os_helper_thread
int start_io_thread(struct os_helper_thread **td_out, int *fd_out);
static struct os_helper_thread *io_td;
int start_io_thread(struct os_helper_thread **td_out, int *fd_out)
struct os_helper_thread;
int os_run_helper_thread(struct os_helper_thread **td_out,
void os_kill_helper_thread(struct os_helper_thread *td);
int os_run_helper_thread(struct os_helper_thread **td_out,
struct os_helper_thread *td;
void os_kill_helper_thread(struct os_helper_thread *td)
static struct os_helper_thread *write_sigio_td;