Question

Given two structure in c:

typedef struct _X_
{
   int   virtual_a;
   int   virtual_b;
   void *virstual_c;
   int   a;
   int   b;
   void *c;

    /* More fields to follow */
}X;

typedef struct _Y_
{
   int a;
   int b;
   void *c;

   /* Same fields as in X structure */
}Y;

Q : Is it safe to say that ?

void foo_low( Y *y )
{
   y->a = 1;
   y->b = 2;
}

 void foo( X *x )
 {
   Y *y = (Y *)(&(x->a) )

   foo_low( y );
 }

Is it standard C ? will it work on all compilers ? Is there any problem with padding ?

Answer 1

That should work. But since you need to access the same fields in two distinct ways (y->a and x->a are different), I would use union:

typedef struct _Y_
{
    int a;
    int b;
    void *c;

   /* Same fields as in X structure */
}Y;


typedef struct _X_
{
     int   virtual_a;
     int   virtual_b;
     void *virstual_c;
     Y    y_fields;     
}X;


typedef union {
    X x;
    Y y;
} Z;

Now x.virtual_a and y.a are in the same memory address.

And you can rewrite your code as follows:

void foo_low( Z *z )
{
   z->y.a = 1;
   z->y.b = 2;
}

void foo( Z *z )
{
   Z *w = z;
   w->y = z->x.y_fields;
   foo_low( w );
}

The only clumsy part is adding Y inside X.