C questions in placement tests and university exams are rarely about syntax alone. Most of what you will find here is output prediction, pointer arithmetic, arrays and strings, structures and unions, storage classes, recursion, and file handling. The explanations walk through memory behaviour step by step, which is usually where the confusion sits rather than in the code itself.
"Hello World" is 11 bytes (including null terminator). Writing to 10-byte buffer causes buffer overflow, corrupting adjacent memory.
Q.66Hard
For implementing a dynamic stack in competitive programming, which memory management approach is best?
Answer: B
Single allocation with index tracking minimizes malloc/free overhead, suitable for competitive programming time constraints.
Q.67Easy
Which function in C is used to allocate memory dynamically at runtime?
Answer: A
malloc() is the standard C library function for dynamic memory allocation. It takes the number of bytes to allocate as an argument and returns a pointer to the allocated memory.
Q.68Easy
Which header file must be included to use malloc() and free() functions?
Answer: A
<stdlib.h> is the standard header file containing declarations for malloc(), calloc(), realloc(), and free() functions.
Q.69Easy
What happens when malloc() fails to allocate memory?
Answer: A
When malloc() cannot allocate the requested memory, it returns NULL. The programmer must check for NULL before using the pointer to avoid undefined behavior.
Q.70Medium
In a competitive programming problem, you need to allocate a 2D array of size 5x5 dynamically. Which approach is most efficient?
Answer: A
Allocating a single contiguous block is more cache-efficient and requires only one malloc() call, making it better for competitive programming where memory access speed matters.
Q.71Medium
What is the critical issue with this code snippet? int *ptr = (int*)malloc(10); ptr[11] = 5;
Answer: A
malloc(10) allocates 10 bytes, but accessing ptr[11] goes beyond the allocated boundary, causing a buffer overflow which leads to undefined behavior and potential segmentation fault.
Q.72Medium
Which of the following is a memory leak?
Answer: A
In option A, the first malloc(100) block becomes inaccessible after p is reassigned, but it's not freed, causing a memory leak. The original 100 bytes are lost.
Q.73Medium
In the 2024-2025 competitive exam pattern, a dynamic array needs to grow from 100 to 200 elements. Using realloc(), what should be the concern?
Answer: A
If realloc() needs to relocate the memory block to a different address, all old pointers to the array become invalid. Only the returned pointer should be used.
Q.74Easy
What is the result of calling free() on a NULL pointer?
Answer: A
Calling free(NULL) is safe in C and does nothing. This is by design, allowing you to write code like free(ptr) without checking if ptr is NULL first.
Q.75Medium
Which scenario represents a use-after-free error?
Answer: A
In option A, after free(p), the memory is deallocated. Accessing it with p[0] = 5 is a use-after-free error, leading to undefined behavior.
Q.76Medium
For storing variable-length strings dynamically in a competitive program, what's the safest approach?
Answer: A
Strings in C are null-terminated, so allocating strlen(s)+1 bytes ensures space for the null terminator. This is the standard safe approach.
Q.77Medium
What potential issue occurs with this code in a large-scale system? while(1) { int *arr = malloc(1000000); }
Answer: A
This creates an infinite loop that allocates 1MB each iteration without freeing, rapidly exhausting available memory. This is a classic memory leak scenario.
Q.78Medium
In graph algorithms with dynamic adjacency lists, what's the memory requirement for a sparse graph with V vertices and E edges?
Answer: A
Using dynamic adjacency lists, you allocate V list pointers and E edge nodes total, giving O(V + E) space complexity, which is optimal for sparse graphs.
Q.79Medium
What is the advantage of dynamic allocation over static allocation in competitive programming?
Answer: A
Dynamic allocation allows using memory proportional to actual input size, preventing TLE from stack overflow with large static arrays and enabling solutions for variable-sized problems.
Q.80Medium
Which function should be used to allocate memory for an array of 50 structures of size 'sizeof(struct Node)' and initialize to zero?
Answer: A
calloc(50, sizeof(struct Node)) allocates memory for 50 elements of struct Node size and initializes all bytes to zero, which is ideal when you need initialized memory.
