Since you need a block of memory, you should, well, allocate that block of memory. In principle, C requires certain alignment so you should also allocate it with the max alignment you might need, in this case, the size is at least 34bytes and the alignment is 4 so you should use a Int32 tuple/array that is at least 36bytes in size (i.e. 9 elements). Either Vector{Int32}(9) or Ref{NTuple{9,Int32}}() works with Ptr{Void} as the argument type.