GATE CS 2017 Set 1 Q.13 (CS Q.13): Structures, Unions and Dynamic Memory Allocation

Consider the following C code:

#include <stdio.h>
#include <stdlib.h>

int *assignval(int *x, int val) {
    *x = val;
    return x;
}

void main() {
    int *x = malloc(sizeof(int));
    if (NULL == x) return;
    x = assignval(x, 0);
    if (x) {
        x = (int *)malloc(sizeof(int));
        if (NULL == x) return;
        x = assignval(x, 10);
    }
    printf("%d\n", *x);
    free(x);
}

The code suffers from which one of the following problems:

compiler error as the return of malloc is not typecast appropriately

compiler error because the comparison should be made as x == NULL and not as shown

compiles successfully but execution may result in dangling pointer

compiles successfully but execution may result in memory leak

Official answer and solution

Official answer: D
From: Official GATE 2017 answer key

Solution

Trace the memory allocations and pointer assignments in the program to identify resource management issues.

Trace the execution of main():

int *x = malloc(sizeof(int));: A dynamic block of memory (say Block 1) is allocated on the heap, and its address is stored in x.
if (NULL == x) return;: Valid check; since allocation succeeds, execution proceeds.
x = assignval(x, 0);: Stores value 0 into Block 1 and returns x.
if (x): Since x is non-NULL, the condition is true.
Inside the if block:
x = (int *)malloc(sizeof(int));
A second block of memory (say Block 2) is allocated on the heap. The returned address is assigned to x, completely overwriting the address of Block 1!

Because no other pointer holds the address of Block 1, Block 1 is now orphaned and cannot be accessed or deallocated. This is the definition of a memory leak.

At the end of main(), free(x); only frees Block 2. Block 1 remains allocated until the process terminates. Hence, the code compiles successfully but results in a memory leak.

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: 13 of 65, printed as CS Q.13
Section: Computer Science and Information Technology

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