Chapter 2. API Reference
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// ...
if( xHigherPrioritytaskWoken == pdTRUE )
{
// Writing to the queue caused a task to unblock and the unblocked task
// has a priority higher than or equal to the priority of the currently
// executing task (the task this interrupt interrupted). Perform a␣
,→context
// switch so this interrupt returns directly to the unblocked task.
portYIELD_FROM_ISR(); // or portEND_SWITCHING_ISR() depending on the port.
}
}
Return xQueueOverwriteFromISR() is a macro that calls xQueueGenericSendFromISR(), and therefore has
the same return values as xQueueSendToFrontFromISR(). However, pdPASS is the only value that can
be returned because xQueueOverwriteFromISR() will write to the queue even when the queue is already
full.
Parameters
• xQueue: The handle to the queue on which the item is to be posted.
• pvItemToQueue: A pointer to the item that is to be placed on the queue. The size of the items
the queue will hold was defined when the queue was created, so this many bytes will be copied from
pvItemToQueue into the queue storage area.
• [out] pxHigherPriorityTaskWoken: xQueueOverwriteFromISR() will set *pxHigher-
PriorityTaskWoken to pdTRUE if sending to the queue caused a task to unblock, and the unblocked
task has a priority higher than the currently running task. If xQueueOverwriteFromISR() sets this
value to pdTRUE then a context switch should be requested before the interrupt is exited.
xQueueSendFromISR(xQueue, pvItemToQueue, pxHigherPriorityTaskWoken)
This is a macro that calls xQueueGenericSendFromISR(). It is included for backward compatibility with
versions of FreeRTOS.org that did not include the xQueueSendToBackFromISR() and xQueueSendToFront-
FromISR() macros.
Post an item to the back of a queue. It is safe to use this function from within an interrupt service routine.
Items are queued by copy not reference so it is preferable to only queue small items, especially when called
from an ISR. In most cases it would be preferable to store a pointer to the item being queued.
Example usage for buffered IO (where the ISR can obtain more than one value per call):
void vBufferISR( void )
{
char cIn;
BaseType_t xHigherPriorityTaskWoken;
// We have not woken a task at the start of the ISR.
xHigherPriorityTaskWoken = pdFALSE;
// Loop until the buffer is empty.
do
{
// Obtain a byte from the buffer.
cIn = portINPUT_BYTE( RX_REGISTER_ADDRESS );
// Post the byte.
xQueueSendFromISR( xRxQueue, &cIn, &xHigherPriorityTaskWoken );
} while( portINPUT_BYTE( BUFFER_COUNT ) );
// Now the buffer is empty we can switch context if necessary.
if( xHigherPriorityTaskWoken )
{
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