Lib5K and Multithreading

A standard Lib5k robot application is multithreaded, with two primary threads:

  • Main thread
    • Handles FPGA and HAL setup, along with initialization of both WPILib and Lib5k
  • "Robot" thread
    • Notifier-driven
    • Contains the command scheduler
    • Handles system watchdog calls
    • Periodically interacts with the DriverStation socket
    • Handles I/O

Along with these, Lib5k introduces more threads through some library features:

  • Logging thread
    • Notifier-driven
    • Very I/O heavy
    • Handles logging to multiple outputs simultaneously
      • Over the network
      • To the local filesystem
      • To the session folder on the robot USB drive
  • FaultReporter thread
    • Notifier-driven
    • Periodically requests "health reports" from all system I/O
    • Reports any unusual issues to the log
    • Is very useful for diagnosing hard-to-find issues
      • This was used at an event in 2020 to find an MXP fault that was causing incorrect gyroscope data to be read


Notifiers are interrupt-driven threads that run at precisely-timed intervals. These threads are run by the RoboRIO FPGA's internal clock.

When running on-robot, a notifier is a reliable way to have a task run at an exact interval, but when run in a simulation, then timing will rely on a simulated FPGA, which is less accurate. This is why Lib5k handles all mathematical calculations with an extra dt value, that scales the result based on the reliability of the notifier.

The scheduler

The main robot tasks are running in a time-shared main thread. The model of this thread is: every scheduled task gets to run one iteration per 20ms. This is where Commands and Subsystems run.