Comment by groundzeros2015

> The closure problem can be neatly described by as “how do I get extra data to use within this qsort call?”

    _Thread_local struct {
      void *data;
      int (*compare)(const void *a, const void*, void*);
    } _qsort2_closure ; 

    static int _qsort2_helper(const void *a, const void *b) {
        return _qsort2_closure.compare(a, b, _qsort2_closure.data);
    }

    void qsort2(void *base, size_t elements, size_t width, int (*compare)(const void *a, const void*, void*), void *userData) 
    {
        _qsort2_closure.data = userData;
        _qsort2_closure.compare = compare;
        qsort(base, elements, width, _qsort2_helper);
    }