Segmentation fault - Implementing Stack using Queues - c

Question

I'm implementing a stack using two queues. I'm quite familiar with the algorithm, and have prepared the following code where the push operation is costly:
#include <stdio.h>
#include <stdlib.h>
struct queue_struct{
int ele;
struct queue_struct *next;
};
struct Queue{
struct queue_struct *front, *rear;
};
struct Stack{
struct Queue *q1, *q2;
};
void enqueue(struct Queue *q, int x){
struct queue_struct *temp=malloc(sizeof(*temp));
temp->ele=x;
temp->next=NULL;
if(q->rear!=NULL){
q->rear->next=temp;
}
q->rear=temp;
if(q->front==NULL)
q->front=temp;
//printf("The item %d has been enqueued into the queue\n", x);
}
void dequeue(struct Queue *q){
struct queue_struct *temp=q->front;
if(temp==NULL){
//printf("The queue is already empty. No more elements can be removed!\n");
return;
}
printf("The item %d has been popped from the stack\n", temp->ele);
q->front=temp->next;
if(q->rear==temp)
q->rear=NULL;
free(temp);
}
void push(struct Stack *s, int x){
enqueue(s->q2, x);
while(!(s->q1->front==NULL)){
enqueue(s->q2, s->q1->front->ele);
dequeue(s->q1);
}
struct Queue *q=s->q1;
s->q1=s->q2;
s->q2=q;
printf("The item %d has been pushed into the stack\n", x);
}
void pop(struct Stack *s){
if(s->q1->front==NULL){
printf("The stack is already empty. No more elements can be removed!\n");
return;
}
dequeue(s->q1);
}
void display(struct Stack *s){
struct queue_struct *temp=s->q1->front;
printf("The contents of the queue are:\n");
if(temp==NULL){
printf("Nothing to be shown, the queue is empty.\n");
return;
}
for(int i=0;temp!=NULL;temp=temp->next){
if(i){
printf(" ------ \n");
}
printf("| %d |\n", temp->ele);
i=1;
}
}
int main()
{
int choice, element;
struct Stack *s=malloc(sizeof(*s));
s->q1->front=s->q1->rear=NULL;
s->q2->front=s->q2->rear=NULL;
printf("LET'S START WITH AN EMPTY STACK\n\n");
while(1){
printf("MENU\n");
printf("----\n");
printf("\t1. Push an element\n");
printf("\t2. Pop an element\n");
printf("\t3. Display stack\n");
printf("\t4. Exit\n");
printf("Enter your choice: ");
scanf("%d", &choice);
switch(choice){
case 1: printf("Enter the element to be pushed: ");
scanf("%d", &element);
push(s, element);
break;
case 2: pop(s);
break;
case 3: display(s);
break;
case 4: printf("Program terminated successfully!");
return 0;
default: printf("Invalid input");
}
}
}
However, I'm getting a Segmentation fault for the line(s): s->q1->front=s->q1->rear=NULL;. I'm not really sure why it is happening and how to fix it. Any help is appreciated.

Answer 1

You invoked undefined behavior by using values in buffer allocated via malloc() and not initialized.
You have to initialize s->q1 and s->q2 before dereferencing them.
struct Stack *s=malloc(sizeof(*s));
if (s == NULL) return 1; /* check if allocation succeeded */
s->q1 = malloc(sizeof(*s->q1)); /* allocate for s->q1 */
s->q2 = malloc(sizeof(*s->q2)); /* allocate for s->q2 */
if (s->q1 == NULL || s->q2 == NULL) return 1; /* check if allocations succeeded */
s->q1->front=s->q1->rear=NULL;
s->q2->front=s->q2->rear=NULL;