Fortran-C++ interoperability: Passing array via void pointer

Question

I roughly have the following situation. I have a C++ function which is called from Fortran code and takes a function pointer and a void pointer as arguments like this

int STDCALL FORTRAN_NAME(CPPFunction, CPPFUNCTION)(
    int (*userFunction)(const int *object,
                        const void *userFunctionUserData),
    const void *userData)
{
  // ...
    int index;
    int result;
  // ...
    result = userFunction(&index, userData);
  // ...
}

This is called from Fortran like this

! ...
DOUBLE PRECISION, ALLOCATABLE :: data(:,:)
INTEGER :: n, result

! ...

ALLOCATE(data(3,n)); data = 0.0

! ... fill data with something

result = CPPFUNCTION(FORTRANFUNCTION, data)
! ...

The Fortran function which I want to pass via the function pointer looks like

INTEGER FUNCTION FORTRANFUNCTION(idx, data)
IMPLICIT NONE

INTEGER, INTENT(IN) :: idx
DOUBLE PRECISION, INTENT(IN) :: data(*)
INTEGER :: i, offset

offset = 3 * (idx - 1)
WRITE(*,*) 'data(offset + 1) = ', data(offset + 1)
WRITE(*,*) 'data(offset + 2) = ', data(offset + 2)
WRITE(*,*) 'data(offset + 3) = ', data(offset + 3)

END FUNCTION FORTRANFUNCTION

If I start the whole thing CPPFunction seems to be correctly called, it calls FORTRANFUNCTION and I get an exception code

c0000005 ACCESS_VIOLATION

exactly in the line in FORTRANFUNCTION where the first access to the array data is done.

WRITE(*,*) 'data(offset + 1) = ', data(offset + 1)

Can somebody tell me where my mistake is? I should also mention that the C++-function is not mine. It is not impossible to change it but it would affect lots of other code. The Fortan part is completely under my control and I can do what I want with it.

Thank you.