vector & print tree

typedef struct tree_node_s {
    struct tree_node_s *left;
    struct tree_node_s *right;
    long value;
} tree_node_t;

void write_tree_to_file(FILE *fp, const tree_node_t *root)
{
    if (root == NULL) {
        fprintf(fp, "# ");
    }
    else {
        fprintf(fp, "%ld ", root->value);
        write_tree_to_file(fp, root->left);
        write_tree_to_file(fp, root->right);
    }
}

int main(int argc, char *argv[])
{
    tree_node_t n4 = { NULL, NULL, 4 };
    tree_node_t n5 = { NULL, NULL, 5 };

    tree_node_t n2 = { &n4, &n5,   2 };
    tree_node_t n3 = { NULL, NULL, 3 };

    tree_node_t root = { &n2, &n3,   1 };

    write_tree_to_file(stdout, &root);
    printf("\n");
}

//////////

 double abstand_3D(int *a, int *b, int *vector) {
     double d;

     vector[0] = b[0] - a[0];
     vector[1] = b[1] - a[1];
     vector[2] = b[2] - a[2];

     d = sqrt(pow(vector[0], 2)+ pow(vector[1], 2) + pow(vector[2], 2)) ;
     return d;
 }

int main()
{
    int vector[] = {0,  0,  0};
    int p1[3];
    int p2[3];

    double d = abstand_3D(p1, p2, vector);
    printf("\n distance = %2.2f \n", d);

    return 0;
}