Which preprocessor directive is used to conditionally compile code based on whether a macro is defined?

The correct answer is D: Both B and C. #ifdef checks if a macro is defined, #ifndef checks if it's not defined. Both are used for conditional compilation based on macro definition status.

Which of the following is a memory leak?

The correct answer is A: int *p = malloc(100); p = malloc(50);. 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.

What is the output? void func(int *arr) { arr[0] = 100; } int main() { int a[5] = {1,2,3,4,5}; func(a); printf("%d", a[0]); }

The correct answer is B: 100. Arrays are passed by reference (as pointers). Changes made in func() affect the original array. a[0] becomes 100.

What is the difference between char str[] = "Test" and char *str = "Test"?

The correct answer is B: First is array, second is pointer; first is modifiable, second is read-only. char[] creates a modifiable array with its own storage. char* points to read-only string literal in memory.

How can you determine the size of a file in bytes using standard C functions?

The correct answer is D: All of the above methods work. Multiple methods exist: fseek/ftell repositions and returns position, stat() gets file info including size, and fstat() does similar with file descriptor. All are valid.

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C Programming
Dynamic Memory

C language from basics to advanced placement prep

53 Q 10 Topics Take Mock Test

Difficulty: All Easy Medium Hard 31–40 of 53

Topics in C Programming

All Basics & Syntax 86 Data Types & Variables 97 Control Flow 100 Functions 99 Arrays & Strings 100 Pointers 99 Structures & Unions 100 File Handling 100 Dynamic Memory 98 Preprocessor 99

Q.31 Medium Dynamic Memory

What happens if you use a pointer after calling free() on it (without reallocating)?

A Program runs normally

B Pointer becomes NULL

C Use-after-free: undefined behavior, possible crash or data corruption

D Compiler automatically prevents it

Correct Answer: C. Use-after-free: undefined behavior, possible crash or data corruption

EXPLANATION

After free(), accessing the pointer causes undefined behavior as memory is returned to the heap and may be reused.

Test

Q.32 Medium Dynamic Memory

In dynamic 2D array creation:
int **arr = (int**)malloc(m * sizeof(int*));
what does the first malloc() allocate?

A m rows of integers

B An array of m pointers to int

C m*n integers

D Memory for entire 2D array at once

Correct Answer: B. An array of m pointers to int

EXPLANATION

First malloc allocates an array of m pointers. Each pointer must be individually allocated in a loop for actual row data.

Test

Q.33 Medium Dynamic Memory

Which statement about free() is correct?

A free() must always be called exactly once per malloc()

B free() sets the pointer to NULL automatically

C Calling free() twice causes undefined behavior

D free() returns 1 on success, 0 on failure

Correct Answer: C. Calling free() twice causes undefined behavior

EXPLANATION

Double-free causes undefined behavior (segmentation fault). free() doesn't set pointer to NULL; programmer must do it explicitly.

Test

Q.34 Medium Dynamic Memory

What will be the output of this code?
int *p = (int*)malloc(5 * sizeof(int));
printf("%d", sizeof(p));

A 20

B 4 (or 8 on 64-bit systems)

C 5

D Undefined

Correct Answer: B. 4 (or 8 on 64-bit systems)

EXPLANATION

sizeof(p) returns the size of pointer itself (4 bytes on 32-bit, 8 bytes on 64-bit), not the allocated memory.

Test

Q.35 Medium Dynamic Memory

What does realloc() do if the new size is smaller than the old size?

A Increases the memory block

B Shrinks the memory block and returns a pointer

C Frees extra memory automatically

D Returns NULL

Correct Answer: B. Shrinks the memory block and returns a pointer

EXPLANATION

realloc() shrinks or expands memory. If new size is smaller, it reduces allocation and returns pointer to resized block.

Test

Q.36 Medium Dynamic Memory

Which best practice prevents memory leaks in complex programs?

A Allocate all memory at program start

B Never use dynamic allocation

C Track allocations and ensure every malloc() has a corresponding free()

D Use realloc() instead of free()

Correct Answer: C. Track allocations and ensure every malloc() has a corresponding free()

EXPLANATION

Disciplined memory management with matching alloc/free pairs and proper error handling prevents leaks.

Test

Q.37 Medium Dynamic Memory

What will happen if you try to allocate extremely large memory?
int *p = malloc(INT_MAX);

A It will always succeed

B It will return NULL on failure, and you should check for this

C It will crash immediately

D Compiler error

Correct Answer: B. It will return NULL on failure, and you should check for this

EXPLANATION

malloc() returns NULL if allocation fails (insufficient memory); always check the return value.

Test

Q.38 Medium Dynamic Memory

In the context of dynamic memory, what does 'memory fragmentation' refer to?

A Breaking memory into chunks that cannot be used efficiently

B Allocating too much memory at once

C Using calloc() instead of malloc()

D Accessing freed memory

Correct Answer: A. Breaking memory into chunks that cannot be used efficiently

EXPLANATION

Fragmentation occurs when memory is allocated and freed irregularly, leaving unused gaps that waste space.

Test

Q.39 Medium Dynamic Memory

How can you safely check if malloc() succeeded?

A Check if pointer equals 0 or NULL

B Dereference the pointer immediately

C Use sizeof() on the pointer

D Compare with void pointer

Correct Answer: A. Check if pointer equals 0 or NULL

EXPLANATION

malloc() returns NULL (pointer value 0) on failure; always check before using the pointer.

Test

Q.40 Medium Dynamic Memory

Which allocation method is most suitable for a dynamically growing linked list?

A Static array allocation

B malloc() for each node as needed

C calloc() all nodes upfront

D Stack allocation

Correct Answer: B. malloc() for each node as needed

EXPLANATION

Each node should be allocated individually with malloc() to allow dynamic growth and efficient memory usage.

Test

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Dynamic Memory