Chapter 2. API Reference
TASK_NOTIFICATION_ARRAY_ENTRIES. xTaskNotify() does not have this parameter and al-
ways sends notifications to index 0.
• ulValue: Data that can be sent with the notification. How the data is used depends on the value
of the eAction parameter.
• eAction: Specifies how the notification updates the task’s notification value, if at all. Valid values
for eAction are as follows:
eIncrement - The target notification value is incremented. ulValue is not used and xTaskNotifyIndexed() always
returns pdPASS in this case.
eSetValueWithOverwrite - The target notification value is set to the value of ulValue, even if the task being no-
tified had not yet processed the previous notification at the same array index (the task already had a notification
pending at that index). xTaskNotifyIndexed() always returns pdPASS in this case.
eSetValueWithoutOverwrite - If the task being notified did not already have a notification pending at the same
array index then the target notification value is set to ulValue and xTaskNotifyIndexed() will return pdPASS.
If the task being notified already had a notification pending at the same array index then no action is performed
and pdFAIL is returned.
eNoAction - The task receives a notification at the specified array index without the notification value at that
index being updated. ulValue is not used and xTaskNotifyIndexed() always returns pdPASS in this case.
pulPreviousNotificationValue - Can be used to pass out the subject task’s notification value before any bits
are modified by the notify function.
Return Dependent on the value of eAction. See the description of the eAction parameter.
BaseType_t xTaskGenericNotifyFromISR(TaskHandle_t xTaskToNotify, UBaseType_t uxIn-
dexToNotify, uint32_t ulValue, eNotifyAction eAction,
uint32_t *pulPreviousNotificationValue, BaseType_t
*pxHigherPriorityTaskWoken)
See https://www.FreeRTOS.org/RTOS-task-notifications.html for details.
configUSE_TASK_NOTIFICATIONS must be undefined or defined as 1 for these functions to be available.
A version of xTaskNotifyIndexed() that can be used from an interrupt service routine (ISR).
Each task has a private array of “notification values”(or ‘notifications’), each of which is a 32-bit un-
signed integer (uint32_t). The constant configTASK_NOTIFICATION_ARRAY_ENTRIES sets the number
of indexes in the array, and (for backward compatibility) defaults to 1 if left undefined. Prior to FreeRTOS
V10.4.0 there was only one notification value per task.
Events can be sent to a task using an intermediary object. Examples of such objects are queues, semaphores,
mutexes and event groups. Task notifications are a method of sending an event directly to a task without the
need for such an intermediary object.
A notification sent to a task can optionally perform an action, such as update, overwrite or increment one of
the task’s notification values. In that way task notifications can be used to send data to a task, or be used as
light weight and fast binary or counting semaphores.
A task can use xTaskNotifyWaitIndexed() to [optionally] block to wait for a notification to be pending, or
ulTaskNotifyTakeIndexed() to [optionally] block to wait for a notification value to have a non-zero value. The
task does not consume any CPU time while it is in the Blocked state.
A notification sent to a task will remain pending until it is cleared by the task calling xTaskNotifyWaitIndexed()
or ulTaskNotifyTakeIndexed() (or their un-indexed equivalents). If the task was already in the Blocked state
to wait for a notification when the notification arrives then the task will automatically be removed from the
Blocked state (unblocked) and the notification cleared.
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.
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 notifica-
tions within the array. xTaskNotifyFromISR() is the original API function, and remains backward compatible
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