How to interface an application's GUI with a C program

I'm trying to execute an application (simulator) using a system call from my program. Once the application (simulator) starts, I want my program to perform certain tasks on the GUI of the simulator: fill in certain fields, start the simulation, collect the results, and close the application. In essence, I need to emulate the keyboard stokes on the GUI of the simulator using my C program. I must interface to the GUI since I only have the executable available to me. Can any one recommend a way to perform this? Any help would be greatly appreciated.

 

Ive never done it, but I'd recommend looking into sockets, you can send data between a host program and server program and such.