Question

Possible Duplicate:
undefined C struct forward declaration

How is it possible to declare a pointer to structure even when I do not declare a structure?

#include<stdio.h>

int main(){
    struct s{
     struct p *ptr;
   }; 
}

Why does the above compile successfully?

Answer 1

It’s possible because the compiler doesn’t need to know anything about the structure if it only deals with a pointer to it.

This is a commonly used technique and is usually called an “opaque pointer”.