Lets see the first for loop:
for (index =1; index<=3; index++){
boxN = (struct Node *) malloc (sizeof(struct Node));
boxE -> next = boxN;
boxN -> value = index;
boxE = boxN; }
After this we get a linked list of size $4$ with head pointing to its beginning, and $\textsf{boxE}$ and $\textsf{boxN}$ pointing to the last node and the next pointer of the last node being uninitialized.
Now the second for loop will do printing as follows until the second $\textsf{printf}$ of the final iteration.
- Value at index 0 is 0
- Value at index 1 is 1
- Value at index 1 is 1
- Value at index 2 is 2
- Value at index 2 is 2
- Value at index 3 is 3
- Value at index 3 is 3
After this, the $\text{head}$ pointer being uninitialized will be having random content which gets treated as an address. So, when head -> value
happens it is basically reading data from uninitialized memory location and so can result (not saying will result because by chance the uninitialized memory can be a valid location) in runtime error.
To correct the error, we just have to add an extra line of code as given below:
for (index =1; index<=3; index++){
boxN = (struct Node *) malloc (sizeof(struct Node));
boxE -> next = boxN;
boxN -> value= index;
boxN-> next = NULL;
boxE = boxN; }
Correct option: D