The main function calls two other functions
The code in the main function in Listing 2 calls two other functions:
Before - The hello function is physically defined ahead of the main function in Listing 2.
A function prototype is not required for the main function to call the hello function.
After - The goodbye function is physically defined after of the main function in Listing 2.
Necessary to provide a function prototype to declare the goodbye function in Listing 2 in a location that is physically ahead of the definition of the main function.
Otherwise, the main function would be unable to call the goodbye function.