Question

If I get the time using clock_gettime(CLOCK_MONOTONIC,x), then call yield (e.g. sched_yield()), then get the CLOCK_MONOTONIC time again, will the difference in times include the time the program wasn’t running (having yielded), or does CLOCK_MONOTONIC only track the time during which the program is executing? My tests seem to imply the latter, but I’d like to know for sure.

Also, if CLOCK_MONOTONIC doesn’t include the time yielded, is there another monotonic timer (ie. one not subject to jumps caused by ntp) that does?

Answer 1

The answer Maxim and comments to that answered the second part of your question, I believe. To expand on the answer for the first part, POSIX 2008 states


If the Monotonic Clock option is supported, all implementations shall support a clock_id of CLOCK_MONOTONIC defined in <time.h>. This clock represents the monotonic clock for the system. For this clock, the value returned by clock_gettime() represents the amount of time (in seconds and nanoseconds) since an unspecified point in the past (for example, system start-up time, or the Epoch). This point does not change after system start-up time.


In particular, note “the monotonic clock for the system”. That is, per-system and not per-process, it keeps ticking even though your process is not running. Also, “This point does not change after system start-up time.”, which again implies that it keeps ticking regardless of whether a particular process is running or sleeping.

So, either you have found a bug in the Linux implementation, or more likely, in your test program.