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the saved set-user-id will be set to the new effective user id.
.pp
completely analogously,
.br setregid ()
sets real and effective group id's of the calling process,
and all of the above holds with "group" instead of "user".
.sh return value
on success, zero is returned.
on error, \-1 is returned, and
.i errno
is set to indicate the error.
.pp
.ir note :
there are cases where
.br setreuid ()
can fail even when the caller is uid 0;
it is a grave security error to omit checking for a failure return from
.br setreuid ().
.sh errors
.tp
.b eagain
the call would change the caller's real uid (i.e.,
.i ruid
does not match the caller's real uid),
but there was a temporary failure allocating the
necessary kernel data structures.
.tp
.b eagain
.i ruid
does not match the caller's real uid and this call would
bring the number of processes belonging to the real user id
.i ruid
over the caller's
.b rlimit_nproc
resource limit.
since linux 3.1, this error case no longer occurs
(but robust applications should check for this error);
see the description of
.b eagain
in
.br execve (2).
.tp
.b einval
one or more of the target user or group ids
is not valid in this user namespace.
.tp
.b eperm
the calling process is not privileged
(on linux, does not have the necessary capability in its user namespace:
.b cap_setuid
in the case of
.br setreuid (),
or
.b cap_setgid
in the case of
.br setregid ())
and a change other than (i)
swapping the effective user (group) id with the real user (group) id,
or (ii) setting one to the value of the other or (iii) setting the
effective user (group) id to the value of the
saved set-user-id (saved set-group-id) was specified.
.sh conforming to
posix.1-2001, posix.1-2008, 4.3bsd
.rb ( setreuid ()
and
.br setregid ()
first appeared in 4.2bsd).
.sh notes
setting the effective user (group) id to the
saved set-user-id (saved set-group-id) is
possible since linux 1.1.37 (1.1.38).
.pp
posix.1 does not specify all of the uid changes that linux permits
for an unprivileged process.
for
.br setreuid (),
the effective user id can be made the same as the
real user id or the saved set-user-id,
and it is unspecified whether unprivileged processes may set the
real user id to the real user id, the effective user id, or the
saved set-user-id.
for
.br setregid (),
the real group id can be changed to the value of the saved set-group-id,
and the effective group id can be changed to the value of
the real group id or the saved set-group-id.
the precise details of what id changes are permitted vary
across implementations.
.pp
posix.1 makes no specification about the effect of these calls
on the saved set-user-id and saved set-group-id.
.pp
the original linux
.br setreuid ()
and
.br setregid ()
system calls supported only 16-bit user and group ids.
subsequently, linux 2.4 added
.br setreuid32 ()
and