Memory pool (fixed-size block allocator) in Embedded C - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a memory pool (fixed-size block allocator)?

A memory pool is a way to manage memory by dividing a big chunk into many small fixed-size blocks. It helps quickly allocate and free memory without using the general heap.

beginner

Why use fixed-size blocks in a memory pool?

Fixed-size blocks make allocation and deallocation very fast and simple because each block is the same size. This avoids fragmentation and complex bookkeeping.

intermediate

How does a free list work in a memory pool?

A free list is a linked list of free blocks. When a block is freed, it is added back to the free list. When allocating, a block is taken from the free list.

intermediate

What happens if the memory pool runs out of free blocks?

If no free blocks are left, allocation fails or returns NULL. The program must handle this case to avoid crashes or memory errors.

intermediate

Show a simple C code snippet to initialize a memory pool with fixed-size blocks.

#define BLOCK_SIZE 32 #define BLOCK_COUNT 10 char pool[BLOCK_SIZE * BLOCK_COUNT]; void* free_list = NULL; void init_pool() { for (int i = 0; i < BLOCK_COUNT; i++) { void* block = &pool[i * BLOCK_SIZE]; *(void**)block = free_list; free_list = block; } }

What is the main advantage of using a fixed-size block memory pool?

A Automatically grows memory size

B Supports variable block sizes

C Uses less memory than heap

D Fast allocation and deallocation

Fixed-size blocks allow very fast allocation and freeing because the size is constant and bookkeeping is simple.

In a memory pool, what data structure usually tracks free blocks?

A Queue

B Binary tree

C Free list (linked list)

D Stack

A free list is a linked list that keeps track of all free blocks for quick allocation.

What should a program do if the memory pool has no free blocks left?

A Allocate from heap automatically

B Return NULL or error

C Overwrite existing blocks

D Ignore and continue

The program must handle allocation failure by returning NULL or an error to avoid crashes.

Which of these is NOT a benefit of fixed-size block allocators?

A Allows variable block sizes

B Simplifies allocation logic

C Avoids fragmentation

D Fast allocation speed

Fixed-size block allocators do NOT support variable block sizes; all blocks are the same size.

How is a block added back to the free list when freed?

A By linking it to the head of the free list

B By copying data to it

C By resizing the block

D By moving it to the end of the pool

The freed block is linked to the head of the free list for quick reuse.

Explain how a fixed-size block memory pool manages allocation and deallocation.

Think about how blocks are taken from and returned to the free list.

Describe what happens when the memory pool runs out of free blocks and how a program should handle it.

Consider what the allocator returns when no blocks are free.

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