C vs C++ compilation incompatibility - does not name a type

teststruct defines a scope in C++. You can form the qualified id teststruct::u64. So the language rules for name lookup account for that, allowing members of classes and unions to hide identifiers in outer scope. Once u64 u64; is introduced, the unqualified u64 cannot refer to the global ::u64, only the member. And the member is not a type.

In C union teststruct does not define a scope. The field can only be used in member access, so there can never arise a conflict. As such the field need not hide the file scope type identifier.

There is nothing, as far as I can tell, that you may do in order to easily work around it. This library (which is a perfectly valid C library), is not a valid C++ library. No different than if it used new or try as variable names. It needs to be adapted.

It seems that you have a header file that is illegal in C++, so you cannot #include it in code compiled as C++. If you cannot effect a change in the library header file (e.g. by complaining to your library supplier) then the most straightforward option is to write a thin C++-compatible wrapper around the library:

To isolate your C++ code against the C header, create a Wrapper.h and Wrapper.c, where the .h is valid for inclusion in C++, does not include header.h, and provides all types and functions that you need for library interaction. Then, in the .c, you can #include "header.h" and implement all the calls (and whatever you need to do to safely convert between the types). This would obviously have to be compiled as C, not C++.

If your mentioned incompatibility between C and C++ is the only one, you should be able to convert header.h to C++ compatible header file programmatically, name it something like header.hpp. And then you can convert newer versions the same way.

Compiler errors tell you everything about what and where should be changed:

header.h:11:9: error: ‘u64’ does not name a type

1. Open header.h;
2. Seek position 11:9;
3. Insert :: there;
4. Repeat for all does not name a type error.

Some string processing and it's done.

PS: C to C++ converters may able to do that too.