C++ Module with C-Wrapped Output Message

C++ Basilisk modules typically use C++ message input and output objects. However, if the module is a flight software module, it is convenient to provide both a C++ message object and a C-wrapped message output. The reason is that some flight software scripts have multiple modules write to a single external gateway message. For this to work, all modules must write to a message object that is of the same wrapper type (either C or C++). Thus, if using both C++ and C Basilisk module, we need the module output to be provided in both formats.

How to add a C++ message output object is already covered in C++ Message Object. This page describes how to add a C-wrapped output message. This requires very little extra overhead. The module is already compute the output message content. We simply have to write this output to a second message type.

Changes in the Module Header File

The C output messages setup steps are mostly the same as outlined in Module Header File. Below we highlight the steps and reasons.

Assume the output message is of type SomeMsg. Replace the import statement:

#include "architecture/msgPayloadDefC/SomeMsgPayload.h"

with:

#include "cMsgCInterface/SomeMsg_C.h"

This single import will now include the message payload definition that the C++ message will need, and it includes the C message structure definition.

In the Basilisk module class definition, we will need to add the SelfInit method which is required to self-initialized the C wrapped output message:

void SelfInit();

Next, add the C-wrapped output message variable as a public variable:

SomeMsg_C someOutMsgC = {};

Note that this line is slightly different than when setup in a C module in Module Header File. First, use the ={} statement to create a zero’d copy of the C message structure. Otherwise the msg payload and header pointers in this structure are not zero on setup, and the message initialization will not function properly.

Second, to distinguish between the regular module C++ message object someOutMsg, we end the message name with C.

Changes in the Module Definition File

First, we need to define the module SelfInit() method to initialize the C-wrapped output message:

void ModuleClass::SelfInit()
{
    SomeMsg_C_init(&this->someOutMsgC);
}

Second, when you write the output message payload this->someOutBuffer to the C++ message object, add this line to write the same content to the C-wrapped output message:

SomeMsg_C_write(&this->someOutBuffer, &this->someOutMsgC, this->moduleID, CurrentClock);

That is it. You now have a C++ FSW module that can write to both message container types.