Question

qsort(bt->rw[t], bt->num[t], 
      sizeof(TRELLIS_ATOM *), 
      (int (*)(const void *,const void *))compare_wid);

bt->rw[t] is a pointer to struct pointer, bt->[num] is an int, I don’t understand what that fourth parameter is, except that compare_wid is a function defined somewhere as follows:

static int compare_wid( TRELLIS_ATOM* a, TRELLIS_ATOM* b )
{
   ...
   return x;
}

Answer 1

(int (*)(const void *,const void *)) means “treat what follows as a pointer to a function that takes two parameters of type const void* and returns an int“. compare_wid is indeed a function that can be treated this way.

qsort will call this function to perform comparisons between items when sorting: if the integer it returns is zero, the items are assumed to be equal, otherwise the sign of the integer is used to order them.