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xSemaphoreCreateBinaryStatic
[Semaphores]

semphr. h

SemaphoreHandle_t xSemaphoreCreateBinaryStatic(
                          StaticSemaphore_t *pxSemaphoreBuffer );
	

Creates a binary semaphore, and returns a handle by which the semaphore can be referenced. configSUPPORT_STATIC_ALLOCATION must be set to 1 in FreeRTOSConfig.h for this RTOS API function to be available.

Each binary semaphore require a small amount of RAM that is used to hold the semaphore's state. If a binary semaphore is created using xSemaphoreCreateBinary() then the required RAM is automatically allocated from the FreeRTOS heap. If a binary semaphore is created using xSemaphoreCreateBinaryStatic() then the RAM is provided by the application writer, which requires an additional parameter, but allows the RAM to be statically allocated at compile time. See the Static Vs Dynamic allocation page for more information.

The semaphore is created in the 'empty' state, meaning the semaphore must first be given using the xSemaphoreGive() API function before it can subsequently be taken (obtained) using the xSemaphoreTake() function.

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. Note the same functionality can often be achieved in a more efficient way using a direct to task notification.

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 referenced by variables of type SemaphoreHandle_t and can be used in any task level API function that takes a parameter of that type. Unlike mutexes, binary semaphores can be used in interrupt service routines.

Parameters:
pxSemaphoreBuffer   Must point to a variable of type StaticSemaphore_t, which will be used to hold the semaphore's state.
Return values:
NULL   The semaphore could not be created because pxSemaphoreBuffer was NULL.
Any other value   The semaphore was created successfully. The returned value is a handle by which the semaphore can be referenced.
Example usage:
 SemaphoreHandle_t xSemaphore = NULL;
 StaticSemaphore_t xSemaphoreBuffer;

 void vATask( void * pvParameters )
 {
    /* Create a binary semaphore without using any dynamic memory
    allocation.  The semaphore's data structures will be saved into
    the xSemaphoreBuffer variable. */
    xSemaphore = xSemaphoreCreateBinaryStatic( &xSemaphoreBuffer );

    /* The pxSemaphoreBuffer was not NULL, so it is expected that the
    handle will not be NULL. */
    configASSERT( xSemaphore );

    /* Rest of the task code goes here. */
 }





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