A function prototype declares the parameter types and return type that determine how a C function may be called. For example, `bool leapyear(unsigned year);` tells the compiler that `leapyear` receives an `unsigned` argument and returns `bool`, without providing its definition. section 7.1
bool leapyear(unsigned year);
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Read the declaration from left to right: the return type is `bool`, the function is `leapyear`, and its parameter has type `unsigned`; the semicolon marks a declaration rather than a definition. section 7.1
TAKEAWAY: prototypes make calls type-aware.
- A declaration may omit parameter names, and `extern` may be added; the compiler still needs the argument types and return type. section 7.1
- Use `void` as the parameter list for a function that accepts no arguments, or as the return type for a function that returns no value. section 7.1
- At a call such as `fbar(2)`, a visible prototype tells the compiler that `fbar` expects `double`; the `signed int` argument can therefore be converted to `2.0`, and the caller knows the result is `double`. section 7.1
- Treat `main` as a special case: portable programs use one of its two official prototypes, return `EXIT_SUCCESS` or `EXIT_FAILURE`, and may omit an explicit return at the end. section 7.2
Declaration versus definition
The definition `bool leapyear(unsigned year) { ... }` supplies the function body. The separate declaration `bool leapyear(unsigned year);` supplies the calling interface. Both agree on the parameter type and return type, but only the definition contains executable behavior. section 7.1
