#include <sys/types.h>
       #include <unistd.h>

       int seteuid(uid_t euid);
       int setegid(gid_t egid);

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

       seteuid(), setegid():
           _BSD_SOURCE || _POSIX_C_SOURCE >= 200112L || _XOPEN_SOURCE >= 600

       seteuid()  sets the effective user ID of the calling process.  Unprivi-
       leged user processes may only set the effective user  ID  to  the  real
       user ID, the effective user ID or the saved set-user-ID.

       Precisely the same holds for setegid() 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 seteuid(), or the CAP_SET-
              GID capability in the case of setegid()) and euid (respectively,
              egid)  is  not  the  real  user  (group)  ID, the effective user
              (group) ID, or the saved set-user-ID (saved set-group-ID).

       4.3BSD, POSIX.1-2001.

       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).  On an arbitrary
       system one should check _POSIX_SAVED_IDS.

       Under libc4,  libc5  and  glibc  2.0  seteuid(euid)  is  equivalent  to
       setreuid(-1,  euid)  and hence may change the saved set-user-ID.  Under
       glibc 2.1 and later it is equivalent to  setresuid(-1,  euid,  -1)  and
       hence  does  not  change the saved set-user-ID.  Analogous remarks hold
       for setegid(), with the difference that the  change  in  implementation
       from  setregid(-1,  egid)  to setresgid(-1, egid, -1) occurred in glibc
       2.2 or 2.3 (dependeing on the hardware architecture).

       According to POSIX.1, seteuid() (setegid()) need not permit euid (egid)
       to be the same value as the current effective user (group) ID, and some
       implementations do not permit this.

       geteuid(2), setresuid(2), setreuid(2), setuid(2), capabilities(7), cre-
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