xch[ug]id

[utzoo!decvax!ucbvax!ucsfcgl!katzung]

The idea of having xchuid() and xchgid() sounds reasonable to me.
Would there be some advantage to using a single routine called,
perhaps, useeid(x) where x = EUID and/or EGID, or 0 (no change)?
The return value would be the old x.  The reasons for this would be:

1)  This would reduce the overhead to a single system call, and
2)  A program would not lose track of how many times it had switched
	Es and Rs.  (suppose a suid-not-root catches an interrupt...)

Reason 2 can be ignored if xch[ug]id returns an indicator of some
sort, but reason 1 still remains.

				Brian Katzung