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semphr. h
xSemaphoreGiveFromISR ( SemaphoreHandle_t xSemaphore, signed BaseType_t *pxHigherPriorityTaskWoken )

Macro to release a semaphore. The semaphore must have previously been created with a call to xSemaphoreCreateBinary() or xSemaphoreCreateCounting().

Mutex type semaphores (those created using a call to xSemaphoreCreateMutex()) must not be used with this macro.

This macro can be used from an ISR.

xSemaphore A handle to the semaphore being released. This is the handle returned when the semaphore was created.
pxHigherPriorityTaskWoken xSemaphoreGiveFromISR() will set *pxHigherPriorityTaskWoken to pdTRUE if giving the semaphore caused a task to unblock, and the unblocked task has a priority higher than the currently running task. If xSemaphoreGiveFromISR() sets this value to pdTRUE then a context switch should be requested before the interrupt is exited.

From FreeRTOS V7.3.0 pxHigherPriorityTaskWoken is an optional parameter and can be set to NULL.

pdTRUE if the semaphore was successfully given, otherwise errQUEUE_FULL.
Example usage:

Note the functionality shown below can often be achieved in a more efficient way by using a direct to task notification in place of a semaphore.

#define LONG_TIME 0xffff #define TICKS_TO_WAIT 10 SemaphoreHandle_t xSemaphore = NULL; /* Repetitive task. */ void vATask( void * pvParameters ) { /* We are using the semaphore for synchronisation so we create a binary semaphore rather than a mutex. We must make sure that the interrupt does not attempt to use the semaphore before it is created! */ xSemaphore = xSemaphoreCreateBinary(); for( ;; ) { /* We want this task to run every 10 ticks of a timer. The semaphore was created before this task was started. Block waiting for the semaphore to become available. */ if( xSemaphoreTake( xSemaphore, LONG_TIME ) == pdTRUE ) { /* It is time to execute. */ ... /* We have finished our task. Return to the top of the loop where we will block on the semaphore until it is time to execute again. Note when using the semaphore for synchronisation with an ISR in this manner there is no need to 'give' the semaphore back. */ } } } /* Timer ISR */ void vTimerISR( void * pvParameters ) { static unsigned char ucLocalTickCount = 0; static signed BaseType_t xHigherPriorityTaskWoken; /* A timer tick has occurred. */ ... Do other time functions. /* Is it time for vATask() to run? */ xHigherPriorityTaskWoken = pdFALSE; ucLocalTickCount++; if( ucLocalTickCount >= TICKS_TO_WAIT ) { /* Unblock the task by releasing the semaphore. */ xSemaphoreGiveFromISR( xSemaphore, &xHigherPriorityTaskWoken ); /* Reset the count so we release the semaphore again in 10 ticks time. */ ucLocalTickCount = 0; } /* If xHigherPriorityTaskWoken was set to true you we should yield. The actual macro used here is port specific. */ portYIELD_FROM_ISR( xHigherPriorityTaskWoken ); }

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