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SETREUID(3P)               POSIX Programmer's Manual              SETREUID(3P)
PROLOG
       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.
NAME
       setreuid -- set real and effective user IDs
SYNOPSIS
       #include <unistd.h>
       int setreuid(uid_t ruid, uid_t euid);
DESCRIPTION
       The setreuid() function shall set the real and effective  user  IDs  of
       the  current process to the values specified by the ruid and euid argu-
       ments. 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.
RETURN VALUE
       Upon successful completion, 0 shall be returned. Otherwise, -1 shall be
       returned and errno set to indicate the error.
ERRORS
       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  per-
              mitted by the implementation.
       The following sections are informative.
EXAMPLES
   Setting the Effective User ID to the Real User ID
       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());
           ...
APPLICATION USAGE
       None.
RATIONALE
       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 fea-
       ture. 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().
FUTURE DIRECTIONS
       None.
SEE ALSO
       getegid(), geteuid(), getgid(), getuid(),  setegid(),  seteuid(),  set-
       gid(), setregid(), setuid()
       The Base Definitions volume of POSIX.1-2008, <unistd.h>
COPYRIGHT
       Portions  of  this text are reprinted and reproduced in electronic form
       from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology
       --  Portable  Operating  System  Interface (POSIX), The Open Group Base
       Specifications Issue 7, Copyright (C) 2013 by the Institute of Electri-
       cal  and  Electronics  Engineers,  Inc  and  The  Open Group.  (This is
       POSIX.1-2008 with the 2013 Technical Corrigendum  1  applied.)  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.unix.org/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.ker-
       nel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group                  2013                         SETREUID(3P)