it supports only CLOCK_REALTIME
though CLOCK_PROCESS_CPUTIME_ID and CLOCK_THREAD_CPUTIME_ID are defined too in bits/time.h, but clock_gettime() gives an error "Invalid argument" for these...
------from bits/time.h-----
/* Identifier for system-wide realtime clock. */
# define CLOCK_REALTIME 0
/* High-resolution timer from the CPU. */
# define CLOCK_PROCESS_CPUTIME_ID 2
/* Thread-specific CPU-time clock. */
# define CLOCK_THREAD_CPUTIME_ID 3
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