xMessageBufferSendFromISR

message_buffer.h

    size_t xMessageBufferSendFromISR( MessageBufferHandle_t xMessageBuffer,
                                      const void *pvTxData,
                                      size_t xDataLengthBytes,
                                      BaseType_t *pxHigherPriorityTaskWoken );

       Interrupt safe version of the API function that sends a discrete message to
       the message buffer.  The message can be any length that fits within the
       buffer's free space, and is copied into the 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 xMessageBufferSend()) inside a critical section and set the send
       block time to 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 xMessageBufferRead()) inside a critical section and set the receive
       block time to 0.

       Use xMessageBufferSend() to write to a message buffer from a task.  Use
       xMessageBufferSendFromISR() to write to a message buffer from an interrupt
       service routine (ISR).

    Parameters

        xMessageBufferThe handle of the message buffer to which a message is
       being sent.
        pvTxDataA pointer to the message that is to be copied into the
       message buffer.
        xDataLengthBytesThe length of the message.  That is, the number of
       bytes to copy from pvTxData into the message buffer.  When a message is
       written to the message buffer an additional sizeof( size_t ) bytes are also
       written to store the message's length.  sizeof( size_t ) is typically 4 bytes
       on a 32-bit architecture, so on most 32-bit architecture setting
       xDataLengthBytes to 20 will reduce the free space in the message buffer by 24
       bytes (20 bytes of message data and 4 bytes to hold the message length).
        pxHigherPriorityTaskWokenIt is possible that a message buffer will
       have a task blocked on it waiting for data.  Calling
       xMessageBufferSendFromISR() can make data available, and so cause a task that
       was waiting for data to leave the Blocked state.  If calling
       xMessageBufferSendFromISR() 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, xMessageBufferSendFromISR()
       will set *pxHigherPriorityTaskWoken to pdTRUE.  If
       xMessageBufferSendFromISR() 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 code example below for an example.

    ReturnsThe number of bytes actually written to the message buffer.  If the
       message buffer didn't have enough free space for the message to be stored
       then 0 is returned, otherwise xDataLengthBytes is returned.
    Example use:

    // A message buffer that has already been created.
    MessageBufferHandle_t xMessageBuffer;

    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 message buffer.
        xBytesSent = xMessageBufferSendFromISR( xMessageBuffer,
                                                ( void * ) pcStringToSend,
                                                strlen( pcStringToSend ),
                                                &xHigherPriorityTaskWoken );

        if( xBytesSent != strlen( pcStringToSend ) )
        {
            // The string could not be added to the message buffer because there was
            // not enough free space in the buffer.
        }

        // If xHigherPriorityTaskWoken was set to pdTRUE inside
        // xMessageBufferSendFromISR() 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 );
    }