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Re: strftime(3) and TZ

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Francis Moreau <francis.moro@gmail.com> writes:

> Le jeudi 29 octobre 2015 20:53:51 UTC+1, Ben Bacarisse a écrit :
>> Francis Moreau <francis.moro@gmail.com> writes:
>> 
>> > I'm probably missing something but can'f find what.
>> >
>> > So my question is: strftime(3) isnt supposed to read TZ and to display
>> > time according to its value ?
>> >
>> > From the manpage:
>> >
>> >    ENVIRONMENT
>> >           The environment variables TZ and LC_TIME are used.
>> 
>> But does it say that they will override an explicit setting?  I would
>> hope they would not (and they don't).
>> 
>> > But I cant verify it with my dumb prog:
>> >
>> > $ cat strftime.c
>> > #include <time.h>
>> > #include <stdio.h>
>> > #include <stdlib.h>
>> >
>> > int
>> > main(int argc, char *argv[])
>> > {
>> >     char outstr[200];
>> >     time_t t;
>> >     struct tm tmp = {
>> > 	    .tm_sec = 2,
>> > 	    .tm_min = 54,
>> > 	    .tm_hour = 19,
>> > 	    .tm_mday = 29,
>> > 	    .tm_mon = 9,
>> > 	    .tm_year = 115,
>> > 	    .tm_wday = 4,
>> > 	    .tm_yday = 301,
>> > 	    .tm_isdst = 0,
>> > 	    .tm_gmtoff = 3600,
>> > 	    .tm_zone = "CET",
>> 
>> These last two are not standard C, and they don't even seem to be
>> standard POSIX extensions (I've not checked all versions) but they are,
>> I think, the root of the problem.  If you don't set them, the code works
>> as you want.
>
> From the manpage of mktime(3):
>
>        The glibc version of struct tm has additional fields
>
>               long tm_gmtoff;           /* Seconds east of UTC */
>               const char *tm_zone;      /* Timezone abbreviation */
>
>        defined when _BSD_SOURCE was set before including <time.h>.  This is a
>        BSD extension, present in 4.3BSD-Reno.
>
> So you're right those fields are not standard C nor POSIX.
>
> I actually set them because I thought that strftime(3) could
> automatically modify the time given by tm struct if a different
> timezone was specified in TZ than the one given by the extra fields
> (tm_zone, ...).

The documentation is not very clear but I would not expect an
environment variable to override an explicit setting.  you were
surprised because you though it would.

> Another strange point is that LC_TIME isn't took into account too:
>
> $ LC_TIME=fr_FR /tmp/strftime "%a %T %Z"
> Thu 19:54:02 CET
>
> which is not correct: 'Thu' should be 'jeu.'

The local documentation is not clear, I agree.  You need to set the
locale for the environment by calling:

  setlocale(LC_ALL, "");

first (include <locale.h> of course).

It's actually clearer in the C standard which explicitly states that
programs execute as if a call to

  setlocale(LC_ALL, "C");

had been made at startup.  The text "Thu" is required from %a in the "C"
locale.

-- 
Ben.

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Thread

strftime(3) and TZ Francis Moreau <francis.moro@gmail.com> - 2015-10-29 12:10 -0700
  Re: strftime(3) and TZ Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-10-29 19:53 +0000
    Re: strftime(3) and TZ Francis Moreau <francis.moro@gmail.com> - 2015-10-30 00:26 -0700
      Re: strftime(3) and TZ Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-10-30 11:19 +0000
        Re: strftime(3) and TZ Francis Moreau <francis.moro@gmail.com> - 2015-10-30 09:06 -0700

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