setresuid, setresgid - set real, effective, and saved user or group ID
#define _GNU_SOURCE /* See feature_test_macros(7) */
#include <unistd.h>
int setresuid(uid_t ruid, uid_t euid, uid_t suid);
int setresgid(gid_t rgid, gid_t egid, gid_t sgid);
setresuid() sets the real user ID, the effective user ID, and the saved set-user-ID of the calling process.
An unprivileged process may change its real UID, effective UID, and saved set-user-ID, each to one of: the current real UID, the current effective UID, or the current saved set-user-ID.
A privileged process (on Linux, one having the CAP_SETUID capability) may set its real UID, effective UID, and saved set-user-ID to arbitrary values.
If one of the arguments equals -1, the corresponding value is not changed.
Regardless of what changes are made to the real UID, effective UID, and saved set-user-ID, the filesystem UID is always set to the same value as the (possibly new) effective UID.
Completely analogously, setresgid() sets the real GID, effective GID, and saved set-group-ID of the calling process (and always modifies the filesystem GID to be the same as the effective GID), with the same restrictions for unprivileged processes.
getresuid(2), getuid(2), setfsgid(2), setfsuid(2), setreuid(2), setuid(2), capabilities(7), credentials(7), user_namespaces(7)