[RTOS Message Buffer API]


size_t xMessageBufferSend( MessageBufferHandle_t xMessageBuffer,
                           const void *pvTxData,
                           size_t xDataLengthBytes,
                           TickType_t xTicksToWait );

Sends a discrete message to a 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 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 xMessageBufferSend() to write to a message buffer from a task. Use xMessageBufferSendFromISR() to write to 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 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).
xTicksToWait   xTicksToWait The maximum amount of time the calling task should remain in the Blocked state to wait for enough space to become available in the message buffer, should the message buffer have insufficient space when xMessageBufferSend() is called. The calling task will never block if xTicksToWait is zero. 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 number of bytes written to the message buffer. If the call to xMessageBufferSend() times out before there was enough space to write the message into the message buffer then zero is returned. If the call did not time out then xDataLengthBytes is returned.

Example usage:

void vAFunction( MessageBufferHandle_t xMessageBuffer )
size_t xBytesSent;
uint8_t ucArrayToSend[] = { 0, 1, 2, 3 };
char *pcStringToSend = "String to send";
const TickType_t x100ms = pdMS_TO_TICKS( 100 );

    /* Send an array to the message buffer, blocking for a maximum of 100ms to
    wait for enough space to be available in the message buffer. */
    xBytesSent = xMessageBufferSend( xMessageBuffer,
                                     ( void * ) ucArrayToSend,
                                     sizeof( ucArrayToSend ),
                                     x100ms );

    if( xBytesSent != sizeof( ucArrayToSend ) )
        /* The call to xMessageBufferSend() times out before there was enough
        space in the buffer for the data to be written. */

    /* Send the string to the message buffer.  Return immediately if there is
    not enough space in the buffer. */
    xBytesSent = xMessageBufferSend( xMessageBuffer,
                                    ( void * ) pcStringToSend,
                                    strlen( pcStringToSend ), 0 );

    if( xBytesSent != strlen( pcStringToSend ) )
        /* The string could not be added to the message buffer because there was
        not enough free space in the buffer. */

Copyright (C) Amazon Web Services, Inc. or its affiliates. All rights reserved.

Latest News

FreeRTOS v10.2.1 is available for immediate download. MIT licensed, includes 64-bit RISC-V, NXP Cortex-M33 demo & Nuvoton Cortex-M23 demo & STM32H745 dual core (AMP) demo.

New Updates to MIT licensed IoT MQTT and POSIX libraries are now available. New library IoT HTTPS is available.  Includes demo projects.

View a recording of the “OTA Update Security and Reliability” webinar, presented by TI and AWS.


FreeRTOS and other embedded software careers at AWS.

FreeRTOS Partners
ARM Connected RTOS partner for all ARM microcontroller cores
Espressif ESP32
IAR Partner
Microchip Premier RTOS Partner
STMicro RTOS partner supporting ARM7, ARM Cortex-M3, ARM Cortex-M4 and ARM Cortex-M0
Texas Instruments MCU Developer Network RTOS partner for ARM and MSP430 microcontrollers
OpenRTOS and SafeRTOS
Xilinx Microblaze and Zynq partner