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       setreuid, setregid - set real and/or effective user or group ID


       #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():


       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

       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_SETGID 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


       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  does  not  specify  all of possible ID changes that are permitted on Linux for an
       unprivileged process.  For setreuid(), the effective user ID can be made the same  as  the
       real user ID or the save set-user-ID, and it is unspecified whether unprivileged processes
       may set the real user ID to the real user ID, the effective user ID,  or  the  saved  set-
       user-ID.   For setregid(), the real group ID can be changed to the value of the saved set-
       group-ID, and the effective group ID can be changed to the value of the real group  ID  or
       the  saved set-group-ID.  The precise details of what ID changes are permitted vary across

       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), capabilities(7)


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