A function is an independent piece of code that takes input variables, performs a specific computation, and returns a result. Functions help break down large programs into smaller, manageable, and reusable modules. This makes code easier to maintain, debug, and extend.

Defining a function involves specifying its characteristics in a function prototype. The prototype describes the return type, function name, and the datatypes of input variables:

  return_type Function_name(datatype1, datatype2, ...);
          

For example, a function to compute the total payable amount after applying sales tax may have the following prototype:

  float compute_total(float subtotal, float tax_rate);
          

The function definition specifies how the computation is performed:

  float compute_total(float subtotal, float tax_rate) {
                  float total;
                  total = subtotal * (1 + (tax_rate / 100));
                  return total;
            }
          

Here, the function accepts two float variables as input and returns a float value. The function can be called from the main program as:

  total = compute_total(1000, 8);
          

A complete C program using functions typically looks like:

  // Function prototypes
            float compute_total(float, float);
          
            int main() {
                  float subtotal = 1000;
                  float tax_rate = 8;
                  float total = compute_total(subtotal, tax_rate);
                  printf("Total payable: %f\n", total);
                  return 0;
            }
          
            // Function definitions
            float compute_total(float subtotal, float tax_rate) {
                  float total = subtotal * (1 + (tax_rate / 100));
                  return total;
            }