Critical section with taskENTER_CRITICAL in FreeRTOS - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to enter a critical section using FreeRTOS.

FreeRTOS

taskENTER_[1]();
// critical code here
taskEXIT_CRITICAL();

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

AENTER

BSECTION

CLOCK

DCRITICAL

Attempts:

3 left

💡 Hint

Common Mistakes

Using taskENTER_SECTION() instead of taskENTER_CRITICAL()

Using taskENTER_LOCK() which does not exist

The correct macro to enter a critical section is taskENTER_CRITICAL().

2fill in blank

medium

Complete the code to exit a critical section using FreeRTOS.

FreeRTOS

taskENTER_CRITICAL();
// critical code
taskEXIT_[1]();

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

ACRITICAL

BSECTION

CENTER

DLOCK

Attempts:

3 left

💡 Hint

Common Mistakes

Using taskEXIT_SECTION() instead of taskEXIT_CRITICAL()

Using taskEXIT_LOCK() which does not exist

The correct macro to exit a critical section is taskEXIT_CRITICAL().

3fill in blank

hard

Fix the error in the code to properly protect the critical section.

FreeRTOS

taskENTER_CRITICAL();
shared_var += 1;
[1]

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

AtaskLOCK()

BtaskEXIT_SECTION()

CtaskEXIT_CRITICAL()

DtaskENTER_CRITICAL()

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting to exit the critical section causing deadlocks

Calling taskENTER_CRITICAL() twice without exit

After the critical code, you must call taskEXIT_CRITICAL() to exit the critical section.

4fill in blank

hard

Fill both blanks to create a safe critical section that increments a variable.

FreeRTOS

task[1]_CRITICAL();
count += 1;
task[2]_CRITICAL();

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

AENTER

BEXIT

CLOCK

DUNLOCK

Attempts:

3 left

💡 Hint

Common Mistakes

Using taskLOCK and taskUNLOCK which are not FreeRTOS macros

Swapping enter and exit macros

Use taskENTER_CRITICAL() to start and taskEXIT_CRITICAL() to end the critical section.

5fill in blank

hard

Fill all three blanks to correctly enter and exit a critical section from within an ISR in FreeRTOS.

FreeRTOS

[1] uxSavedStatus;
port[2]_CRITICAL_FROM_ISR( &uxSavedStatus );
// ISR critical code here
port[3]_CRITICAL_FROM_ISR( uxSavedStatus );

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

AUBaseType_t

BENTER

CEXIT

DDISABLE

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'taskENTER_CRITICAL_FROM_ISR()' instead of 'portENTER_CRITICAL_FROM_ISR()'

Forgetting the '&' before uxSavedStatus in the enter macro

Using 'DISABLE' instead of 'ENTER' or 'EXIT'

In FreeRTOS ISRs, declare UBaseType_t uxSavedStatus, then portENTER_CRITICAL_FROM_ISR(&uxSavedStatus), critical code, then portEXIT_CRITICAL_FROM_ISR(uxSavedStatus).

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