Sorting an array of struct pointers using qsort

I'm getting weird results from trying to use qsort on this array of structs.

I have this struct:

struct access_data{
    int sector;
    int arrival_time;
    int checked;
    int processed;
};

I construct an array of access_data pointers from a file such that they are sorted by arrival_time, but I need to sort them by sector later, so I have the following:

int compare_data(const void* a, const void* b){
    if (((access_data*)a)->sector < ((access_data*)b)->sector)
        return 1;
    else if (((access_data*)a)->sector > ((access_data*)b)->sector)
        return -1;
    else
        return 0;
}

void scan(access_data* data[], int len, int sec_to_sec_seek){
    qsort(data, len, sizeof(access_data*), &compare_data);

    show_data(data, len);
}

show_data simply prints the data, but I get the following on a sample input; again, sorted already by arrival time:

data[0]: arrival_time: 7, sector: 3
data[1]: arrival_time: 6, sector: 8
data[2]: arrival_time: 5, sector: 6
data[3]: arrival_time: 4, sector: 5
data[4]: arrival_time: 3, sector: 12
data[5]: arrival_time: 2, sector: 10
data[6]: arrival_time: 1, sector: 1
data[7]: arrival_time: 0, sector: 2

It is simply not sorting by sector, but by reverse arrival time. I'm really at a complete loss at to what could be causing this behavior.

Your code suggests that you are actually trying sort an array of pointers to struct.

In this case you are missing a level of indirection. You also have your directions reversed.

Your compare_data routine would be fine for reverse sorting an array of structs, but you wish to sort an array of pointers based on what they point to.

int compare_pointed_to_data(const void* a, const void* b) {
    // a is a pointer into the array of pointers
    struct access_data *ptr_to_left_struct = *(access_data**)a;
    struct access_data *ptr_to_right_struct = *(access_data**)b;

    if ( ptr_to_left_struct->sector < ptr_to_right_struct->sector)
        return -1;
    else if (ptr_to_left_struct->sector > ptr_to_right_struct->sector)
        return 1;
    else
        return 0;
}