BaseType_t xTaskNotifyGive( TaskHandle_t xTaskToNotify );
BaseType_t xTaskNotifyGiveIndexed( TaskHandle_t xTaskToNotify,
UBaseType_t uxIndexToNotify );
Each task has an array of ‘task notifications’ (or just ‘notifications’), each
of which has a state and a 32-bit value. A direct to task notification is an event sent
directly to a task that can unblock the receiving task, and
optionally update one of the receiving task’s notification values in a number of different ways.
For example, a notification may overwrite one of the receiving task’s notification values, or just set one or
more bits in one of the receiving task’s notification values.
xTaskNotifyGive() is a macro intended for use when a task notification
is being used as a light weight
and faster binary or counting semaphore alternative.
FreeRTOS semaphores are given using the xSemaphoreGive() API function, xTaskNotifyGive()
is the equivalent that uses one of the receiving RTOS task’s notification values in place of a semaphore.
xTaskNotifyGive() and xTaskNotifyGiveIndexed() are equivalent macros – the only difference
being xTaskNotifyGiveIndexed() can operate on any task notification within the
array and xTaskNotifyGive() always operates on the task notification at array index 0.
When a task notification value is being used as a binary or counting semaphore
equivalent then the task being notified should wait for the notification
using the ulTaskNotifyTake() API
function rather than the xTaskNotifyWait()
Note: Each notification within the array operates independently – a task can only block on one notification within the array at a time and will not be unblocked by a notification sent to any other array index.
xTaskNotifyGive() must not be called from an interrupt service routine. Use
must set to 1 in FreeRTOSConfig.h (or be left undefined) for these macros to be available.
configTASK_NOTIFICATION_ARRAY_ENTRIES sets the number of indexes in
each task’s array of task notifications.
Backward compatibility information:
Prior to FreeRTOS V10.4.0 each task had a single “notification value”, and
all task notification API functions operated on that value. Replacing the
single notification value with an array of notification values necessitated a
new set of API functions that could address specific notifications within the
array. xTaskNotifyGive() is the original API function, and remains backward
compatible by always operating on the notification value at index 0 in the
array. Calling xTaskNotifyGive() is equivalent to calling
xTaskNotifyGiveIndexed() with the uxIndexToNotify parameter set to 0.
The handle of the RTOS task being notified, and having its
notification value incremented.
To obtain a task’s handle create the task using
xTaskCreate() and make use of the
pxCreatedTask parameter, or create the task using
and store the returned value, or use the task’s name in a
call to xTaskGetHandle().
The handle of the currently executing RTOS task is returned by
The index within the target task’s array of
notification values to which the notification is to be sent.
uxIndexToNotify must be less than configTASK_NOTIFICATION_ARRAY_ENTRIES.
xTaskNotifyGive() does not have this parameter and always sends notifications to index 0.
xTaskNotifyGiveIndexed() is a macro that calls xTaskNotifyIndexed() with the
eAction parameter set to eIncrement, so all calls returning pdPASS.
[More examples are referenced from the main RTOS task notifications page]
/* Prototypes of the two tasks created by main(). */
static void prvTask1( void *pvParameters );
static void prvTask2( void *pvParameters );
/* Handles for the tasks create by main(). */
static TaskHandle_t xTask1 = NULL, xTask2 = NULL;
/* Create two tasks that send notifications back and forth to each other,
then start the RTOS scheduler. */
void main( void )
xTaskCreate( prvTask1, “Task1”, 200, NULL, tskIDLE_PRIORITY, &xTask1 );
xTaskCreate( prvTask2, “Task2”, 200, NULL, tskIDLE_PRIORITY, &xTask2 );
/* prvTask1() uses the ‘indexed’ version of the API. */
static void prvTask1( void *pvParameters )
for( ;; )
/* Send notification to prvTask2(), bringing it out of the
Blocked state. */
xTaskNotifyGiveIndexed( xTask2, 0 );
/* Block to wait for prvTask2() to notify this task. */
ulTaskNotifyTakeIndexed( 0, pdTRUE, portMAX_DELAY );
/* prvTask2() uses the original version of the API (without the
static void prvTask2( void *pvParameters )
for( ;; )
/* Block to wait for prvTask1() to notify this task. */
ulTaskNotifyTake( pdTRUE, portMAX_DELAY );
/* Send a notification to prvTask1(), bringing it out of the
Blocked state. */
xTaskNotifyGive( xTask1 );
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