Difference between CLOCK_REALTIME and CLOCK_MONOTONIC?
Solution 1:
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.
Solution 2:
Robert Love's book LINUX System Programming 2nd Edition, specifically addresses your question at the beginning of Chapter 11, pg 363:
The important aspect of a monotonic time source is NOT the current value, but the guarantee that the time source is strictly linearly increasing, and thus useful for calculating the difference in time between two samplings
That said, I believe he is assuming the processes are running on the same instance of an OS, so you might want to have a periodic calibration running to be able to estimate drift.
Solution 3:
CLOCK_REALTIME is affected by NTP, and can move forwards and backwards. CLOCK_MONOTONIC is not, and advances at one tick per tick.
Solution 4:
In addition to Ignacio's answer, CLOCK_REALTIME can go up forward in leaps, and occasionally backwards. CLOCK_MONOTONIC does neither; it just keeps going forwards (although it probably resets at reboot).
A robust app needs to be able to tolerate CLOCK_REALTIME leaping forwards occasionally (and perhaps backwards very slightly very occasionally, although that is more of an edge-case).
Imagine what happens when you suspend your laptop - CLOCK_REALTIME jumps forwards following the resume, CLOCK_MONOTONIC does not. Try it on a VM.