.. _makingModules-2:

Creating New Message Definitions
================================

Msg File Location
-----------------
The preferred message definition uses only C code to define a ``struct`` which contains the message data.  This is written in a stand-alone ``*.h`` file located in the ``src/architecture/msgPayloadDefC`` folder.

If the message ``struct`` definition needs to include C++ code, then the stand-alone message definition file is stored in ``src/architecture/msgPayloadDefCpp`` folder.

The ``*.h`` file name need to be upper camel case (start with a capital letter) and end with ``...MsgPayload.h``.  An example is ``SomeMsgPayload.h``.

Msg Data Definition
-------------------
The message data is defined through a ``struct`` definition.  For example, assume the new message is to be of type ``SomeMsg``:

.. code:: cpp

    #ifndef SOME_MSG_H
    #define SOME_MSG_H

    /*! @brief Brief description of what this message contains */
    typedef struct {
        int variable1;              //!< [units] variable description
        double variable2[3];        //!< [units] variable description
    }SomeMsgPayload;

    #endif

The ``#ifndef`` statement ensures this header file is only included once when compiling.  The ``struct`` definition needs to have the message type name followed by ``MsgPayload``.  In this case it is ``SomeMsgPayload``.  The msg payload is the data of the message.  This is in contrasts to the message object which contains a copy of this message payload, as well as a message header.


Creating the Msg C/C++ Interface Files
--------------------------------------
To create message objects in C and C++ modules, you need to run ``python3 conanfile.py`` again from in the Terminal or command line window.  This script auto-creates the corresponding C module interface files and stores them in ``basilisk/dist3/autoSource/cMsgCInterface``.  The C++ message objects are automatically created as a template class using the ``*.h`` definitions in ``msgPayloadDefC``.  Naturally no C module interface files are created for ``*.h`` files in ``msgPayloadDefCpp``.

Running ``python3 conanfile.py`` re-creates the IDE project file that will now include access to the new message definition.