Just curious, was there any response? On Mon, 18 Jul 2022 at 18:19, Paul Eggert <eggert@cs.ucla.edu> wrote:
On 7/18/22 09:03, Paul Eggert wrote:
I suppose someone should file a defect report with the C standardization committee.
I found what I hope is the correct email address for that (it's not well-advertised) and submitted the following bug report:
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Subject: strftime %z and %Z depend on more than just tm_isdst
A recent discussion in the Time Zone Database mailing list <https://mm.icann.org/pipermail/tz/2022-July/031674.html> has prompted me to file this bug report against the C standard.
Draft N2912, section 7.28.3.5 "The strftime function", pages 365-6, paragraph 3 says that the %z and %Z conversion specifiers examine only the tm_isdst members of the passed-in structure. However, this is not how many implementations actually behave.
Some implementations (e.g., AIX, Solaris) infer %z and %Z output from tm_year, tm_mon, tm_mday, tm_hour, tm_min, tm_sec, and tm_isdst, and therefore sometimes guess %z or %Z output incorrectly when the clock moves back from one standard time to another (e.g., Iran at the end of the year 1978). These implementations typically also depend on the current setting of the TZ environment variable.
Other implementations (e.g., FreeBSD, GNU/Linux) get the UT offset and abbreviation from struct tm members tm_gmtoff and tm_zone that are not specified by the C standard. These implementations do not have to guess %z and %Z output; however, they require that the nonstandard members be filled in correctly by localtime or equivalent.
Implementations that conform to the standard's current wording cannot handle anything more complicated than a time zone with just one standard time offset and abbreviation, which means they cannot handle timestamps for locations like Iran, Portugal, etc., that have changed their standard-time offset. The C standard should not require implementations to be so limited that they cannot handle common timekeeping situations.
A simple fix for this issue is to require applications to initialize a struct tm as if by localtime or equivalent, before passing the struct tm to strftime. This is what applications need to do anyway, if they want to be portable to real-world systems such as AIX, FreeBSD, GNU/Linux, Solaris, etc.
Proposed fix:
1. Remove the two instances of "[tm_isdst]" in the %z and %Z entries in Draft N2912, section 7.28.3.5 "The strftime function", pages 365-6, paragraph 3.
2. Add the following text to that paragraph:
If the %z and %Z conversion specifiers are used, the broken-down time structure pointed to by timeptr shall contain values generated by a successful previous call to gmtime, gmtime_r, gmtime_s, localtime, localtime_r, localtime_s, or mktime.