Trace hooks and FreeRTOS+Trace - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of a Trace Hook Callback

Consider a FreeRTOS trace hook function that logs task switches. What will be the output when the following trace hook is called during a context switch from TaskA to TaskB?

FreeRTOS

void traceTASK_SWITCHED_IN(void) {
    static char *lastTask = NULL;
    char *currentTask = pcTaskGetName(NULL);
    if (lastTask != currentTask) {
        printf("Switching from %s to %s\n", lastTask ? lastTask : "None", currentTask);
        lastTask = currentTask;
    }
}

// Assume TaskA runs first, then TaskB is switched in.

A Switching from None to TaskA\nSwitching from TaskA to TaskB

B Switching from TaskA to TaskB

C Switching from None to TaskA

D No output

💡 Hint

Remember that the static variable retains its value between calls.

The first time the hook runs, lastTask is NULL, so it prints switching from None to TaskA. Then lastTask is set to TaskA. When TaskB is switched in, the hook detects the change and prints switching from TaskA to TaskB.

🧠 Conceptual

intermediate

Purpose of Trace Hooks in FreeRTOS

What is the main purpose of using trace hooks in FreeRTOS?

A To modify the scheduler's behavior dynamically at runtime

B To increase the priority of tasks automatically

C To monitor and record kernel events such as task switches and interrupts

D To allocate memory for new tasks

💡 Hint

Think about what trace hooks help you observe.

Trace hooks allow developers to monitor kernel events like task switches, queue operations, and interrupts for debugging and performance analysis.

🔧 Debug

advanced

Debugging a Trace Hook Causing System Crash

A developer added a trace hook to log task names on every context switch. After adding it, the system crashes intermittently. Which of the following is the most likely cause?

A Using a static variable inside the trace hook

B Forgetting to call vTaskStartScheduler() after adding the trace hook

C Not defining configUSE_TRACE_FACILITY as 1 in FreeRTOSConfig.h

D Using printf inside the trace hook which is not thread-safe and may cause reentrancy issues

💡 Hint

Consider what functions are safe to call inside interrupt or trace hooks.

printf is not thread-safe and can cause reentrancy problems when called inside trace hooks that run in critical sections or interrupts, leading to crashes.

🚀 Application

advanced

Using FreeRTOS+Trace to Analyze Task Behavior

You want to analyze task execution times using FreeRTOS+Trace. Which of the following steps is NOT required to correctly collect trace data?

A Manually insert calls to traceTASK_SWITCHED_IN() in your application code

B Call vTraceEnable() before starting the scheduler to start trace recording

C Enable trace macros in FreeRTOSConfig.h such as configUSE_TRACE_FACILITY and configUSE_STATS_FORMATTING_FUNCTIONS

D Use the FreeRTOS+Trace recorder to export and visualize the collected data

💡 Hint

Think about how trace hooks are integrated with the kernel.

traceTASK_SWITCHED_IN() is a trace hook macro called automatically by the kernel during context switches; manually calling it in application code is unnecessary and incorrect.

❓ Predict Output

expert

Output of Custom Trace Hook with Task Priorities

Given the following trace hook code that logs the priority of the task being switched in, what will be the output if Task1 (priority 3) is switched in, then Task2 (priority 5) is switched in?

FreeRTOS

void traceTASK_SWITCHED_IN(void) {
    TaskHandle_t current = xTaskGetCurrentTaskHandle();
    UBaseType_t priority = uxTaskPriorityGet(current);
    printf("Task switched in: %s with priority %u\n", pcTaskGetName(current), (unsigned int)priority);
}

// Assume Task1 runs first, then Task2 is switched in.

A Task switched in: Task2 with priority 5\nTask switched in: Task1 with priority 3

B Task switched in: Task1 with priority 3\nTask switched in: Task2 with priority 5

C Task switched in: Task1 with priority 5\nTask switched in: Task2 with priority 3

D Task switched in: Task1 with priority 3

💡 Hint

The hook runs every time a task is switched in, printing its name and priority.

The hook prints the current task's name and priority each time it runs. Since Task1 runs first, it prints its info, then when Task2 switches in, it prints Task2's info.