@StefanKarpinski and @yuyichao
Thanks for all the explanation.
From a user point of view (my own 
), I think we can improve the documentation about the Julia immutable types being changed by the C code. If I did not miss anything, then the docs are not clear.
Maybe a small warning at the end of this section:
https://docs.julialang.org/en/v1/manual/calling-c-and-fortran-code/#man-bits-types-1
telling that the user must take care about these things.
The same can happen if I pass a pointer to an immutable structure to C and then C function modifies it right?