Multiple tasks running concurrently 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 created with the same priority in FreeRTOS. Each task prints a message and then delays for 100 ticks. What will be the output sequence?

FreeRTOS

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

void Task1(void *pvParameters) {
    while(1) {
        printf("Task 1 running\n");
        vTaskDelay(100);
    }
}

void Task2(void *pvParameters) {
    while(1) {
        printf("Task 2 running\n");
        vTaskDelay(100);
    }
}

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

A Task 1 running\nTask 2 running\nTask 1 running\nTask 2 running\n... (alternating)

B Task 1 running\nTask 1 running\nTask 1 running\nTask 2 running\nTask 2 running\nTask 2 running\n...

C Task 2 running\nTask 1 running\nTask 2 running\nTask 1 running\n... (alternating)

D Only Task 1 runs repeatedly; Task 2 never runs.

💡 Hint

Tasks with the same priority and equal delay times will share CPU time in a round-robin manner.

Since both tasks have the same priority and call vTaskDelay(100), the scheduler switches between them, resulting in alternating output.

🧠 Conceptual

intermediate

Which FreeRTOS function is used to create a new task?

In FreeRTOS, what is the correct function to create a new task that will run concurrently?

A vTaskRun()

B xTaskCreate()

C vTaskStart()

D xCreateTask()

💡 Hint

Look for the function starting with 'x' that creates tasks.

The function xTaskCreate() is the official FreeRTOS API to create a new task.

🔧 Debug

advanced

Why does this FreeRTOS code cause a stack overflow?

Examine the following task code. Why might it cause a stack overflow?

FreeRTOS

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

int main(void) {
    xTaskCreate(Task, "Task", 100, NULL, 1, NULL);
    vTaskStartScheduler();
    return 0;
}

A The stack size (100) is too small for the 1024-byte buffer, causing overflow.

B The task priority is too high, causing stack overflow.

C The task function is missing a return statement, causing overflow.

D vTaskDelay(10) causes the task to never yield, leading to overflow.

💡 Hint

Consider the size of local variables versus the stack size allocated.

The task stack size is set to 100 words, which is too small to hold the 1024-byte local buffer, causing stack overflow.

📝 Syntax

advanced

Identify the syntax error in this FreeRTOS task creation code

Which option contains the correct syntax to create a FreeRTOS task?

A xTaskCreate(TaskFunction, Task, 256, NULL, 2, NULL);

B xTaskCreate(TaskFunction, "Task", 256, NULL, 2);

C xTaskCreate("TaskFunction", "Task", 256, NULL, 2, NULL);

D xTaskCreate(TaskFunction, "Task", 256, NULL, 2, NULL);

💡 Hint

Check the parameter types and count for xTaskCreate.

The first parameter must be a function pointer, the second a string name, and the last parameter is a pointer to receive the task handle (can be NULL). Option D matches this.

🚀 Application

expert

How to ensure two FreeRTOS tasks do not access a shared variable simultaneously?

You have two tasks that both read and write a shared variable. Which method ensures safe concurrent access?

A Increase task priorities so one runs before the other.

B Use vTaskDelay() in both tasks to avoid overlap.

C Use a mutex to lock the variable during access.

D Declare the variable as volatile to prevent conflicts.

💡 Hint

Think about synchronization tools FreeRTOS provides.

A mutex ensures only one task accesses the shared variable at a time, preventing race conditions.