[racket] math/matrix <--> FFI/array

Another FFI question:

I want to interface a C function which has the following signature:

void iauA2tf(int ndp, double angle, char *sign, int ihmsf[4])

The "char *sign" parameter is a single char, not a string, output only, and
is giving me problems.

The C code uses this parameter simply:

   *sign = (char) ( ( days >= 0.0 ) ? '+' : '-' );

If I use:

(define-sofa iauA2tf
  (_fun (ndp : _int)
        (angle : _double)
        (sign : (_ptr o _int))  ; <====
        (idmsf : (_ptr o (_array _int 4)))
        -> _void
        -> (values sign idmsf)))

It works (for a sufficiently pragmatic value of "works"), but  the returned
sign is a number composed of one good byte and 3 garbage bytes.
I can get the correct result with (bitwise-and sign 255).

I tried:

(define-sofa iauA2tf
  (_fun (ndp : _int)
        (angle : _double)
        (sign : (_ptr o _int8))  ; <====
        (idmsf : (_ptr o (_array _int 4)))
        -> _void
        -> (values sign idmsf)))

But this crashes DrRacket.

I think I must be missing something simple.  Please give me a hint.

Cheers
P.
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