Ada to C Interface: Passing Pointers Via System.Address

["ReversedBias" <thehumblizer@hotmail.com>]

I am relatively new to Ada.  I am constructing an API so that my Ada
clients can talk to my C code.  My C functions return pointers to data
structures (very large structures).  I need to then pass the pointer to
the data struct to the Ada code and back into other C functions.  There is
no interaction with the data structure in the Ada code except to pass the
pointer from one C function to another.  I have been trying to use the C
interface pragma and return the pointers from C into an Ada System.Address
and then pass that System.Address into the another pragma back down to the
C functions.  My hope is that the C will be able to type cast the address
back into the correct pointer type.  So far I have not been able to get
this to work properly.  Please advise on another approach or post an
example of how I could do this using the System.Address

Thanks,