链表-Free（）节点导致程序崩溃

Question

I am new to linked lists in c and wrote a little program in c, creating nodes and when I try to free the allocated memory my program crashes. I don't know what is causing this error, so I hope you can help me. My code:

#include <stdio.h>
#include <stdlib.h>
#include <ctype.h>
typedef struct list
{
    int data;
    struct list *link;
} node;
node *createnode();
int main()
{
    node *a = createnode(); //create first node
    node *start = a;
    printf("Value of first node: ");
    scanf("%d", &a->data);
    for(int j = 0; j<3; j++) //create three nodes
    {
        a->link = createnode();
        a = a -> link;
        printf("Value of node %d: ", j+1);
        scanf("%d", &a->data);
        if(j==2)
        {
            a -> link = NULL;
        }
    }
    a = start;
    while(a != NULL)
    {
        printf("%d -> ", a->data);
        a = a -> link;
        if(a==NULL)
        {
            printf("null");
        }
    }
    a = start;
    while(a != NULL)
    {
        a = start;
        start = a -> link;
        free(a);
    }



    return 0;
}
node *createnode()
{
    return malloc(sizeof(node));
}

Answer 1

while(a != NULL)
{
    a = start;
    start = a -> link;
    free(a);
}

must be

while(start != NULL)
{
    a = start;
    start = a -> link;
    free(a);
}

and of course a = start; just before is useless

{edit}

additional remark, to do if(j==2) { a -> link = NULL; } if(j==2) { a -> link = NULL; } is a bad choice, because just after you test again the value of j and if you have to change the number of loops you have to do 2 changes. Better to remove it and to add a -> link = NULL; directly after the loop (no test needed)

Answer 2

Replace this:

    a = start;
    while(a != NULL)
    {
        a = start;
        start = a -> link;
        free(a);
    }

with:

    while(start != NULL)
    {
        a = start;
        start = a -> link;
        free(a);
    }

In the original loop, a was being set to the new value without being tested.