Learning Perl, Second Edition

16.2. Packing and Unpacking Binary Data

The password and group information is nicely represented in textual form. Other system databases are more naturally represented in other forms. For example, the IP address of an interface is internally managed as a four-byte number. While it is frequently decoded into a textual representation consisting of four small integers separated by periods, this encoding and decoding is wasted effort if a human is not interpreting the data in the meantime.

Because of this, the network routines in Perl that expect or return an IP address use a four-byte string that contains one character for each sequential byte in memory. While constructing and interpreting such a byte string is fairly straightforward using chr and ord (not presented here), Perl provides a short cut that is equally applicable to more difficult structures.

The pack function works a bit like sprintf, taking a format control string and a list of values, and creating a single string from those values. The pack format string is geared towards creating a binary data structure, however. For example, here's how to take four small integers and pack them as successive unsigned bytes in a composite string:

$buf = pack("CCCC", 140, 186, 65, 25);

Here, the pack format string is four C's. Each C represents a separate value taken from the following list (similar to what a % field does in sprintf). The C format (according to the Perl manpages, the reference card, Programming Perl, the HTML files, ...