How to check if a void* pointer can be safely cast to something else?

Question

Let's say I have this function, which is part of some gui toolkit:

typedef struct _My_Struct My_Struct;
/* struct ... */

void paint_handler( void* data )
{
   if ( IS_MY_STRUCT(data) ) /* <-- can I do something like this? */
   {
      My_Struct* str = (My_Struct*) data;
   }
}

/* in main() */
My_Struct s;
signal_connect( SIGNAL_PAINT, &paint_handler, (void*) &s ); /* sent s as a void* */

Since the paint_handler will also be called by the GUI toolkit's main loop with other arguments, I cannot always be sure that the parameter I am receiving will always be a pointer to s.

Can I do something like IS_MY_STRUCT in the paint_handler function to check that the parameter I am receiving can be safely cast back to My_Struct* ?

Answer 1

Your void pointer looses all its type information, so by that alone, you cannot check if it can be cast safely. It's up to the programmer to know if a void* can be cast safely to a type.