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[RTOS Message Buffer API]


size_t xMessageBufferReceiveFromISR( MessageBufferHandle_t xMessageBuffer,
                                     void *pvRxData,
                                     size_t xBufferLengthBytes,
                                     BaseType_t *pxHigherPriorityTaskWoken );

An interrupt safe version of the API function that receives a discrete message from a message buffer. Messages can be of variable length and are copied out of 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 xMessageBufferReceive() to read from a message buffer from a task. Use xMessageBufferReceiveFromISR() to read from 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 from which a message is being received.
pvRxData   A pointer to the buffer into which the received message will be copied.
xBufferLengthBytes   The length of the buffer pointed to by the pvRxData parameter. This sets the maximum length of the message that can be received. If xBufferLengthBytes is too small to hold the next message then the message will be left in the message buffer and 0 will be returned.
pxHigherPriorityTaskWoken   It is possible that a message buffer will have a task blocked on it waiting for space to become available. Calling xMessageBufferReceiveFromISR() can make space available, and so cause a task that is waiting for space to leave the Blocked state. If calling xMessageBufferReceiveFromISR() 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, xMessageBufferReceiveFromISR() will set *pxHigherPriorityTaskWoken to pdTRUE. If xMessageBufferReceiveFromISR() sets this value to pdTRUE, then normally a context switch should be performed before the interrupt is exited. That will ensure 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 length, in bytes, of the message read from the message buffer, if any.

Example usage:

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

void vAnInterruptServiceRoutine( void )
uint8_t ucRxData[ 20 ];
size_t xReceivedBytes;
BaseType_t xHigherPriorityTaskWoken = pdFALSE;  /* Initialised to pdFALSE. */

    /* Receive the next message from the message buffer. */
    xReceivedBytes = xMessageBufferReceiveFromISR( xMessageBuffer,
                                                  ( void * ) ucRxData,
                                                  sizeof( ucRxData ),
                                                  &xHigherPriorityTaskWoken );

    if( xReceivedBytes > 0 )
        /* A ucRxData contains a message that is xReceivedBytes long.  Process
        the message here.... */

    /* If xHigherPriorityTaskWoken was set to pdTRUE inside
    xMessageBufferReceiveFromISR() 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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