setreuid(2)?

[Mouse <mouse%Rodents-Montreal.ORG@localhost>]

I have a program, running with ruid=euid=0, that wants to set its real
and effective IDs to two other, different, IDs, neither one privileged.

What is the proper way to do this?  I first reached for setreuid(2),
but its manpage says that it is "made obsolete" by the saved-ID
functionality of setuid(2) and seteuid(2) and that it "should not be
used in new code".  But I must be missing something, because I can't
see any way to exploit the functionality described there, including the
saved IDs, to get the effect I want...short of creating an executable
setuid to one of the IDs, then switching to the other and execing that
executable.  I would hardly say this makes setreuid() obsolete, since
it requires writable filesystem space with set-ID functionality turned
on, a whole lot more syscalls, *and* MD code to construct a suitable
executable, none of which setreuid() needs to do the same job.

What am I missing?

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