[PATCH 2/4] libstdc++: Add _Streambuf_sink for direct streambuf formatting
Tomasz Kaminski
tkaminsk@redhat.com
Tue Aug 11 10:08:17 GMT 2026
On Thu, Jul 16, 2026 at 2:04 PM Anlai Lu <agicy@qq.com> wrote:
> 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).
>
> The _Sink_state enum is omitted in favor of _M_on_stack() which
> tests whether the current span points at the stack buffer.
>
> libstdc++-v3/ChangeLog:
>
> * include/bits/streambuf_iterator.h (ostreambuf_iterator):
> Add internal _M_get_sbuf() and _M_set_failed() members.
> * include/std/format: Include <bits/streambuf_iterator.h>.
> (__format::_Streambuf_sink): New class template.
> * include/std/streambuf: Forward-declare __format::_Streambuf_sink,
> add friend declaration to basic_streambuf.
>
> Signed-off-by: Anlai Lu <agicy@qq.com>
> ---
> .../include/bits/streambuf_iterator.h | 11 ++
> libstdc++-v3/include/std/format | 132 ++++++++++++++++++
> libstdc++-v3/include/std/streambuf | 4 +
> 3 files changed, 147 insertions(+)
>
> diff --git a/libstdc++-v3/include/bits/streambuf_iterator.h
> b/libstdc++-v3/include/bits/streambuf_iterator.h
> index 095928ca4..d919fdbb8 100644
> --- a/libstdc++-v3/include/bits/streambuf_iterator.h
> +++ b/libstdc++-v3/include/bits/streambuf_iterator.h
> @@ -318,6 +318,17 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> failed() const _GLIBCXX_USE_NOEXCEPT
> { return _M_failed; }
>
> + /// @cond internal
> + _GLIBCXX_NODISCARD
> + streambuf_type*
> + _M_get_sbuf() const _GLIBCXX_USE_NOEXCEPT
> + { return _M_sbuf; }
> +
> + void
> + _M_set_failed() _GLIBCXX_USE_NOEXCEPT
> + { _M_failed = true; }
> + /// @endcond
> +
> ostreambuf_iterator&
> _M_put(const _CharT* __ws, streamsize __len)
> {
> diff --git a/libstdc++-v3/include/std/format
> b/libstdc++-v3/include/std/format
> index 9bf5e7857..b676629b1 100644
> --- a/libstdc++-v3/include/std/format
> +++ b/libstdc++-v3/include/std/format
> @@ -59,6 +59,7 @@
> #include <bits/ranges_util.h> // subrange
> #include <bits/ranges_algobase.h> // ranges::copy
> #include <bits/stl_iterator.h> // counted_iterator
> +#include <bits/streambuf_iterator.h> // ostreambuf_iterator,
> basic_streambuf
> #include <bits/stl_pair.h> // __is_pair
> #include <bits/unicode.h> // __is_scalar_value, _Utf_view, etc.
> #include <bits/utility.h> // tuple_size_v
> @@ -3530,6 +3531,137 @@ 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>
> + {
> + using _Buf_sink<_CharT>::_M_buf;
> +
> + protected:
> + basic_streambuf<_CharT, _Traits>* _M_sbuf;
> + bool _M_write_failed = false;
> +
> + _GLIBCXX_CONSTEXPR_FORMAT bool
> + _M_on_stack() const noexcept
> + { return this->_M_used().data() == this->_M_buf; }
> +
> + // Switch to _M_buf when the streambuf has no put area
> + // or we need to stop using it.
> + _GLIBCXX_CONSTEXPR_FORMAT void
> + _M_use_stackbuf()
> + { this->_M_reset(_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.
> + _GLIBCXX_CONSTEXPR_FORMAT bool
> + _M_use_put_area(size_t __n = 0)
> + {
> + if (auto __p = _M_sbuf->pptr())
> + {
> + auto __e = _M_sbuf->epptr();
> + if (__e && __e > __p
> + && static_cast<size_t>(__e - __p) >= __n)
> + {
> + this->_M_reset(
> + span<_CharT>{__p, static_cast<size_t>(__e - __p)});
> + return true;
> + }
> + }
> + return false;
> + }
> +
> + // Allow derived classes to commit to the put area
> + // without needing friendship to basic_streambuf.
> + _GLIBCXX_CONSTEXPR_FORMAT void
> + _M_pbump(streamsize __n)
> + { _M_sbuf->__safe_pbump(__n); }
>
Is tehre any reason to use internal __safe_pbump instead of simply pbump
here?
> +
> + // 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). Does not refresh the
> + // span; callers use _M_use_put_area/stackbuf.
> + _GLIBCXX_CONSTEXPR_FORMAT void
> + _M_flush(span<_CharT> __s)
> + {
> + if (__s.empty() || _M_write_failed) [[unlikely]]
> + return;
> + __try {
> + if (_M_on_stack())
> + {
> + if (_M_sbuf->sputn(__s.data(), __s.size())
> + != static_cast<streamsize>(__s.size()))
> + _M_write_failed = true;
> + }
> + else
> + _M_pbump(__s.size());
> + }
> + __catch(const __cxxabiv1::__forced_unwind&)
> + { throw; }
> + __catch(...)
> + { _M_write_failed = true; }
> + }
> +
> + _GLIBCXX_CONSTEXPR_FORMAT void
> + _M_overflow() override
> + {
> + _M_flush(this->_M_used());
> + if (!_M_use_put_area())
> + _M_use_stackbuf();
> + }
> +
> + public:
> + [[__gnu__::__always_inline__]]
> + _GLIBCXX_CONSTEXPR_FORMAT explicit
> + _Streambuf_sink(basic_streambuf<_CharT, _Traits>* __sbuf) noexcept
> + : _M_sbuf(__sbuf)
> + { }
> +
> + using _Sink<_CharT>::out;
> +
> + // _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).
> +
> + _GLIBCXX_CONSTEXPR_FORMAT typename _Sink<_CharT>::_Reservation
> + _M_reserve(size_t __n) override
> + {
> + if (__n <= this->_M_unused().size())
> + return { this };
> +
> + if (!this->_M_used().empty())
> + _M_overflow();
> +
> + // Try to write directly into the streambuf's put area.
> + if (_M_use_put_area(__n))
> + return { this };
> +
> + // Otherwise reset to the stack buffer.
> + _M_use_stackbuf();
> + if (__n <= this->_M_unused().size())
> + return { this };
> +
> + return { nullptr };
> + }
> +
> + _GLIBCXX_CONSTEXPR_FORMAT bool
> + _M_discarding() const override
> + { return _M_write_failed; }
> +
> + _GLIBCXX_CONSTEXPR_FORMAT void
> + _M_finish() &&
> + { _M_flush(this->_M_used()); }
> +
> + _GLIBCXX_CONSTEXPR_FORMAT 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 616e44f74..22b505228 100644
> --- a/libstdc++-v3/include/std/streambuf
> +++ b/libstdc++-v3/include/std/streambuf
> @@ -57,6 +57,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> __copy_streambufs_eof(basic_streambuf<_CharT, _Traits>*,
> basic_streambuf<_CharT, _Traits>*, bool&);
>
> + namespace __format { template<typename, typename> class
> _Streambuf_sink; }
> +
> /**
> * @brief The actual work of input and output (interface).
> * @ingroup io
> @@ -149,6 +151,8 @@ _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>;
> + template<typename, typename>
> + friend class __format::_Streambuf_sink;
>
> friend streamsize
> __copy_streambufs_eof<>(basic_streambuf*, basic_streambuf*, bool&);
> --
> 2.34.1
>
>
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