c - Sorting an array of struct pointers using qsort

Question

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.

Answer 1

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;
}