KernelQueues

Updated Jul 2025

xQueueReceiveFromISR

[Queue Management]

queue. h

1 BaseType_t xQueueReceiveFromISR
2           (
3               QueueHandle_t xQueue,
4               void *pvBuffer,
5               BaseType_t *pxHigherPriorityTaskWoken
6           );

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Receive an item from a queue. It is safe to use this function from within an interrupt service routine.

Parameters:

• xQueue

  The handle to the queue from which the item is to be received.

• pvBuffer

  Pointer to the buffer into which the received item will be copied.

• pxHigherPriorityTaskWoken

  A task may be blocked waiting for space to become available on the queue. If

  xQueueReceiveFromISR

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  causes such a task to unblock

  *pxHigherPriorityTaskWoken

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  will get set to

  pdTRUE

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  , otherwise

  *pxHigherPriorityTaskWoken

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  will remain unchanged. From FreeRTOS V7.3.0

  pxHigherPriorityTaskWoken

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  is an optional parameter and can be set to NULL.

Returns:

• pdPASS if an item was successfully received from the queue,
• pdFAIL otherwise.

Example usage:

1QueueHandle_t xQueue;
2
3/* Function to create a queue and post some values. */
4void vAFunction( void *pvParameters )
5{
6    char cValueToPost;
7    const TickType_t xTicksToWait = ( TickType_t )0xff;
8
9    /* Create a queue capable of containing 10 characters. */
10    xQueue = xQueueCreate( 10, sizeof( char ) );
11    if( xQueue == 0 )
12    {
13        /* Failed to create the queue. */
14    }
15
16    /* ... */
17
18    /* Post some characters that will be used within an ISR. If the queue
19       is full then this task will block for xTicksToWait ticks. */
20    cValueToPost = 'a';
21    xQueueSend( xQueue, ( void * ) &cValueToPost, xTicksToWait );
22    cValueToPost = 'b';
23    xQueueSend( xQueue, ( void * ) &cValueToPost, xTicksToWait );
24
25    /* ... keep posting characters ... this task may block when the queue
26       becomes full. */
27
28    cValueToPost = 'c';
29    xQueueSend( xQueue, ( void * ) &cValueToPost, xTicksToWait );
30}
31
32/* ISR that outputs all the characters received on the queue. */
33void vISR_Routine( void )
34{
35BaseType_t xTaskWokenByReceive = pdFALSE;
36char cRxedChar;
37
38    while( xQueueReceiveFromISR( xQueue,
39                                ( void * ) &cRxedChar,
40                                &xTaskWokenByReceive) )
41    {
42        /* A character was received. Output the character now. */
43        vOutputCharacter( cRxedChar );
44
45        /* If removing the character from the queue woke the task that was
46           posting onto the queue xTaskWokenByReceive will have been set to
47           pdTRUE. No matter how many times this loop iterates only one
48           task will be woken. */
49    }
50
51    if( xTaskWokenByReceive != pdFALSE )
52    {
53        /* We should switch context so the ISR returns to a different task.
54           NOTE: How this is done depends on the port you are using. Check
55           the documentation and examples for your port. */
56        taskYIELD ();
57    }
58}

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