C程序，用于从链表中的升序添加和删除节点

Question

I get segmentation fault when I execute the following code. The segmentation fault occurs only when I add an item in a node whose data is greater than that of 1st node item.

In this code when I try to add 20 then I encounter a segmentation fault because 20 is greater than the item in the first node. Why does this occur and how to prevent this error?

Code:

#include <stdio.h>

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

int main()
{
    struct node *p;
    p = NULL;

    add(&p,10);
    add(&p,9);
    add(&p,1);
    add(&p,20);

    display(p);
    printf("\nNo of elements in Linked List=%d",count(p));

    delete(&p,7);
    delete(&p,4);
    delete(&p,5);
    delete(&p,9);

    display(p);
    printf("\nNo of elements in Linked List=%d",count(p));

}
/* adds a node to an ascending order linked list */
add(struct node **q,int num)
{

    struct node *r,*temp=*q;
    r = malloc(sizeof(struct node));
    r->data = num;

    /* If list is empty or if new node is to be inserted before */
    if(*q==NULL || (((*q)->data) > num))
    {
        *q = r;
        (*q)->link = temp;
    }
    else
    {
            /* traverse the list to search the position to insert the new node */
        while(temp!=NULL)
        {

            if(temp->data <=num && (temp->link->data > num || temp->link == NULL))
            {
                r->link = temp->link;
                temp->link = r;
                return;
            }
            temp = temp->link; /* go to next node */
        }

    }       
}

display(struct node *q)
{
    printf("\n");

    while(q!=NULL)
    {
        printf("%d ",q->data);
        q = q->link;
    }
}

count(struct node *q)
{
    int c=0;
    while(q!=NULL)
    {
        q = q->link;
        c++;
    }
    return c;
}

delete(struct node **q,int num)
{
    struct node *old,*temp;
    temp = *q;

    while(temp!=NULL)
    {
        if(temp->data == num)
        {
            if(temp == *q)
            {
                *q =temp->link;
                free(temp);
                return;
            }
            else
            {
                old->link = temp->link;
                free(temp);
                return;

            }   
        }
        else
        {
            old = temp;
            temp = temp->link;
        }
    }
    printf("\nElement %d not found",num); 
}

Answer 1

In your add method try changing the line:

if(temp->data <=num && (temp->link->data > num || temp->link == NULL))

to

if(temp->data <=num && (temp->link == NULL || temp->link->data > num))

Since C is short cutting boolean expressions, NULL check will be evaluated before the other operator and stop execution when true.

Answer 2

Change the line

if(temp->data <=num && (temp->link->data > num || temp->link == NULL))

into

if(temp->data <=num && (temp->link == NULL || temp->link->data > num))

That should solve your problem (it will not try to access data from a node that does not exist)