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.