get_tick_count overflow after 24 days - Script Writing - CHDK Forum
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get_tick_count overflow after 24 days

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Offline vincasr

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get_tick_count overflow after 24 days
« on: 02 / March / 2019, 14:32:02 »
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What will CHDK LUA get_tick_count() return after 24+ days?

My thoughts:
Return value should be 32 signed integer
Most probably answer is just 2147483647 + 1 -> -2147483648
But it could also cap at last integer second and wrap to 0 or some other stuff

Just want to confirm this...  without running camera for few weeks :)
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Offline reyalp

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Re: get_tick_count overflow after 24 days
« Reply #1 on: 02 / March / 2019, 15:51:45 »
Quote from: vincasr on 02 / March / 2019, 14:32:02
What will CHDK LUA get_tick_count() return after 24+ days?

My thoughts:
Return value should be 32 signed integer
Most probably answer is just 2147483647 + 1 -> -2147483648
But it could also cap at last integer second and wrap to 0 or some other stuff

Just want to confirm this...  without running camera for few weeks :)
FWIW, I wouldn't assume that the Canon firmware handles this situation correctly, so my recommendation would be to reboot before that. Even if it appears to work, there could be subtle impacts.

I think some people may have run cameras this long, but I don't recall any detailed reports of exactly what happened. Assuming the camera keeps incrementing (which would be my guess), the tick count in lua would indeed go negative.
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Offline c_joerg

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Re: get_tick_count overflow after 24 days
« Reply #2 on: 02 / March / 2019, 16:54:25 »
If you had asked the question a month ago, then I could give you an answer. I stopped my camera after 23 days. get_tick_count () can be found in the third column (tick) in the logfile.
May be next time  ;)
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Offline reyalp

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Re: get_tick_count overflow after 24 days
« Reply #3 on: 02 / March / 2019, 18:24:05 »
Quote from: c_joerg on 02 / March / 2019, 16:54:25
If you had asked the question a month ago, then I could give you an answer. I stopped my camera after 23 days. get_tick_count () can be found in the third column (tick) in the logfile.
May be next time  ;)
Yes, if you do another one of these it would definitely be good to know what happens.

One other thing to watch out for is that calculations can overflow before the actual wraparound time, like
Code: [Select]
time_for_next_shot=get_tick_count() + some_value
wait until get_tick_count() >= time_for_next_shot
time_for_next_shot becomes negative immediately, less than the still positive get_tick_count.

The firmware likely does some calculations of this sort, though I'd expect most are on the order of seconds rather than days.
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Offline waterwingz

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Re: get_tick_count overflow after 24 days
« Reply #4 on: 02 / March / 2019, 23:52:55 »
Quote from: vincasr on 02 / March / 2019, 14:32:02
Just want to confirm this...  without running camera for few weeks :)
Or yo=u could do like this script :  Ultimate Intervalometer and automatically reboot the camera every couple of days.
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Offline c_joerg

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Re: get_tick_count overflow after 24 days
« Reply #5 on: 03 / March / 2019, 01:36:27 »
Quote from: reyalp on 02 / March / 2019, 18:24:05
One other thing to watch out for is that calculations can overflow before the actual wraparound time, like
Code: [Select]
time_for_next_shot=get_tick_count() + some_value
wait until get_tick_count() >= time_for_next_shot
time_for_next_shot becomes negative immediately, less than the still positive get_tick_count.
I could do the same think with os.time…
Where is the advantage of this method over os.time?
https://chdk.setepontos.com/index.php?topic=13675.0

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Offline reyalp

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Re: get_tick_count overflow after 24 days
« Reply #6 on: 03 / March / 2019, 16:37:46 »
Quote from: c_joerg on 03 / March / 2019, 01:36:27
Quote from: reyalp on 02 / March / 2019, 18:24:05
One other thing to watch out for is that calculations can overflow before the actual wraparound time, like
Code: [Select]
time_for_next_shot=get_tick_count() + some_value
wait until get_tick_count() >= time_for_next_shot
time_for_next_shot becomes negative immediately, less than the still positive get_tick_count.
I could do the same think with os.time…
Where is the advantage of this method over os.time?
https://chdk.setepontos.com/index.php?topic=13675.0
os.time only has second resolution, which may or may not matter to you, but my example was just meant to show a kind of calculation that would have problems before the actual wraparound.

Script logic could be adjusted to deal with all this, but the risk is that the Canon firmware (or CHDK C code) does calculations with tick values internally that could start to have problems. So for people who are really worried about long runs, I'd recommend ensuring the camera is rebooted some time (maybe a day or so) before the overflow unless they've actually verified with testing.

edit:
In fact, the action_stack code used for blocking script functions in CHDK does something very much like this, but uses unsigned values so might work up to ~49 days instead of ~24.
« Last Edit: 03 / March / 2019, 19:16:27 by reyalp »
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Offline vincasr

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Re: get_tick_count overflow after 24 days
« Reply #7 on: 04 / March / 2019, 11:02:31 »
Quote from: waterwingz on 02 / March / 2019, 23:52:55
Quote from: vincasr on 02 / March / 2019, 14:32:02
Just want to confirm this...  without running camera for few weeks :)
Or yo=u could do like this script :  Ultimate Intervalometer and automatically reboot the camera every couple of days.

This is probably safest approach, to avoid any non-script bugs as well. Also thanks for noting about ultimate.lua, this has most and more features that I implemented on my own in past couple of years, will take a look!
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