unable to create new node where head pointer points to in c? - c

Question

say I have the following code for inserting a node at the end to a linkedlist:
int main() {
struct Node* head = NULL;
newNode(head, 1);
newNode(head, 2);
newNode(head, 3);
print(head);
return 0;
}
void newNode(struct Node* head, int val) {
struct Node* curr_p = head;
struct Node* new_p = malloc(sizeof(struct Node));
new_p->data = val;
new_p->next = NULL;
if (head == NULL) {
curr_p = new_p;
// printf("head %d \n", head->data);
}
else{
while (curr_p->next != NULL) {
curr_p = curr_p->next;
}
curr_p->next = new_p;
}
}
void print(struct Node* head) {
struct Node* curr_p = head;
while (curr_p != NULL) {
printf("%d\n", curr_p->data);
curr_p = curr_p->next;
}
}
It appears what causes the error is in the if statement block where head == NULL, the head node pointer seems to unable to point to new node. I always ended up with a segmentation fault. any reason for this?

Answer 1

I guess that your fault is that you are passing the head pointer in value not in reference so the code doesn't change it's value because it just copies the pointer
so your code should look like this
newNode(struct Node** head , int val ) {
struct Node* curr_p = *head;
/*
*/
if(curr_p == NULL ) {
*head = new_p;
}/*....*/
}
and in the main
newNode(&head,1)
and the finale code would be
#include <stdio.h>
#include <stdlib.h>
struct Node{
int data;
struct Node * next;
};
void newNode(struct Node** head, int val) {
struct Node* curr_p = *head;
struct Node* new_p =malloc(sizeof(struct Node));
new_p->data = val;
new_p->next = NULL;
if ( curr_p == NULL) {
*head = new_p;
// printf("head %d \n", head->data);
}
else{
while (curr_p->next != NULL) {
curr_p = curr_p->next;
}
curr_p->next = new_p;
}
}
void print(struct Node* head) {
struct Node* curr_p = head;
while (curr_p != NULL) {
printf("%d\n", curr_p->data);
curr_p = curr_p->next;
}
}
int main() {
struct Node* head = NULL;
newNode(&head, 1);
newNode(&head, 2);
newNode(&head, 3);
print(head);
return 0;
}
and actually you don't have to use curr_p in the print function because the head pointer in the print function it's just a copy of the head pointer of the main function so it wouldn't change it's value if you do something like head = head->next ; in the print function .

Answer 2

You have not specified that where you are getting segmentation fault but my strong guess is that you would be getting at - printf("head %d \n", head->data);. Why? Because with your present code head is NULL (that's what you are passing from your main method) and then you try to de-reference a NULL then you WILL segmentation fault.
You must only de-reference a valid pointer. If the pointer variable is holding NULL or some uninitialized/default value then de-referencing it will result in segmentation fault.
Below is the fixed IF block.
if (head == NULL) {
head = new_p;
printf("head %d \n", head->data);
}