C23 defines library macros __STDC_VERSION_<header>_H__ to indicate
that a header has support for new / changed features from C23. Now
that all the required library features are implemented in glibc,
define these macros. I'm not sure this is sufficiently much of a
user-visible feature to be worth a mention in NEWS.
Tested for x86_64.
There are various optional C23 features we don't yet have, of which I
might look at the Annex H ones (floating-point encoding conversion
functions and _Float16 functions) next.
* Optional time bases TIME_MONOTONIC, TIME_ACTIVE, TIME_THREAD_ACTIVE.
See
<https://sourceware.org/pipermail/libc-alpha/2023-June/149264.html>
- we need to review / update that patch. (I think patch 2/2,
inventing new names for all the nonstandard CLOCK_* supported by the
Linux kernel, is rather more dubious.)
* Updating conform/ tests for C23.
* Defining the rounding mode macro FE_TONEARESTFROMZERO for RISC-V (as
far as I know, the only architecture supported by glibc that has
hardware support for this rounding mode for binary floating point)
and supporting it throughout glibc and its tests (especially the
string/numeric conversions in both directions that explicitly handle
each possible rounding mode, and various tests that do likewise).
* Annex H floating-point encoding conversion functions. (It's not
entirely clear which are optional even given support for Annex H;
there's some wording applied inconsistently about only being
required when non-arithmetic interchange formats are supported; see
the comments I raised on the WG14 reflector on 23 Oct 2025.)
* _Float16 functions (and other header and testcase support for this
type).
* Decimal floating-point support.
* Fully supporting __int128 and unsigned __int128 as integer types
wider than intmax_t, as permitted by C23. Would need doing in
coordination with GCC, see GCC bug 113887 for more discussion of
what's involved.
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