size_t xMessageBufferReceiveFromISR( MessageBufferHandle_t xMessageBuffer,
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).
The handle of the message buffer from which a message is being received.
A pointer to the buffer into which the received message will be
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.
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
/* A message buffer that has already been created. */
void vAnInterruptServiceRoutine( void )
uint8_t ucRxData[ 20 ];
BaseType_t xHigherPriorityTaskWoken = pdFALSE; /* Initialised to pdFALSE. */
/* Receive the next message from the message buffer. */
xReceivedBytes = xMessageBufferReceiveFromISR( xMessageBuffer,
( void * ) ucRxData,
sizeof( ucRxData ),
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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