Common RTOS bugs and debugging strategies in FreeRTOS - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this FreeRTOS task scheduling code?

Consider two tasks with different priorities. What will be printed when both tasks run?

FreeRTOS

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

void Task1(void *pvParameters) {
    printf("Task1 start\n");
    vTaskDelay(pdMS_TO_TICKS(100));
    printf("Task1 end\n");
    vTaskDelete(NULL);
}

void Task2(void *pvParameters) {
    printf("Task2 start\n");
    vTaskDelay(pdMS_TO_TICKS(50));
    printf("Task2 end\n");
    vTaskDelete(NULL);
}

int main() {
    xTaskCreate(Task1, "Task1", 1000, NULL, 2, NULL);
    xTaskCreate(Task2, "Task2", 1000, NULL, 1, NULL);
    vTaskStartScheduler();
    return 0;
}

A Task1 start\nTask1 end\nTask2 start\nTask2 end\n

B Task1 start\nTask2 start\nTask2 end\nTask1 end\n

C Task2 start\nTask1 start\nTask1 end\nTask2 end\n

D Task2 start\nTask2 end\nTask1 start\nTask1 end\n

💡 Hint

Remember that higher priority tasks preempt lower priority ones, and vTaskDelay causes the task to block for the specified time.

Task1 has higher priority (2) than Task2 (1). Task1 runs first and prints "Task1 start". Then it delays for 100ms, allowing Task2 to run and print its messages. Task2 delays for 50ms, so it finishes before Task1 resumes and prints "Task1 end".

🧠 Conceptual

intermediate

Which common RTOS bug causes a system to freeze due to tasks waiting forever?

In FreeRTOS, what is the typical cause of a system freeze when tasks never resume?

A Incorrect task notification value

B Priority inversion without priority inheritance

C Stack overflow in a task

D Deadlock caused by improper mutex usage

💡 Hint

Think about tasks waiting on each other indefinitely.

Deadlock happens when two or more tasks wait for resources held by each other, causing all to block forever. This freezes the system.

🔧 Debug

advanced

Identify the bug causing stack overflow in this FreeRTOS task

Examine the task code below. What causes the stack overflow?

FreeRTOS

void TaskExample(void *pvParameters) {
    char buffer[1024];
    while(1) {
        // Do some work
        vTaskDelay(pdMS_TO_TICKS(10));
    }
}

A vTaskDelay is called inside the loop causing stack to grow

B Missing vTaskDelete causes stack overflow

C The large local buffer uses too much stack space causing overflow

D Infinite loop without break causes stack overflow

💡 Hint

Consider how much memory the stack has and what local variables consume.

The large local array 'buffer' (1024 bytes) consumes a lot of stack memory. If the task stack size is not large enough, this causes stack overflow.

📝 Syntax

advanced

What error does this FreeRTOS queue code produce?

What error occurs when running this code snippet?

FreeRTOS

QueueHandle_t queue = xQueueCreate(5, sizeof(int));
int value = 10;
xQueueSend(queue, &value, 0);
xQueueSend(queue, NULL, 0);

A Runtime error due to NULL pointer passed to xQueueSend

B Compile-time error due to wrong argument type

C No error, both sends succeed

D Deadlock because queue is full

💡 Hint

Check the parameters passed to xQueueSend carefully.

xQueueSend expects a valid pointer to data. Passing NULL causes undefined behavior and likely runtime error.

🚀 Application

expert

How many items are in the queue after this sequence?

Given a FreeRTOS queue created with length 3, what is the number of items in the queue after these operations?

QueueHandle_t q = xQueueCreate(3, sizeof(int));
int a=1,b=2,c=3,d=4;
xQueueSend(q, &a, 0);
xQueueSend(q, &b, 0);
xQueueSend(q, &c, 0);
xQueueSend(q, &d, 0);

A 3 items, last send fails because queue is full

B 4 items, queue expands automatically

C 2 items, first send overwritten

D 0 items, all sends fail

💡 Hint

Remember the fixed size of FreeRTOS queues and behavior when full.

The queue has fixed length 3. The first three sends succeed. The fourth send fails because the queue is full, so only 3 items remain.