Rendezvous (sync) pattern in FreeRTOS - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a binary semaphore for synchronization.

FreeRTOS

SemaphoreHandle_t syncSemaphore = xSemaphoreCreate[1]();

Drag options to blanks, or click blank then click option'

AMutex

BCountingSemaphore

CBinary

DQueue

Attempts:

3 left

💡 Hint

Common Mistakes

Using a counting semaphore instead of a binary semaphore.

Using a mutex which is for mutual exclusion, not rendezvous.

Trying to create a queue instead of a semaphore.

The rendezvous pattern uses a binary semaphore to synchronize tasks. The function xSemaphoreCreateBinary() creates such a semaphore.

2fill in blank

medium

Complete the code to wait indefinitely for the semaphore to be given.

FreeRTOS

if (xSemaphoreTake(syncSemaphore, [1]) == pdTRUE) {
    // Proceed after synchronization
}

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A0

BpdFALSE

C100

DportMAX_DELAY

Attempts:

3 left

💡 Hint

Common Mistakes

Using 0 which means no wait and causes immediate failure if semaphore not available.

Using a finite number which may timeout before synchronization.

Using pdFALSE which is not a valid wait time.

Using portMAX_DELAY makes the task wait indefinitely until the semaphore is available, which is typical in rendezvous synchronization.

3fill in blank

hard

Fix the error in the code to give the semaphore from an ISR correctly.

FreeRTOS

BaseType_t xHigherPriorityTaskWoken = pdFALSE;
xSemaphoreGiveFromISR(syncSemaphore, [1]);
portYIELD_FROM_ISR(xHigherPriorityTaskWoken);

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A&xHigherPriorityTaskWoken

BxHigherPriorityTaskWoken

CNULL

DpdTRUE

Attempts:

3 left

💡 Hint

Common Mistakes

Passing the variable instead of its address.

Passing NULL which disables context switch notification.

Passing pdTRUE which is not a pointer.

The function xSemaphoreGiveFromISR() requires a pointer to xHigherPriorityTaskWoken to correctly manage context switching after ISR.

4fill in blank

hard

Fill both blanks to create a rendezvous where two tasks wait and signal each other using semaphores.

FreeRTOS

void Task1(void *pvParameters) {
    // Wait for Task2
    xSemaphoreTake(syncSemaphore, [1]);
    // Signal Task2
    xSemaphoreGive([2]);
}

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AportMAX_DELAY

BsyncSemaphore

CotherSemaphore

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Using 0 as wait time causing immediate timeout.

Giving the same semaphore it waits on, causing deadlock.

Using incorrect semaphore names.

Task1 waits indefinitely for syncSemaphore and then signals otherSemaphore to synchronize with Task2.

5fill in blank

hard

Fill all three blanks to implement a rendezvous pattern where TaskA and TaskB synchronize using two semaphores.

FreeRTOS

void TaskA(void *pvParameters) {
    xSemaphoreGive([1]);
    xSemaphoreTake([2], [3]);
    // Continue after rendezvous
}

Drag options to blanks, or click blank then click option'

AsemA

BsemB

CportMAX_DELAY

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Using the same semaphore for both give and take causing deadlock.

Using 0 as wait time causing immediate timeout.

Mixing up semaphore names.

TaskA signals semA to notify TaskB, then waits indefinitely on semB to complete the rendezvous synchronization.

Practice

1. What is the main purpose of the Rendezvous (sync) pattern in FreeRTOS?

easy

A. To synchronize two tasks so they wait for each other before proceeding

B. To create multiple independent tasks running without coordination

C. To prioritize one task over another permanently

D. To allocate memory dynamically for tasks

Solution

1. Step 1: Understand the Rendezvous pattern concept

   The Rendezvous pattern is designed to synchronize two tasks so that each waits for the other to reach a certain point before continuing.

2. Step 2: Identify the purpose in FreeRTOS context

   In FreeRTOS, this synchronization is often done using semaphores to signal readiness and wait for the other task.

3. Final Answer:

   To synchronize two tasks so they wait for each other before proceeding -> Option A

4. Quick Check:

   Rendezvous = Synchronize tasks [OK]

Hint: Rendezvous means tasks meet and wait for each other [OK]

Common Mistakes:

• Thinking it creates independent tasks without coordination
• Confusing with task prioritization
• Assuming it manages memory allocation

2. Which FreeRTOS primitive is typically used to implement the Rendezvous pattern?

easy

A. Mutex

B. Queue

C. Semaphore

D. Event Group

Solution

1. Step 1: Recall synchronization primitives in FreeRTOS

   FreeRTOS provides mutexes, semaphores, queues, and event groups for task synchronization and communication.

2. Step 2: Identify which primitive signals readiness and waits

   The Rendezvous pattern requires signaling and waiting between two tasks, which is best done using semaphores.

3. Final Answer:

   Semaphore -> Option C

4. Quick Check:

   Rendezvous uses semaphores [OK]

Hint: Semaphores signal and wait between tasks [OK]

Common Mistakes:

• Choosing mutex which is for mutual exclusion, not sync
• Confusing queues with direct signaling
• Using event groups which are for multiple event flags

3. Consider two tasks using binary semaphores for Rendezvous. Task A signals semaphore S1 and waits on S2. Task B signals S2 and waits on S1. What happens if Task A runs first and signals S1 before Task B waits on it?

medium

A. Both tasks will run without blocking

B. Task B will block indefinitely waiting on S1

C. Task A will block indefinitely waiting on S2

D. Task B will unblock immediately when it waits on S1

Solution

1. Step 1: Understand semaphore signaling and waiting

   If Task A signals semaphore S1 before Task B waits on it, the semaphore count becomes available.

2. Step 2: Analyze Task B's wait on S1

   When Task B waits on S1, it finds the semaphore already signaled and unblocks immediately without waiting.

3. Final Answer:

   Task B will unblock immediately when it waits on S1 -> Option D

4. Quick Check:

   Signaled semaphore unblocks waiting task immediately [OK]

Hint: Semaphore signals persist until taken by waiting task [OK]

Common Mistakes:

• Assuming tasks must wait even if semaphore is signaled
• Thinking Task A blocks waiting on S2 first
• Believing both tasks run without blocking always

4. Given this snippet for Rendezvous synchronization, what is the error?

void TaskA(void *pvParameters) {
  xSemaphoreGive(SemA);
  xSemaphoreTake(SemB, portMAX_DELAY);
  // Continue...
}

void TaskB(void *pvParameters) {
  xSemaphoreTake(SemA, portMAX_DELAY);
  xSemaphoreGive(SemB);
  // Continue...
}

medium

A. Tasks should not use semaphores for synchronization

B. Semaphores SemA and SemB are not created before use

C. xSemaphoreGive should be called after xSemaphoreTake in TaskA

D. portMAX_DELAY is invalid for xSemaphoreTake

Solution

1. Step 1: Check semaphore initialization

   Before using semaphores, they must be created with xSemaphoreCreateBinary or similar functions.

2. Step 2: Identify missing creation

   The code snippet does not show creation of SemA and SemB, which will cause runtime errors or undefined behavior.

3. Final Answer:

   Semaphores SemA and SemB are not created before use -> Option B

4. Quick Check:

   Semaphores must be created before giving or taking [OK]

Hint: Always create semaphores before using them [OK]

Common Mistakes:

• Thinking order of give/take is wrong here
• Believing semaphores are not for synchronization
• Misunderstanding portMAX_DELAY usage

5. You want two tasks to synchronize at multiple points using the Rendezvous pattern. Which approach ensures no deadlock if one task signals twice before the other waits?

hard

A. Use counting semaphores instead of binary semaphores

B. Use mutexes to lock and unlock tasks

C. Use queues to send messages instead of semaphores

D. Use event groups with a single bit flag

Solution

1. Step 1: Understand semaphore types and their behavior

   Binary semaphores can only hold one signal at a time, so signaling twice before waiting can cause lost signals and deadlock.

2. Step 2: Identify solution to handle multiple signals

   Counting semaphores can hold multiple signals, allowing tasks to signal multiple times without losing signals, preventing deadlock.

3. Final Answer:

   Use counting semaphores instead of binary semaphores -> Option A

4. Quick Check:

   Counting semaphores handle multiple signals safely [OK]

Hint: Counting semaphores count signals, preventing lost sync [OK]

Common Mistakes:

• Using mutexes which do not count signals
• Using queues which add complexity
• Using event groups with single bits only