FreeRTOS stream & message buffers

Introduction

Stream buffers are an RTOS task to RTOS task, and interrupt to task communication primitives. Unlike most other FreeRTOS communications primitives, they are optimised for single reader single writer scenarios, such as passing data from an interrupt service routine to a task, or from one microcontroller core to another on dual core CPUs. Data is passed by copy - the data is copied into the buffer by the sender and out of the buffer by the read.

Stream buffers pass a continuous stream of bytes. Message buffers pass variable sized but discrete messages. Message buffers use stream buffers for data transfer.

IMPORTANT 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 serialize calls to writing API functions (such as xStreamBufferSend()). Likewise, if there are to be multiple different readers then the application writer must serialize calls to reading API functions (such as xStreamBufferReceive()). One way to achieve such serialization in single core or SMP kernel is to place each API call inside a critical section and use a block time of 0.

FreeRTOS stream & message buffers

Introduction

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