On 11-Jun-2002, Alastair Reid <[EMAIL PROTECTED]> wrote: > > If I understand you correctly, your concern is that a pointer > argument/result might be passed differently depending on what it is a > pointer to. > > We already have a special case for FunPtr because they can be > different sizes from normal Ptrs (especially on a Harvard > architecture). What I think you're suggesting is that we might > need to make a special case for other pointer types too. > > This feels like a very unlikely thing to happen and I don't understand > it yet so I propose the following (btw note that I don't speak for the > whole FFI group). > > Put off acting on this until we find a platform like this. I believe > that all currently supported platyforms are not like this because if > they were we'd have noticed when we ported the compiler to this > platform. > > On finding a platform like this, we can do one of: > > 1) Define a new calling convention ccall-plus (or whatever) which is > just like the ccall convention except that the extloc strings can > contain extra information to resolve the problem. > > 2) Require any affected functions to go through C wrappers which do the > coercion. > > My feeling is that this will be viable because I suspect there > would be something very unusual about the pointers which are passed > differently (e.g., they will be pointers into I/O space or pointers > to 32-bit aligned memory) so they will be rare and easy to find.
> 3) Find a way to add the missing type information to the Haskell type. > I think this is much more than being careful to use > > Ptr (Ptr DB) > > instead of > > Ptr DB > > because I would be hugely surprised if void* and void** had different > representations. On word-addressed machines, `void *' will most likely have a different representation than `void **'. `void *' will require additional bits to distinguish which byte within the word is being pointed to, but `void **' is guaranteed to point to a word-aligned `void *', so these additional bits are not needed. On word-addressed machines, there are usually two representations for pointers: most pointers are represented using the native machine representation, but pointers of type `char *', `signed char *', `unsigned char *', and `void *' need a different (and probably larger) representation, because they can point to bytes within a word. -- Fergus Henderson <[EMAIL PROTECTED]> | "I have always known that the pursuit The University of Melbourne | of excellence is a lethal habit" WWW: <http://www.cs.mu.oz.au/~fjh> | -- the last words of T. S. Garp. _______________________________________________ FFI mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/ffi
