xTaskNotifyGive() as lightweight semaphore in FreeRTOS

Introduction

xTaskNotifyGive() lets tasks signal each other quickly and simply, like passing a small note to say 'I'm done' or 'Go ahead'. It works like a lightweight semaphore to control access or sync tasks without heavy overhead.

When one task needs to tell another task that some work is finished.

When you want to control access to a shared resource without using full semaphores.

When you want a simple way to count events or signals between tasks.

When you want faster task synchronization with less memory use.

When you want to avoid complex semaphore setup but still need task coordination.

Syntax

FreeRTOS

void xTaskNotifyGive(TaskHandle_t xTaskToNotify);

xTaskToNotify is the handle of the task you want to notify.

This function sends a notification that increments the notified task's notification value by 1.

Examples

Notify the task identified by xTaskHandle that an event occurred.

FreeRTOS

xTaskNotifyGive(xTaskHandle);

Notify a task from an interrupt service routine (ISR), with a flag to request a context switch if needed.

FreeRTOS

xTaskNotifyGiveFromISR(xTaskHandle, &xHigherPriorityTaskWoken);

Sample Program

This program creates two tasks. Task 1 does some work, then notifies Task 2 using xTaskNotifyGive(). Task 2 waits for this notification using ulTaskNotifyTake() and runs when notified. This shows how xTaskNotifyGive() acts like a lightweight semaphore to sync tasks.

FreeRTOS

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

TaskHandle_t xTask1Handle = NULL;
TaskHandle_t xTask2Handle = NULL;

void vTask1(void *pvParameters) {
    while (1) {
        printf("Task 1: Doing work...\n");
        vTaskDelay(pdMS_TO_TICKS(1000));
        printf("Task 1: Notifying Task 2\n");
        xTaskNotifyGive(xTask2Handle); // Signal Task 2
    }
}

void vTask2(void *pvParameters) {
    while (1) {
        ulTaskNotifyTake(pdTRUE, portMAX_DELAY); // Wait for notification
        printf("Task 2: Received notification, running...\n");
    }
}

int main(void) {
    xTaskCreate(vTask1, "Task1", 1000, NULL, 1, &xTask1Handle);
    xTaskCreate(vTask2, "Task2", 1000, NULL, 1, &xTask2Handle);
    vTaskStartScheduler();
    return 0;
}

Important Notes

ulTaskNotifyTake() is used by the receiving task to wait for the notification and optionally clear it.

This method is lighter and faster than traditional semaphores but only supports one notification count per task.

Use xTaskNotifyGiveFromISR() if notifying from an interrupt.

Summary

xTaskNotifyGive() sends a simple signal from one task to another.

It works like a lightweight semaphore to sync tasks with less overhead.

Use ulTaskNotifyTake() to wait for the signal and continue work.

Practice

1. What is the primary purpose of xTaskNotifyGive() in FreeRTOS?

easy

A. To send a simple signal from one task to another as a lightweight semaphore

B. To create a new task in the system

C. To delete a task permanently

D. To allocate memory dynamically

Solution

1. Step 1: Understand the function role

   xTaskNotifyGive() is designed to send a notification signal from one task to another.

2. Step 2: Compare with other options

   Creating or deleting tasks and memory allocation are unrelated to xTaskNotifyGive().

3. Final Answer:

   To send a simple signal from one task to another as a lightweight semaphore -> Option A

4. Quick Check:

   Signal sending = To send a simple signal from one task to another as a lightweight semaphore [OK]

Hint: Remember: xTaskNotifyGive() signals tasks like a lightweight semaphore [OK]

Common Mistakes:

• Confusing it with task creation or deletion functions
• Thinking it manages memory
• Assuming it blocks tasks directly

2. Which of the following is the correct syntax to send a notification using xTaskNotifyGive() to a task handle named xTaskHandle?

easy

A. xTaskNotifyGive(xTaskHandle, 1);

B. xTaskNotifyGive(&xTaskHandle);

C. xTaskNotifyGive(xTaskHandle);

D. xTaskNotifyGive();

Solution

1. Step 1: Recall function signature

   xTaskNotifyGive() takes a task handle as a parameter, not a pointer or extra arguments.

2. Step 2: Check each option

   xTaskNotifyGive(xTaskHandle); correctly passes the task handle. xTaskNotifyGive(&xTaskHandle); passes address (wrong), C adds an extra argument (wrong), D has no argument (wrong).

3. Final Answer:

   xTaskNotifyGive(xTaskHandle); -> Option C

4. Quick Check:

   Correct syntax = xTaskNotifyGive(xTaskHandle); [OK]

Hint: Pass the task handle directly without extra arguments [OK]

Common Mistakes:

• Passing pointer to task handle instead of handle itself
• Adding extra parameters
• Calling without any argument

3. Consider the following code snippet:

void Task1(void *pvParameters) {
    // Wait for notification
    ulTaskNotifyTake(pdTRUE, portMAX_DELAY);
    // Continue after notification
    printf("Task1 resumed\n");
}

void Task2(void *pvParameters) {
    // Notify Task1
    xTaskNotifyGive(Task1Handle);
}

What will be printed when Task2 calls xTaskNotifyGive()?

medium

A. Task2 resumes instead of Task1

B. Task1 resumed

C. Syntax error at compile time

D. No output, Task1 blocks forever

Solution

1. Step 1: Understand notification flow

   Task1 waits for a notification with ulTaskNotifyTake(). When Task2 calls xTaskNotifyGive(), it sends the notification.

2. Step 2: Effect of notification

   The notification unblocks Task1, so it prints "Task1 resumed".

3. Final Answer:

   Task1 resumed -> Option B

4. Quick Check:

   Notification received = Task1 prints [OK]

Hint: xTaskNotifyGive() wakes waiting task to continue [OK]

Common Mistakes:

• Assuming Task1 never resumes
• Confusing which task prints output
• Thinking code causes compile error

4. Identify the error in this code snippet using xTaskNotifyGive():

void TaskA(void *pvParameters) {
    ulTaskNotifyTake(pdFALSE, portMAX_DELAY);
    printf("TaskA running\n");
}

void TaskB(void *pvParameters) {
    xTaskNotifyGive(&TaskAHandle);
}

medium

A. Passing address of task handle instead of task handle to xTaskNotifyGive()

B. Using pdFALSE in ulTaskNotifyTake() causes immediate return

C. Missing delay in TaskB before notify

D. printf statement is incorrect syntax

Solution

1. Step 1: Check parameter passed to xTaskNotifyGive()

   The function expects a task handle, but &TaskAHandle passes its address, which is incorrect.

2. Step 2: Validate other parts

   Using pdFALSE is valid to clear notification count, no syntax error in printf, and delay is optional.

3. Final Answer:

   Passing address of task handle instead of task handle to xTaskNotifyGive() -> Option A

4. Quick Check:

   Correct parameter type needed = Passing address of task handle instead of task handle to xTaskNotifyGive() [OK]

Hint: Pass task handle directly, not its address [OK]

Common Mistakes:

• Passing pointer to handle instead of handle
• Misunderstanding pdFALSE effect
• Assuming printf syntax error

5. You want to use xTaskNotifyGive() as a lightweight semaphore to synchronize two tasks: Producer and Consumer. The Consumer waits for the notification before processing data. Which sequence correctly implements this synchronization?

hard

A. Producer calls ulTaskNotifyTake(pdTRUE, portMAX_DELAY);, Consumer calls xTaskNotifyGive(ProducerHandle);

B. Consumer calls xTaskNotifyGive() and Producer calls ulTaskNotifyTake()

C. Both tasks call xTaskNotifyGive() simultaneously without waiting

D. Consumer calls ulTaskNotifyTake(pdTRUE, portMAX_DELAY); to wait, Producer calls xTaskNotifyGive(ConsumerHandle); to signal

Solution

1. Step 1: Identify roles of tasks

   The Consumer should wait for a signal before processing, so it uses ulTaskNotifyTake() to block until notified.

2. Step 2: Producer signals Consumer

   The Producer sends the notification using xTaskNotifyGive() with the Consumer's task handle.

3. Final Answer:

   Consumer calls ulTaskNotifyTake(pdTRUE, portMAX_DELAY); to wait, Producer calls xTaskNotifyGive(ConsumerHandle); to signal -> Option D

4. Quick Check:

   Wait then signal = Consumer calls ulTaskNotifyTake(pdTRUE, portMAX_DELAY); to wait, Producer calls xTaskNotifyGive(ConsumerHandle); to signal [OK]

Hint: Consumer waits, Producer signals with xTaskNotifyGive() [OK]

Common Mistakes:

• Reversing roles of Producer and Consumer
• Both tasks signaling without waiting
• Using wrong task handles in calls