I made a start on writing an FFI howto at: http://www.reid-consulting-uk.ltd.uk/docs/ffi.html It is very rough, doesn't answer your specific question, has many typos and omissions, etc. but some of the links in the Tips and Tricks section (section 4) might help. To answer your question directly, I think it all depends on what you're doing. Sometimes the semantics of the operations make it easy. For example, if there is any notion of object identity (and it often isn't stated whether there is or not), you usually have to keep the master copy in the C world and not duplicate. If there is no notion of identity, no modification to the object, etc., it is often easiest to keep it on the Haskell side and copy over to the C side on each operation because it's easier. There may be a performance hit in doing this depending on the number of times it is copied in each direction but I don't think we have any numbers for a range of implementations (ghc numbers may be available). Without any effort at measurement, I'd say that using malloc is pretty slow (because C's malloc is pretty slow) and that ForeignPtr's are fairly low overhead but more than just copying a small object over. -- Alastair Reid alastair@reid-consulting-uk.ltd.uk Reid Consulting (UK) Limited http://www.reid-consulting-uk.ltd.uk/alastair/