=over =item unpack TEMPLATE,EXPR X C does the reverse of C: 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, or the characters of the string represent a C structure of some kind. The TEMPLATE has the same format as in the C 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 % to indicate that you want a -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 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("%32W*",<>) % 65535; }; The following efficiently counts the number of set bits in a bit vector: $setbits = unpack("%32b*", $selectmask); The C

and C

formats should be used with care. Since Perl has no way of checking whether the value passed to C corresponds to a valid memory location, passing a pointer value that's not known to be valid is likely to have disastrous consequences. If there are more pack codes or if the repeat count of a field or a group is larger than what the remainder of the input string allows, the result is not well defined: in some cases, the repeat count is decreased, or C will produce null strings or zeroes, or terminate with an error. If the input string is longer than one described by the TEMPLATE, the rest is ignored. See L for more examples and notes. =back