CLOCK_REALTIME
CLOCK_MONOTONIC
CLOCK_PROCESS_CPUTIME_ID
CLOCK_THREAD_CPUTIME_ID
CLOCK_MONOTONIC_RAW
CLOCK_REALTIME_COARSE
CLOCK_MONOTONIC_COARSE
CLOCK_BOOTTIME
CLOCK_REALTIME_ALARM
CLOCK_BOOTTIME_ALARM
CLOCK_SGI_CYCLE
CLOCK_TAI