array pointer change after calling FORTRAN subroutine in C++

[Qianqian Fang]

hi

I am working on a library written in F90 and compiled
by gfortran. Now I am trying to write a C++ header to
use the fortran library (.so/.a). I am very new to interfacing
C/Fortran and found a strange issue when calling the
fortran code:

Here is a sample of my header file: DBLQMRPrep
is a function defined in a fortran module:

...
#define DBLQMRPrep      __blit_blqmr_real_MOD_blqmrprep

extern void DBLQMRPrep(BLQMRSolver *qmr, int *Ap, double *Ax, int *nz);

template <class T>
class MyClass{
     DBLQMRPrep qmr;
     int *Ap;
     T *Ax;
public:
     void Prepare(int *App, T *Axx, int nz){
             Ap=App; Ax=Axx;
printf("%XL",(unsigned long)Ap);

             DBLQMRPrep(&qmr,Ap,Ax,&nz); // here I call FORTRAN

printf("%XL",(unsigned long)Ap);
     }
}

You see I put two printf in the Prepare() function,
I found that the pointer addresses for Ap/Ax
became different after calling DBLQMRPrep().

Inside the fortran function, the arrays Ap/Ax are
automatic(or dynamic) arrays. Something like:

    subroutine BLQMRPrep(this, Ap, Ax, nnz)
    implicit none

    type(BLQMRSolver), intent(inout) :: this
    integer, intent(in) :: nnz, Ap(this%n+1)
    MTYPE(kind=Kdouble), intent(in) :: Ax(nnz)
    ...

Can someone explain to me what happened after
calling DBLQMRPrep()? how can I prevent FORTRAN to
mess up with the array pointer?

thanks

Qianqian