So i am trying to understand simple OS system calls and stumbled upon this line of code witch i don't know what it means : *__errno()= msg.error.number;. Can somebody help me please?
It is very likely a multithreaded version of errno defined in errno.h.
__errno() call will return a pointer to the thread local int,
and *__errno()= msg.error.number; will write into that int.
That should normally be wrapped in a macro, so you don't see the call or dereference.