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



PPRROOLLOOGG
       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.

NNAAMMEE
       setreuid -- set real and effective user IDs

SSYYNNOOPPSSIISS
       #include <unistd.h>

       int setreuid(uid_t _r_u_i_d, uid_t _e_u_i_d);

DDEESSCCRRIIPPTTIIOONN
       The _s_e_t_r_e_u_i_d() function shall set the real and effective user IDs of
       the current process to the values specified by the _r_u_i_d and _e_u_i_d
       arguments. If _r_u_i_d or _e_u_i_d 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 _e_u_i_d
       argument is equal to either the real, effective, or saved user ID of
       the process.

       If the real user ID is being set (_r_u_i_d 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.

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

EERRRROORRSS
       The _s_e_t_r_e_u_i_d() function shall fail if:

       EEIINNVVAALL The value of the _r_u_i_d or _e_u_i_d argument is invalid or out-of-
              range.

       EEPPEERRMM  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.

       _T_h_e _f_o_l_l_o_w_i_n_g _s_e_c_t_i_o_n_s _a_r_e _i_n_f_o_r_m_a_t_i_v_e_.

EEXXAAMMPPLLEESS
   SSeettttiinngg tthhee EEffffeeccttiivvee UUsseerr IIDD ttoo tthhee RReeaall UUsseerr IIDD
       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());
           ...

AAPPPPLLIICCAATTIIOONN UUSSAAGGEE
       None.

RRAATTIIOONNAALLEE
       Earlier versions of this standard did not specify whether the saved
       set-user-ID was affected by _s_e_t_r_e_u_i_d() 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 _s_e_t_u_i_d(), but
       for non-privileged applications the only standard method available is
       to use this feature of _s_e_t_r_e_u_i_d().

FFUUTTUURREE DDIIRREECCTTIIOONNSS
       None.

SSEEEE AALLSSOO
       _g_e_t_e_g_i_d(), _g_e_t_e_u_i_d(), _g_e_t_g_i_d(), _g_e_t_u_i_d(), _s_e_t_e_g_i_d(), _s_e_t_e_u_i_d(),
       _s_e_t_g_i_d(), _s_e_t_r_e_g_i_d(), _s_e_t_u_i_d()

       The Base Definitions volume of POSIX.1-2017, <<uunniissttdd..hh>>

CCOOPPYYRRIIGGHHTT
       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)
