GATE CS 2017 Set 1 — Question 8

MCQ+1 / -0.33EasySingly Linked ListsLinked ListsProgramming & Data Structures

Programming & Data Structures → Linked Lists → Singly Linked Lists

Last updated 5 September 2026

Question

Consider the C code fragment given below. typedef struct node { int data; node* next; } node; void join(node* m, node* n) { node* p = n; while (p->next != NULL) { p = p->next; } p->next = m; } Assuming that m and n point to valid NULL-terminated linked lists, invocation of join will

Correct answer (B) either cause a null pointer dereference or append list m to the end of list n. Solution The function join attempts to append list m to the end of list n . 1. node* p = n; initializes p to the head of list n . 2. The loop while (p->next != NULL) is intended to traverse to the last node of n . 3. p->next = m; links the last node of n to the head of m . Scenario 1: n is not NULL (non-empty list) The code works as intended. p traverses to the last node, and m is appended. Result: m appended to n . Scenario 2: n is NULL (empty list) p becomes NULL . The condition p->next in the while loop attempts to dereference a NULL pointer, causing a crash (segmentation fault). Since n being NULL is a valid state for a "valid NULL-terminated linked list" (an empty list), the code is not safe for all inputs. Therefore, it will either cause a null pointer dereference (if n is NULL) or append list m to n (if n is not NULL). Turn this into a strength. Explore AI-powered practice and doubt support with Success Tracker.

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Correct answer (B) either cause a null pointer dereference or append list m to the end of list n. Solution The function join attempts to append list m to the end of list n . 1. node* p = n; initializes p to the head of list n . 2. The loop while (p->next != NULL) is intended to traverse to the last node of n . 3. p->next = m; links the last node of n to the head of m . Scenario 1: n is not NULL (non-empty list) The code works as intended. p traverses to the last node, and m is appended. Result: m appended to n . Scenario 2: n is NULL (empty list) p becomes NULL . The condition p->next in the while loop attempts to dereference a NULL pointer, causing a crash (segmentation fault). Since n being NULL is a valid state for a "valid NULL-terminated linked list" (an empty list), the code is not safe for all inputs. Therefore, it will either cause a null pointer dereference (if n is NULL) or append list m to n (if n is not NULL).

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