N1256 5.1.2.2.3 p1: If the return type of the main function is a type compatible with int, a return from the initial call to the main function is equivalent to calling the exit function with the value returned by the main function as its argument; reaching the } that terminates the main function returns a value of 0. If the return type is not compatible with int, the termination status returned to the host environment is unspecified.
main() is __cdecl__ or __stdcall__ on the major platforms.
That calling convention on x86 specifies that the return value is red from the EAX register. So, when your main() function exits, the return value is red from EAX, it's that simple.
The compilers may add boilerplate code around the main, in fact main() is rarely the real main() function, but that doesn't change the spec.
The two comments are not incompatible, they are just very different worldviews. One tells you what the standard says (that the termination status is unspecified if main does not return an int), the other tells you what usually happens (you get what happened to be in AX).
And as I've just tested, gcc doesn't return zero termination status if you reach the } at the end of main.
Calling conventions are as much a standard as the C spec.
The main() is called like a regular function, by the system thing that executes programs.
Depending on the compiler and the flags, the main() is not the real entry point of the program. It can add another entry point to do some magic, like setting the return code.
