Chapter 2. API Reference
• pvBuffer: Pointer to the buffer into which the received item will be copied.
• xTicksToWait: The maximum amount of time the task should block waiting for an item to
receive should the queue be empty at the time of the call. xQueueReceive() will return immediately
if xTicksToWait is zero and the queue is empty. The time is defined in tick periods so the constant
portTICK_PERIOD_MS should be used to convert to real time if this is required.
UBaseType_t uxQueueMessagesWaiting(const QueueHandle_t xQueue)
Return the number of messages stored in a queue.
Return The number of messages available in the queue.
Parameters
• xQueue: A handle to the queue being queried.
UBaseType_t uxQueueSpacesAvailable(const QueueHandle_t xQueue)
Return the number of free spaces available in a queue. This is equal to the number of items that can be sent to
the queue before the queue becomes full if no items are removed.
Return The number of spaces available in the queue.
Parameters
• xQueue: A handle to the queue being queried.
void vQueueDelete(QueueHandle_t xQueue)
Delete a queue - freeing all the memory allocated for storing of items placed on the queue.
Parameters
• xQueue: A handle to the queue to be deleted.
BaseType_t xQueueGenericSendFromISR(QueueHandle_t xQueue, const void *const pvItemTo-
Queue, BaseType_t *const pxHigherPriorityTaskWoken,
const BaseType_t xCopyPosition)
It is preferred that the macros xQueueSendFromISR(), xQueueSendToFrontFromISR() and xQueueSendTo-
BackFromISR() be used in place of calling this function directly. xQueueGiveFromISR() is an equivalent for
use by semaphores that don’t actually copy any data.
Post an item on 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 xHigherPriorityTaskWokenByPost;
// We have not woken a task at the start of the ISR.
xHigherPriorityTaskWokenByPost = pdFALSE;
// Loop until the buffer is empty.
do
{
// Obtain a byte from the buffer.
cIn = portINPUT_BYTE( RX_REGISTER_ADDRESS );
// Post each byte.
xQueueGenericSendFromISR( xRxQueue, &cIn, &xHigherPriorityTaskWokenByPost,
,→ queueSEND_TO_BACK );
} while( portINPUT_BYTE( BUFFER_COUNT ) );
// Now the buffer is empty we can switch context if necessary. Note that the
// name of the yield function required is port specific.
if( xHigherPriorityTaskWokenByPost )
{
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Espressif Systems 887
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