simple linked list implementation logical error? - c

Question

I have implemented a linked list code, but it seems that there is somewhere a logic error in my code, can anyone help me to fix this problem?
struct Node{
int val;
struct Node *next;
};
void add(struct Node *new_node, struct Node *head){
struct Node *new_n;
if( head == NULL){
head = new_node;
}
else{
new_n = head;
while(new_n){
new_n = new_n->next;
}
new_n = new_node;
}
}
void print(struct Node*n){
while(n != NULL){
fprintf(stderr, "val:%d addr%p \t next%p\n",n->val, n, n->next);
n=n->next;
}
}
void main (){
struct Node *head;
struct Node *node;
int i ;
for( i =1; i< 5; i++){
struct Node *node = malloc(sizeof(*node));
bzero(node,sizeof(*node));
node->val = i;
node->next =NULL;
add(node, head);
}
print(head);
}
This code doesn't print any values? What are the problems with this code?
[ I used: $gcc filename.c -o filename.o]

Answer 1

I see three problems:
1) head is never initialized
Use:
struct Node *head = NULL;
2) Changes to head inside the add function does not change head in main
Try
void add(struct Node *new_node, struct Node **head){
^^
and use *head in the function and call it like add(node, &head);
3) New elements are not added to the list.
Try:
void add(struct Node *new_node, struct Node **head) {
struct Node *new_n;
if (*head == NULL) {
*head = new_node;
}
else {
new_n = *head;
while (new_n->next) { // Notice - iterate until the next pointer is NULL
new_n = new_n->next;
}
new_n->next = new_node; // Notice
}
}

Answer 2

in doing this you return a pointer to the new head node and thus can iterate the new list containing your newer element at the end of the list.
struct Node{
int val;
struct Node *next;
};
*Node add(struct Node *new_node, struct Node *head)
{
struct Node *new_n;
if( head == NULL)
{
head = new_node;
}
else
{
new_n = head;
while(new_n)
{
new_n = new_n->next;
}
new_n = new_node;
}
return new_n;
}
void print(struct Node*n){
while(n != NULL){
fprintf(stderr, "val:%d addr%p \t next%p\n",n->val, n, n->next);
n=n->next;
}
}
void main (){
struct Node *head;
struct Node *node;
int i ;
for( i =1; i< 5; i++){
struct Node *node = malloc(sizeof(*node));
bzero(node,sizeof(*node));
node->val = i;
node->next =NULL;
*head = add(node, head);
}
print(head);
}