The difference between void definition function and int, double and __ function

A function preceded by void, which cannot return any data, cannot be used, and such functions should implement all functions that should be implemented in this function. However, it is not possible to exchange data with the outside, but it is still possible to pass data through reference parameters, only when the call cannot directly accept the return value, because there is no return value.
A function preceded by an int, the return value is an integer, perhaps the result is a number of integers, or it may be a running state, a successful or failed identity, and function calls can be directly used to implement some of the functions of the return information.