[RTOS Message Buffer API]
size_t xMessageBufferReceive( MessageBufferHandle_t xMessageBuffer,
TickType_t xTicksToWait );
Receives a discrete message from an RTOS 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 is to 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. xTicksToWait The maximum amount of time the task should remain in the Blocked state to wait for a message, should the message buffer be empty when xMessageBufferReceive() was called. xMessageBufferReceive() will return immediately if xTicksToWait is zero and the message buffer is empty. The block time is specified 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.
- The length, in bytes, of the message read from the message buffer, if any. If xMessageBufferReceive() times out before a message became available then zero is returned. If the length of the message is greater than xBufferLengthBytes then the message will be left in the message buffer and zero is returned.
void vAFunction( MessageBuffer_t xMessageBuffer )
uint8_t ucRxData[ 20 ];
const TickType_t xBlockTime = pdMS_TO_TICKS( 20 );
/* Receive the next message from the message buffer. Wait in the Blocked
state (so not using any CPU processing time) for a maximum of 100ms for
a message to become available. */
xReceivedBytes = xMessageBufferReceive( xMessageBuffer,
( void * ) ucRxData,
sizeof( ucRxData ),
if( xReceivedBytes > 0 )
/* A ucRxData contains a message that is xReceivedBytes long. Process
the message here.... */