Return_Values

   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.