Generic Compare

Generic functions like qsort and bsearch in C's standard library need a way to compare elements without knowing their types. The solution is a comparison function — a function you write that the generic algorithm calls whenever it needs to compare two items.

The standard signature for comparison functions is:

int compare(const void* a, const void* b);

Both parameters are const void* because the generic function doesn't know what type you're sorting. The return value follows a specific convention:

Inside your comparison function, you cast the void* pointers to the actual type and compare the values. Here's how to compare integers:

int compare_ints(const void* a, const void* b) {
    int val_a = *(const int*)a;
    int val_b = *(const int*)b;
    return val_a - val_b;
}

The subtraction trick works well for integers: if a is smaller, the result is negative; if equal, zero; if larger, positive. This function can now be passed to qsort to sort an integer array in ascending order.