Chapter 2. API Reference
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).
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 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 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.
Parameters
• 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: It is possible that a message buffer will have a task blocked
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