You must define your own implementation of the abstract data type for the heap. A C header file describing this abstract data type is provided in rt_heap.h. You must provide the interior definition of the structure so that the other functions can find the heap data. Typical contents are given in Example 5.17.

struct __Heap_Descriptor {	
  void *my_first_free_block;
  void *my_heap_limit;
}

Your heap descriptor is set by __Heap_Initialize() and is passed to the other heap functions, for example __Heap_Alloc() and __Heap_Free().