Priority numbering in FreeRTOS - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Understanding FreeRTOS priority numbering

Consider a FreeRTOS system with configMAX_PRIORITIES set to 5. If a task is created with priority 3, what is the relative priority of this task compared to a task with priority 1?

A Both tasks have the same priority.

B The task with priority 3 has lower priority than the task with priority 1.

C The task with priority 3 has higher priority than the task with priority 1.

D Priority numbers do not determine task priority in FreeRTOS.

💡 Hint

Remember, in FreeRTOS, higher numeric priority means higher task priority.

In FreeRTOS, priority numbers start at 0 (lowest) and go up to configMAX_PRIORITIES - 1 (highest). So priority 3 is higher than priority 1.

❓ Predict Output

intermediate

Effect of configMAX_PRIORITIES on task creation

If configMAX_PRIORITIES is set to 4, what happens when you try to create a task with priority 5?

A The task is created with priority 5 without any error.

B The system crashes immediately.

C The task is created with priority 0 instead.

D The task creation fails or priority is capped at the maximum allowed priority.

💡 Hint

Check the allowed range of priorities based on configMAX_PRIORITIES.

FreeRTOS only allows priorities from 0 up to configMAX_PRIORITIES - 1. Trying to use a priority outside this range results in failure or priority being capped.

🔧 Debug

advanced

Why does a higher priority task not run immediately?

In FreeRTOS, a task with priority 4 is created while a task with priority 3 is running. However, the higher priority task does not start running immediately. What could be the reason?

A The scheduler is not started or preemption is disabled.

B The task with priority 3 has a higher priority than 4.

C FreeRTOS does not support priority-based scheduling.

D The higher priority task is blocked or suspended.

💡 Hint

Think about scheduler state and preemption settings.

If the scheduler is not started or preemption is disabled, higher priority tasks will not preempt running lower priority tasks.

🧠 Conceptual

advanced

Priority inversion and FreeRTOS priority numbering

Which statement best describes how FreeRTOS priority numbering helps to avoid priority inversion?

A Priority inversion is avoided by disabling all interrupts.

B Higher numeric priority tasks always preempt lower priority tasks, reducing priority inversion.

C FreeRTOS uses random priority assignment to avoid priority inversion.

D FreeRTOS does not handle priority inversion at all.

💡 Hint

Consider how task priorities affect scheduling and preemption.

FreeRTOS assigns higher numeric values to higher priorities, so higher priority tasks preempt lower ones, helping to reduce priority inversion.

❓ Predict Output

expert

Determining the number of priority levels in FreeRTOS

Given configMAX_PRIORITIES is set to 7, how many distinct priority levels can tasks have in FreeRTOS?

A 7 distinct priority levels, numbered 0 to 6.

B 6 distinct priority levels, numbered 1 to 6.

C 8 distinct priority levels, numbered 0 to 7.

D Only 1 priority level regardless of configMAX_PRIORITIES.

💡 Hint

Remember how FreeRTOS counts priority levels starting from zero.

FreeRTOS priority levels start at 0 and go up to configMAX_PRIORITIES - 1, so with 7, priorities are 0 to 6, totaling 7 levels.