Generic Quick Sort Algorithm in C

Quicksort
-----
 int partition(Item a[], int l, int r);
void quicksort(Item a[], int l, int r)
  { int i;
    if (r <= l) return;
    i = partition(a, l, r);
    quicksort(a, l, i-1);
    quicksort(a, i+1, r);
  }
-----
int partition(Item a[], int l, int r)
  { int i = l-1, j = r; Item v = a[r];
    for (;;)
      { 
        while (less(a[++i], v)) ;
        while (less(v, a[--j])) if (j == l) break;
        if (i >= j) break;
        exch(a[i], a[j]);
      }
    exch(a[i], a[r]);
    return i;
  }
-----
#define push2(A, B)  push(B); push(A);
void quicksort(Item a[], int l, int r)
  { int i;
    stackinit(); push2(l, r);
    while (!stackempty())
      {
        l = pop(); r = pop(); 
        if (r <= l) continue;
        i = partition(a, l, r);
        if (i-l > r-i)
          { push2(l, i-1); push2(i+1, r); }
        else
          { push2(i+1, r); push2(l, i-1); }
      }
  }
-----
#define M 10
void quicksort(Item a[], int l, int r)
  { int i; 
    if (r-l <= M) return;
    exch(a[(l+r)/2], a[r-1]);
    compexch(a[l], a[r-1]); 
      compexch(a[l], a[r]); 
        compexch(a[r-1], a[r]);
    i = partition(a, l+1, r-1);
    quicksort(a, l, i-1);
    quicksort(a, i+1, r);
  } 
void sort(Item a[], int l, int r)
  { 
    quicksort(a, l, r);
    insertion(a, l, r);
  }
-----
#define eq(A, B) (!less(A, B) && !less(B, A))
void quicksort(Item a[], int l, int r)
  { int i, j, k, p, q; Item v;
    if (r <= l) return;
    v = a[r]; i = l-1; j = r; p = l-1; q = r;
    for (;;)
      { 
        while (less(a[++i], v)) ;
        while (less(v, a[--j])) if (j == l) break;
        if (i >= j) break;
        exch(a[i], a[j]);
        if (eq(a[i], v)) { p++; exch(a[p], a[i]); }
        if (eq(v, a[j])) { q--; exch(a[q], a[j]); }
      }
    exch(a[i], a[r]); j = i-1; i = i+1;
    for (k = l  ; k < p; k++, j--) exch(a[k], a[j]);
    for (k = r-1; k > q; k--, i++) exch(a[k], a[i]);
    quicksort(a, l, j);
    quicksort(a, i, r); 
  }
-----
select(Item a[], int l, int r, int k)
  { int i;
    if (r <= l) return;
    i = partition(a, l, r);
    if (i > k) select(a, l, i-1, k);
    if (i < k) select(a, i+1, r, k);
  }
-----
select(Item a[], int l, int r, int k)
  { 
    while (r > l)
      { int i = partition(a, l, r);
        if (i >= k) r = i-1;
        if (i <= k) l = i+1;
      } 
  }