z-distribution function in C

I'm doing some simple confidence interval statistics. I need to calculate normal curve areas. Where can I find a C function that calculates the z-distribution?

Thanks,
Peter

Most people start with google

I did check google and always do. Should I state that at the top of a new question? It is not easy to find C code for the inverse error function.

-Peter

Yeah, but did you find anything describing the algorithm?