The function int pthread_condattr_setclock(pthread_condattr_t *attr, clockid_t clock_id); shall set the 'clock' attribute in an initialized attributes object referenced by 'attr'. If it is called with a clock_id argument that refers to a CPU-time clock, the call shall fail. It returns 0 upon success, and an error code if it fails: It may fail if: [EINVAL] - the 'attr' value is invalid [EINVAL] - 'clock_id' doesn't refer to a known clock or is a CPU-time clock.