Could you explain the difference between CLOCK_REALTIME and CLOCK_MONOTONIC clocks returned by clock_gettime() on Linux?
Which is a better choice if I need to compute elapsed time between timestamps produced by an external source and the current time?
Lastly, if I have an NTP daemon periodically adjusting system time, how do these adjustments interact with each of CLOCK_REALTIME and CLOCK_MONOTONIC?
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CLOCK_REALTIME represents the machine's best-guess as to the current wall-clock, time-of-day time. As Ignacio and MarkR say, this means that CLOCK_REALTIME can jump forwards and backwards as the system time-of-day clock is changed, including by NTP.
CLOCK_MONOTONIC represents the absolute elapsed wall-clock time since some arbitrary, fixed point in the past. It isn't affected by changes in the system time-of-day clock.
If you want to compute the elapsed time between two events observed on the one machine without an intervening reboot, CLOCK_MONOTONIC is the best option.
Note that on Linux, CLOCK_MONOTONIC does not measure time spent in suspend, although by the POSIX definition it should. You can use the Linux-specific CLOCK_BOOTTIME for a monotonic clock that keeps running during suspend.