=over

=item unpack TEMPLATE,EXPR

C<unpack> does the reverse of C<pack>: it takes a string
and expands it out into a list of values.
(In scalar context, it returns merely the first value produced.)

The string is broken into chunks described by the TEMPLATE.  Each chunk
is converted separately to a value.  Typically, either the string is a result
of C<pack>, or the bytes of the string represent a C structure of some
kind.

The TEMPLATE has the same format as in the C<pack> function.
Here's a subroutine that does substring:

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

and then there's

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

In addition to fields allowed in pack(), you may prefix a field with
a %<number> to indicate that
you want a <number>-bit checksum of the items instead of the items
themselves.  Default is a 16-bit checksum.  Checksum is calculated by
summing numeric values of expanded values (for string fields the sum of
C<ord($char)> is taken, for bit fields the sum of zeroes and ones).

For example, the following
computes the same number as the System V sum program:

    $checksum = do {
	local $/;  # slurp!
	unpack("%32C*",<>) % 65535;
    };

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

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

The C<p> and C<P> formats should be used with care.  Since Perl
has no way of checking whether the value passed to C<unpack()>
corresponds to a valid memory location, passing a pointer value that's
not known to be valid is likely to have disastrous consequences.

If the repeat count of a field is larger than what the remainder of
the input string allows, repeat count is decreased.  If the input string
is longer than one described by the TEMPLATE, the rest is ignored.

See L</pack> for more examples and notes.

=back