ANN: H98 FFI Addendum 1.0, Release Candidate 9

[Manuel M T Chakravarty]

Ross Paterson <ross at soi.city.ac.uk> wrote,

> On Wed, Jun 04, 2003 at 11:09:43PM +1000, Manuel M T Chakravarty wrote:
> > StablePtr are used to export references to Haskell values to
> > C, where they are treated as abstract data.  In C one
> > traditionally uses (void *) for that purpose (see "man
> > qsort(3)").  We want to make sure HsStablePtr is not to wide
> > to be passed as an argument to C functions expecting such
> > abstract types (such as qsort(3)).
> 
> How does HsStablePtr differ from HsDouble in this respect?
> (except that the C side can do even less with
> HsStablePtr.)

HsDouble is guaranteed to be one of C's floating types;
hence, all operations on floating types can be applied on
values of type HsDouble.  If a value of type "float" is
passed to a function argument of type "double", the C
compiler will insert automatic coercion code by way of C's
argument promotion rules automatically.  We cannot rely on
such promotion rules for pointers.

> You know it somehow corresponds to a pointer on the Haskell side,
> but that doesn't seem relevant.

There is no guarantee whatsoever that HsStablePtr is a
pointer on the Haskell side.  In GHC, stable pointers are
integral indexes into a table of stable values.

Cheers,
Manuel