Note that this is a change in C99 - in C90 spec just says: "A return from the initial call to the main function is equivalent to calling the exit function with

Note that this is a change in C99 - in C90 spec just says: "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. If the main function executes a return that specifies no value, the termination status returned to the host environment is undefined."