[math-fun] Custom number crunching programs --- expectations?

Hello, I've used a GNFS factoring package, and it seemed to have been modeled after a C program targeting POSIX, and emitting output to stdout. I also wrote similar programs with the same mode of operation. Have you had experience with custom number crunching programs like that? How did you interacting with them? What would you expect? Thanks in advance. Andres.