To access some structures (such as GValue) I manually allocate the correct size with cmalloc, and to access the structure i use low level functions (such as inc_pt) wich make assumptions on the size of the structure. I program on a 32 bit machine where pointers are smaller than on a 64 bit machine, so this could (and probably is) a problem. I definetly need a better way to access C structures, but this would mean to extend Arc FFI capabilities.
Edit: i've tried mzscheme 360 and it works. The problem then is with the 64 bit.
