.. include:: <mmlalias.txt>


Datalogger: A Semi-Realistic Project (Events Version)
=====================================================

.. topic:: See also

   * :doc:`../polling/index`
   * :doc:`../timer/index`

.. topic:: Trainer's note

   * Continue from :ref:`why-datalogger-snapshot-button`
   * Copy over :download:`datalogger-snapshot-button.cpp
     <../datalogger/code/datalogger-snapshot-button.cpp>`

Polling-Thread To Consume Both Message Queues
---------------------------------------------

* Two message queues: sensor data, snapshot requests
* Collapse both threads into one

  * Rip out the two thread while-loop bodies
  * Create respective lambdas/callback for event notification

.. important::

   No need to protect CSV writing against snapshot rotation
   |longrightarrow| remove lock

.. literalinclude:: code/datalogger-polling-queues.cpp
   :caption: :download:`code/datalogger-polling-queues.cpp`
   :language: c++

Watchdog Timer Into Polling-Thread (Semaphore Version)
------------------------------------------------------

* Currently, we do a (blocking) sleep in main loop
* Plan:

  * Start a :doc:`timer <../timer/index>`
  * In the timer callback function - *in ISR context* -, give a
    semaphore
  * In event loop, watch semaphore for takeability
  * Feed watchdog in event callback

.. literalinclude:: code/datalogger-polling-timer-sem.cpp
   :caption: :download:`code/datalogger-polling-timer-sem.cpp`
   :language: c++

Watchdog Timer Into Polling-Thread (Poll-Signal Version)
--------------------------------------------------------

.. topic:: Documentation

   * :doc:`../why-os/pollsignal`

* Semaphore is heavyweight: can be waited for which is extra logic
* Poll signal is rather lightweight
* Specific to event driven applications

.. literalinclude:: code/datalogger-polling-pollsignal.cpp
   :caption: :download:`code/datalogger-polling-pollsignal.cpp`
   :language: c++


And Single-Threaded Programs?
-----------------------------

* Main thread is useless now
* All logic in the poller thread
* Why not eliminate that thread altogether?

.. literalinclude:: code/datalogger-polling-singlethreaded.cpp
   :caption: :download:`code/datalogger-polling-singlethreaded.cpp`
   :language: c++