| SETREUID(2) | System Calls Manual | SETREUID(2) | 
setreuid —
#include <unistd.h>
int
  
  setreuid(uid_t
    ruid, uid_t
  euid);
The real and effective user ID's of the current process are set according to the arguments. If the real user ID is changed, the saved user ID is changed to the new value of the effective user ID.
If ruid or euid is -1, the current uid is filled in by the system. Unprivileged users may change the real user ID to the effective user ID, and may change the effective user ID to the real user ID or the saved user ID; only the super-user may make other changes.
The setreuid() function has been used to
    swap the real and effective user IDs in set-user-ID programs to temporarily
    relinquish the set-user-ID value. This purpose is now better served by the
    use of the seteuid() function (see
    setuid(2)).
When setting the real and effective user IDs to the same value,
    this function is equivalent to the setuid()
    function. When setting only the effective user ID, this function is
    equivalent to the seteuid() function.
EPERM]setreuid() function call appeared in
  4.2BSD. An incompatible version was implemented in
  4.4BSD. It was reimplemented in
  NetBSD 1.2 in a way compatible with
  4.3BSD, SunOS and Linux, but should not be used in new
  code.
| January 5, 2001 | NetBSD 9.3 |