Quality RTOS & Embedded Software

AWS Reference Integrations:
Embedded World track keynote:

[RTOS Stream Buffer API]


size_t xStreamBufferSendFromISR( StreamBufferHandle_t xStreamBuffer,
                                 const void *pvTxData,
                                 size_t xDataLengthBytes,
                                 BaseType_t *pxHigherPriorityTaskWoken );

Interrupt safe version of the API function that sends a stream of bytes to the stream buffer.

NOTE: Uniquely among FreeRTOS objects, the stream buffer implementation (so also the message buffer implementation, as message buffers are built on top of stream buffers) assumes there is only one task or interrupt that will write to the buffer (the writer), and only one task or interrupt that will read from the buffer (the reader). It is safe for the writer and reader to be different tasks or interrupts, but, unlike other FreeRTOS objects, it is not safe to have multiple different writers or multiple different readers. If there are to be multiple different writers then the application writer must place each call to a writing API function (such as xStreamBufferSend()) inside a critical section and use a send block time of 0. Likewise, if there are to be multiple different readers then the application writer must place each call to a reading API function (such as xStreamBufferReceive()) inside a critical section and use a receive block time of 0.

Use xStreamBufferSend() to write to a stream buffer from a task. Use xStreamBufferSendFromISR() to write to a stream buffer from an interrupt service routine (ISR).


xStreamBuffer   The handle of the stream buffer to which a stream is being sent.
pvTxData   A pointer to the buffer that holds the bytes to be copied into the stream buffer.
xDataLengthBytes   The maximum number of bytes to copy from pvTxData into the stream buffer.
pxHigherPriorityTaskWoken   It is possible that a stream buffer will have a task blocked on it waiting for data. Calling xStreamBufferSendFromISR() can make data available, and so cause a task that was waiting for data to leave the Blocked state. If calling xStreamBufferSendFromISR() causes a task to leave the Blocked state, and the unblocked task has a priority higher than the currently executing task (the task that was interrupted), then, internally, xStreamBufferSendFromISR() will set *pxHigherPriorityTaskWoken to pdTRUE. If xStreamBufferSendFromISR() sets this value to pdTRUE, then normally a context switch should be performed before the interrupt is exited. This will ensure that the interrupt returns directly to the highest priority Ready state task. *pxHigherPriorityTaskWoken should be set to pdFALSE before it is passed into the function. See the example code below for an example.
The number of bytes written to the stream buffer. If a task times out before it can write all xDataLengthBytes into the buffer it will still write as many bytes as possible.

Example usage:

/* A stream buffer that has already been created. */
StreamBufferHandle_t xStreamBuffer;

void vAnInterruptServiceRoutine( void )
size_t xBytesSent;
char *pcStringToSend = "String to send";
BaseType_t xHigherPriorityTaskWoken = pdFALSE; /* Initialised to pdFALSE. */

    /* Attempt to send the string to the stream buffer. */
    xBytesSent = xStreamBufferSendFromISR( xStreamBuffer,
                                           ( void * ) pcStringToSend,
                                           strlen( pcStringToSend ),
                                           &xHigherPriorityTaskWoken );

    if( xBytesSent != strlen( pcStringToSend ) )
        /* There was not enough free space in the stream buffer for the entire
        string to be written, ut xBytesSent bytes were written. */

    /* If xHigherPriorityTaskWoken was set to pdTRUE inside
    xStreamBufferSendFromISR() then a task that has a priority above the
    priority of the currently executing task was unblocked and a context
    switch should be performed to ensure the ISR returns to the unblocked
    task.  In most FreeRTOS ports this is done by simply passing
    xHigherPriorityTaskWoken into taskYIELD_FROM_ISR(), which will test the
    variables value, and perform the context switch if necessary.  Check the
    documentation for the port in use for port specific instructions. */
    taskYIELD_FROM_ISR( xHigherPriorityTaskWoken );
