[plt-scheme] FFI and pointer manipulations

Can you pass the pointer to the front and a fixnum to the C code and
let it do the addition?

Robby

On 2/10/07, Jens Axel Søgaard <jensaxel at soegaard.net> wrote:
> Eli Barzilay wrote:
>
> >On Feb  9, Jens Axel Søgaard wrote:
> >
> >
> >>With the FFi how do I add an integer to a c-pointer?
> >>
> >>More specifically I am interested in getting a pointer
> >>into the middle of a byte-string.
> >>
> >>
> >
> >It's possible to do arbitrary conversions using the foreign interface,
> >just like you do in C -- except that you have to put something in a
> >malloced space, and reference it as something else.  No "typecast"
> >function yet.  (Partly because foreign types are not completely first
> >class, which is yet to be hacked.)  But:
> >
> >
> I'm not sure, I follow. Here is an example of what I want to do:
>
> (let ([bs (bytes 0 1 2 3 4 5)])
>   (md5 <cpointer-to-bs+2> 3))
>
> should calculate the md5 of the 3 bytes 2 3 4.
>
> I can't figure out what to write instead of <cpointer-to-bs+2>.
>
> >>I couldn't find anything on pointer arithmetic besides
> >>ptr-equal? in the FFI manual.
> >>
> >>
> >
> >This is intentional.  Having a pointer to the middle of a malloced
> >object will not work right with 3m.  In fact, I think that it may
> >cause damage even if you use it in some harmless way.  One way to deal
> >with this would be to provide a kind of pointer-with-offset, and
> >another would be to add a new kind of "dangerous pointer" type.  Both
> >will not be easy to understand and to use, and nothing is implemented
> >yet.
> >
> >
> It is the pointer-with-offset I need. In my case the C code doesn't
> store the pointer, so
> it ought to be safe.
>
> --
> Jens Axel Søgaard
>
>
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