getresuid, getresgid - get real, effective, and saved user/group IDs
#define _GNU_SOURCE /* See feature_test_macros(7) */
#include <unistd.h>
int getresuid(uid_t *ruid, uid_t *euid, uid_t *suid);
int getresgid(gid_t *rgid, gid_t *egid, gid_t *sgid);
On success, zero is returned. On error, -1 is returned, and
errno
is set to indicate the error.
getuid(2), setresuid(2), setreuid(2), setuid(2), credentials(7)