Access to variables in a union within a structure

Question

Can someone explain why the first method of accessing the nested structural element inside the union in the structure works, and the second does not?

typedef struct element Node;
struct element
{
    int type;
    union
    {
        int value;
        Node *child[2];
    } u;
};
int main()
{
    Node n;
    Node *p;    
    n.type = 0;
    p = n.u.child[0];
    p->type = 10;  // 1st method
    (n.u.child[1])->type = 24;   // 2nd method
    return 0;
}

Answer 1

Try the following:

int main()
{
    Node n;
    Node *p;    
    n.type = 0;

    // allocate memory for child nodes
    n.u.child[0] = (Node *)malloc(sizeof(Node));

    if (n.u.child[0] == NULL)
    {
        return 1;
    }

    n.u.child[1] = (Node *)malloc(sizeof(Node));

    if (n.u.child[1] == NULL)
    {
        free(n.u.child[0]);
        return 1;
    }

    p = n.u.child[0];
    p->type = 10;  // 1st method
    (n.u.child[1])->type = 24;   // 2nd method

    // release dynamically allocated memory
    free(n.u.child[0]);
    free(n.u.child[1]);

    return 0;
}

NOTE. Do not modify n.u.valueNode if you have already assigned its child [] pointers. You will overwrite one of the pointers and skip this memory, as well as crash, if after that you try to access the child [] array. Unions are complex - it’s best to avoid this kind of arrangement.

Answer 2

, , , [0] [1]. ( , "1- " .)

Answer 3

, union.

. . , , -, , , .

, à la C99:

Node n = { .type = 0 };

Node n = { 0 };

à la C89, . , 0, . .