You can sort of get that behavior on Linux using clone(..., ~CLONE_THREAD|~CLONE_SIGHAND|CLONE_VM, ...), which creates otherwise distinct processes which share an address space.
You can do all sorts of weird things like create threads which don't share file descriptors, threads which chdir() independently... except that CLONE_THREAD|~CLONE_SIGHAND and CLONE_SIGHAND|~CLONE_VM are disallowed.
You can do all sorts of weird things like create threads which don't share file descriptors, threads which chdir() independently... except that CLONE_THREAD|~CLONE_SIGHAND and CLONE_SIGHAND|~CLONE_VM are disallowed.