How do you use a constant to size an array event argument

[JackNWhite]

My F' instance is interfacing with external code which is the authority on the size of a particular array, and therefore also the size of the telemetry packet used to relate it to ground.

I want to do this:

<event id="9"
       name="MY_ARRAY"
       severity="ACTIVITY_HI"
       format_string="%s">
    <comment>The thing of the jobbler.</comment>
    <args>
        <arg name="my_array" type="I32" size="MY_ALREADY_DEFINED_ARRAY_LENGTH" />
    </args>
</event>

... where MY_ALREADY_DEFINED_ARRAY_LENGTH is a preprocessor constant.

When generating the code, however, I get this difficulty:

ERROR:RELAXNGV:RELAXNG_ERR_INTEREXTRA: Extra element args in interleave
ERROR:RELAXNGV:RELAXNG_ERR_INTEREXTRA: Extra element events in interleave
/path/to/MyComponentAi.xml:1:0:ERROR:RELAXNGV:RELAXNG_ERR_CONTENTVALID: Element component failed to validate content

What is the remedy here?

[timcanham]

The XML expects an integer for the size in the attribute, so it fails validation. That explains the error. The XML specification doesn't allow for references to external code, so you'll have to manually keep them in sync.

[JackNWhite]

Can I make it a constant within F', so I can define my external constant against that?

[timcanham]

The old XML doesn't have constants, but the new FPP does. @bocchino: Does an FPP constant end up in a header somewhere that can be used by C++ code?

[bocchino]

Yes, each F Prime build module that defines FPP constants should have a generated file FppConstantsAc.hpp that you can include. See https://fprime-community.github.io/fpp/fpp-users-guide.html#Analyzing-and-Translating-Models_Generating-C-Plus-Plus_Constant-Definitions.