C API - Authentication Question

I'm very impressed so far with what I've seen of PostgreSQL,
however I have a nagging question which is absolutely vital for the
project I have in mind... How do I tell PostgreSQL what
username:password to use to authenticate myself to the database?

I'm going to have clients(Linux) on changing IPs that must be
authenicated with user:pw, not simply by host, I understand how to
change the config file to allow connections but I can't find sufficient
documentation to tell me how to give a username:pw pair...

Thank you in advance...

--David