Using qsort / casting in C

Question

I'm attempting to use qsort() to sort an array of mine, but coming up with a weird error...

error: passing argument 4 of ‘qsort’ from incompatible pointer type
/usr/include/stdlib.h:761: note: expected ‘__compar_fn_t’ but argument is of 
type ‘int (*)(struct Record *, struct Record *)’

Here is my function call:

qsort(arr, (sizeof(arr)/sizeof(struct Record)), sizeof(struct Record), compare);

And here is my compare method:

int compare (struct Record *a, struct Record *b){

return (a->key - b->key);
}

I know this is likely a casting issue - but I thought I could just pass the name of the comparative function itself..?

Answer 1

If you look, the error is quite clear: __compar_fn_t has type int(*)(const void*, const void*) but your function is int(*)(struct Record*, struct Record*). Quicksort doesn't know about the type of your array, so you have to use void* for your comparator arguments and then cast them to the appropriate types inside it:

int compare (const void *a, const void *b){
    struct Record const* a_r = (struct Record const*)a,
                       * b_r = (struct Record const*)b;

    return (a_r->key - b_r->key);
}

Answer 2

int compare (const void * a, const void * b)
{
  return ( *(struct Record*)a -  *(struct Record*)b );
}