It was maybe asked here before, but I couldn't find anything about this. Anyway is it possible to call BASIC functions/commands and/or calculate Run-Matrix expressions from C? The expression parser at least must be shared in the system somehow, every built-in plugins use it

Yes this is possible. But you have to use the calculator's BCD-floating point types. F. i. see the keywords "BCD" and "Alpha" in fx_calculators_SuperH_based.chm for a start. There are a lot of syscalls related to this, which are not verified yet (this would be a lot of work). Instead of using the calculators BCD-math-library, you could as well transfer BCD-values to C-double and do the calculations with the C-math library.

Thank you! My problem was that my help file was too old, but now I downloaded the new version. I'll make a post here if I can find out anything new or useful