algorithm - Segmentation Default

Question

Creation of node:

struct Node
{
    int data;
    struct Node *link;
};

struct Node *head = NULL;

Append Function

int append()
{
    struct Node *temp;
    struct Node *p;
    temp = (struct Node *)malloc(sizeof(struct Node));
    printf("Enter the data");
    scanf("%d", &temp->data);
    
    if (head == NULL)
    {   temp->link = NULL;
        head = temp;
        
    }
    else
    {
         
        p = head;
        while (p != NULL)
        {
            p = p->link;
        }
     p->link=NULL;
    }
    return p->data;
}

Main Function

void main()
{
    int append();
    int insert();
    int insert_begin();
    int display();
    int delete ();
    int del_between();
    int k = 1, ch ,d;
    while (k)
    {
        printf("
Enter choice
");
        printf("1.Append
");
        printf("2.In between
");
        printf("3.At the beginning
");
        printf("4.Display
");
        printf("5.Delete
");
        printf("6.Delete from between
");
        printf("7.Quit
");
        scanf("%d", &ch);
        switch (ch)
        {
        case 1:
            d = append();
            printf("pushed %d",d);
            break;
        case 2:
            insert();
            break;
        case 3:
            insert_begin();
            break;
        case 4:
            display();
            break;
        case 5:
            delete ();
            break;
        case 6:
            del_between();
            break;
        case 7:
            k = 0;
            break;
        default:
            printf("wrong choice");
        }
    }
}

I have been trying to append a node at the end of a linked list but as soon as I enter the data to be added Segmentation default error occurs.

Output screen

Enter choice
1.Append
2.In between
3.At the beginning
4.Display
5.Delete
6.Delete from between
7.Quit
1
Enter the data23
Segmentation fault

.........................................................................................................

What is the meaning of Segmentation fault ?How to get rid of it? Where am I going wrong? ................................................................................................................

Thanks.

question from:https://stackoverflow.com/questions/65882918/segmentation-default

Answer 1

Segmentation fault occurs when you try to access some memory which is restricted.

The error came because in your case linked list is empty and first time when you call append function it goes to your if statement, and in your if head = temp; so you are updating your head with temp data. Further when the if condition ends you are returning the value of p return p->data; but this wont work.

As p is only initialized but with no data value thus accessing something which is not assigned giving SIGSEGV.

you can resolve the error by editing :

if (head == NULL)
{   temp->link = NULL;
    head = temp;
    return head->data;
}
else
{
     
    p = head;
    while (p != NULL)
    {
        p = p->link;
    }
 p->link=temp;
 p=temp;
 temp->link=NULL;
}
return p->data;