Re: [Chicken-users] Re: object-evict, string ports, safe-foreign-wrapper, foreign-primitive, Cheney on the Victoria Line, etc (was: What happens to a (non-simple) Scheme object sent to a foreign function?)

[felix winkelmann]

On 2/5/07, Tony Sidaway <address@hidden> wrote:
> >
> > Would it be best to write something to do that in C as a
> > foreign-primitive?  Basically a C function, declared
> > foreign-primitive, that takes a C-pointer and an int, and allocates a
> > byte vector of the appropriate size.  Presumably this would eat into
> > the nursery until such time as a minor garbage collection takes place.
>
> I suppose I mean something like this:
>
> (define produce-byte-vector
>   (foreign-primitive scheme-object
>     ((int size) (c-pointer data))
>     "C_word *p =
> C_alloc(sizeof(C_header)+C_align(size));C_return(C_bytevector(&p,
> size, data));"))
>
> I expect I've got the arithmetic wrong, but this is the basic idea.
>
> This is a nice easy implementation because you can then use
> (byte-vector->u8vector), (byte-vector->s16vector) or whatever you want
> to coerce the resulting byte vector to whatever you want.

Yes, that should work (with the caveat you mentioned: if the size of the
data is large or the nursery is already used up, the latter might not be
sufficiently big). An alternative is to create a suitably sized
byte-vector beforehand
(in Scheme, using the heap) and copying the data inside a block of foreign
code (or use "move-memory!").


cheers,
felix