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[RTOS Message Buffer API]


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 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 xMessageBufferRead()) inside a critical section and use a receive block time of 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).

Message buffer functionality is enabled by including the FreeRTOS/source/stream_buffer.c source file in the build (as message buffers use stream buffers).

xMessageBuffer   The handle of the message buffer to which a message is being sent.
pvTxData   A pointer to the message that is to be copied into the message buffer.
xDataLengthBytes   The 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).
pxHigherPriorityTaskWoken   (This is an optional parameter that can be set to NULL.) It 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.
The 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 usage:

/* 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 );

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