Hello, Tech-Talkers.
Is there a way to get monotonic time from EPICS Base libCom?
By "monotonic time," I mean a system time that increases at a steady
rate and is not affected by a change to the system's RTC. The origin of
the time is unspecified, but the difference between two monotonic times
can be used to compute elapsed time. [1]
I think the answer is no, but I thought I'd ask. I'd like a simple way
to implement a timeout in a C function like this:
start_time = epicsTimeGetMonotonic();
for (;;) {
if (is_data_ready()) goto out;
elapsed_time = epicsTimeGetMonotonic() - start_time;
if (elapsed_time > timeout) goto timeout;
epicsThreadSleep(0.1);
}
Without monotonic time, the closest I can get to this in a simple way
is to use epicsTimeGetCurrent and epicsTimeDiffInSeconds. This is
suboptimal because I have to deal with time that can advance backward
or forward in an unpredictable way, and it's not possible to correctly
compensate for that. For example, if the elapsed time is negative, time
has advanced backward, and I need to set the start time to the new time.
This has the undesirable effect of extending the timeout to an actual
amount of time longer than the caller requested. If time is slowly
advancing backward, this could have the undesirable effect of extending
the actual timeout indefinitely. And if time has advanced forward
faster than normal (either by being set or due to a system suspend), it
would have the undesirable effect of reducing the actual timeout to an
amount of time shorter than the caller requested.
Thank you!
Lewis
[1] It seems that most, if not all, implementations of monotonic time
will suspend the monotonic clock during a system suspend. This
way, programs using time from such a monotonic clock will function
normally through a system suspend-resume cycle. However, if
the process itself is suspended, when it resumes, it will see a
monotonic time that has advanced "faster" than it expected. In
other words, the monotonic time is per-system, not per-process. (I
see that Linux has a CLOCK_PROCESS_CPUTIME_ID clock type that would
overcome this, but I'm not sure something like this is available on
the other OSes supported by EPICS.)
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- Re: Monotonic time from EPICS Base libCom? Kasemir, Kay
- Re: Monotonic time from EPICS Base libCom? Michael Davidsaver
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