int setreuid(uid_t ruid, uid_t euid);
int setregid(gid_t rgid, gid_t egid);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
Supplying a value of -1 for either the real or effective user ID forces the system to leave that ID unchanged.
Unprivileged processes may only set the effective user ID to the real user ID, the effective user ID, or the saved set-user-ID.
Unprivileged users may only set the real user ID to the real user ID or the effective user ID.
If the real user ID is set or the effective user ID is set to a value not equal to the previous real user ID, the saved set-user-ID will be set to the new effective user ID.
Completely analogously, setregid() sets real and effective group ID's of the calling process, and all of the above holds with "group" instead of "user".
POSIX.1 does not specify all of possible ID changes that are permitted on Linux for an unprivileged process. For setreuid(), the effective user ID can be made the same as the real user ID or the save 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 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.
POSIX.1 makes no specification about the effect of these calls on the saved set-user-ID and saved set-group-ID.
The original Linux setreuid() and setregid() system calls supported only 16-bit user and group IDs. Subsequently, Linux 2.4 added setreuid32() and setregid32(), supporting 32-bit IDs. The glibc setreuid() and setregid() wrapper functions transparently deal with the variations across kernel versions.