“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.