“Core” function in Postgres

Hi experts,

I want to know what's the "core" function used in Postgres server? I am looking for something corresponding to main() in a simple C program. I want to know the file path and the function name. I am using Postgres 9.3.5, however I assume the "core" function will be unchanged between different revisions.

Please let me know if you are confused by my question.

Thanks

Zack

PS: I have the same post on stackoverflow. Since no one answered there, I just report here.