Referring to - http://www.mkssoftware.com/docs/man3/malloc.3.asp

I would be obliged to see explanation for this

"Unlike many UNIX platforms, the heap that is used by malloc(), calloc(), and realloc() is not guaranteed to be one contiguous piece of memory. ok!

Thus it is invalid to assume that all memory between two pointers returned by these functions is accessible, -- didnt get it

and it is invalid to compare pointers returned by these functions to determine the total size of the heap." -- how can i compare pointers to determine size of heap?