Generic binary search in C

binsearch.c

/*
 * This differs from the standard function `bsearch` in the following important ways:
 *
 * 1. Parameter ordering: this version groups entities with their sizes;
 * 2. Return value type: this version returns an index, or -1 if the key isn't present. This is more useful;
 * 3. Speed: this version is likely slower in an implementation-defined way;
 * 4. Consistence: this version has not been thoroughly tested.
 *
 */

int binsearch(void *elt, size_t size, void *arr, size_t length, int (*compare)(void *, void *))
{
    size_t i = length / 2;
    char *array = arr;

    while (i < length)
    {
        int comparison = compare(array + i * size, elt);

        if (comparison == 0)
        {
            return i;
        }

        if (comparison < 0)
        {
            i += (length - i + 1) / 2;
        }
        else
        {
            length = i;
            i /= 2;
        }
    }

    return -1;
}