Re: 32 bits time_t and Y2038 issue

Il 12/03/2025 19:18, David Brown ha scritto:

On 12/03/2025 18:13, pozz wrote:

Il 12/03/2025 17:39, David Brown ha scritto:

On 12/03/2025 16:48, pozz wrote:

Il 12/03/2025 10:33, David Brown ha scritto:

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For all of this, the big question is /why/ you are doing it.  What are you doing with your times?  Where are you getting them?  Are you actually doing this in a sensible way because they suit your application, or are you just using these types and structures because they are part of the standard C library - which is not good enough for your needs here?

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When the user wants to set the current date and time, I fill a struct tm with user values. Next I call mktime() to calculate time_t that is been incrementing every second.
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When I need to show the current date and time to the user, I call localtime() to convert time_t in struct tm. And I have day of the week too.
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Consider that mktime() and localtime() take into account timezone, that is important for me. In Italy we have daylight savings time with not so simple rules. Standard time functions work well with timezones.
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Maybe you are going about it all the wrong way.  If you need to be able to display and set the current time and date, and to be able to conveniently measure time differences for alarms, repetitive tasks, etc., then you probably don't need any correlation between your monotonic seconds counter and your time/date tracker.  All you need to do is add one second to each, every second.  I don't know the details of your application (obviously), but often no conversion is needed either way.

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I'm talking about *wall* clock only. Internally I have a time_t variable that is incremented every second. But I need to show it to the user and I can't show the seconds from the epoch.
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The sane way to do this - the way it has been done for decades on small embedded systems - is to track both a human-legible date/time structure (ignore standard struct tm - make your own) /and/ to track a monotonic seconds counter (or milliseconds counter, or minutes counter - whatever you need).  Increment both of them every second.  Both operations are very simple - far easier than any conversions.

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If I got your point, adding one second to struct mytm isn't reduced to a ++ on one of its member. I should write something similar to this:
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if (mytm.tm_sec < 59) {
   mytm.tm_sec += 1;
} else {
   mytm.tm_sec = 0;
   if (mytm.tm_min < 59) {
     mytm.tm_min += 1;
   } else {
     mytm.tm_min = 0;
     if (mytm.tm_hour < 23) {
       mytm.tm_hour += 1;
     } else {
       mytm.tm_hour = 0;
       if (mytm.tm_mday < days_in_month(mytm.tm_mon, mytm.tm_year)) {
         mytm.tm_mday += 1;
       } else {
         mytm.tm_mday = 1;
         if (mytm.tm_mon < 12) {
           mytm.tm_mon += 1;
         } else {
           mytm.tm_mon = 0;
           mytm.tm_year += 1;
         }
       }
     }
   }
}
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 Yes, that's about it.
 

However taking into account dst is much more complex. The rule is the last sunday of March and last sunday of October (if I'm not wrong).

 No, it is not complex.  Figure out the rule for your country (I'm sure Wikipedia well tell you if you are not sure) and then apply it.  It's just a comparison to catch the right time and date, and then you add or subtract an extra hour.
 

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All can be coded manually from the scratch, but there are standard functions just to avoid reinventing the wheel.

 You've just written the code!  You have maybe 10-15 more lines to add to handle daylight saving.
 

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Tomorrow I could install my device in another country in the world and it could be easy to change the timezone with standard function.

 How many countries are you targeting?  Europe all uses the same system.
 <https://en.wikipedia.org/wiki/Daylight_saving_time_by_country>
 

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Adding or subtracting an hour on occasion is also simple.

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Yes, but the problem is *when*. You need to know the rules and you need to implement them. localtime() just works.
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 You are getting ridiculous.  This is not rocket science.

Ok, but I don't understand why you prefer to write your own code (yes, you're an exper programmer, but you can introduce some bugs, you have to write  some tests), while there are standard functions that make the job for you.
I could rewrite memcpy, strcat, strcmp, they aren't rocket science, but why? IMHO there is no sense.
In my case standard functions aren't good (because of Y2038 issue) and rewriting them can be a valid solution. But if I had a 64 bits time_t, I would live with standard functions very well.

Besides, any fixed system is at risk from changes - and countries have in the past and will in the future change their systems for daylight saving.  (Many have at least vague plans of scraping it.)  So if a simple fixed system is not good enough for you, use the other method I suggested - handle it by regular checks from a server that you will need anyway for keeping an accurate time, or let the user fix it for unconnected systems.

My users like the automatic dst changes on my connected and unconnected devices. The risk of a future changes in the dst rules doesn't seem to me a good reason to remove that feature.

If your system is connected to the internet, then occasionally pick up the current wall-clock time (and unix epoch, if you like) from a server, along with the time of the next daylight savings change.

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What do you mean with "next daylight savings change"? I'm using NTP (specifically SNTP from a public server) and I'm able to retrive the current UTC time in seconds from Unix epoch.
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I just take this value and overwrite my internal counter.
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In other application, I retrive the current time from incoming SMS. In this case I have a local broken down time.
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If it is not connected, then the user is going to have to make adjustments to the time and date occasionally anyway, as there is always drift

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Drifts? By using a 32.768kHz quartz to generate a 1 Hz clock that increases the internal counter avoid any drifts.

 There is no such thing as a 32.768 kHz crystal - there are only approximate crystals.  If you don't update often enough from an accurate time source, you will have drift.  (How much drift you have, and what effect it has, is another matter.)

Of course, the quartz has an accuracy that changes with life, temperature an so on. However the real accuracy doesn't allow the time drifting so much the user needs to reset the time.

- they can

do the daylight saving change at the same time as they change their analogue clocks, their cooker clock, and everything else that is not connected.

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I think you can take into account dst even if the device is not connected.
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 You certainly can.  But then you have to have a fixed algorithm known in advance.
 

I bet Windows is able to show the correct time (with dst changes) even if the PC is not connected.

 I bet it can't, in cases where the date system for the daylight savings time has changed or been removed.  Other than that, it will just use a table of date systems such as on the Wikipedia page.  Or perhaps MS simply redefined what they think other people should use.
 Older Windows needed manual changes for the date and time, even when it was connected - their support for NTP was late.

Maybe Windows is not able, but I'm reading Linux is. It saves the time as UTC on the hw RTC and shows it to the user as localtime, of course applying dst and timezone rules from a database of rules.
So, as long as the timezone/dst info for my timezone is correct, I think Linux could manage dst changes automatically without user activity.
My approach is identical to what Linux does.

Or you can get the sources for a modern version of newlib, and pull the routines from there.

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It's a very complex code. time functions are written for whatever timezone is set at runtime (TZ env variable), so their complexity are higher.
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So find a simpler standard C library implementation.  Try the avrlibc, for example.
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But I have no doubt at all that you can make all this yourself easily enough.

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I think timezone rules are not so simple to implement.
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You don't need them.  That makes them simple.