Question

I have a class, in which i declare a static function. I register this function as the callback function for another library. Now, inside this function, i dereference the callback data to the class pointer and invoke a particular non-static member.

The invocation succeeds and the control comes into the non-static member function. But inside this function at the point where i access a member variable of the same class, i get a segmentation fault. This is strange. Can anybody help me with the possible mistake i have made and the solution?

Here is how the code looks like.

class test
{
   int count;
   int callback(const char * a, const char *b)
   {
      print(a);
      print(b);
      count++; //// HERE IS WHERE I GET THE SEGMENTATION FAULT
   }

   public:
   static int callbackwrapper(const char*a, const char *b, void *ptr)
   {
      test *p = (test *)ptr; 
      p->callback(a, b);
   }

   test():count(0) {}
   ~test() {}
   Register()
   {
      registercallback(callbackwrapper);
   }
}

Answer 1

The ptr passed to callackwrapper is most likely an invalid pointer or not an object of the class test. The invocation still succeeds because as long as you don’t access a member variable, a class method is the same as a normal C++ function. Where you access count is when you actually dereference the object, hence the segmentation fault.