return value of main() [was: need some explanation about this..]

If you want your program portable, you must use int main(). Because some compiler will complain you if you use void main() (g++).
An why you should return value from main ?? That's because OS expected that return value to determine whether that your application exit normally or not.

int main() is the Unix standard and also ANSI standard.
void main() is nonstandard.

any integer is legal. 0 means 'exited ok' while any other number means 'error'

Renaming a constant to explain what it is makes for readable code. I am all about doing that! I also like to tie constants to whatever they are used for with a namespace or class membership so they are "bound" to the stuff that uses them instead of leaving the reader wondering what all will be broken if the constant is changed (recompile of course).

first of all, you don't really have to return anything explicitly from main. When there's no explicit return statement, the compiler implicitly inserts a return 0; at the end. 0 is an agreed-upon code for success. You can return any other value between 1-127 to indicate a failure of some sort. This technique is used by launchers or parent processes that check the return status of their child process. EXIT_SUCCESS and EXIT_FAILURE are standard constants that you can use to make the code more readable but as I said, EXIT_SUCCESS is redundant.
As for the prototype: void main() has never been a standard form. In earlier versions of C, it was just main() which implies int main() (the famous defualt to int rule).