Functions in C

float average(const int *values, int count);   // prototype (declaration)

float average(const int *values, int count) {  // definition
    long sum = 0;
    for (int i = 0; i < count; i++) sum += values[i];
    return (float)sum / count;
}

Prototypes go in .h files so other files can call the function. static on a function makes it private to its .c file.