Re: [PATCH v2 2/4] libstdc++: Add _Streambuf_sink for direct streambuf formatting

On Mon, Aug 10, 2026 at 5:40 PM Tomasz Kamiński <[email protected]> wrote:

> From: Anlai Lu <[email protected]>
>
> Introduce _Streambuf_sink that writes directly to basic_streambuf
> via sputn, preferring zero-copy writes into the streambuf put area
> (pptr/epptr/pbump) and falling back to the stack buffer and bulk
> sputn.
>
> Streambuf I/O errors are tracked internally (_M_write_failed) and
> reported after formatting completes, so that format-phase exceptions
> (format_error, bad_alloc) propagate without setting badbit per
> [ostream.formatted.print]/(4.2).
>
> libstdc++-v3/ChangeLog:
>
>         * include/std/format (__format::_Streambuf_sink): New class
>         template.
>         * include/std/streambuf: Include bits/version.h.
>         (__format::_Streambuf_sink) [__glibcxx_format]: Declare.
>         (std::basic_streambuf) [__glibcxx_format]: Befriend
>         __format::_Streambuf_sink.
>
> Co-authored-by: Tomasz Kamiński <[email protected]>
> Signed-off-by: Anlai Lu <[email protected]>
> Signed-off-by: Tomasz Kamiński <[email protected]>
> ---
> Changes in v2:
>  - call _M_reserve_put_area in _Streambuf_sink constructor,
>    poiting buffer to put area
>  - ostreambuf_iterator: Remove the changes, that were unrelated
>    to standard
>  - remove _GLIBCXX_CONSTEXPR_FORMAT, as they classes cannot
>    be used at compile time
>  - remove and inline _M_pbump and _M_use_stackbuf, rename
>    _M_use_put_area to _M_reserve_put_area
>  - done some formatting changes
>
> Testing on powerprc64. Tested on x86_64, additionally tested
> *format* and *print* in all standard modes, assertions and
> debug. OK for trunk?
>
>  libstdc++-v3/include/std/format    | 110 +++++++++++++++++++++++++++++
>  libstdc++-v3/include/std/streambuf |   9 +++
>  2 files changed, 119 insertions(+)
>
> diff --git a/libstdc++-v3/include/std/format
> b/libstdc++-v3/include/std/format
> index 729cb89ec60..9223cd2e78b 100644
> --- a/libstdc++-v3/include/std/format
> +++ b/libstdc++-v3/include/std/format
> @@ -3532,6 +3532,116 @@ namespace __format
>        { }
>      };
>
> +  // A format sink that writes directly to a basic_streambuf.
> +  // Prefers zero-copy writes into the streambuf's put area
> +  // (pptr/epptr/pbump), falling back to the stack buffer
> +  // (_M_buf) and bulk sputn.
> +  template<typename _CharT, typename _Traits = char_traits<_CharT>>
> +    class _Streambuf_sink : public _Buf_sink<_CharT>
> +    {
> +    protected:
> +      basic_streambuf<_CharT, _Traits>* _M_sbuf;
> +      bool _M_write_failed = false;
> +
> +      bool
> +      _M_on_stack() const noexcept
> +      { return this->_M_used().data() == this->_M_buf; }
> +
> +      // Try to point our span directly into the streambuf's put
> +      // area for zero-copy writes.  Caller must fall back to
> +      // _M_use_stackbuf() on failure.
> +      bool
> +      _M_reserve_put_area(size_t __n = 0)
> +      {
> +       if (auto __p = _M_sbuf->pptr())
> +         if (auto __e = _M_sbuf->epptr(); __e > __p)
> +           if (size_t __a(__e - __p); __a >= __n)
> +             {
> +               this->_M_reset(span<_CharT>(__p, __a));
> +               return true;
> +             }
> +       return false;
> +      }
> +
> +      // Write characters to the streambuf, tracking errors.
> +      // I/O exceptions from sputn are caught and converted to
> +      // _M_write_failed so they do not propagate as vformat exceptions
> +      // per [ostream.formatted.print]/(4.2).
> +      void
> +      _M_flush()
> +      {
> +       span<const _CharT> __s = this->_M_used();
> +       if (__s.empty() || _M_write_failed) [[unlikely]]
> +         return;
> +
> +       __try
> +         {
> +           if (!_M_on_stack())
> +             _M_sbuf->__safe_pbump(__s.size());
> +           else if (_M_sbuf->sputn(__s.data(), __s.size())
> +                      != static_cast<streamsize>(__s.size()))
> +             _M_write_failed = true;
> +         }
> +       __catch(const __cxxabiv1::__forced_unwind&)
> +         { throw; }
> +       __catch(...)
> +         { _M_write_failed = true; }
> +      }
> +
> +      void
> +      _M_overflow() override
> +      {
> +       _M_flush();
> +       if (!_M_reserve_put_area() || _M_write_failed)
>
We should check _M_write_failed first and make it unlikely.

> +         this->_M_reset(this->_M_buf);
> +      }
> +
> +    public:
> +      [[__gnu__::__always_inline__]]
> +      explicit
> +      _Streambuf_sink(basic_streambuf<_CharT, _Traits>* __sbuf) noexcept
> +      : _M_sbuf(__sbuf)
> +      { _M_reserve_put_area(); }
> +
> +      // _M_bump is not overridden: the default advances _M_next within
> +      // the current span (stack buffer or put area).  Actual commit
> +      // to the streambuf happens via _M_flush, called from _M_overflow
> +      // (buffer full) or _M_finish (end of formatting).
> +
> +      typename _Sink<_CharT>::_Reservation
> +      _M_reserve(size_t __n) override
> +      {
> +       if (__n <= this->_M_unused().size())
> +         return { this };
> +
> +       if (!this->_M_used().empty())
> +         _M_flush();
> +
> +       // Try to write directly into the streambuf's put area.
> +       if (_M_reserve_put_area(__n))
> +         return { this };
> +
> +       // Otherwise reset to the stack buffer.
> +       this->_M_reset(this->_M_buf);
> +       if (__n <= this->_M_unused().size())
> +         return { this };
> +
> +       return { nullptr };
> +      }
> +
> +      bool
> +      _M_discarding() const override
> +      { return _M_write_failed; }
> +
> +      void
> +      _M_finish()
> +      { _M_flush(); }
> +
> +      bool
> +      _M_failed() const noexcept
> +      { return _M_write_failed; }
> +    };
> +
>    using _GLIBCXX_STD_C::vector;
>
>    // A sink that fills a sequence (e.g. std::string, std::vector,
> std::deque).
> diff --git a/libstdc++-v3/include/std/streambuf
> b/libstdc++-v3/include/std/streambuf
> index 616e44f74a7..d273db2835f 100644
> --- a/libstdc++-v3/include/std/streambuf
> +++ b/libstdc++-v3/include/std/streambuf
> @@ -39,6 +39,8 @@
>
>  #include <bits/requires_hosted.h> // iostreams
>
> +#include <bits/version.h>
> +
>  #include <bits/c++config.h>
>  #include <bits/iosfwd.h>
>  #include <bits/localefwd.h>
> @@ -57,6 +59,10 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>      __copy_streambufs_eof(basic_streambuf<_CharT, _Traits>*,
>                           basic_streambuf<_CharT, _Traits>*, bool&);
>
> +#ifdef __glibcxx_format // C++ >= 20 && HOSTED
> +  namespace __format { template<typename, typename> class
> _Streambuf_sink; }
> +#endif
> +
>    /**
>     *  @brief  The actual work of input and output (interface).
>     *  @ingroup io
> @@ -149,6 +155,9 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        friend class basic_ostream<char_type, traits_type>;
>        friend class istreambuf_iterator<char_type, traits_type>;
>        friend class ostreambuf_iterator<char_type, traits_type>;
> +#ifdef __glibcxx_format // C++ >= 20 && HOSTED
> +      friend class __format::_Streambuf_sink<char_type, traits_type>;
> +#endif
>
>        friend streamsize
>        __copy_streambufs_eof<>(basic_streambuf*, basic_streambuf*, bool&);
> --
> 2.55.0
>
>