setresuid, setresgid - set real, effective, and saved user / group ID
#include <unistd.h>
int setresuid(uid_t, uid_t, uid_t);
int setresgid(gid_t, gid_t, gid_t);
Sets all of real, effective, and saved user or group ID to the given values.
An argument of -1 keeps the corresponding ID unchanged.
For non-superusers, each of the given values has to be equal to any of the current real, effective, or saved IDs for the call to succeed.
If the call was set successful, returns 0.
Otherwise, returns -1 and sets errno to describe the error.
EPERM: At least one of the passed IDs was not equal to the current real, effective, or saved ID, and the user is not superuser.