Function prototype in C is used by the compiler to ensure
whether the function call matches the return type
and the correct number of arguments or parameters with its data type of the called function.
In the absence of the function prototype,
a coder might call function improperly without the compiler detecting errors
that may lead to fatal execution-time errors that are difficult to detect.
return_type function_name( type argument1, type argument2, ...);
Function prototype in C programming
原文：https://www.cnblogs.com/JasperZhao/p/12897974.html