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

A version of xSemaphoreTake() that can be called from an ISR. Unlike xSemaphoreTake(), xSemaphoreTakeFromISR() does not permit a block time to be specified.

xSemaphore The semaphore being 'taken'. A semaphore is referenced by a variable of type SemaphoreHandle_t and must be explicitly created before being used.
pxHigherPriorityTaskWoken It is possible (although unlikely, and dependent on the semaphore type) that a semaphore will have one or more tasks blocked on it waiting to give the semaphore. Calling xSemaphoreTakeFromISR() will make a task that was blocked waiting to give the semaphore leave the Blocked state. If calling the API function causes a task to leave the Blocked state, and the unblocked task has a priority equal to or higher than the currently executing task (the task that was interrupted), then, internally, the API function will set *pxHigherPriorityTaskWoken to pdTRUE.

If xSemaphoreTakeFromISR() sets *pxHigherPriorityTaskWoken to pdTRUE, then a context switch should be performed before the interrupt is exited. This will ensure that the interrupt returns directly to the highest priority Ready state task. The mechanism is identical to that used in the xQueueReceiveFromISR() function, and readers are referred to the xQueueReceiveFromISR() documentation for further explanation.

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

pdTRUE if the semaphore was successfully taken. pdFALSE if the semaphore was not successfully taken because it was not available.

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