Is It Necessary to Return a Value in Main()?

Question

I was just writing a quick program for calculating some things when I came across the return statement/exit statement for the C program.

I declared main() to be of type int, so I would have to put in a return of an integer, or my program would not compile correctly. However, is it acceptable to make main a Boolean or even void?

I know the standard way to create a C program is to return a value so any problems can be sorted out, among other things, but wouldn't a Boolean work the same way? Also, could I get away with declaring it void and not having problems with the operating system still running my program after it has been terminated?

Thanks for any and all help.

Answer 1

The C99 standard says: (§5.1.2.2.1 Program startup)

The function called at program startup is named main. The implementation declares no prototype for this function. It shall be defined with a return type of int and with no parameters:

int main(void) { /* ... */ }

or with two parameters (referred to here as argc and argv, though any names may be used, as they are local to the function in which they are declared):

int main(int argc, char *argv[]) { /* ... */ }

or equivalent; or in some other implementation-defined manner.

So in a hosted environment, int is the only valid, standard return type. Implementations can define other entry points though.

Note that section §5.1.2.2.3 Program Termination has this:

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.

So you omitting a return from main is legal in C99, as long as your main returns an int.
(But previous versions of the C standard didn't have that exception for main - returning no value (or reaching the final } without a return statement) causes "the termination status returned to the host environment [to be] undefined.".)