Question

What is the correct way, according to the latest C standard, to define functions without parameters: int main() or int main(void)?

Answer 1

Both forms of definition are valid (the one without void is an invalid prototype and an incomplete (albeit valid) declaration).

The form int main(void) { /* whetever */ } also provides a prototype for the function.
The form int main() { /* whatever */ } does not provide a prototype (and the compiler cannot check if it is called correctly).

See the Standard (PDF)

6.7.5.3/14

An empty list in a function declarator that is part of a definition of that function specifies that the function has no parameters.

difference between definition: int main() { /* whatever */ }
and declaration: int main();
and prototype: int main(void);.

The definition does not provide a prototype;
the declaration is valid but specifies no information about the number or types of parameters;
the prototype is ok and compatible with the definition.