RTOS intertasks communication mechanism description

• fast
• lightweight
• simple

In RTOS some mechanism to share data between tasks is needed. Commonly there is two kind of doing it: exclusive sections and queues.

Queue seems more preferable because provides non-blocking mechanism of doing it. Low priority tasks in this case do not consume time of tasks with higher priority. Also exclusive sections lead to priority inversion of course this situation is fixed in all well known RTOSes. But any way in case of using exclusive sections the low priority tasks spent CPU time when they entering these exclusive sections. As result this leads to performance degradation.

The one problem with queue appears when sharing data structures are large. In that case lot of CPU time is spent by copying data into/from queues. To avoid this situation sharing data pointers instead of actual data seems reasonable. But to avoid memory leaks and data collisions sharing mechanism should be well clarified.

Mechanish clarification:

• Sender task(s) creates data and sent it. This task do not worry about this data after sent. It can’t reuse this data or update. Each time when task has new data it creates this data and sent.
• Receiver task retrieve data from queue and process received data. After processing receiver delete this data.

C++ compare to pure C provides easy and clear mechanism to distinguish messages retrieved from queue without explicitly adding additional data to message, like message ID or so on. So we have some base message polymorphic class which is empty (does not contain any data). All messages are inherited from this base class with adding data field(s). It provides possibility to send/receive data using pointer to base class. After receiving message its type can be restored using dynamic_cast mechanism.

Provided code demonstrates this intertasks communication mechanism. As OS chosen posix interface as some common implementation. In real cases should be replaced with particular RTOS queue implementation.

Note:
For sure std::cout is not thread safe and used only for demo. More complex tracing mechanism is not used in order to simplify code
and make idea more clear.