What is the reason you want to do this, and what would be your intended behavior? It may be possible, but maybe the result / effect is still not what you are hoping for.

This is for PLC code. Global variables associated to I/O are defined in a file which is not C code. But they are available for C code. The freestanding C compiler for the PLC deals with that. It is possible to have different configurations in the build environment. And those configurations may include or not some files C files. Which results in some global variables not being used in some configurations. Which results in a warning. And removing that warning is what I wish to achieve. But only for variables known to be used in at least one configuration.

This goes then in a different direction:

• From a pure C perspective, there is no issue with global variables being uninitialzed. Global variables are always initialized. If you don't given an explicit initializer, they are implicitly initialized to 0.

• In your case, you are dealing with global variables defined in a different language. This is then not a matter of being classified as undefined or not by the C standard - the standard won't make any statements about such situations at all. It's only a practical question of "does it work" in my setup of languages and tools.