GATE CS 2021 Set 2 Q.45 (CS Q.35): Pointers, Arrays and Strings

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?

Upon execution, the program creates a linked-list of five nodes.

Upon execution, the program goes into an infinite loop.

It has a missing return which will be reported as an error by the compiler.

It dereferences an uninitialized pointer that may result in a run-time error.

Official answer and solution

Official answer: D
From: Official GATE 2021 answer key

Solution

Trace the dynamic memory allocations and pointer traversals in the C program.

In main(), the program initializes a linked list:

boxE = head = malloc(...) creates the first node with value = 0.
The loop for (index = 1; index <= 3; index++) allocates 3 more nodes (with values 1, 2, and 3), linking them via boxE->next = boxN; boxE = boxN;.
Note that after the loop, the last node's next pointer boxE->next is NEVER set to NULL. Since memory from malloc contains uninitialized indeterminate values, the last node's next pointer holds garbage.

Next, the traversal loop executes:
for (index = 0; index <= 3; index++)

index = 0: prints node 0, sets head = head->next (node 1), prints node 1.
index = 1: prints node 1, sets head = head->next (node 2), prints node 2.
index = 2: prints node 2, sets head = head->next (node 3), prints node 3.
index = 3: prints node 3, then executes head = head->next (dereferencing the uninitialized next pointer of node 3), followed by head->value.

Because the next pointer of the fourth node was never initialized to NULL or valid memory, dereferencing it causes undefined behavior, typically triggering a segmentation fault (run-time error). This matches statement D.

Final answer

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

Source

Paper: GATE 2021 Computer Science and Information Technology - Session 2 (CS2)
Question: 45 of 65, printed as CS Q.35
Section: Computer Science and Information Technology

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