GATE CS 2021 Set 2 — Question 45

MCQ+2 / -0.67MediumSingly Linked ListsLinked ListsProgramming & Data StructuresDynamic Memory AllocationC ProgrammingPointers & Pointer Arithmetic

Programming & Data Structures → C Programming → Dynamic Memory Allocation

Last updated 5 September 2026

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?

Correct answer(D) It dereferences an uninitialized pointer that may result in a run-time error.Solution1.The first loop creates a linked list with 4 nodes (values 0, 1, 2, 3). 2.boxE points to the last node (value 3). Note that boxE->next is never initialized to NULL after the loop finishes.3.The second loop iterates for index = 0, 1, 2, 3.4.In the final iteration (index = 3):printf("Value at index 3 is %d\n", 3, head->value); executes correctly (head is at node 3).head = head->next; sets head to the uninitialized next pointer of the last node.printf("Value at index 4 is %d\n", 4, head->value); attempts to dereference this uninitialized pointer, which leads to undefined behavior and typically a run-time error (segmentation fault).Turn this into a strength.Explore AI-powered practice and doubt support with Success Tracker.

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Correct answer(D) It dereferences an uninitialized pointer that may result in a run-time error.Solution1.The first loop creates a linked list with 4 nodes (values 0, 1, 2, 3). 2.boxE points to the last node (value 3). Note that boxE->next is never initialized to NULL after the loop finishes.3.The second loop iterates for index = 0, 1, 2, 3.4.In the final iteration (index = 3):printf("Value at index 3 is %d\n", 3, head->value); executes correctly (head is at node 3).head = head->next; sets head to the uninitialized next pointer of the last node.printf("Value at index 4 is %d\n", 4, head->value); attempts to dereference this uninitialized pointer, which leads to undefined behavior and typically a run-time error (segmentation fault).

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