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Date: Tue, 25 Apr 2023 13:19:16 +0200
From: Florian Weimer <>
To: Jonathan Wakely <>
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.


| 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 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.


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