clock_add_backend
clock_add_backend(CLOCK_HIGHRES, &clock_highres);
clock_add_backend(CLOCK_PROCESS_CPUTIME_ID, &clock_process);
clock_add_backend(CLOCK_REALTIME, &clock_realtime);
clock_add_backend(__CLOCK_REALTIME0, &clock_realtime);
clock_add_backend(CLOCK_VIRTUAL, &clock_thread_usr);
clock_add_backend(CLOCK_THREAD_CPUTIME_ID, &clock_thread_usrsys);
extern void clock_add_backend(clockid_t clock, clock_backend_t *backend);