Segmentation fault in C while implementing deque - c

Question

I am trying to implement a deque in C. I am learning C, so the error might seem very trivial. Here's the entire program
#include <stdio.h>
#include <stdlib.h>
typedef struct node{
int data;
struct node *prev;
struct node *next;
}Node;
Node *getNode(int data){
Node *temp = (Node *)malloc(sizeof(Node));
temp -> next = temp -> prev = NULL;
temp -> data = data;
return temp;
}
void push_right(Node **head, Node **tail, int data){
Node *newNode = getNode(data);
Node *temp = (*head);
if (temp == NULL){
*head = newNode;
}
else{
while(temp -> next != NULL){
temp = temp -> next;
}
temp -> next = newNode;
newNode -> prev = temp;
*tail = newNode;
}
}
void push_left(Node **head, Node **tail, int data){
Node *newNode = getNode(data);
Node *temp = (*head);
if (temp == NULL){
*head = newNode;
}
else{
newNode -> next = temp;
newNode -> prev = NULL;
(*head) = newNode;
}
}
void remove_right(Node **head, Node **tail, int *val){
Node *temp = (*tail);
if (temp == NULL)
{
printf("Cannot be removed doesn't point to anything\n");
return;
}
else{
*val = temp -> data;
temp = temp -> prev;
(*tail) = temp;
}
free(temp);
}
void remove_left(Node **head, Node **tail, int *val){
Node *temp = (*head);
if (temp == NULL)
{
printf("Cannot be removed doesn't point to anything\n");
return;
}
else{
*val = temp -> data;
temp = temp -> next;
(*tail) = temp;
}
free(temp);
}
void print_all(Node *head){
Node *temp = head;
printf("\n");
while(temp != NULL){
printf("%d\n",temp->data);
temp = temp -> next;
}
}
int main(int argc, char const *argv[])
{
int *val = NULL;
Node *head = NULL;
Node *tail = NULL;
for (int i = 0; i < 10; ++i){
push_right(&head, &tail,i);
push_left(&head, &tail,i);
}
remove_left(&head, &tail, val);
print_all(head);
return 0;
}
The problem seems to arise when remove_left() is called. I have spend a significant amount of time to understand where the problem is coming from but nothing seems to work. Please help.
This is the output on the Terminal screen.
lib2s-iMac:queue admin$ ./a.out
Segmentation fault: 11

Answer 1

Problem is here :
*val = temp -> data;
Val is NULL, so trying to dereference it will result in segmentation fault.
If you change val type to an int instead of a pointer to an int. And then call remove_left like this :
int main(int argc, char const *argv[])
{ int val = 0;
Node *head = NULL;
Node *tail = NULL;
for (int i = 0; i < 10; ++i){
push_right(&head, &tail,i);
push_left(&head, &tail,i);
}
remove_left(&head, &tail, &val);
print_all(head);
return 0;
}
This should work.

Answer 2

In your program have 3 main mistakes .
At int *val = NULL; Here it should be int val not int *val.
At function remove_left() . Here You should edit (*head) not (*tail).Also pointer (next,perv) are not managed properly.
Same in the case of remove_right(). Mistakes in managing pointers.
Modified Functions :-
void remove_right(Node **head, Node **tail, int *val) // modified This function.
{
Node *temp = (*tail);
if (temp == NULL)
{
printf("Cannot be removed doesn't point to anything\n");
return;
}
else
{
*val = temp->data;
temp->prev->next = NULL;
(*tail) = temp->prev;
}
free(temp);
}
void remove_left(Node **head, Node **tail, int *val) // modified This function.
{
Node *temp = (*head);
if (temp == NULL)
{
printf("Cannot be removed doesn't point to anything\n");
return;
}
else
{
*val = temp->data;
temp->next->prev = NULL;
(*head) = temp->next;
}
free(temp);
}
Modified main() :-
int main(int argc, char const *argv[])
{
int val = -1; // Modified here
Node *head = NULL;
Node *tail = NULL;
for (int i = 0; i < 10; ++i)
{
push_right(&head, &tail, i);
push_left(&head, &tail, i);
}
remove_left(&head, &tail, &val);
print_all(head);
return 0;
}
Complete code Online.
Output :-
8
7
6
5
4
3
2
1
0
0
1
2
3
4
5
6
7
8
9