如何将const void *转换为struct元素？

Question

I have my comparison function to use in qsort() like this:

int compar(const void *p, const void *q){
    interval a,b;

    *p = (interval)a
    *q = (interval)b;

    if(a.extrem[1] < b.extrem[0])
        return -1;
    if(b.extrem[1] < a.extrem[0] )
        return 1;

    return 0;
}

My structure is as follows:

typedef struct interval{
    double extrem[2];
    } interval;

I've tried many variations of "casting" in function compar, which all failed. My question, as it is apparent, how can I cast a const void* to my struct element? I know it seems to be a very basic question but I could not find a clear answer anywhere, also I'm new to this. Any help is gladly appreciated.

Answer 1

You were close...

typedef struct interval {
  double extrem[2];
} interval;

int compar(const void *p, const void *q) {
  const interval *a = p, *b = q;

  if(a->extrem[1] < b->extrem[0])
    return -1;
  if(b->extrem[1] < a->extrem[0])
    return 1;
  return 0;
}

BTW, without a single cast, this will be perfectly clean under gcc -Wall.

Update... Tavian makes a good point, this is not a transitive ordering, so your set technically has no partial order. But depending on your data and your objective, qsort may return a useful result even so.

Answer 2

The qsort() function is going to call your callback with pointers to the elements.

Assuming the array is an actual array of interval , not one of interval pointers, you'd do:

static int interval_compare(const void *a, const void *b)
{
  const interval *ia = a, *ib = b;

  if(ia->extrem[1] < ib->extrem[0]) return -1;
  return ib->extrem[1] > ia->extrem[0];
}

I don't understand the extrem indexes, but that's what you used. No casts are necessary when converting void * to a more specific type. See this answer .