GATE CS 2017 Set 1 Q.8 (CS Q.8): Pointers, Arrays and Strings

Consider the C code fragment given below.

typedef struct node {
    int data;
    struct 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

append list $m$ to the end of list $n$ for all inputs.

either cause a null pointer dereference or append list $m$ to the end of list $n$ .

cause a null pointer dereference for all inputs.

append list $n$ to the end of list $m$ for all inputs.

Official answer and solution

Official answer: B
From: Official GATE 2017 answer key

Solution

Trace the behavior of join(m, n) under both empty and non-empty list inputs for $n$ .

Consider the function:

void join(node* m, node* n) {
    node* p = n;
    while (p->next != NULL) {
        p = p->next;
    }
    p->next = m;
}

Case 1: $n$ is an empty list ( $n == \text{NULL}$ ). Then p = n becomes p = NULL. When the while condition checks p->next != NULL, it attempts to dereference a NULL pointer (NULL->next), resulting in a runtime null pointer dereference error (segmentation fault).

Case 2: $n$ is a non-empty list ( $n \ne \text{NULL}$ ). The pointer p advances until p->next == NULL, which is the last node of list $n$ . Then p->next = m; sets the next pointer of the last node of $n$ to point to the head of list $m$ . This successfully appends list $m$ to the end of list $n$ .

Therefore, invocation of join will either cause a null pointer dereference (when $n$ is NULL) or append list $m$ to the end of list $n$ (when $n$ is non-empty).

Final answer

The answer is the one in the official GATE answer key. The solution is this site's own explanation.

Source

Paper: GATE 2017 Computer Science and Information Technology (CS) Session 1
Question: 8 of 65, printed as CS Q.8
Section: Computer Science and Information Technology

Report an error in this question

GATE questions are the work of the organising institutes. This site is independent and is not affiliated with them.

Question

Solution

Source