Why does the program keeps giving me segmentation fault when I'm trying to make a user defined queue?

Harshit Singh

Please help me find the mistake.

I was trying to make a circular queue in which the size of the queue as well as the elements inside of the queue are user defined. The code works fine when I am not using loop but as soon as I use the loop it starts showing segmentation fault. enter image description here

#include <stdio.h>
#include <stdlib.h>

struct node{
    int data;
    struct node *next;
};
typedef struct node node;

typedef struct{
    node *head;
    node *tail;
} queue;

void createqueue(queue *q){
    q->head = NULL;
    q->tail = NULL;
    return;
}
int enqueue(queue *q, int value){
    node *newnode = malloc(sizeof(node));
    if (newnode==NULL) return 0;
    newnode->data = value;
    newnode->next = NULL;
    if(q->tail!=NULL){
        q->tail->next = newnode;
    }
    q->tail = newnode;
    q->tail->next = q->head;
    if(q->head == NULL){
        q->head=newnode;
    }
    return 1;
}

int dequeue(queue *q){
    if (q->head==NULL) return 0;
    q->head = q->head->next;
    q->tail->next = q->head;
    return 1;
}

int display(queue *q){
    node *printval;
    printval = q->head;
    if(printval==NULL) return 0;
    printf("\n");
    do
    {
        printf("%d-->",printval->data);
        printval = printval->next;
    } while (printval!=q->head);
    
}

int main(){
    queue *root;
    createqueue(root);
    printf("Enter the size of the queue");
    int size;
    scanf("%d",&size);
    for(int i=0;i<size;i++){
        int value;
        printf("Enter the value of the %d element",i);
        scanf("%d",&value);
        enqueue(root,value);
    }
    display(root);

}

SokolovVadim

In a function createqueue you use a pointer to an object for which memory was not allocated. You need to call malloc first and then set head and tail to a null.

queue* createQueue()
{
    queue* q = malloc(sizeof(queue));
    if (q != NULL) {
        q->head = NULL;
        q->tail = NULL;
    }
    return q;
}

// ...

int main()
{
    queue* q = createQueue();
    // ...
}

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