Interprocess communication can be achieved with pipes (
http://www.cs.cf.ac.uk/Dave/C/node23.html ). Basically a pipe is a buffer managed by the operating system, you can read from it and write to it. Each operation (read/write) has an associated file descriptor.
http://en.wikipedia.org/wiki/Pipes_and_filters
http://www.tin.org/bin/man.cgi?section=2&topic=pipe