fcntl()
function lose the locks, making it harder to write servers.

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

=item fork

Does a fork(2) system call to create a new process running the
same program at the same point.  It returns the child pid to the
parent process, C<0> to the child process, or C<undef> if the fork is
unsuccessful.  File descriptors (and sometimes locks on those descriptors)
are shared, while everything else is copied.  On most systems supporting
fork(), great care has gone into making it extremely efficient (for
example, using copy-on-write technology on data pages), making it the
dominant paradigm for multitasking over the last few decades.

All files opened for output are flushed before forking the child process.

If you C<fork()> without ever waiting on your children, you will
accumulate zombies.  On some systems, you can avoid this by setting
C<$SIG{CHLD}> to C<"IGNORE">.  See also L<perlipc> for more examples of
forking and reaping moribund children.

Note that if your forked child inherits system file descriptors like
STDIN and STDOUT that are actually connected by a pipe or socket, even
if you exit, then the remote server (such as, say, a CGI script or a
backgrounded job launced from a remote shell) won't think you're done.
You should reopen those to F</dev/null> if it's any issue.

=item format

Declare a picture format for use by the C<write()> function.  For
example:

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

    $str = "widget";
    $num = $cost/$quantity;
    $~ = '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<$^A> (or C<$ACCUMULATOR> in English).
Eventually, when a C<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 C<"">.  Note that a format typically
does one C<formline()> per line of form, but the C<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, because an "C<@>"
character may be taken to mean the beginning of an array name.
C<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 the undefined value at end of file, or if there was an error.
If FILEHANDLE is omitted, reads from STDIN.  This is not particularly
efficient.  However, it cannot be used by itself to fetch single
characters without waiting for the user to hit enter.  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 $BSD_STYLE should be set
is left as an exercise to the reader.

The C<POSIX::getattr()> function can do this more portably on
systems purporting POSIX compliance.  See also the C<Term::ReadKey>
module from your nearest CPAN site; details on CPAN can be found on
L<perlmodlib/CPAN>.

=item getlogin

Implements the C library function of the same name, which on most
systems returns the current login from F</etc/utmp>, if any.  If null,
use C<getpwuid()>.

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

Do not consider C<getlogin()> for authentication: it is not as
secure as C<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.  Use
a PID of C<0> to get the current process group 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.  Note that the POSIX version of C<getpgrp()>
does not accept a PID argument, so only C<PID==0> is truly portable.

=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.  In list context, the return values from the
various get routines are as follows:

    ($name,$passwd,$uid,$gid,
       $quota,$comment,$gcos,$dir,$shell,$expire) = 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.)

In 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);
    $name  = getpwuid($num);
    $name  = getpwent();
    $gid   = getgrnam($name);
    $name  = getgrgid($num;
    $name  = getgrent();
    #etc.

In I<getpw*()> the fields C<$quota>, C<$comment>, and C<$expire> are
special cases in the sense that in many systems they are unsupported.
If the C<$quota> is unsupported, it is an empty scalar.  If it is
supported, it usually encodes the disk quota.  If the C<$comment>
field is unsupported, it is an empty scalar.  If it is supported it
usually encodes some administrative comment about the user.  In some
systems the $quota field may be C<$change> or C<$age>, fields that have
to do with password aging.  In some systems the C<$comment> field may
be C<$class>.  The C<$expire> field, if present, encodes the expiration
period of the account or the password.  For the availability and the
exact meaning of these fields in your system, please consult your
getpwnam(3) documentation and your F<pwd.h> file.  You can also find
out from within Perl what your C<$quota> and C<$comment> fields mean
and whether you have the C<$expire> field by using the C<Config> module
and the values C<d_pwquota>, C<d_pwage>, C<d_pwchange>, C<d_pwcomment>,
and C<d_pwexpire>.  Shadow password files are only supported if your
vendor has implemented them in the intuitive fashion that calling the
regular C library routines gets the shadow versions if you're running
under privilege.  Those that incorrectly implement a separate library
call are not supported.

The C<$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
C<@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]);

The Socket library makes this slightly easier:

    use Socket;
    $iaddr = inet_aton("127.1"); # or whatever address
    $name  = gethostbyaddr($iaddr, AF_INET);

    # or going the other way
    $straddr = inet_ntoa($iaddr");

If you get tired of remembering which element of the return list contains
which return value, by-name interfaces are also provided in modules:
C<File::stat>, C<Net::hostent>, C<Net::netent>, C<Net::protoent>, C<Net::servent>,
C<Time::gmtime>, C<Time::localtime>, and C<User::grent>.  These override the
normal built-in, replacing them with versions that return objects with
the appropriate names for each field.  For example:

   use File::stat;
   use User::pwent;
   $is_his = (stat($filename)->uid == pwent($whoever)->uid);

Even though it looks like they're the same method calls (uid), 
they aren't, because a C<File::stat> object is different from a C<User::pwent> object.

=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 undef if there is an error.

=item glob EXPR

=item glob

Returns the value of EXPR with filename expansions such as the
standard Unix shell F</bin/csh> would do.  This is the internal function
implementing the C<E<lt>*.cE<gt>> operator, but you can use it directly.
If EXPR is omitted, C<$_> is used.  The C<E<lt>*.cE<gt>> operator is
discussed in more detail in L<perlop/"I/O Operators">.

=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 time zone.
Typically used as follows:

    #  0    1    2     3     4    5     6     7     8
    ($sec,$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 C<$mon> has the range C<0..11> and C<$wday>
has the range C<0..6> with sunday as day C<0>.  Also, C<$year> is the
number of years since 1900, that is, C<$year> is C<123> in year 2023,
I<not> simply the last two digits of the year.  If you assume it is,
then you create non-Y2K-compliant programs--and you wouldn't want to do
that, would you?

If EXPR is omitted, does C<gmtime(time())>.

In scalar context, returns the ctime(3) value:

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

Also see the C<timegm()> function provided by the C<Time::Local> module,
and the strftime(3) function available via the POSIX module.

This scalar value is B<not> locale dependent (see L<perllocale>), but
is instead a Perl builtin.  Also see the C<Time::Local> module, and the
strftime(3) and mktime(3) functions available via the POSIX module.  To
get somewhat similar but locale dependent date strings, set up your
locale environment variables appropriately (please see L<perllocale>)
and try for example:

    use POSIX qw(strftime);
    $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;

Note that the C<%a> and C<%b> escapes, which represent the short forms
of the day of the week and the month of the year, may not necessarily
be three characters wide in all locales.

=item goto LABEL

=item goto EXPR

=item goto &NAME

The C<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 C<foreach> loop.  It
also can't be used to go into a construct that is optimized away,
or to get out of a block or subroutine given to C<sort()>.
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 C<last> or C<die()>.  The author of Perl has never felt the
need to use this form of C<goto> (in Perl, that is--C is another matter).

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

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

The C<goto-&NAME> form is highly magical, and substitutes a call to the
named subroutine for the currently running subroutine.  This is used by
C<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 C<@_> in the current subroutine are
propagated to the other subroutine.)  After the C<goto>, not even C<caller()>
will be able to tell that this routine was called first.

=item grep BLOCK LIST

=item grep EXPR,LIST

This is similar in spirit to, but not the same as, grep(1) and its
relatives.  In particular, it is not limited to using regular expressions.

Evaluates the BLOCK or EXPR for each element of LIST (locally setting
C<$_> to each element) and returns the list value consisting of those
elements for which the expression evaluated to TRUE.  In 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, because C<$_> 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.
Similarly, grep returns aliases into the original list, much as a for
loop's index variable aliases the list elements.  That is, modifying an
element of a list returned by grep (for example, in a C<foreach>, C<map()>
or another C<grep()>) actually modifies the element in the original list.
This is usually something to be avoided when writing clear code.

See also L</map> for an array composed of the results of the BLOCK or EXPR.

=item hex EXPR

=item hex

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

    print hex '0xAf'; # prints '175'
    print hex 'aF';   # same

=item import

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

=item index STR,SUBSTR,POSITION

=item index STR,SUBSTR

The index function searches for one string within another, but without
the wildcard-like behavior of a full regular-expression pattern match.
It 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 C<0> (or whatever
you've set the C<$[> variable to--but don't do that).  If the substring
is not found, returns one less than the base, ordinarily C<-1>.

=item int EXPR

=item int

Returns the integer portion of EXPR.  If EXPR is omitted, uses C<$_>.
You should not use this function for rounding: one because it truncates
towards C<0>, and two because machine representations of floating point
numbers can sometimes produce counterintuitive results.  For example,
C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
because it's really more like -268.99999999999994315658 instead.  Usually,
the C<sprintf()>, C<printf()>, or the C<POSIX::floor> and C<POSIX::ceil>
functions will serve you better than will int().

=item ioctl FILEHANDLE,FUNCTION,SCALAR

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

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

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 that
may help you in this, but it's nontrivial.)  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 C<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 C<0> to the scalar before using it.)  The C<pack()> and C<unpack()>
functions are useful for manipulating the values of structures used by
C<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 C<ioctl()> (and C<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(...) || -1;
    printf "System returned %d\n", $retval;

The special string "C<0> but true" is exempt from B<-w> complaints
about improper numeric conversions.

=item join EXPR,LIST

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

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

See L</split>.

=item keys HASH

Returns a list consisting of all the keys of the named hash.  (In a
scalar context, returns the number of keys.)  The keys are returned in
an apparently random order.  The actual random order is subject to
change in future versions of perl, but it is guaranteed to be the same
order as either the C<values()> or C<each()> function produces (given
that the hash has not been modified).  As a side effect, it resets
HASH's iterator.

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 a hash 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;
    }

As an lvalue C<keys()> allows you to increase the number of hash buckets
allocated for the given hash.  This can gain you a measure of efficiency if
you know the hash is going to get big.  (This is similar to pre-extending
an array by assigning a larger number to $#array.)  If you say

    keys %hash = 200;

then C<%hash> will have at least 200 buckets allocated for it--256 of them,
in fact, since it rounds up to the next power of two.  These
buckets will be retained even if you do C<%hash = ()>, use C<undef
%hash> if you want to free the storage while C<%hash> is still in scope.
You can't shrink the number of buckets allocated for the hash using
C<keys()> in this way (but you needn't worry about doing this by accident,
as trying has no effect).

See also C<each()>, C<values()> and C<sort()>.

=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 L<perlipc/"Signals"> 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
	#...
    }

C<last> cannot be used to exit a block which returns a value such as
C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
a grep() or map() operation.

See also L</continue> for an illustration of how C<last>, C<next>, and
C<redo> work.

=item lc EXPR

=item lc

Returns an lowercased version of EXPR.  This is the internal function
implementing the C<\L> escape in double-quoted strings.
Respects current LC_CTYPE locale if C<use locale> in force.  See L<perllocale>.

If EXPR is omitted, uses C<$_>.

=item lcfirst EXPR

=item lcfirst

Returns the value of EXPR with the first character lowercased.  This is
the internal function implementing the C<\l> escape in double-quoted strings.
Respects current LC_CTYPE locale if C<use locale> in force.  See L<perllocale>.

If EXPR is omitted, uses C<$_>.

=item length EXPR

=item length

Returns the length in characters of the value of EXPR.  If EXPR is
omitted, returns length of C<$_>.  Note that this cannot be used on 
an entire array or hash to find out how many elements these have.
For that, use C<scalar @array> and C<scalar keys %hash> respectively.

=item link OLDFILE,NEWFILE

Creates a new filename linked to the old filename.  Returns TRUE for
success, FALSE otherwise. 

=item listen SOCKET,QUEUESIZE

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

=item local EXPR

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

A local modifies the listed variables to be local to the enclosing
block, file, or eval.  If more than one value is listed, the list must
be placed in parentheses.  See L<perlsub/"Temporary Values via local()">
for details, including issues with tied arrays and hashes.

=item localtime EXPR

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

    #  0    1    2     3     4    5     6     7     8
    ($sec,$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 C<$mon> has the range C<0..11> and C<$wday>
has the range C<0..6> with sunday as day C<0>.  Also, C<$year> is the
number of years since 1900, that is, C<$year> is C<123> in year 2023,
and I<not> simply the last two digits of the year.  If you assume it is,
then you create non-Y2K-compliant programs--and you wouldn't want to do
that, would you?

If EXPR is omitted, uses the current time (C<localtime(time)>).

In scalar context, returns the ctime(3) value:

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

This scalar value is B<not> locale dependent, see L<perllocale>, but
instead a Perl builtin.  Also see the C<Time::Local> module, and the
strftime(3) and mktime(3) function available via the POSIX module.  To
get somewhat similar but locale dependent date strings, set up your
locale environment variables appropriately (please see L<perllocale>)
and try for example:

    use POSIX qw(strftime);
    $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;

Note that the C<%a> and C<%b>, the short forms of the day of the week
and the month of the year, may not necessarily be three characters wide.

=item log EXPR

=item log

Returns the natural logarithm (base I<e>) of EXPR.  If EXPR is omitted,
returns log of C<$_>.  To get the log of another base, use basic algebra:
The base-N log of a number is is equal to the natural log of that number
divided by the natural log of N.  For example:

    sub log10 {
	my $n = shift;
	return log($n)/log(10);
    } 

See also L</exp> for the inverse operation.

=item lstat FILEHANDLE

=item lstat EXPR

=item lstat

Does the same thing as the C<stat()> function (including setting the
special C<_> filehandle) but stats a symbolic link instead of the file
the symbolic link points to.  If symbolic links are unimplemented on
your system, a normal C<stat()> is done.

If EXPR is omitted, stats C<$_>.

=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 C<$_> 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.

In scalar context, returns the total number of elements so generated.

    @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) {
	$hash{getkey($_)} = $_;
    }

Note that, because C<$_> 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.
Using a regular C<foreach> loop for this purpose would be clearer in
most cases.  See also L</grep> for an array composed of those items of
the original list for which the BLOCK or EXPR evaluates to true.

=item mkdir FILENAME,MODE

Creates the directory specified by FILENAME, with permissions
specified by MODE (as modified by C<umask>).  If it succeeds it
returns TRUE, otherwise it returns FALSE and sets C<$!> (errno).

In general, it is better to create directories with permissive MODEs,
and let the user modify that with their C<umask>, than it is to supply
a restrictive MODE and give the user no way to be more permissive.
The exceptions to this rule are when the file or directory should be
kept private (mail files, for instance).  The perlfunc(1) entry on
C<umask> discusses the choice of MODE in more detail.

=item msgctl ID,CMD,ARG

Calls the System V IPC function msgctl(2).  You'll probably have to say

    use IPC::SysV;

first to get the correct constant definitions.  If CMD is C<IPC_STAT>,
then ARG must be a variable which will hold the returned C<msqid_ds>
structure.  Returns like C<ioctl()>: the undefined value for error, "C<0> but
true" for zero, or the actual return value otherwise.  See also
C<IPC::SysV> and C<IPC::Semaphore::Msg> documentation.

=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.  See also C<IPC::SysV>
and C<IPC::SysV::Msg> documentation.

=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.  See also C<IPC::SysV>
and C<IPC::SysV::Msg> documentation.

=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.  See also C<IPC::SysV> and C<IPC::SysV::Msg> documentation.

=item my EXPR

A C<my()> declares the listed variables to be local (lexically) to the
enclosing block, file, or C<eval()>.  If
more than one value is listed, the list must be placed in parentheses.  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.

C<next> cannot be used to exit a block which returns a value such as
C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
a grep() or map() operation.

See also L</continue> for an illustration of how C<last>, C<next>, and
C<redo> work.

=item no Module LIST

See the L</use> function, which C<no> is the opposite of.

=item oct EXPR

=item oct

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

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

If EXPR is omitted, uses C<$_>.  This function is commonly used when
a string such as C<644> needs to be converted into a file mode, for
example. (Although perl will automatically convert strings into
numbers as needed, this automatic conversion assumes base 10.)

=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.
(Note that lexical variables--those declared with C<my()>--will not work
for this purpose; so if you're using C<my()>, specify EXPR in your call
to open.)  See L<perlopentut> for a kinder, gentler explanation of opening
files.

If the filename begins with C<'E<lt>'> or nothing, the file is opened for input.
If the filename begins with C<'E<gt>'>, the file is truncated and opened for
output, being created if necessary. If the filename begins with C<'E<gt>E<gt>'>,
the file is opened for appending, again being created if necessary. 
You can put a C<'+'> in front of the C<'E<gt>'> or C<'E<lt>'> to indicate that
you want both read and write access to the file; thus C<'+E<lt>'> is almost
always preferred for read/write updates--the C<'+E<gt>'> mode would clobber the
file first.  You can't usually use either read-write mode for updating
textfiles, since they have variable length records.  See the B<-i>
switch in L<perlrun> for a better approach.  The file is created with
permissions of C<0666> modified by the process' C<umask> value.

The prefix and the filename may be separated with spaces.
These various prefixes correspond to the fopen(3) modes of C<'r'>, C<'r+'>, C<'w'>,
C<'w+'>, C<'a'>, and C<'a+'>.

If the filename begins with C<'|'>, the filename is interpreted as a
command to which output is to be piped, and if the filename ends with a
C<'|'>, the filename is interpreted as a command which pipes output to
us.  See L<perlipc/"Using open() for IPC">
for more examples of this.  (You are not allowed to C<open()> to a command
that pipes both in I<and> out, but see L<IPC::Open2>, L<IPC::Open3>,
and L<perlipc/"Bidirectional Communication"> for alternatives.)

Opening C<'-'> opens STDIN and opening C<'E<gt>-'> opens STDOUT.  Open returns
nonzero upon success, the undefined value otherwise.  If the C<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 C<binmode()>
and those that don't is their text file formats.  Systems like Unix, MacOS, and
Plan9, which delimit lines with a single character, and which encode that
character in C as C<"\n">, do not need C<binmode()>.  The rest need it.

When opening a file, it's usually a bad idea to continue normal execution
if the request failed, so C<open()> is frequently used in connection with
C<die()>. Even if C<die()> won't do what you want (say, in a CGI script,
where you want to make a nicely formatted error message (but there are
modules that can help with that problem)) you should always check
the return value from opening a file. The infrequent exception is when
working with an unopened filehandle is actually what you want to do.

Examples:

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

    open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
    # if the open fails, output is discarded

    open(DBASE, '+<dbase.mine')		    # open for update
	or die "Can't open 'dbase.mine' for update: $!";

    open(ARTICLE, "caesar <$article |")     # decrypt article
	or die "Can't start caesar: $!";

    open(EXTRACT, "|sort >/tmp/Tmp$$")      # $$ is our process id
	or die "Can't start sort: $!";

    # process argument list of files along with any includes

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

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

	local $_;
	while (<$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 C<'E<gt>&'>, in which case the rest of the string is interpreted as the
name of a filehandle (or file descriptor, if numeric) to be
duped and opened.  You may use C<&> after C<E<gt>>, C<E<gt>E<gt>>, C<E<lt>>, C<+E<gt>>,
C<+E<gt>E<gt>>, and C<+E<lt>>.  The
mode you specify should match the mode of the original filehandle.
(Duping a filehandle does not take into account any existing contents of
stdio buffers.)
Here is a script that saves, redirects, and restores STDOUT and
STDERR:

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

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

If you specify C<'E<lt>&=N'>, where C<N> is a number, then Perl will do an
equivalent of C's C<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 C<'-'>, i.e., either C<'|-'> or C<'-|'>, then
there is an implicit fork done, and the return value of open is the pid
of the child within the parent process, and C<0> within the child
process.  (Use C<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.

NOTE: On any operation that may do a fork, all files opened for output
are flushed before the fork is attempted.  On systems that support a
close-on-exec flag on files, the flag will be set for the newly opened
file descriptor as determined by the value of $^F.  See L<perlvar/$^F>.

Closing any piped filehandle causes the parent process to wait for the
child to finish, and returns the status value in C<$?>.

The filename passed to open will have leading and trailing
whitespace deleted, and the normal redirection characters
honored.  This property, known as "magic open", 
can often be used to good effect.  A user could specify a filename of
F<"rsh cat file |">, or you could change certain filenames as needed:

    $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
    open(FH, $filename) or die "Can't open $filename: $!";

However, to open a file with arbitrary weird characters in it, it's
necessary to protect any leading and trailing whitespace:

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

If you want a "real" C C<open()> (see L<open(2)> on your system), then you
should use the C<sysopen()> function, which involves no such magic.  This is
another way to protect your filenames from interpretation.  For example:

    use IO::Handle;
    sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
	or die "sysopen $path: $!";
    $oldfh = select(HANDLE); $| = 1; select($oldfh);
    print HANDLE "stuff $$\n");
    seek(HANDLE, 0, 0);
    print "File contains: ", <HANDLE>;

Using the constructor from the C<IO::Handle> package (or one of its
subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous
filehandles that have the scope of whatever variables hold references to
them, and automatically close whenever and however you leave that scope:

    use IO::File;
    #...
    sub read_myfile_munged {
	my $ALL = shift;
	my $handle = new IO::File;
	open($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.
    }

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

=item opendir DIRHANDLE,EXPR

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

=item ord EXPR

=item ord

Returns the numeric (ASCII or Unicode) value of the first character of EXPR.  If
EXPR is omitted, uses C<$_>.  For the reverse, see L</chr>.
See L<utf8> for more about Unicode.

=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	A string with arbitrary binary data, will be null padded.
    A	An ascii string, will be space padded.
    Z	A null terminated (asciz) 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.  Only does bytes.  See U for Unicode.

    s	A signed short value.
    S	An unsigned short value.
	  (This 'short' is _exactly_ 16 bits, which may differ from
	   what a local C compiler calls 'short'.)

    i	A signed integer value.
    I	An unsigned integer value.
	  (This 'integer' is _at least_ 32 bits wide.  Its exact
           size depends on what a local C compiler calls 'int',
           and may even be larger than the 'long' described in
           the next item.)

    l	A signed long value.
    L	An unsigned long value.
	  (This 'long' is _exactly_ 32 bits, which may differ from
	   what a local C compiler calls 'long'.)

    n	A short in "network" (big-endian) order.
    N	A long in "network" (big-endian) order.
    v	A short in "VAX" (little-endian) order.
    V	A long in "VAX" (little-endian) order.
	  (These 'shorts' and 'longs' are _exactly_ 16 bits and
	   _exactly_ 32 bits, respectively.)

    q	A signed quad (64-bit) value.
    Q	An unsigned quad value.
	  (Available only if your system supports 64-bit integer values
	   _and_ if Perl has been compiled to support those.
           Causes a fatal error otherwise.)

    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.
    U	A Unicode character number.  Encodes to UTF-8 internally.
	Works even if C<use utf8> is not in effect.

    w	A BER compressed integer.  Its bytes represent an unsigned
	integer in base 128, most significant digit first, with as
        few digits as possible.  Bit eight (the high bit) is set
        on each byte except the last.

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

The following rules apply:

=over 8

=item *

Each letter may optionally be followed by a number giving a repeat
count.  With all types except C<"a">, C<"A">, C<"Z">, C<"b">, C<"B">, C<"h">,
C<"H">, and C<"P"> the pack function will gobble up that many values from
the LIST.  A C<*> for the repeat count means to use however many items are
left.

=item *

The C<"a">, C<"A">, and C<"Z"> types gobble just one value, but pack it as a
string of length count, padding with nulls or spaces as necessary.  When
unpacking, C<"A"> strips trailing spaces and nulls, C<"Z"> strips everything
after the first null, and C<"a"> returns data verbatim.

=item *

Likewise, the C<"b"> and C<"B"> fields pack a string that many bits long.

=item *

The C<"h"> and C<"H"> fields pack a string that many nybbles long.

=item *

The C<"p"> type packs a pointer to a null-terminated string.  You are
responsible for ensuring the string is not a temporary value (which can
potentially get deallocated before you get around to using the packed result).
The C<"P"> type packs a pointer to a structure of the size indicated by the
length. A NULL pointer is created if the corresponding value for C<"p"> or
C<"P"> is C<undef>.

=item *

The integer types C<"s">, C<"S">, C<"l">, and C<"L"> may be
immediately followed by a C<"!"> to signify native shorts or longs--as
you can see from above for example a bare C<"l"> does mean exactly 32
bits, the native C<long> (as seen by the local C compiler) may be
larger.  This is an issue mainly in 64-bit platforms.  You can see
whether using C<"!"> makes any difference by

	print length(pack("s")), " ", length(pack("s!")), "\n";
	print length(pack("l")), " ", length(pack("l!")), "\n";

C<"i!"> and C<"I!"> also work but only because of completeness;
they are identical to C<"i"> and C<"I">.

The actual sizes (in bytes) of native shorts, ints, and longs on
the platform where Perl was built are also available via L<Config>:

	use Config;
	print $Config{shortsize}, "\n";
	print $Config{intsize}, "\n";
	print $Config{longsize}, "\n";

=item *

The integer formats C<"s">, C<"S">, C<"i">, C<"I">, C<"l">, and C<"L">
are inherently non-portable between processors and operating systems
because they obey the native byteorder and endianness.  For example a
4-byte integer 0x87654321 (2271560481 decimal) be ordered natively
(arranged in and handled by the CPU registers) into bytes as
 
 	0x12 0x34 0x56 0x78	# little-endian
 	0x78 0x56 0x34 0x12	# big-endian
 
Basically, the Intel, Alpha, and VAX CPUs and little-endian, while
everybody else, for example Motorola m68k/88k, PPC, Sparc, HP PA,
Power, and Cray are big-endian.  MIPS can be either: Digital used it
in little-endian mode, SGI uses it in big-endian mode.

The names `big-endian' and `little-endian' are joking references to
the classic "Gulliver's Travels" (via the paper "On Holy Wars and a
Plea for Peace" by Danny Cohen, USC/ISI IEN 137, April 1, 1980) and
the egg-eating habits of the lilliputs.
 
Some systems may even have weird byte orders such as
 
 	0x56 0x78 0x12 0x34
 	0x34 0x12 0x78 0x56
 
You can see your system's preference with

 	print join(" ", map { sprintf "%#02x", $_ }
                            unpack("C*",pack("L",0x12345678))), "\n";

The byteorder on the platform where Perl was built is also available
via L<Config>:

	use Config;
	print $Config{byteorder}, "\n";

Byteorders C<'1234'> and C<'12345678'> are little-endian, C<'4321'>
and C<'87654321'> are big-endian.

If you want portable packed integers use the formats C<"n">, C<"N">,
C<"v">, and C<"V">, their byte endianness and size is known.

=item *

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", $foo)>) will not in general
equal C<$foo>).

=back

Examples:

    $foo = pack("CCCC",65,66,67,68);
    # foo eq "ABCD"
    $foo = pack("C4",65,66,67,68);
    # same thing
    $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);
    # same thing with Unicode circled letters

    $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)

    $utmp_template = "Z8 Z8 Z16 L";
    $utmp = pack($utmp_template, @utmp1);
    # a struct utmp (BSDish)

    @utmp2 = unpack($utmp_template, $utmp);
    # "@utmp1" eq "@utmp2"

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

The same template may generally also be used in unpack().

=item package 

=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, file, or eval (the same as the C<my()> operator).
All further unqualified dynamic identifiers will be in this namespace.
A package statement affects only dynamic variables--including those
you've used C<local()> on--but I<not> lexical variables, which are created
with C<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<$main::sail> (as well as to C<$main'sail>,
still seen in older code).

If NAMESPACE is omitted, then there is no current package, and all
identifiers must be fully qualified or lexicals.  This is stricter
than C<use strict>, since it also extends to function names.

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 C<$|> to flush your WRITEHANDLE
after each command, depending on the application.

See L<IPC::Open2>, L<IPC::Open3>, and L<perlipc/"Bidirectional Communication">
for examples of such things.

On systems that support a close-on-exec flag on files, the flag will be set
for the newly opened file descriptors as determined by the value of $^F.
See L<perlvar/$^F>.

=item pop ARRAY

=item pop

Pops and returns the last value of the array, shortening the array by
one element.  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 C<@ARGV> array in the main program, and
the C<@_> array in subroutines, just like C<shift()>.

=item pos SCALAR

=item pos

Returns the offset of where the last C<m//g> search left off for the variable
is in question (C<$_> is used when the variable is not specified).  May be
modified to change that offset.  Such modification will also influence
the C<\G> zero-width assertion in regular expressions.  See L<perlre> and
L<perlop>.

=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 C<+> or put parentheses around the arguments.)
If FILEHANDLE is omitted, prints by default to standard output (or to the
last selected output channel--see L</select>).  If LIST is also omitted,
prints C<$_> to the currently selected output channel.  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 list context, and any subroutine that you call will have one or
more of its expressions evaluated in 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 C<+> or put parentheses 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 FORMAT, LIST

=item printf FORMAT, LIST

Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that C<$\>
(the output record separator) is not appended.  The first argument
of the list will be interpreted as the C<printf()> format.  If C<use locale> is
in effect, the character used for the decimal point in formatted real numbers
is affected by the LC_NUMERIC locale.  See L<perllocale>.

Don't fall into the trap of using a C<printf()> when a simple
C<print()> would do.  The C<print()> is more efficient and less
error prone.

=item prototype FUNCTION

Returns the prototype of a function as a string (or C<undef> if the
function has no prototype).  FUNCTION is a reference to, or the name of,
the function whose prototype you want to retrieve.

If FUNCTION is a string starting with C<CORE::>, the rest is taken as a
name for Perl builtin.  If the builtin is not I<overridable> (such as
C<qw//>) or its arguments cannot be expressed by a prototype (such as
C<system()>) returns C<undef> because the builtin does not really behave
like a Perl function.  Otherwise, the string describing the equivalent
prototype is returned.

=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 qr/STRING/

=item qx/STRING/

=item qw/STRING/

Generalized quotes.  See L<perlop/"Regexp Quote-Like Operators">.

=item quotemeta EXPR

=item quotemeta

Returns the value of EXPR with all non-alphanumeric
characters backslashed.  (That is, all characters not matching
C</[A-Za-z_0-9]/> will be preceded by a backslash in the
returned string, regardless of any locale settings.)
This is the internal function implementing
the C<\Q> escape in double-quoted strings.

If EXPR is omitted, uses C<$_>.

=item rand EXPR

=item rand

Returns a random fractional number greater than or equal to C<0> and less
than the value of EXPR.  (EXPR should be positive.)  If EXPR is
omitted, the value C<1> is used.  Automatically calls C<srand()> unless
C<srand()> has already been called.  See also C<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.)

=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,
C<0> at end of file, 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(3)
call.  To get a true read(2) system call, see C<sysread()>.

=item readdir DIRHANDLE

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

If you're planning to filetest the return values out of a C<readdir()>, you'd
better prepend the directory in question.  Otherwise, because we didn't
C<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 readline EXPR

Reads from the filehandle whose typeglob is contained in EXPR.  In scalar
context, each call reads and returns the next line, until end-of-file is
reached, whereupon the subsequent call returns undef.  In list context,
reads until end-of-file is reached and returns a list of lines.  Note that
the notion of "line" used here is however you may have defined it
with C<$/> or C<$INPUT_RECORD_SEPARATOR>).  See L<perlvar/"$/">.

When C<$/> is set to C<undef>, when readline() is in scalar
context (i.e. file slurp mode), and when an empty file is read, it
returns C<''> the first time, followed by C<undef> subsequently.

This is the internal function implementing the C<E<lt>EXPRE<gt>>
operator, but you can use it directly.  The C<E<lt>EXPRE<gt>>
operator is discussed in more detail in L<perlop/"I/O Operators">.

    $line = <STDIN>;
    $line = readline(*STDIN);		# same thing

=item readlink EXPR

=item readlink

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 C<$!> (errno).  If EXPR is
omitted, uses C<$_>.

=item readpipe EXPR

EXPR is executed as a system command.
The collected standard output of the command is returned.
In scalar context, it comes back as a single (potentially
multi-line) string.  In list context, returns a list of lines
(however you've defined lines with C<$/> or C<$INPUT_RECORD_SEPARATOR>).
This is the internal function implementing the C<qx/EXPR/>
operator, but you can use it directly.  The C<qx/EXPR/>
operator is discussed in more detail in L<perlop/"I/O Operators">.

=item recv SOCKET,SCALAR,LENGTH,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 C<recvfrom()>, so that it can return 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;
    }

C<redo> cannot be used to retry a block which returns a value such as
C<eval {}>, C<sub {}> or C<do {}>, and should not be used to exit
a grep() or map() operation.

See also L</continue> for an illustration of how C<last>, C<next>, and
C<redo> work.

=item ref EXPR

=item ref

Returns a TRUE value if EXPR is a reference, FALSE otherwise.  If EXPR
is not specified, C<$_> will be used.  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 C<ref()> as a C<typeof()> operator.

    if (ref($r) eq "HASH") {
	print "r is a reference to a hash.\n";
    }
    unless (ref($r)) {
	print "r is not a reference at all.\n";
    }
    if (UNIVERSAL::isa($r, "HASH")) {  # for subclassing
	print "r is a reference to something that isa hash.\n";
    } 

See also L<perlref>.

=item rename OLDNAME,NEWNAME

Changes the name of a file.  Returns C<1> for success, C<0> otherwise.
Behavior of this function varies wildly depending on your system
implementation.  For example, it will usually not work across file system
boundaries, even though the system I<mv> command sometimes compensates
for this.  Other restrictions include whether it works on directories,
open files, or pre-existing files.  Check L<perlport> and either the
rename(2) manpage or equivalent system documentation for details.

=item require EXPR

=item require

Demands some semantics specified by EXPR, or by C<$_> if EXPR is not
supplied.  If EXPR is numeric, demands that the current version of Perl
(C<$]> 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 C<eval()>.  Has semantics similar to the following
subroutine:

    sub require {
	my($filename) = @_;
	return 1 if $INC{$filename};
	my($realfilename,$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;
	return $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 "C<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 bareword, the require assumes a "F<.pm>" extension and
replaces "F<::>" with "F</>" in the filename for you,
to make it easy to load standard modules.  This form of loading of
modules does not risk altering your namespace.

In other words, if you try this:

        require Foo::Bar;    # a splendid bareword 

The require function will actually look for the "F<Foo/Bar.pm>" file in the 
directories specified in the C<@INC> array.

But if you try this:

        $class = 'Foo::Bar';
        require $class;	     # $class is not a bareword
    #or
        require "Foo::Bar";  # not a bareword because of the ""

The require function will look for the "F<Foo::Bar>" file in the @INC array and 
will complain about not finding "F<Foo::Bar>" there. In this case you can do:

        eval "require $class";

For a yet-more-powerful import facility, see 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 C<??> 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 (C<?pattern?>) are reset to match again.  Resets
only 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 ?one-time? searches

Resetting C<"A-Z"> is not recommended because you'll wipe out your
C<@ARGV> and C<@INC> arrays and your C<%ENV> hash.  Resets only package
variables--lexical variables are unaffected, but they clean themselves
up on scope exit anyway, so you'll probably want to use them instead.
See L</my>.

=item return EXPR

=item return

Returns from a subroutine, C<eval()>, or C<do FILE> with the value 
given in EXPR.  Evaluation of EXPR may be in list, scalar, or void
context, depending on how the return value will be used, and the context
may vary from one execution to the next (see C<wantarray()>).  If no EXPR
is given, returns an empty list in list context, the undefined value in
scalar context, and (of course) nothing at all in a void context.

(Note that in the absence of a explicit C<return>, a subroutine, eval,
or do FILE will automatically return the value of the last expression
evaluated.)

=item reverse LIST

In list context, returns a list value consisting of the elements
of LIST in the opposite order.  In scalar context, concatenates the
elements of LIST and returns a string value with all characters
in the opposite order.

    print reverse <>;		# line tac, last line first

    undef $/;			# for efficiency of <>
    print scalar reverse <>;	# character tac, last line tsrif

This operator is also handy for inverting a hash, although there are some
caveats.  If a value is duplicated in the original hash, only one of those
can be represented as a key in the inverted hash.  Also, this has to
unwind one hash and build a whole new one, which may take some time
on a large hash, such as from a DBM file.

    %by_name = reverse %by_address;	# Invert the hash

=item rewinddir DIRHANDLE

Sets the current position to the beginning of the directory for the
C<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

=item rmdir

Deletes the directory specified by FILENAME if that directory is empty.  If it
succeeds it returns TRUE, otherwise it returns FALSE and sets C<$!> (errno).  If
FILENAME is omitted, uses C<$_>.

=item s///

The substitution operator.  See L<perlop>.

=item scalar EXPR

Forces EXPR to be interpreted in 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 list context because in practice, this is 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.

Wince C<scalar> is unary operator, if you accidentally use for EXPR a
parenthesized list, this behaves as a scalar comma expression, evaluating
all but the last element in void context and returning the final element
evaluated in scalar context.  This is seldom what you want.

The following single statement:

	print uc(scalar(&foo,$bar)),$baz;

is the moral equivalent of these two:

	&foo;
	print(uc($bar),$baz);

See L<perlop> for more details on unary operators and the comma operator.

=item seek FILEHANDLE,POSITION,WHENCE

Sets FILEHANDLE's position, just like the C<fseek()> call of C<stdio()>.
FILEHANDLE may be an expression whose value gives the name of the
filehandle.  The values for WHENCE are C<0> to set the new position to
POSITION, C<1> to set it to the current position plus POSITION, and C<2> to
set it to EOF plus POSITION (typically negative).  For WHENCE you may
use the constants C<SEEK_SET>, C<SEEK_CUR>, and C<SEEK_END> from either the
C<IO::Seekable> or the POSIX module.  Returns C<1> upon success, C<0> otherwise.