0                  Indicates success.
    -1                 Indicates an error. The signal mask of the
                       process is unchanged. errno is set to one of
                       the following values:

                       o  EINVAL - The value of the how argument is
                          not equal to one of the defined values.

                       o  EFAULT - The set or o_set argument points
                          to a location outside the allocated address
                          space of the process.