#include <sys/types.h>
       #include <unistd.h>

       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)):

       setreuid(), setregid():
           _BSD_SOURCE || _XOPEN_SOURCE >= 500 ||

       setreuid() sets real and effective user IDs of the calling process.

       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".

       On  success,  zero is returned.  On error, -1 is returned, and errno is
       set appropriately.

       EPERM  The calling process is not privileged (Linux: does not have  the
              CAP_SETUID capability in the case of setreuid(), or the CAP_SET-
              GID capability in the case of setregid())  and  a  change  other
              than  (i)  swapping  the effective user (group) ID with the real
              user (group) ID, or (ii) setting one to the value of  the  other
              or  (iii)  setting the effective user (group) ID to the value of
              the saved set-user-ID (saved set-group-ID) was specified.

       POSIX.1-2001, 4.3BSD (the  setreuid()  and  setregid()  function  calls
       first appeared in 4.2BSD).

       Setting  the  effective user (group) ID to the saved set-user-ID (saved
       set-group-ID) is possible since Linux 1.1.37 (1.1.38).
       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.

       getgid(2), getuid(2), seteuid(2), setgid(2),  setresuid(2),  setuid(2),

       This  page  is  part of release 3.54 of the Linux man-pages project.  A
       description of the project, and information about reporting  bugs,  can
       be found at

Linux                             2010-11-22                       SETREUID(2)
Man Pages Copyright Respective Owners. Site Copyright (C) 1994 - 2019 Hurricane Electric. All Rights Reserved.