Static memory allocation patterns in Embedded C - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is static memory allocation in embedded C?

Static memory allocation means reserving memory at compile time before the program runs. The size and location of memory are fixed and do not change during execution.

beginner

How does static memory allocation differ from dynamic memory allocation?

Static memory allocation reserves memory at compile time with fixed size, while dynamic memory allocation reserves memory at runtime and can change size during execution.

beginner

Give an example of static memory allocation in embedded C.

Example:

int buffer[100]; // Array of 100 integers allocated statically

This array size and memory location are fixed at compile time.

intermediate

Why is static memory allocation preferred in embedded systems?

Because embedded systems often have limited memory and no operating system, static allocation avoids runtime overhead and fragmentation, making programs more predictable and reliable.

intermediate

What are common static memory allocation patterns in embedded C?

Common patterns include:
- Global variables
- Static local variables
- Fixed-size arrays
- Constant data in read-only memory
These ensure memory is reserved at compile time.

Which of the following is an example of static memory allocation?

A int *ptr = malloc(50 * sizeof(int));

B int arr[50];

C Using realloc() to resize memory

D Allocating memory inside a function with malloc

int arr[50]; allocates memory at compile time (static). malloc() and realloc() allocate memory at runtime (dynamic).

Why is static memory allocation safer in embedded systems?

A It requires an operating system

B It allows flexible memory resizing

C It uses more CPU cycles

D It avoids runtime memory fragmentation

Static allocation avoids fragmentation and unpredictable behavior because memory is fixed at compile time.

Which keyword in C declares a variable with static storage duration inside a function?

A static

B register

C auto

D extern

The 'static' keyword inside a function keeps the variable alive for the program's lifetime with fixed memory.

What happens if you declare a global array in embedded C?

A Memory is allocated statically at compile time

B Memory is not allocated until first use

C Memory is allocated on the stack

D Memory is allocated dynamically at runtime

Global variables and arrays are allocated statically at compile time in embedded C.

Which of these is NOT a benefit of static memory allocation in embedded systems?

A Predictable memory usage

B No runtime allocation overhead

C Ability to resize memory dynamically

D Avoids memory fragmentation

Static allocation cannot resize memory dynamically; that is a feature of dynamic allocation.

Explain static memory allocation and why it is important in embedded C programming.

Think about how embedded devices have limited memory and no OS.

List common static memory allocation patterns used in embedded C and describe their purpose.

Consider where memory is reserved before the program runs.

Practice

1. What is the main characteristic of static memory allocation in embedded C?

easy

A. Memory size is fixed at compile time and cannot change during execution.

B. Memory is allocated dynamically during program execution.

C. Memory is allocated only when a function is called.

D. Memory size can grow or shrink based on input data.

Solution

1. Step 1: Understand static memory allocation

   Static memory allocation means reserving memory before the program runs, with a fixed size.

2. Step 2: Compare with dynamic allocation

   Dynamic allocation changes size during execution, but static does not.

3. Final Answer:

   Memory size is fixed at compile time and cannot change during execution. -> Option A

4. Quick Check:

   Static memory = fixed size [OK]

Hint: Static means fixed size known before running [OK]

Common Mistakes:

• Confusing static with dynamic allocation
• Thinking static memory can resize
• Assuming static memory is allocated at runtime

2. Which of the following declarations uses static memory allocation correctly in embedded C?

easy

A. int *arr; arr = calloc(10, sizeof(int));

B. int *arr = malloc(10 * sizeof(int));

C. int arr[];

D. int arr[10];

Solution

1. Step 1: Identify static allocation syntax

   Static arrays are declared with fixed size like int arr[10];

2. Step 2: Recognize dynamic allocation

   malloc and calloc allocate memory dynamically at runtime, not static.

3. Final Answer:

   int arr[10]; -> Option D

4. Quick Check:

   Static array declaration = int arr[10]; [OK]

Hint: Static arrays use fixed size brackets [] [OK]

Common Mistakes:

• Using malloc or calloc for static allocation
• Not specifying array size for static arrays
• Confusing pointer declarations with arrays

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

static int buffer[5] = {1, 2, 3, 4, 5};

void main() {
    for(int i = 0; i < 5; i++) {
        buffer[i] += 1;
        printf("%d ", buffer[i]);
    }
}

medium

A. Compilation error due to static keyword

B. 2 3 4 5 6

C. 0 1 2 3 4

D. 1 2 3 4 5

Solution

1. Step 1: Understand static array initialization

   The static array buffer starts with values {1, 2, 3, 4, 5} fixed at compile time.

2. Step 2: Trace the loop increment and print

   Each element is increased by 1, so values become {2, 3, 4, 5, 6} and printed.

3. Final Answer:

   2 3 4 5 6 -> Option B

4. Quick Check:

   Increment each element by 1 = 2 3 4 5 6 [OK]

Hint: Static arrays behave like normal arrays with fixed size [OK]

Common Mistakes:

• Thinking static causes compile error
• Forgetting to increment values before printing
• Confusing static with uninitialized variables

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

void main() {
    static int data[];
    data[0] = 10;
    printf("%d", data[0]);
}

medium

A. Cannot assign value to static array elements.

B. Static keyword cannot be used inside functions.

C. Array size is not specified for static array.

D. Missing return type for main function.

Solution

1. Step 1: Check static array declaration

   Static arrays must have size or initializer; data[] without size or init is invalid.

2. Step 2: Verify other code parts

   Static inside functions is allowed; assigning values and printing is valid; main return type is void (allowed in embedded C).

3. Final Answer:

   Array size is not specified for static array. -> Option C

4. Quick Check:

   Static array needs size or initializer [OK]

Hint: Static arrays need size or initializer [OK]

Common Mistakes:

• Assuming static arrays can omit size without initializer
• Thinking static is disallowed inside functions
• Confusing main return type errors with static errors

5. You need to store sensor readings in a fixed-size buffer that must retain values between function calls in embedded C. Which static memory allocation pattern is best suited?

hard

A. Declare a static array inside the function to hold readings.

B. Use dynamic memory allocation with malloc inside the function.

C. Declare a local array inside the function without static keyword.

D. Use global variables without static keyword.

Solution

1. Step 1: Understand requirement for retaining values

   Values must persist between calls, so memory must not be reinitialized each time.

2. Step 2: Choose static array inside function

   Static array inside function keeps values between calls and has fixed size, perfect for sensor buffer.

3. Step 3: Exclude other options

   Dynamic allocation is flexible but not static; local array resets each call; global without static may cause namespace issues.

4. Final Answer:

   Declare a static array inside the function to hold readings. -> Option A

5. Quick Check:

   Static inside function retains data between calls [OK]

Hint: Static inside function keeps data persistent [OK]

Common Mistakes:

• Using local arrays that reset each call
• Choosing dynamic allocation when fixed size needed
• Ignoring static keyword for persistence