Question

When using realloc is the memory automatically freed? Or is it necessary to use free with realloc? Which of the following is correct?

//Situation A
ptr1 = realloc(ptr1, 3 * sizeof(int));

//Situation B
ptr1 = realloc(ptr2, 3 * sizeof(int));
free(ptr1);
ptr1 = ptr2;

Answer 1

Neither is correct. realloc() can return a pointer to newly allocated memory or NULL on error. What you should do is check the return value:

ptr1 = realloc(ptr2, 3 * sizeof(int));
if (!ptr1) {
    /* Do something here to handle the failure */
    /* ptr2 is still pointing to allocated memory, so you may need to free(ptr2) here */
}

/* Success! ptr1 is now pointing to allocated memory and ptr2 was deallocated already */
free(ptr1);