This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface

       None.

       Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1-2017, Standard
       for Information  Technology  --  Portable  Operating  System  Interface  (POSIX),  The  Open  Group  Base
       Specifications  Issue  7, 2018 Edition, Copyright (C) 2018 by the Institute of Electrical and Electronics
       Engineers, Inc and The Open Group.  In the event of any discrepancy between this version and the original
       IEEE and The Open Group Standard, the original IEEE and The Open Group Standard is the referee  document.
       The original Standard can be obtained online at http://www.opengroup.org/unix/online.html .

       Any  typographical  or formatting errors that appear in this page are most likely to have been introduced
       during  the  conversion  of  the  source  files  to  man  page  format.  To  report  such   errors,   see
       https://www.kernel.org/doc/man-pages/reporting_bugs.html .

IEEE/The Open Group                                   2017                                      SETREUID(3POSIX)

       The  setreuid()  function  shall set the real and effective user IDs of the current process to the values
       specified by the ruid and euid arguments. If ruid or euid is -1, the corresponding effective or real user
       ID of the current process shall be left unchanged.

       A process with appropriate privileges can set either ID to any value.  An unprivileged process  can  only
       set  the  effective user ID if the euid argument is equal to either the real, effective, or saved user ID
       of the process.

       If the real user ID is being set (ruid is not -1), or the effective user ID is being set to a  value  not
       equal  to  the  real user ID, then the saved set-user-ID of the current process shall be set equal to the
       new effective user ID.

       It is unspecified whether a process without appropriate privileges is permitted to change the  real  user
       ID to match the current effective user ID or saved set-user-ID of the process.

       The setreuid() function shall fail if:

       EINVAL The value of the ruid or euid argument is invalid or out-of-range.

       EPERM  The current process does not have appropriate privileges, and either an attempt was made to change
              the effective user ID to a value other than the real user  ID  or  the  saved  set-user-ID  or  an
              attempt was made to change the real user ID to a value not permitted by the implementation.

       Thefollowingsectionsareinformative.

SettingtheEffectiveUserIDtotheRealUserID
       The  following  example  sets  the  effective user ID of the calling process to the real user ID, so that
       files created later will be owned by the current user. It also sets the saved  set-user-ID  to  the  real
       user ID, so any future attempt to set the effective user ID back to its previous value will fail.

           #include <unistd.h>
           #include <sys/types.h>
           ...
           setreuid(getuid(), getuid());
           ...

       None.

       setreuid — set real and effective user IDs

       This  manual  page  is part of the POSIX Programmer's Manual.  The Linux implementation of this interface
       may differ (consult the corresponding Linux manual page for details of Linux behavior), or the  interface
       may not be implemented on Linux.

       Earlier  versions  of  this  standard  did  not  specify  whether  the  saved set-user-ID was affected by
       setreuid() calls. This version specifies common existing practice that constitutes an important  security
       feature.  The  ability to set both the effective user ID and saved set-user-ID to be the same as the real
       user ID means that any security weakness in code that is executed  after  that  point  cannot  result  in
       malicious  code being executed with the previous effective user ID. Privileged applications could already
       do this using just setuid(), but for non-privileged applications the only standard method available is to
       use this feature of setreuid().

       Upon  successful  completion,  0  shall  be  returned.  Otherwise,  -1 shall be returned and errno set to
       indicate the error.

getegid(), geteuid(), getgid(), getuid(), setegid(), seteuid(), setgid(), setregid(), setuid()

       The Base Definitions volume of POSIX.1‐2017, <unistd.h>

       #include <unistd.h>

       int setreuid(uid_t ruid, uid_t euid);