Neither one really works in the case of pid's. Once you move to
a 32-bit pid in the kernel, there is nothing sensible one can
return for getppid, when the parent's id is 100001.
One could make a case for using personalities (discovered at exec
time) for controling the pid range that a particular new process
will be allocated to, and for using personalities for determining
the pid range for fork. But the ppid case of new execing old
would seem to be a show-stopper.
r~