Best way to call external functions from a different process

Hello, I created a basic Qt UI that has buttons. I have an external application that I created in C++ from MSVS2008. I am using QSharedMemory for IPC communication. I have successfully binded the data from my app to the UI and can read the outputs. I am a little bit stuck as to what the best way would be to communicate back to my external application. For example, how would I call a function in my external app once a button is pressed in my GUI? Thanks ahead of time!