Chapter 2. API Reference
in tick periods, so the absolute time it represents is dependent on the tick frequency. The macro
pdMS_TO_TICKS() can be used to convert a time specified in milliseconds into a time specified in
ticks. Setting xTicksToWait to portMAX_DELAY will cause the task to wait indefinitely (without
timing out), provided INCLUDE_vTaskSuspend is set to 1 in FreeRTOSConfig.h. Tasks do not use
any CPU time when they are in the Blocked state.
xMessageBufferReceiveFromISR(xMessageBuffer, pvRxData, xBufferLengthBytes, pxHigherPri-
orityTaskWoken)
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 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 xMessageBufferReceive() to read from a message buffer from a task. Use xMessageBufferReceive-
FromISR() to read from a message buffer from an interrupt service routine (ISR).
Example use:
// 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 portYIELD_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.
portYIELD_FROM_ISR( xHigherPriorityTaskWoken );
}
Return The length, in bytes, of the message read from the message buffer, if any.
Parameters
• 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 is to be copied.
• xBufferLengthBytes: The length of the buffer pointed to by the pvRxData parameter. This
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