Passing the function *cmp(const *void, const *void) as a parameter in C and using it to create a queue?

Question

I have an assignment from a professor that I don't fully understand. This is our specified function header:

PQueue createQueue( int (*cmp)(const void*a, const void*b) ) {

I can't find the documentation for cmp to understand what the function itself does; what does it do?

And beyond that, how can I refer to it within createQueue when I write the code to create the queue?

Answer 1

cmp is parameter of createQueue, it is a function pointer passed in by the caller. Typically cmp it will return:

• a negative integer if *a is less than *b
• 0 if *a is equal to *b
• a positive integer if *a is greater than *b

This is the convention used by the standard library qsort, which sorts an array when given some comparison function.

As a concrete example, we can get the behavior

int a = 1, b = 2, c = 2;
cmp(&a, &b); // returns < 0
cmp(&b, &c); // returns 0
cmp(&b, &a); // returns > 0

if we define cmp as

int cmp(const void * a, const void * b) {
    const int * a_int_ptr = (const int *)a;
    const int * b_int_ptr = (const int *)b;

    int a_value = *a_int_ptr;
    int b_value = *b_int_ptr;

    return a_value - b_value;
}