xSemaphoreGiveFromISR

semphr. h

xSemaphoreGiveFromISR(
                         SemaphoreHandle_t xSemaphore,
                         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.

Parameters

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.

Returns

pdTRUE if the semaphore was successfully given, otherwise errQUEUE_FULL.

Example usage:

#define LONG_TIME 0xffff
#define TICKS_TO_WAIT  10
SemaphoreHandle_t xSemaphore = NULL;

// Repetitive task.
void vATask( void * pvParameters )
{
   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 uint8_t ucLocalTickCount = 0;
static 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 != pdFALSE )
   {
       // We can force a context switch here.  Context switching from an
       // ISR uses port specific syntax.  Check the demo task for your port
       // to find the syntax required.
   }
}