clock_gettime() returns the current value of timespec tp for the clock parameter clock_id If the clock parameter clock_id is CLOCK_REALTIME, the value returned by clock_gettime() is the seconds (or nanoseconds) since the Epoch. If the clock parameter clock_id is CLOCK_MONOTONIC, the value returned by clock_gettime() is the seconds (or nanoseconds) since an unspecified point, which cannot change after system startup, in the past. If the clock parameter clock_id is CLOCK_PROCESS_CPUTIME_ID, the value returned by clock_gettime() is the amount of execution time of the process associated with the clock. If the clock parameter clock_id is CLOCK_THREAD_CPUTIME_ID, the value returned by clock_gettime() is the amount of execution time of the thread associated with the clock. clock_gettime() returns 0 on success. clock_gettime() returns -1 on failure. clock_gettime() sets errno to EINVAL if the clock_id argument does not specify a known clock