Description

   The setreuid function sets 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 is left unchanged.

   A process with the IMPERSONATE privilege can set either ID to any
   value. An unprivileged process can set the effective user ID only
   if the euid argument is equal to either the real, effective, or
   saved user ID of the process.

   It is unspecified whether a process without the IMPERSONATE
   privilege is permitted to change the real user ID to match the
   current real, effective, or saved user ID of the process.

   This function requires that long (32-bit) UID/GID support be
   enabled. See 32-Bit UID and GID Macro (Integrity servers, Alpha)
   for more information.

   See also getuid to know how UIC is represented.