Memory allocation flow - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to allocate memory for an integer pointer.

C

int *ptr = (int *)[1](sizeof(int));

Drag options to blanks, or click blank then click option'

Afree

Bmalloc

Csizeof

Dcalloc

Attempts:

3 left

💡 Hint

Common Mistakes

Using free instead of malloc causes errors because free releases memory.

Using sizeof alone does not allocate memory.

The malloc function allocates memory dynamically. Here, it allocates enough space for one integer.

2fill in blank

medium

Complete the code to release the allocated memory.

C

free([1]);

Drag options to blanks, or click blank then click option'

Aptr

BNULL

Cmalloc

Dsizeof

Attempts:

3 left

💡 Hint

Common Mistakes

Passing sizeof or malloc to free causes runtime errors.

Passing NULL to free does nothing but is not useful here.

The free function releases the memory pointed to by ptr.

3fill in blank

hard

Fix the error in the code to allocate memory for 5 integers.

C

int *arr = (int *)malloc([1] * sizeof(int));

Drag options to blanks, or click blank then click option'

Asizeof(arr)

Bsizeof(int *)

C5

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Using sizeof(arr) gives size of pointer, not the number of elements.

Using 0 allocates no memory.

To allocate memory for 5 integers, multiply 5 by the size of an integer.

4fill in blank

hard

Fill both blanks to create a loop that initializes the allocated array to zero.

C

for (int [1] = 0; [2] < 5; [1]++) {
    arr[[1]] = 0;
}

Drag options to blanks, or click blank then click option'

Ai

Bj

Ck

Dn

Attempts:

3 left

💡 Hint

Common Mistakes

Using different variable names in the loop causes errors.

Using a variable not declared causes compilation errors.

The variable i is used as the loop counter consistently in all parts of the for loop.

5fill in blank

hard

Fill all three blanks to safely allocate memory and check if allocation succeeded.

C

int *ptr = (int *)[1](sizeof(int) * [2]);
if ([3] == NULL) {
    printf("Memory allocation failed\n");
    return 1;
}

Drag options to blanks, or click blank then click option'

Amalloc

B10

Cptr

Dfree

Attempts:

3 left

💡 Hint

Common Mistakes

Using free instead of malloc causes errors.

Not checking if pointer is NULL can cause crashes.

Using wrong variable name in the if condition causes errors.

Use malloc to allocate memory for 10 integers. Then check if ptr is NULL to detect failure.

Practice

1. What does the malloc function do in C programming?

easy

A. Frees previously allocated memory

B. Allocates a block of memory on the heap

C. Declares a variable

D. Copies data from one memory location to another

Solution

1. Step 1: Understand the purpose of malloc

   malloc is used to allocate memory dynamically on the heap during program execution.

2. Step 2: Differentiate from other memory functions

   free releases memory, declaration creates variables, and copying data is done by functions like memcpy.

3. Final Answer:

   Allocates a block of memory on the heap -> Option B

4. Quick Check:

   malloc = Allocate memory [OK]

Hint: Remember: malloc = get memory, free = release memory [OK]

Common Mistakes:

• Confusing malloc with free
• Thinking malloc declares variables
• Assuming malloc copies data

2. Which of the following is the correct syntax to allocate memory for 10 integers using malloc?

easy

A. int *ptr = malloc(sizeof(int));

B. int *ptr = malloc(10);

C. int ptr = malloc(10 * sizeof(int));

D. int *ptr = malloc(10 * sizeof(int));

Solution

1. Step 1: Calculate total memory needed

   To allocate memory for 10 integers, multiply 10 by the size of an int: 10 * sizeof(int).

2. Step 2: Check pointer type and assignment

   The pointer must be of type int * and assigned the result of malloc. int *ptr = malloc(10 * sizeof(int)); matches this.

3. Final Answer:

   int *ptr = malloc(10 * sizeof(int)); -> Option D

4. Quick Check:

   Correct malloc size and pointer type = int *ptr = malloc(10 * sizeof(int)); [OK]

Hint: Multiply count by sizeof(type) for malloc size [OK]

Common Mistakes:

• Not multiplying by sizeof(int)
• Using wrong pointer type
• Allocating too little memory

3. What will be the output of this C code snippet?

int *ptr = malloc(4 * sizeof(int));
if (ptr == NULL) {
    printf("Allocation failed\n");
} else {
    ptr[0] = 10;
    ptr[1] = 20;
    printf("%d %d\n", ptr[0], ptr[1]);
    free(ptr);
}

medium

A. 10 20

B. 0 0

C. Allocation failed

D. Compilation error

Solution

1. Step 1: Check memory allocation and NULL condition

   The code allocates memory for 4 integers. If allocation fails, it prints "Allocation failed". Otherwise, it proceeds.

2. Step 2: Assign and print values

   Since allocation likely succeeds, ptr[0] = 10 and ptr[1] = 20 are set, then printed as "10 20".

3. Final Answer:

   10 20 -> Option A

4. Quick Check:

   Successful malloc prints assigned values [OK]

Hint: Check if malloc returns NULL before using pointer [OK]

Common Mistakes:

• Ignoring NULL check
• Assuming uninitialized values print zero
• Forgetting to free memory

4. Identify the error in this code snippet related to memory allocation:

char *str = malloc(5);
strcpy(str, "Hello World");
free(str);

medium

A. Memory allocated is too small for the string

B. Missing check if malloc returned NULL

C. Using strcpy without including

D. No error, code is correct

Solution

1. Step 1: Check allocated memory size

   Memory allocated is 5 bytes, but the string "Hello World" needs 12 bytes (11 chars + 1 null terminator).

2. Step 2: Understand consequences of insufficient memory

   Copying a larger string into smaller allocated space causes buffer overflow and undefined behavior.

3. Final Answer:

   Memory allocated is too small for the string -> Option A

4. Quick Check:

   Allocated size < string length = Memory allocated is too small for the string [OK]

Hint: Allocate enough space including null terminator [OK]

Common Mistakes:

• Ignoring string length vs allocated size
• Not checking malloc return value
• Assuming strcpy auto-expands memory

5. You want to allocate memory for an array of 5 integers, initialize them to zero, and then safely release the memory. Which code snippet correctly implements this flow?

hard

A. int *arr = malloc(5 * sizeof(int));
for(int i=0; i<5; i++) arr[i] = 0;

B. int arr[5];
for(int i=0; i<5; i++) arr[i] = 0;

C. int *arr = calloc(5, sizeof(int));
free(arr);

D. int *arr = malloc(5);
memset(arr, 0, 5 * sizeof(int));
free(arr);

Solution

1. Step 1: Understand memory allocation and initialization

   calloc allocates and initializes memory to zero automatically, perfect for this task.

2. Step 2: Check safe memory release

   free(arr); correctly releases the allocated memory after use.

3. Final Answer:

   int *arr = calloc(5, sizeof(int)); free(arr); -> Option C

4. Quick Check:

   calloc zero-initializes and malloc does not [OK]

Hint: Use calloc for zero-initialized memory allocation [OK]

Common Mistakes:

• Using malloc without initializing memory
• Allocating too little memory
• Not freeing dynamically allocated memory