Memory pool (fixed-size block allocator) in Embedded C - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define the size of each block in the memory pool.

Embedded C

const size_t block_size = [1];

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A128

B64

C256

D512

Attempts:

3 left

💡 Hint

Common Mistakes

Choosing a block size too small causes overhead.

Choosing a block size too large wastes memory.

The block size is set to 128 bytes, which is a common fixed size for memory pool blocks.

2fill in blank

medium

Complete the code to initialize the memory pool array with the correct total size.

Embedded C

static unsigned char memory_pool[[1]];

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Ablock_size * 10

Bblock_size * 40

Cblock_size * 20

Dblock_size * 30

Attempts:

3 left

💡 Hint

Common Mistakes

Using too few blocks limits pool capacity.

Using too many blocks wastes memory.

The memory pool is sized to hold 30 blocks, each of block_size bytes.

3fill in blank

hard

Fix the error in the function that allocates a block from the pool by completing the condition to check if a free block is available.

Embedded C

if (free_list_head [1] NULL) {
    // allocate block
}

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A<=

B>=

C!=

D==

Attempts:

3 left

💡 Hint

Common Mistakes

Checking if free_list_head == NULL causes no allocation.

Using comparison operators like <= or >= is incorrect here.

The condition checks if the free list head is not NULL, meaning there is a free block to allocate.

4fill in blank

hard

Fill both blanks to correctly update the free list head after allocating a block.

Embedded C

allocated_block = free_list_head;
free_list_head = [1];
allocated_block->next = [2];

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Afree_list_head->next

BNULL

Callocated_block

Dmemory_pool

Attempts:

3 left

💡 Hint

Common Mistakes

Setting free_list_head to allocated_block causes a loop.

Not setting allocated_block->next to NULL can cause errors.

After allocation, the free list head moves to the next free block, and the allocated block's next pointer is set to NULL.

5fill in blank

hard

Fill all three blanks to correctly add a freed block back to the free list.

Embedded C

void free_block(Block* block) {
    // Optional: clear block data
    memset(block, 0, [3]);
    block->next = [1];
    [2] = block;
}

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

Afree_list_head

Csizeof(Block)

Dblock

Attempts:

3 left

💡 Hint

Common Mistakes

Not updating free_list_head causes lost free blocks.

Using wrong size in memset causes memory corruption.

The freed block's next pointer is set to the current free list head, then the free list head is updated to the freed block. Clearing the block data uses sizeof(Block).

Practice

1.

What is the main advantage of using a memory pool with fixed-size blocks in embedded systems?

easy

A. Unlimited memory usage

B. Automatic garbage collection

C. Faster and predictable memory allocation

D. Dynamic resizing of blocks

Solution

1. Step 1: Understand memory pool purpose

   A memory pool divides memory into fixed-size blocks to allocate memory quickly without fragmentation.

2. Step 2: Identify advantage in embedded systems

   Embedded systems need fast and predictable memory allocation, which memory pools provide.

3. Final Answer:

   Faster and predictable memory allocation -> Option C

4. Quick Check:

   Memory pool advantage = Faster and predictable memory allocation [OK]

Hint: Memory pools speed up allocation by fixed block sizes [OK]

Common Mistakes:

• Confusing memory pools with dynamic resizing
• Thinking memory pools provide garbage collection
• Assuming memory pools allow unlimited memory

2.

Which of the following is the correct way to declare a fixed-size memory pool array for 10 blocks of 32 bytes each in C?

?

easy

A. char pool[10][32];

B. char pool[32][10];

C. char *pool[10];

D. char pool[10*32];

Solution

1. Step 1: Understand array dimensions

   We want 10 blocks, each block is 32 bytes, so array should be 10 rows and 32 columns.

2. Step 2: Match declaration syntax

   char pool[10][32]; declares 10 blocks of 32 bytes each correctly.

3. Final Answer:

   char pool[10][32]; -> Option A

4. Quick Check:

   Array size = 10 blocks x 32 bytes [OK]

Hint: Rows = blocks, columns = block size in bytes [OK]

Common Mistakes:

• Swapping dimensions (32 and 10)
• Using pointer array instead of fixed blocks
• Declaring a single large array without blocks

3.

Given this simplified memory pool allocator code snippet, what will be the output?

char pool[3][16];
int free_blocks = 3;

char* allocate() {
    if (free_blocks > 0) {
        return pool[--free_blocks];
    }
    return NULL;
}

int main() {
    char* p1 = allocate();
    char* p2 = allocate();
    char* p3 = allocate();
    char* p4 = allocate();
    printf("%p %p %p %p\n", p1, p2, p3, p4);
    return 0;
}

medium

A. Three valid pointers and one NULL pointer

B. All pointers are NULL

C. All pointers point to the same block

D. Compilation error due to missing headers

Solution

1. Step 1: Trace allocation calls

   Initially free_blocks=3. Each allocate() call decrements free_blocks and returns pool[free_blocks]. So p1=pool[2], p2=pool[1], p3=pool[0], p4=NULL because free_blocks=0.

2. Step 2: Understand output

   First three pointers are valid addresses, last pointer is NULL.

3. Final Answer:

   Three valid pointers and one NULL pointer -> Option A

4. Quick Check:

   3 blocks allocated, 4th returns NULL [OK]

Hint: Allocation decreases free_blocks; NULL when none left [OK]

Common Mistakes:

• Assuming pointers are all NULL
• Thinking pointers point to same block
• Forgetting to decrement free_blocks

4.

Identify the bug in this memory pool free function and select the correct fix.

void free_block(char* block) {
    if (free_blocks < MAX_BLOCKS) {
        pool[free_blocks++] = block;
    }
}

medium

A. No bug; function works correctly

B. Incrementing free_blocks before storing pointer causes overwrite

C. Should check if block is NULL before freeing

D. Store pointer in pool array is wrong; should mark block as free differently

Solution

1. Step 1: Analyze free_block logic

   The pool array holds actual memory blocks, not pointers. Storing a pointer in pool overwrites data.

2. Step 2: Correct way to free block

   Freeing should mark the block as available in a separate structure or use a free list, not overwrite pool data.

3. Final Answer:

   Store pointer in pool array is wrong; should mark block as free differently -> Option D

4. Quick Check:

   Freeing must not overwrite pool memory [OK]

Hint: Freeing must not overwrite pool data blocks [OK]

Common Mistakes:

• Overwriting pool memory with pointers
• Not checking block validity
• Assuming pool holds pointers, not blocks

5.

You want to implement a memory pool that supports 5 blocks of 64 bytes each. You also want to track which blocks are free using a bitmap (an integer where each bit represents a block's status: 0 = free, 1 = used). Which of the following code snippets correctly initializes the bitmap and allocates a block?

?

hard

A. int bitmap = 0xFF; // all used
char pool[5][64];
char* allocate() {
for (int i = 0; i < 5; i++) {
if ((bitmap & (1 << i)) == 0) {
bitmap &= ~(1 << i);
return pool[i];
}
}
return NULL;
}

B. int bitmap = 0; // all free
char pool[5][64];
char* allocate() {
for (int i = 0; i < 5; i++) {
if ((bitmap & (1 << i)) == 0) {
bitmap |= (1 << i);
return pool[i];
}
}
return NULL;
}

C. int bitmap = 0; // all used
char pool[5][64];
char* allocate() {
for (int i = 0; i < 5; i++) {
if ((bitmap & (1 << i)) != 0) {
bitmap |= (1 << i);
return pool[i];
}
}
return NULL;
}

D. int bitmap = 0xFF; // all free
char pool[5][64];
char* allocate() {
for (int i = 0; i < 5; i++) {
if ((bitmap & (1 << i)) != 0) {
bitmap &= ~(1 << i);
return pool[i];
}
}
return NULL;
}

Solution

1. Step 1: Understand bitmap meaning

   Bitmap bit 0 means free, 1 means used. So initializing bitmap=0 means all blocks free.

2. Step 2: Check allocation logic

   Allocation finds first free bit (0), sets it to 1 (used), and returns that block.

3. Final Answer:

   Bitmap initialized to 0; allocate sets bit to 1 on allocation -> Option B

4. Quick Check:

   0 = free, set bit to 1 when used [OK]

Hint: 0 bits free, set bit to 1 when allocated [OK]

Common Mistakes:

• Initializing bitmap with all bits set (all used)
• Checking wrong bit condition for free blocks
• Setting bits incorrectly on allocation