Critical section with taskENTER_CRITICAL in FreeRTOS - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of taskENTER_CRITICAL() in FreeRTOS?

taskENTER_CRITICAL() is used to start a critical section by disabling interrupts. This prevents other tasks or interrupts from interrupting the current code, ensuring exclusive access to shared resources.

beginner

How do you properly exit a critical section started by taskENTER_CRITICAL()?

You use taskEXIT_CRITICAL() to re-enable interrupts and end the critical section. This allows other tasks and interrupts to run again.

beginner

Why should critical sections be kept as short as possible?

Because interrupts are disabled during critical sections, keeping them short avoids delaying important system tasks and maintains system responsiveness.

intermediate

What happens if you forget to call taskEXIT_CRITICAL() after taskENTER_CRITICAL()?

Interrupts remain disabled, which can cause the system to become unresponsive or miss important events.

intermediate

Can taskENTER_CRITICAL() be nested? What does that mean?

Yes, taskENTER_CRITICAL() can be nested. This means you can enter multiple critical sections one inside another, and interrupts will only be re-enabled after the matching number of taskEXIT_CRITICAL() calls.

What does taskENTER_CRITICAL() do in FreeRTOS?

A Starts a new task

B Enables interrupts after a critical section

C Suspends the current task

D Disables interrupts to start a critical section

taskENTER_CRITICAL() disables interrupts to protect a critical section.

Which function should be called to end a critical section started by taskENTER_CRITICAL()?

A taskDISABLE_INTERRUPTS()

B taskSTART_CRITICAL()

C taskEXIT_CRITICAL()

D taskSUSPEND()

taskEXIT_CRITICAL() re-enables interrupts and ends the critical section.

Why is it important to keep critical sections short?

A To avoid blocking interrupts for too long

B To use more CPU time

C To increase task priority

D To create more tasks

Long critical sections block interrupts and reduce system responsiveness.

What happens if taskEXIT_CRITICAL() is not called after taskENTER_CRITICAL()?

A The system resets automatically

B Interrupts stay disabled, causing system issues

C Nothing happens

D The task ends immediately

Interrupts remain disabled, which can freeze the system.

Can you nest multiple taskENTER_CRITICAL() calls?

A Yes, interrupts are re-enabled after matching <code>taskEXIT_CRITICAL()</code> calls

B No, nesting causes errors

C Yes, but interrupts are never re-enabled

D No, nesting is ignored

Nesting is supported; interrupts re-enable only after all critical sections exit.

Explain what a critical section is and how taskENTER_CRITICAL() and taskEXIT_CRITICAL() are used in FreeRTOS.

Think about how to prevent other code from interrupting your current code.

Describe why it is important to keep critical sections short and what could happen if they are too long.

Consider what happens when interrupts are off for too long.

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