> Suppose you create a struct tm _without_ gmtime(3) or localtime(3),
> using designated initializers or memset for zero-initialization, with
> only what is included in POSIX:
> struct tm tm = {
> .tm_sec = 56,
> .tm_min = 34,
> .tm_hour = 12,
> .tm_mday = 1,
> .tm_mon = 12 - 1, /* December */
> .tm_year = 2021 - 1900,
> .tm_wday = 3, /* Wednesday */
> .tm_yday = 334, /* zero-based day of year (%j - 1) */
> .tm_isdst = 0,
> };
This is fine. But using memset is not; if struct tm contains a pointer
or a floating-point value, setting it to all-0-bits may produce a trap
representation - or, possibly worse, a valid value that means something
different from what you intend.
Unless POSIX was stupid enough to mandate that all-bits-0 is nil for
any pointer type and something well-defined for floating-point. (I'd
be surprised by that, but standards bodies have surprised me often
enough in the past.) Certainly C doesn't, at least not as of C99 - I
don't have a copy of anything newer.
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