[MLton-user] FFI and pointer relocation

[Matthew Fluet]

On Mon, 26 Nov 2007, Dave Herman wrote:
>>  GC may move the ref cell.  As a rule of thumb (unless you really know
>>  what you are doing), any data you wish to modify on the C side should
>>  be allocated from the C heap.  If you wish this stuff to work from
>>  both SML/NJ and MLton, I recommend that you use ML-NLFFI (the learning
>>  curve is a bit steep, but it works).
>
> Yeah, I've briefly played with ML-NLFFI before but MLton's native FFI is so 
> easy to use that I've stuck with that for now.
>
> For now I'll just allocate the cell in C and export a C function to check if 
> the cell has changed.

If there is only one cell (which can be statically allocated), then using 
MLton's  _address  (with the MLton.Pointer structure) or  _symbol  FFI may 
be more efficient.  There is a slight overhead in making a C call, and if 
you are simply checking the cell's contents, then the above can all be 
done without leaving (the generated) ML code.  See
   http://mlton.org/ForeignFunctionInterfaceSyntax
for the syntax.