RTOS Stream & Message Buffers
[Inter-task communication and synchronisation]
IntroductionStream 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 place each call to a writing API function (such as xStreamBufferSend()) 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 xStreamBufferReceive()) inside a critical section and use a receive block time of 0.