.. include:: <mmlalias.txt>


Message Queues
==============

.. topic:: Documentation

   * :doc:`../why-os/communication`
   * `Zephyr: Message Queues
     <https://docs.zephyrproject.org/latest/kernel/services/data_passing/message_queues.html>`__

Basic Usage
-----------

* Fixed size data items
* ``get()`` *blocks* when no items available
* ``put()`` *blocks* when full
* ``_nowait()`` variants

.. literalinclude:: code/basic.cpp
   :caption: :download:`code/basic.cpp`
   :language: c++

Communicating From An ISR Into Thread Context
---------------------------------------------

.. topic:: Trainer's note

   * Copy over spinlock example from :doc:`../spinlocks/index`:
     :download:`../spinlocks/code/sensor-enqueue-list-spinlock.cpp`

* Common pattern: in the ISR, quickly transfer work into an easier -
  schedulable - context
* Producer is an ISR: *cannot* wait |longrightarrow| ``put_noblock()``
* Consumer is a thread: *can* wait |longrightarrow| ``get()``
* Main: not a busy loop anymore
* *OS does powermanagement when nothing else happens*

.. literalinclude:: code/sensor-messagequeue.cpp
   :caption: :download:`code/sensor-messagequeue.cpp`
   :language: c++

.. code-block:: console

   $ while true; do echo irq 2; done | \
       why-shell code/why-mq-sensor-messagequeue