[racket] Struct equality for foreign pointers

I am interfacing with a foreign library and have a datatype whose
representation in racket is a cpointer, where equality is ptr-equal?.
I want to make a struct encapsulating this using prop:equal+hash to
make struct equality match the foreign library's equality operator. Is
there a way to get something that I could use to make the hash
function? I wanted to get the actual pointer value, but did not see a
way of extracting that from the cpointer, only the offset.

-Eric