who calls main function in c programming?

That depends on who calls it. When you start a c program from the shell, I'd believe that it the shell that makes the call.

There is some startup module linked into each image. I'm not certain of the details, but roughly:

Some loader code executes first inside the image, loading required .so files and doing some relocations. It transfers control to the startup module (as a jump, not a call).

The startup module does some initialization (a lot of things in C++, I'm not sure how much in C) and pushes the arguments for main and calls main. When main returns, it does a few kinds of cleanup before exiting to the OS.

In many compile/link packages, that startup module is named crtbegin.

Looks like homework to me. Oh well.

AFAIK, C runtime library calls it from real entry point - after it builds argument lists (& etc) using os-specific calls. I'd recommend you to check gcc sources to find out how exactly things work.

It's not in the gcc source, but in the glibc - code for ld-linux.so.

From what I understand the program is the one that calls main in an indirect way. First when the binary is called a jump is made to an entry point but that point is not main() but a runtime library which will first activate a few things stdin/stdout/stderr and then some other things then call main().

Try searching google for c + application + "entry point"