[Ntp] Antw: Re: Antw: [EXT] Timescales, leapseconds and smearing

[Ulrich Windl <[e-mail]>]

>>> Kurt Roeckx <[e-mail]> schrieb am 08.12.2020 um 12:05 in Nachricht
<[e-mail]>:
> On Tue, Dec 08, 2020 at 10:56:24AM +0100, Ulrich Windl wrote:
>> Hi!
>> 
>> I think if we agree to use anything other than UTC in NTPv5, we absolutely
>> need to specify some use-level API how to get the current time:
>> time()
>> gettimeofday()
>> clock_gettime()
>> localtime()
>> strftime()
>> 
>> They all assume UTC... It's rather useless if NTP has the exact time, but no
>> ("standard") program can use it.
> 
> They don't use UTC, they use POSIX/UNIX time. POSIX time, just like
> NTP, ignores the leap seconds. POSIX actually says that the length
> of a second is not specified, an implementation can then say it's
> SI seconds. POSIX has support for a leap second in struct tm, so as
> long as you don't convert it to a time_t you can work in UTC.

Hi!

Can you be more specific? I only see:

           struct tm {
               int tm_sec;    /* Seconds (0-60) */
               int tm_min;    /* Minutes (0-59) */
               int tm_hour;   /* Hours (0-23) */
               int tm_mday;   /* Day of the month (1-31) */
               int tm_mon;    /* Month (0-11) */
               int tm_year;   /* Year - 1900 */
               int tm_wday;   /* Day of the week (0-6, Sunday = 0) */
               int tm_yday;   /* Day in the year (0-365, 1 Jan = 0) */
               int tm_isdst;  /* Daylight saving time */
           };

The practical reason when POSIX does not specify the length of a second seems to allow clocks with errors; demending that  it's an SI second yould pose relly trouble on implementers I'm afraid.

Regards,
Ulrich

> strftime() works with a struct tm, so can work in UTC.
> 
> The linux kernel has support for TAI. You can set a TAI offset, so
> that applications making use of the POSIX API still get the POSIX
> time. The clock_gettime() function you mentioned has TAI support.

Again, can you be more specific. I only see:

       The  res  and  tp  arguments  are  timespec structures, as specified in
       <time.h>:

           struct timespec {
               time_t   tv_sec;        /* seconds */
               long     tv_nsec;       /* nanoseconds */
           };

And the letters TAI appear nowhere in the description.

> The information to convert from TAI to UTC is available, so it's
> possible to work in TAI or UTC on Linux, it's just that most
> applications don't.

And when not using Linux?

Regards,
Ulrich
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