Good hash for pointers

What is a good hash function for pointers to use in portable ANSI C?
The pointers are nodes of a tree, which are read only, and I want to associate read/write data with them. So potentially a lage number of pointers,and they might be consecutively ordered if they are taken from an array, or they might be returned from repeared calls to malloc() with small allocations. Obviously I have no control over pointer size or internal representation.
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