[PATCH v2 2/4] libstdc++: Add _Streambuf_sink for direct streambuf formatting
Tomasz Kaminski
tkaminsk@redhat.com
Tue Aug 11 08:35:54 GMT 2026
On Mon, Aug 10, 2026 at 6:29 PM Tomasz Kaminski <tkaminsk@redhat.com> wrote:
>
>
> On Mon, Aug 10, 2026 at 5:40 PM Tomasz Kamiński <tkaminsk@redhat.com>
> wrote:
>
>> From: Anlai Lu <agicy@qq.com>
>>
>> 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 <tkaminsk@redhat.com>
>> Signed-off-by: Anlai Lu <agicy@qq.com>
>> Signed-off-by: Tomasz Kamiński <tkaminsk@redhat.com>
>> ---
>> 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.
>
Changed it locally to:
_M_overflow() override
{
_M_flush();
- if (!_M_reserve_put_area() || _M_write_failed)
- this->_M_reset(this->_M_buf);
+ if (!_M_write_failed) [[likely]]
+ if (_M_reserve_put_area())
+ return;
+ this->_M_reset(this->_M_buf);
}
> + 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
>>
>>
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