But then I have used exactly this pattern, and it looks something like:

struct protected_stuff { int userid; ... };

void set_userid(const struct protected_stuff prot, int newuserid) { struct protected_stuff backdoor = (struct protected_stuff *)prot; backdoor->userid = newuserid; }

and then the compiler complains if you go fiddling with userid outside this function where you deliberately opened a backdoor to write to it. (and you can wrap pragmas around that function to turn off warnings).