[plt-scheme] "define-cunion" prototype for FFI

Brian Chin wrote:
>
> On Nov 10, 2007, at 12:07 PM, hendrik at topoi.pooq.com wrote:
>
>> On Sat, Nov 10, 2007 at 11:01:11AM -0800, Brian Chin wrote:
>>>
>>> Correct me if my understanding is flawed, but unions are notably
>>> simpler than structs, given that most of the alignment issues can be
>>> avoided since all of the "fields" have a pointer offset of 0. In that
>>> regard, the only remaining issues is ensuring that the union type has
>>> the correct size, which should be the maximum size of it's members.
>>
>> in C, the sizeof() of a type is not the number of bytes it takes but the
>> spacing between adjacent elements if it is packed into an array.  This
>> may be different from the actual size of a value of that type.--
>
> Ah, excellent point. Instead of just using the raw type, I create as a
> new basetype a struct type with a single element (via
> make-cstruct-type), which should solve those alignment issues, unless
> there's a platform where the type with the largest sizeof will not
> have the largest aligned size.
>
It doesn't seem to match gcc.
(define bytes (make-cstruct-type (ctypes _byte 9)))

(define-cstruct _Foo
        ((y bytes)))

(define-cunion _X1
           ((z _pointer)
        (x _int)
        (y _Foo)))

Size of _X1 is 9, but gcc would say 16. I'm not sure what the rules are
for determining the size of a union but gcc does some wierd things
sometimes.

union y{
    struct{
        int x;
        int q;
    };
    struct{
        char x;
        void * p;
    };
    int q;
};
is 16 bytes but

union y{
    struct{
        int x;
        int q;
    };
    struct{
        char x;
        short f;
        short q;
        short p;
        short n;
     };
    int q;
};

is 12 bytes on a 64-bit machine.
> For those who are curious about the code, I've posted my version of
> foreign.ss here. All of the changes are near the definition to
> define-cstruct and define-cunion:
>
You forgot to provide define-cunion ;)
> http://www.cs.ucla.edu/~naerbnic/foreign.ss
>
> Have fun!
> - Brian Chin
>
>
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