Problem with mixing C++ and C code

In a C++ project that also contained legacy C files, I started to get the error "C2664 ...cannot convert argument 3 from void * to const_locale_t". The argument in question was NULL. Changing the include order by including the legacy header after stdio.h solved the problem but I wanted to understand the root problem to avaiod unnecessary technical debt.

One of the C headers contained #define NULL ((void *)0) which is standard for C but not for C++. Normally NULL is defined inside vcruntime.h (part of Visual C++). Using the legacy NULL before any code that depends on vcruntime.h, like string.h, stdio.h, caused this error. Removing the custom definition or changing it to the following solved the problem, see also my answer on StackOverflow:

Another strange error happens if you have defined fmax globally in your own C code because it also exists in cmath. The strange error will be C2039 'fmax': is not a member of 'global namespace'. Removing or renaming your own custom definition will solve the problem.