Remember that functions can't "see" variables declared inside other functions, so you'll need to make use of the return values and parameters to get information from one function and pass it to the next.
Your function prototypes do not have formal parameters, nor do your calls to the functions in main have arguments for the functions. However, in your definitions, you are stating that the functions input, processing and output should have formal parameters. You may also want to think about making some of your parameters in your definitions referenced. Also, you probably will want a return statement in your processing function. For the output function, think about why you want it to be double. A function that is not a void function should return a value.