JD> function should look like this:
 JD> int cmp(const void *a, const void *b)
 JD> {
 JD>     struct foo *struct_a = (struct foo*)a,
 JD>                *struct_b = (struct foo*)b;
 JD> }
Well, not quite.
int cmp(const void* a, const void* b)
{
  const struct foo* left  = (const struct foo*)a;
  const struct foo* right = (const struct foo*)b;
  // remember to compare these.  :-)
  // return  0 for left > right
}
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