[C] error : dereferencing pointer to incomplete type

error message

error: dereferencing pointer to incomplete type
if ( ioctl ( drv->ioctl_sock, IEEE80211_IOCTL_GETCHANINFO, &iwr ) < 0 ) {
               ^

error code

struct atheros_driver_data {
     int     ioctl_sock;     
};

static struct hostapd_hw_modes * get_hw_feature_data( void *priv, u16 *num_modes, u16 *flags )
{
     struct driver_atheros_data *drv = priv;

     if ( ioctl ( drv->ioctl_sock, IEEE80211_IOCTL_GETCHANLIST, &iwr ) < 0 )
     {
          wpa_printf( MSG_DEBUG, "athr: Failed to get channel info: %s", strerror ( errno ) );
          return NULL;
     }
}

explain

宣告的 structure 是 "atheros_driver_data", 
但使用的 structure 是 "driver_atheros_data",
兩個 structure 名稱是不一樣的