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SETREUID(2)                 BSD System Calls Manual                SETREUID(2)

     setreuid -- set real and effective user IDs

     #include <&lt;unistd.h>&gt;

     setreuid(uid_t ruid, uid_t euid);

     The real and effective user IDs of the current process are set according
     to the arguments.  The saved user ID will be set to the new value of the
     real user ID if a real user ID is specified and either the new real user
     ID value is different from the current value or the new value of the
     effective user ID differs from the current saved user ID.

     Unprivileged users may change either user ID to the current value of the
     real, effective, or saved user ID.  Only the superuser may make other

     Supplying a value of -1 for either the real or effective user ID forces
     the system to substitute the current ID in place of the -1 parameter.

     The setreuid() function was intended to allow swapping the real and
     effective user IDs in set-user-ID programs to temporarily relinquish the
     set-user-ID value.  This purpose is now better served by the use of the
     seteuid(2) function.

     When setting the real and effective user IDs to the same value, the
     setuid(2) function is preferred.

     Upon successful completion, the value 0 is returned; otherwise the
     value -1 is returned and the global variable errno is set to indicate the

     [EPERM]            The current process is not the superuser and a change
                        other than changing the effective user ID to the real
                        user ID was specified.

     getuid(2), seteuid(2), setregid(2), setresuid(2), setuid(2)

     The setreuid() function conforms to the IEEE Std 1003.1-2008
     (``POSIX.1'') specification, except that the conditions for changing the
     saved user ID differ and that, if it is changed, the saved user ID is set
     to the real user ID instead of the effective user ID.

     The setreuid() system call first appeared in 4.1cBSD, predating POSIX.  A
     semantically different version appeared in 4.4BSD.  The current version,
     with the original semantics restored, appeared in OpenBSD 3.3.

BSD                            February 5, 2015                            BSD