Critical section with taskENTER_CRITICAL in FreeRTOS

Introduction

We use critical sections to protect parts of code that access shared resources. This stops other tasks or interrupts from changing data at the same time, avoiding errors.

When updating a shared variable that multiple tasks use.

When accessing hardware registers that must not be interrupted.

When performing a sequence of operations that must complete without interruption.

When preventing race conditions in multitasking environments.

Syntax

FreeRTOS

taskENTER_CRITICAL();
// critical code here
taskEXIT_CRITICAL();

taskENTER_CRITICAL disables interrupts to protect the code inside.

Always pair taskENTER_CRITICAL with taskEXIT_CRITICAL to re-enable interrupts.

Examples

This protects incrementing a shared counter from interruption.

FreeRTOS

taskENTER_CRITICAL();
sharedCounter++;
taskEXIT_CRITICAL();

Use critical section to safely write to hardware registers.

FreeRTOS

taskENTER_CRITICAL();
// Access hardware safely
writeRegister(value);
taskEXIT_CRITICAL();

Sample Program

This program shows how to safely increase a shared counter using taskENTER_CRITICAL and taskEXIT_CRITICAL to prevent interruptions.

FreeRTOS

#include "FreeRTOS.h"
#include "task.h"
#include <stdio.h>

volatile int sharedCounter = 0;

void incrementCounter() {
    taskENTER_CRITICAL();
    sharedCounter++;
    taskEXIT_CRITICAL();
}

int main() {
    printf("Counter before: %d\n", sharedCounter);
    incrementCounter();
    printf("Counter after: %d\n", sharedCounter);
    return 0;
}

Important Notes

Do not keep critical sections longer than necessary to avoid blocking interrupts too long.

taskENTER_CRITICAL disables interrupts globally, so use carefully in real-time systems.

Summary

Critical sections protect shared data by disabling interrupts.

Use taskENTER_CRITICAL and taskEXIT_CRITICAL together.

Keep critical sections short to keep system responsive.

Practice

1. What is the main purpose of using taskENTER_CRITICAL() in FreeRTOS?

easy

A. To allocate memory dynamically

B. To disable interrupts and protect shared data during a critical section

C. To start a new task in the scheduler

D. To send a message to a queue

Solution

1. Step 1: Understand what taskENTER_CRITICAL() does

   This function disables interrupts to prevent context switches and protect shared resources.

2. Step 2: Identify the purpose in multitasking

   Disabling interrupts ensures that the critical section code runs without interruption, avoiding data corruption.

3. Final Answer:

   To disable interrupts and protect shared data during a critical section -> Option B

4. Quick Check:

   Critical section protection = disable interrupts [OK]

Hint: Remember: critical section means no interrupts [OK]

Common Mistakes:

• Confusing task creation with critical section
• Thinking it allocates memory
• Assuming it sends messages

2. Which of the following is the correct syntax to start a critical section in FreeRTOS?

easy

A. enter_critical_section();

B. taskENTER_CRITICAL;

C. taskENTER_CRITICAL();

D. task_enter_critical();

Solution

1. Step 1: Recall the exact function name and syntax

   The correct FreeRTOS API function to enter a critical section is taskENTER_CRITICAL() with parentheses.

2. Step 2: Check the options for correct spelling and syntax

   Only taskENTER_CRITICAL(); uses the exact function name with parentheses, which is required to call the function.

3. Final Answer:

   taskENTER_CRITICAL(); -> Option C

4. Quick Check:

   Function call syntax = parentheses needed [OK]

Hint: Function calls always need parentheses () [OK]

Common Mistakes:

• Omitting parentheses when calling functions
• Using incorrect function names
• Confusing with similar sounding functions

3. Consider the following FreeRTOS code snippet:

int sharedCounter = 0;

void incrementCounter() {
    taskENTER_CRITICAL();
    sharedCounter++;
    taskEXIT_CRITICAL();
}

incrementCounter();
printf("%d", sharedCounter);

What will be the output after running this code once?

medium

A. 1

B. 0

C. Compilation error

D. Undefined behavior

Solution

1. Step 1: Analyze the increment inside the critical section

   The function increments sharedCounter by 1 while interrupts are disabled, ensuring safe update.

2. Step 2: Check the output after one call

   After calling incrementCounter() once, sharedCounter becomes 1, so printing it outputs 1.

3. Final Answer:

   1 -> Option A

4. Quick Check:

   Increment once = 1 [OK]

Hint: Increment inside critical section updates safely [OK]

Common Mistakes:

• Assuming no increment due to critical section
• Thinking code won't compile
• Confusing output with initial value

4. Identify the error in this FreeRTOS critical section code:

void updateData() {
    taskENTER_CRITICAL();
    sharedData = 42;
    // Missing taskEXIT_CRITICAL();
}

medium

A. Missing taskEXIT_CRITICAL() causes interrupts to stay disabled

B. sharedData must be declared volatile

C. Using taskENTER_CRITICAL() inside a function is invalid

D. No error, code is correct

Solution

1. Step 1: Check for matching critical section calls

   The code calls taskENTER_CRITICAL() but does not call taskEXIT_CRITICAL(), so interrupts remain disabled.

2. Step 2: Understand consequences of missing exit

   Interrupts staying disabled can freeze the system or cause missed interrupts, which is a serious error.

3. Final Answer:

   Missing taskEXIT_CRITICAL() causes interrupts to stay disabled -> Option A

4. Quick Check:

   Always pair enter and exit critical [OK]

Hint: Always match enter and exit critical calls [OK]

Common Mistakes:

• Forgetting to call taskEXIT_CRITICAL()
• Thinking enter critical is invalid in functions
• Assuming volatile fixes missing exit

5. You have two tasks incrementing a shared variable counter. To avoid race conditions, you use a critical section:

void increment() {
    taskENTER_CRITICAL();
    counter++;
    taskEXIT_CRITICAL();
}

However, the system becomes unresponsive when interrupts are disabled for too long. What is the best way to improve this?

hard

A. Remove the critical section to improve responsiveness

B. Use a busy-wait loop inside the critical section

C. Disable interrupts globally for the entire task duration

D. Keep the critical section as short as possible, only around the increment

Solution

1. Step 1: Understand impact of long critical sections

   Disabling interrupts for too long blocks other important system activities, causing unresponsiveness.

2. Step 2: Optimize critical section length

   Keeping the critical section short, only around the shared variable update, minimizes interrupt blocking and improves responsiveness.

3. Final Answer:

   Keep the critical section as short as possible, only around the increment -> Option D

4. Quick Check:

   Short critical section = better responsiveness [OK]

Hint: Shorten critical sections to keep system responsive [OK]

Common Mistakes:

• Removing critical section causing race conditions
• Disabling interrupts too long
• Using busy-wait inside critical section