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What is the proper declaration of main?

I am working on my C skills and I have noticed that

int main( int argc, char *argv[] )

and

return (EXIT_SUCCESS)

instead of

int main() and return 0

Why is this?

Answer 1

If you are going to ignore the argument list, it is reasonable and sensible to use:

int main(void) { ... }

The standards bless this usage, as well as the one with arguments. You get a warning from GCC if you compile with -Wstrict-prototypes and don’t include the void, so I write the void. C++ is different here.

As for return EXIT_SUCCESS;, there seems to be little benefit to it in general; I continue to write return 0; at the end of a main() function, even though C99 permits you to omit any return there (and it then behaves as if you wrote return 0;).

ISO/IEC 9899:1999

§5.1.2.2.1 Program startup

¶1 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;⁹⁾ or in some other implementation-defined manner.

⁹⁾ Thus, int can be replaced by a typedef name defined as int, or the type of argv can be written as
char ** argv, and so on.

§5.1.2.2.3 Program termination

¶1 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;¹⁰⁾ 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.

¹⁰⁾ In accordance with §6.2.4, the lifetimes of objects with automatic storage duration declared in main
will have ended in the former case, even where they would not have in the latter.

§7.20.4.3 The exit function

¶5 Finally, control is returned to the host environment. If the value of status is zero or
EXIT_SUCCESS, an implementation-defined form of the status successful termination is
returned. If the value of status is EXIT_FAILURE, an implementation-defined form
of the status unsuccessful termination is returned. Otherwise the status returned is
implementation-defined.

Aside: Note that §5.1.2.2.3 clearly indicates that the C standard allows an implementation to permit return types for main() other than int (unlike the C++ standard, which expressly forbids a return type other than int). However, as Jens rightly points out, a non-int return type from main is only allowed if the implementation explicitly documents that it is allowed, and the documentation will probably place limits on what is allowed.