@Nadir
Thanks a lot, now it is clear for me.

But I am still a bit puzzled regarding libnix/ixemul.

For example I want to port a small tool, which uses fts_close, fts_open, etc...
If I use ixemul, the thing just compiles and runs.
If I use libnix, linking fails because of missing fts_close, fts_open, etc...
What options do I have then? I guess I can try to implement the missing functions with "native" (dos, exec,...) functions?
How can I find out which functions are really implemented in libnix?

What about missing functions in case of ixemul? According to the docs I can't use any "native" functions/libs. So how can I implement missing functions? Just by using available POSIX functions of ixemul?

What about ported libs, like SDL? Can I use these with libnix, or ixemul, or both?