Leave other errors or inefficiency here and there in the code. I am a beginner and this is a simple queue using linked list program. I did some debug on gdb and found that there is a segmentation fault at function call create_queue.

#include<stdio.h>
#include<malloc.h>
struct node{
    int data;
    struct node*next;
};
typedef struct node node;

struct queue{
    struct node* front;
    struct node* rear;
};

struct queue* q;
void create_queue(struct queue*);
struct queue* insert(struct queue*, int);
struct queue* delete_element(struct queue*);
struct queue* display(struct queue*);
int peek(struct queue*);

int main(){
    int val, choice;
    create_queue(q);
    do{
        printf("\nChoose an operation :\n1. Insert\n2. Delete\n3. Peek\n4. Display\n5. Exit\n->");
        scanf("%d", &choice);
        switch(choice){
            case 1:
            printf("Enter the element to insert: \n");
            scanf("%d", &val);
            q=insert(q,val);
            break;

            case 2:
            q=delete_element(q);
            break;

            case 3:
            val =peek(q);
            if(val!=-1)
            printf("First number : %d", val);
            break;

            case 4:
            q=display(q);
            break;

            case 5:
            break;

            default:
            printf("Invalid choice !");
            break;
        }
    }while(choice!=5);
    return 0;
}
void create_queue(struct queue *q){
    q->rear= NULL;
    q->front= NULL;
}
struct queue * insert(struct queue *q, int val){
    node * ptr=(node *)malloc(sizeof(node));
    ptr->data=val;
    if(q->front == NULL){
        q->front = ptr;
        q->rear = ptr;
        q->front->next= q->rear->next = NULL;
    }
    else{
        q->rear->next = ptr;
        q->rear = ptr;
        q->rear->next = NULL;
    }
    return q;
}
struct queue * display(struct queue *q){
    node * ptr;
    ptr = q->front;
    if(ptr == NULL){
        printf("queue is empty !");
    }
    else {
        printf("\n");
        while(ptr!=q->rear){
            printf("%d \t ", ptr->data);
            ptr = ptr->next;
        }
        printf("%d \t ", ptr->data);
    }
    return q;
}
struct queue* delete_element(struct queue *q){
    node* ptr;
    ptr = q->front;
    if(q->front==NULL)
    printf("Underflow !");
    else{
        q->front = q->front->next;
        printf("Deleting  %d", ptr->data);
        free(ptr);

    }
    return q;
}
int peek(struct queue *q){
    if(q->front == NULL ){
        printf("queue is empty !");
        return -1;
    }
    else 
    return q->front->data;
}

Also is there an easier way to find segmentation faults and fixing them ?

CodePudding user response：

Your global variable struct queue *q is initialized to NULL, and in create_queue() you dereference q->rear = .. and q->front = ... which cause the segfault. You could make it struct queue q instead and then call it with create_queue(&q). Make q a local variable for good measure, too:

int main() {
    int val, choice;
    struct queue q;
    create_queue(&q);
    do{
        printf("\nChoose an operation :\n1. Insert\n2. Delete\n3. Peek\n4. Display\n5. Exit\n->");
        scanf("%d", &choice);
        switch(choice){
            case 1:
                printf("Enter the element to insert: \n");
                scanf("%d", &val);
                insert(&q,val);
                break;

            case 2:
                delete_element(&q);
                break;

            case 3:
                val = peek(&q);
                if(val!=-1)
                    printf("First number : %d", val);
                break;

            case 4:
                display(&q);
                break;

            case 5:
                break;

            default:
                printf("Invalid choice !");
                break;
        }
    }while(choice!=5);
    return 0;
}