Question

Consider the following ANSI C program: 

#include <stdio.h> 
#include <stdlib.h> 
struct Node{ 
	int value; 
	struct Node *next; }; 
int main(){ 
	struct Node *boxE, *head, *boxN; int index = 0; 
	boxE = head = (struct Node *) malloc(sizeof(struct Node)); 
	head->value = index; 
	for (index = 1; index <= 3; index++){ 
		boxN = (struct Node *) malloc(sizeof(struct Node)); 
		boxE->next = boxN; 
		boxN->value = index; 
		boxE = boxN; } 
	for (index = 0; index <= 3; index++) { 
		printf("Value at index %d is %d\n", index, head->value); 
		head = head->next; 
		printf("Value at index %d is %d\n", index+1, head->value); } } 

Which one of the statements below is correct about the program? 
 

• A. Upon execution, the program creates a linked-list of five nodes.
• B. Upon execution, the program goes into an infinite loop.
• C. It has a missing return which will be reported as an error by the compiler.
• D. It dereferences an uninitialized pointer that may result in a run-time error.

Hint:

Always initialize the \texttt{next} pointer of the last node in a linked list to \texttt{NULL} to avoid undefined behavior.

Answer 1

The Correct Option is D

Step 1: Understand the linked-list construction.
The program allocates the first node and assigns it to both \texttt{boxE} and \texttt{head}. It then creates three more nodes inside the first \texttt{for} loop (for \texttt{index = 1} to \texttt{3}) and links them using the \texttt{next} pointer.
Thus, a total of four nodes are created, not five.

Step 2: Identify the issue with the \texttt{next pointer.}
After the loop finishes, the \texttt{next} pointer of the last node is never initialized. That is, there is no statement like: \[ \texttt{boxE->next = NULL;} \] As a result, the last node's \texttt{next} pointer contains an indeterminate (garbage) value.

Step 3: Analyze the second loop.
In the second \texttt{for} loop, the statement: \[ \texttt{head = head->next;} \] is executed repeatedly. Eventually, when \texttt{head} points to the last node, \texttt{head->next} refers to an uninitialized pointer. Dereferencing it in: \[ \texttt{head->value} \] can lead to undefined behavior or a run-time error (such as a segmentation fault).

Step 4: Eliminate incorrect options.
(A) Incorrect: Only four nodes are created.
(B) Incorrect: There is no infinite loop; both loops have fixed bounds.
(C) Incorrect: In C, reaching the end of \texttt{main} without a \texttt{return} is allowed (implicitly returns 0).

Step 5: Conclusion.
The program dereferences an uninitialized pointer, which may cause a run-time error.

Final Answer: (D)