如何从2个结构指针访问元素？

Question

I have a pointer & 2 different structures. The first structure has a member that is a void* pointer. Now i need to access a member of the second structure using the previous pointer to the first structure.

struct a {
  void *ptrxx;
}

struct b {
  int info;
}

struct a *ptr;

I need to do something like :

 ptr->ptrxx->info;

But i have to do some kind of typecasting to let C know that ptr->ptrxx is a pointer to struct b. How can i do this in one expression (without the need of an extra pointer) ?

PS:I'm getting error: request for member ***** in something not a structure or union. (gcc).

Answer 1

((struct b *)(ptr->ptrxx))->info

Probably don't need all those parens, but it helps to show the order of what's happenning. Take the ptrxx member of struct a (which is a void * ), cast it into a pointer to struct b , then take the info member of that.