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Date: Tue, 25 Apr 2023 13:19:16 +0200
From: Florian Weimer <fweimer@...hat.com>
To: Jonathan Wakely <jwakely@...hat.com>
Cc: libc-coord@...ts.openwall.com
Subject: Re: Relation between C timespec_get time bases and
POSIX clockid_t values.
* Jonathan Wakely:
> C11 added timespec_get and the constant TIME_UTC. POSIX extends this
> by requiring TIME_UTC to use the same clock as
> CLOCK_REALTIME. However, TIME_UTC is required to be non-zero, and
> CLOCK_REALTIME is zero on existing implementations.
Uh-oh.
| If the timespec_get function is successful it returns the nonzero
| value base; otherwise, it returns zero.
Here, “base” is TIME_UTC, so that non-zero assumption is baked pretty
strongly into the API. This is rather unfortunate.
> C23 adds TIME_MONOTONIC, and (optionally supported) TIME_ACTIVE and
> TIME_THREAD_ACTIVE. Presumably a future version of POSIX will say
> those use the same clocks as CLOCK_MONOTONIC, CLOCK_PROCESS_CPUTIME_ID
> and CLOCK_THREAD_CPUTIME_ID respectively.
>
> It would be helpful if POSIX guaranteed that every C time base can be
> converted to a POSIX clockid_t by adding the constant value (TIME_UTC
> - (int)CLOCK_REALTIME). This would allow programs using both
> timespec_get and clock_gettime to use a single value for a given
> clock, and easily convert to the other domain. For example, a program
> using timespec_get in one module (which might be written in portable
> ISO C) might want to pass a clock to pthread_cond_timedwait in another
> module (written in POSIX C). This could also be relevant to a proposed
> C++ extension that would expose the underlying clock used by C++
> clocks such as std::chrono::system_clock and std::chrono::steady_clock
> (see https://lists.isocpp.org/std-proposals/2023/04/6379.php for
> details).
I think the C++ extension should aim at supporting POSIX (plus vendor
extensions) exclusively, and ignore the ISO C clocks.
C libraries don't know what kind of incompatibilities WG14 will come up
next, so we cannot give you any promises for the future behavior of
timespec_get. You probably should ignore it.
Thanks,
Florian
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