[PATCH v3 2/2] libstdc++: Use __chrono_write via _S_empty_fs for chrono ostream insertion

[Jonathan Wakely]

On Wed, 1 Jul 2026 at 13:59, Richard Earnshaw (foss)
<Richard.Earnshaw@arm.com> wrote:
>
> On 01/07/2026 13:33, Tomasz Kaminski wrote:
> >
> >
> > On Wed, Jul 1, 2026 at 2:15 PM Anlai Lu <agicy@qq.com <mailto:agicy@qq.com>> wrote:
> >
> >     Add __detail::__chrono_write which formats a chrono object into a
> >     stack buffer via std::format_to_n with _S_empty_fs(), then writes
> >     through __ostream_insert.  A non-type template parameter _BufSize
> >     (default 128) allows per-type buffer tuning.  Each _BufSize is the
> >     smallest power of two that accommodates the longest possible formatted
> >     output for that type, including error cases.  All chrono operator<<
> >     overloads that previously used std::format, std::vformat, or
> >     basic_stringstream now use this helper.
> >
> >     libstdc++-v3/ChangeLog:
> >
> >             * include/bits/chrono_io.h (__formatter_chrono::_S_empty_fs):
> >             Make public.
> >             (__detail::__chrono_write): New function template.
> >             (operator<<): Use __chrono_write consistently for all chrono
> >             types.
> >
> >     Signed-off-by: Anlai Lu <agicy@qq.com <mailto:agicy@qq.com>>
> >     ---
> >      libstdc++-v3/include/bits/chrono_io.h | 244 ++++++--------------------
> >      1 file changed, 52 insertions(+), 192 deletions(-)
> >
> > LGTM. Thank you for working on this.
> >
>
> I think this is causing:
>
> /home/rearnsha/gnusrc/gcc-cross/misc-fixes/libstdc++-v3/src/c++20/tzdb.cc: In st
> atic member function ‘static const std::chrono::tzdb& std::chrono::tzdb_list::_N
> ode::_S_replace_head(std::shared_ptr<std::chrono::tzdb_list::_Node>, std::shared
> _ptr<std::chrono::tzdb_list::_Node>)’:
> /home/rearnsha/gnusrc/gcc-cross/misc-fixes/libstdc++-v3/src/c++20/tzdb.cc:1617:22: error: ‘struct std::chrono::tzdb_list::_Node::NumLeapSeconds’ has no member named ‘set_locked’
>  1617 |     num_leap_seconds.set_locked(new_head_ptr->db.leap_seconds.size(), lock);
>       |                      ^~~~~~~~~~
> make[9]: *** [Makefile:575: tzdb.lo] Error 1
>
> when building an arm-eabi cross with newlib.

That's a different patch (this one isn't committed yet) but I'll take
care of it.