[tz] Invalid tm handling in %s pattern in strftime

Paul Eggert eggert at cs.ucla.edu
Mon Jun 6 18:27:40 UTC 2022

On 6/6/22 10:35, Almaz Mingaleev wrote:

> 1) According to the man page mktime sets errno if tm is invalid. Why is it
> not safe to just check errno value (saving and restoring it if needed)?

Because mktime can set errno if tm is valid. mktime is like most C 
syscalls - when they fail, they set errno to a value that tells you what 
the error was, but when they succeed they set errno to an unspecified value.

> 2) man page says that in error case mktime "returns (time_t) -1 and does
> not alter the members of the broken-down time structure". Wouldn't it be
> enough to check that tm.tm_yday is >= 0?

That's the man page for TZDB mktime; unfortunately the C standard does 
not guarantee this and strftime.c is attempting to be portable to 
non-TZDB mktime.

Now that I look at the code again, I see that the heuristic was still 
too trusting of non-TZDB mktime implementations, so I installed the 
attached to tighten things up. It's just a heuristic, so it can still do 
the wrong thing on even weirder implementations (though not TZDB).

Perhaps some day we can optimize strftime.c if we know that 
TZDB-compatible mktime is being used.
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