[RTOS Task Notification API]


 BaseType_t xTaskNotifyAndQueryFromISR(
                                 TaskHandle_t xTaskToNotify,
                                 uint32_t ulValue,
                                 eNotifyAction eAction,
                                 uint32_t *pulPreviousNotifyValue,
                                 BaseType_t *pxHigherPriorityTaskWoken );

[If you are using RTOS task notifications to implement binary or counting semaphore type behaviour then use the simpler vTaskNotifyGiveFromISR() API function instead of xTaskNotifyAndQueryFromISR().]

xTaskNotifyAndQuery() is similar to xTaskNotify(), but contains an additional parameter in which the subject task’s previous notification value is returned. xTaskNotifyAndQueryFromISR() is a version of xTaskNotifyAndQuery() that can be called from an interrupt service routine (ISR).

Each RTOS task has a 32-bit notification value which is initialised to zero when the RTOS task is created. xTaskNotifyAndQueryFromISR() is used to send an event directly to and potentially unblock an RTOS task, and optionally update the receiving task’s notification value in one of the following ways:

  • Write a 32-bit number to the notification value
  • Add one (increment) the notification value
  • Set one or more bits in the notification value
  • Leave the notification value unchanged

xTaskToNotify   The handle of the RTOS task being notified. This is the subject task.

To obtain a task’s handle create the task using xTaskCreate() and make use of the pxCreatedTask parameter, or create the task using xTaskCreateStatic() 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 xTaskGetCurrentTaskHandle() API function.

ulValue   Used to update the notification value of the subject task. See the description of the eAction parameter below.
eAction   An enumerated type that can take one of the values documented in the table below in order to perform the associated action.
pulPreviousNotifyValue   Can be used to pass out the subject task’s notification value before any bits are modified by the action of xTaskNotifyAndQueryFromISR().

pulPreviousNotifyValue is an optional parameter, and can be set to NULL if it is not required. If pulPreviousNotifyValue is not used then consider using xTaskNotify() in place of xTaskNotifyAndQueryFromISR().

pxHigherPriorityTaskWoken   *pxHigherPriorityTaskWoken must be initialised to pdFALSE (0).

xTaskNotifyAndQueryFromISR() will set *pxHigherPriorityTaskWoken to pdTRUE if sending the notification caused a task to unblock, and the unblocked task has a priority higher than the currently running task.

If xTaskNotifyAndQueryFromISR() sets this value to pdTRUE then a context switch should be requested before the interrupt is exited. See the example below.

pxHigherPriorityTaskWoken is an optional parameter and can be set to NULL.

eAction Setting Action Performed
eNoAction The subject task receives the event, but its notification value is not updated. In this case ulValue is not used.

eSetBits The notification value of the subject task will be bitwise ORed with ulValue. For example, if ulValue is set to 0x01, then bit 0 will get set within the subject task’s notification value. Likewise if ulValue is 0x04 then bit 2 will get set in the subject task’s notification value. In this way the RTOS task notification mechanism can be used as a light weight alternative to an event group.

eIncrement The notification value of the subject task will be incremented by one, making the call to xTaskNotify() equivalent to a call to xTaskNotifyGive(). In this case ulValue is not used.
eSetValueWithOverwrite The notification value of the subject task is unconditionally set to ulValue. In this way the RTOS task notification mechanism is being used as a light weight alternative to xQueueOverwrite().
eSetValueWithoutOverwrite If the subject task does not already have a notification pending then its notification value will be set to ulValue.

If the subject task already has a notification pending then its notification value is not updated as to do so would overwrite the previous value before it was used. In this case the call to xTaskNotify() fails and pdFALSE is returned.

In this way the RTOS task notification mechanism is being used as a light weight alternative to xQueueSend() on a queue of length 1.

pdPASS is returned in all cases other than when eAction is set to eSetValueWithoutOverwrite and the subject task’s notification value cannot be updated because the subject task already had a notification pending.

Example usage:

void vAnISR( void )
BaseType_t xHigherPriorityTaskWoken = pdFALSE. /* Muse be Initialised to pdFALSE! */
uint32_t ulPreviousValue;

    /* Set bit 8 in the notification value of the task referenced by xTask1Handle.
    Store the task's previous notification value (before bit 8 is set) in
    ulPreviousValue. */
    xTaskNotifyAndQueryFromISR( xTask1Handle,
                                ( 1UL << 8UL ),
                                &xHigherPriorityTaskWoken );

    /* The task's previous notification value is saved in ulPreviousValue. */

    /* If the task referenced by xTask1Handle was in the Blocked state, waiting
    for the notification, then it will now have been moved from the Blocked
    state to the Ready state.  If its priority is higher than the priority of
    the currently executing task (the task this interrupt interrupted) then
    xHigherPriorityTaskWoken will have been set to pdTRUE, and passing the
    variable into a call to portYIELD_FROM_ISR() will result in the interrupt
    returning directly to the unblocked task.  If xHigherPriorityTaskWoken is
    still pdFALSE then passing it into portYIELD_FROM_ISR() will have no
    effect. */
    portYIELD_FROM_ISR( xHigherPriorityTaskWoken );

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