Looking for a program to produce flow charts of C source

[Peter Palmer]

I'm a technical writer a Cray Research with limited C experience.  I'm
looking for some sort of Unix program that will produce flow and control
diagrams for very complex operating system C code.  This program
must be able to take C source (or executables, I guess) and not only
track the contents of individual files, but be able to leave and 
track the code if library routines or functions are called.

Is there such a product out there in the commercial world?

Thanks in advance.  You can reach me at n2450 at cray.com.
.