Question

struct node{  
    struct node next;  
    int id;  
}

gives “field next has incomplete type error “.

what is wrong with this struct ?

Answer 1

When creating a self-referential data type, you need to use pointers to get around problems of circularity:

struct node;

struct node {  
    struct node * next;  
    int id;  
}

…should work, but take care to allocate memory correctly when using it.

Why a pointer? Consider this: the point of a struct definition is so that the compiler can figure out how much memory to allocate and what parts to access when you say node.id. If your node struct contains another node struct, how much memory should the compiler allocate for a given node?

By using a pointer you get around this, because the compiler knows how much space to allocate for a pointer.