Write an adequate function prototype for each of the following functions, so that the data types used allow their desired functionality:

A function called count_numbers that receives a number and prints all the even numbers between 0 and that number.

A function called clean_array that takes an array and sets every element to 0.0.

A function called string_reverse that receives a string and returns a new string that contains the reversed contents. Note: The original string will not be modified.

A function called avg that receives two integer variables and returns the average of both, without rounding.