SETREUID(3)                POSIX Programmer's Manual               SETREUID(3)



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

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
       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.

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
              permitted 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
       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().

FUTURE DIRECTIONS
       None.

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

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

COPYRIGHT
       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(3)
