`bool leapyear(unsigned year);` declares a function without defining its body. Its prototype specifies a return type, `bool`, and a parameter type, `unsigned`. The parameter name may be omitted, as in `extern bool leapyear(unsigned);`; `extern` is optional here. A definition supplies the body as well as the prototype. section 7.1
In a declaration or definition, parentheses enclose parameter declarations; in a call, they enclose arguments. A function taking no parameters uses `(void)`. A function returning no value uses `void` as its return type. These are different positions with different meanings. section 7.1
extern double fbar(double); ... double fbar2 = fbar(2)/2;
The declaration lets the caller convert the integer argument `2` to `double` before calling `fbar`. Because the return type is also `double`, `fbar(2)/2` uses floating-point division. section 7.1
TAKEAWAY: A prototype determines how a function can be called. Standard-library declarations provide the same information: for example, `<string.h>` declares string operations such as `strlen` and `strcmp`. Their parameter types distinguish operations on strings from operations on character arrays more generally. section 7.4 section 6.1.5
`main` is a special case: its prototype is constrained by the C standard, though the programmer defines it. For portable programs, use `int main(void)` or `int main(int argc, char* argv[argc+1])`. The exit status returned from `main` is passed to the runtime system; `EXIT_SUCCESS` and `EXIT_FAILURE` are the portable status choices. By contrast, `[[noreturn]] void exit(int status);` declares a function that does not return to its caller. section 7.2
