Question : getting uid & euid of a process

How is it possible to get the real & effective user id of
a given process?

The idea is to call something like geteuid(), from inside a
c program, to get the effective user id of ANOTHER process
(let's say the father).

Answer : getting uid & euid of a process

You can't.  It can change (setreuid(), setuid()) and you must be root to read kmem on older vmunix's (that do not have /proc).

You can get a guess on systems that support /proc, but it could change in the next instruction.  What are you really trying to do?

If you want a process to prove it is running as a User you _can_ do that with 100% portable code -- but it'll cost you points.