message_passing_example

message_passing_example

This example is a more "complete" example compared to others, and it is a simplified version of what a more "real" program would look like. There are two threads:

• DataLogger: a non RT thread that receives data and logs the data into build/data.csv every 1 second.
• RtThread: a RT CyclicThread that executes at 1 kHz. For demonstration purposes, it simply calculates a cosine function (amplitude = 1, period = 1 second). The output of the cosine function is sent to the `DataLogger``.

This demonstrates two kind of message passing: (1) message passing through a single-producer-single-consumer (SPSC) queue and (2) message passing via an std::atomic.

Message passing through SPSC queue

This is akin to streaming robotics data from a real-time control thread to a non-real-time data logger thread so the robotics data can be saved to disk. In this example, an OutputData struct is sent from the RtThread to the DataLogger thread via the moodycamel::readerwriterqueue, which is a queue cactus-rt links to by default (so there's no "external dependencies"). This is a single-producer-single-consumer queue that's wait-free when using the try_emplace and try_push methods, which makes it suitable for real time. The RtThread is running at 1 kHz and sends one such structure per iteration, mimicking what would happen in a real-world robot controller code. The RtThread is rigged to run for 30 seconds. Once the RtThread quits, the entire program quits.

The data is read by the DataLogger almost continuously and placed into a temporary buffer (backed by a std::vector that has pre-allocated a certain amount of space). Every 1 second, it flushes the buffer to a CSV file located at build/data.csv. It will also flush and write the buffer content when it *terminates.

As noted, this emulates a real-world application where the RT thread is doing work and needs to log per-iteration metric to a data logger in a real-time-safe manner. In fact, any time you need to send ordered messages from a RT thread to a non-RT thread, you can use a similar pattern.

Message passing through std::atomic

A second example of message passing through std::atomic is shown via the message counter (DataLogger.message_count_).

TODO: write more about this case...