KernelSemaphore and Mutexes

Updated Mar 2025

vSemaphoreCreateBinary

[Semaphores]

semphr.h

1vSemaphoreCreateBinary( SemaphoreHandle_t xSemaphore )

Copy

NOTE: The

vSemaphoreCreateBinary()

Copy

Also, in many cases, it is faster and more memory efficient to use a direct to task notification in place of a binary semaphore.

Macro that creates a semaphore by using the existing queue mechanism. The queue length is 1 as this is a binary semaphore. The data size is 0 as we don't want to actually store any data - we just want to know if the queue is empty or full.

Binary semaphores and mutexes are very similar but have some subtle differences: Mutexes include a priority inheritance mechanism, binary semaphores do not. This makes binary semaphores the better choice for implementing synchronisation (between tasks or between tasks and an interrupt), and mutexes the better choice for implementing simple mutual exclusion.

A binary semaphore need not be given back once obtained, so task synchronisation can be implemented by one task/interrupt continuously 'giving' the semaphore while another continuously 'takes' the semaphore. This is demonstrated by the sample code on the xSemaphoreGiveFromISR() documentation page.

The priority of a task that 'takes' a mutex can potentially be raised if another task of higher priority attempts to obtain the same mutex. The task that owns the mutex 'inherits' the priority of the task attempting to 'take' the same mutex. This means the mutex must always be 'given' back - otherwise the higher priority task will never be able to obtain the mutex, and the lower priority task will never 'disinherit' the priority. An example of a mutex being used to implement mutual exclusion is provided on the xSemaphoreTake() documentation page.

Both mutex and binary semaphores are assigned to variables of type

SemaphoreHandle_t

Copy

Parameters:

• xSemaphore

  Handle to the created semaphore. Should be of type

  SemaphoreHandle\_t

  Copy
  .

Example usage:

1 SemaphoreHandle_t xSemaphore;
2
3 void vATask( void * pvParameters )
4 {
5    // Semaphore cannot be used before a call to vSemaphoreCreateBinary ().
6    // This is a macro so pass the variable in directly.
7    vSemaphoreCreateBinary( xSemaphore );
8
9    if( xSemaphore != NULL )
10    {
11        // The semaphore was created successfully.
12        // The semaphore can now be used.
13    }
14 }

Copy