On Fri, Oct 2, 2026 at 12:05 AM Jonathan Wakely <[email protected]> wrote:
> Fix the __builtin_alloca argument to account for wide characters. Also
> consider them when deciding whether to use alloca or the heap.
>
> libstdc++-v3/ChangeLog:
>
> PR libstdc++/124370
> * include/std/bitset (operator>>): Account for sizeof(_CharT) in
> condition and buffer size.
> * testsuite/20_util/bitset/io/wchar_t.cc: New test.
> ---
>
> Tested x86_64-linux.
>
> We previously had no tests for wide character I/O with bitset, so the
> new test covers the basics as well as the bug.
>
> This is a regression so needs to be backported.
>
> libstdc++-v3/include/std/bitset | 8 +++--
> .../testsuite/20_util/bitset/io/wchar_t.cc | 36 +++++++++++++++++++
> 2 files changed, 42 insertions(+), 2 deletions(-)
> create mode 100644 libstdc++-v3/testsuite/20_util/bitset/io/wchar_t.cc
>
> diff --git a/libstdc++-v3/include/std/bitset
> b/libstdc++-v3/include/std/bitset
> index d581606e7dde..c244ee80bbb8 100644
> --- a/libstdc++-v3/include/std/bitset
> +++ b/libstdc++-v3/include/std/bitset
> @@ -1709,7 +1709,11 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
> #pragma GCC diagnostic ignored "-Wc++17-extensions" // if constexpr
> struct _Buffer
> {
> - static _GLIBCXX_CONSTEXPR bool _S_use_alloca() { return _Nb <=
> 256; }
> + static _GLIBCXX_CONSTEXPR size_t
> + _S_size_in_bytes() { return _Nb * sizeof(_CharT); }
> +
> + static _GLIBCXX_CONSTEXPR bool
> + _S_use_alloca() { return _S_size_in_bytes() <= 256; }
>
I would prefer something like:
private:
static const size_t _S_local = _Nb * sizeof(_CharT) <= 256 ?
_S_bytes : 1;
_CharT _M_buf[_S_local];
>
> explicit _Buffer(_CharT* __p) : _M_ptr(__p) { }
>
And then:
_Buffer() : _M_ptr(&__local)
{
if _GLIBCXX_CONSTEXPR(_S_local < _Nb)
_M_ptr = new _CharT[_Nb];
}
~_Buffer()
{
if _GLIBCXX_CONSTEXPR(_S_local < _Nb)
delete[] _M_ptr;
}
> @@ -1723,7 +1727,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
> };
> _CharT* __ptr;
> if _GLIBCXX_CONSTEXPR (_Buffer::_S_use_alloca())
> - __ptr = (_CharT*)__builtin_alloca(_Nb);
> + __ptr = (_CharT*)__builtin_alloca(_Buffer::_S_size_in_bytes());
>
Is this guaranteed to always give us proper aligment for wchar_t? Does not
matter with my proposal,
as we will create array of _CharT on stack.
> else
> __ptr = new _CharT[_Nb];
> const _Buffer __buf(__ptr);
> diff --git a/libstdc++-v3/testsuite/20_util/bitset/io/wchar_t.cc
> b/libstdc++-v3/testsuite/20_util/bitset/io/wchar_t.cc
> new file mode 100644
> index 000000000000..893ebd37352a
> --- /dev/null
> +++ b/libstdc++-v3/testsuite/20_util/bitset/io/wchar_t.cc
> @@ -0,0 +1,36 @@
> +// { dg-do run }
> +
> +// C++98 23.3.5.3 bitset operators
> +
> +#include <bitset>
> +#include <sstream>
> +#include <testsuite_hooks.h>
> +
> +// Bug 124370 - Out-of-bounds write for wistream >> bitset
> +void
> +test_pr124370()
> +{
> + std::wistringstream input(L"10011011001101");
> + std::bitset<10> b;
> + input >> b;
> + VERIFY( b == std::bitset<10>(0b1001101100) );
> +}
> +
> +// Round-trip through a wide stream.
> +template<int N>
> +void
> +test_round_trip()
> +{
> + std::bitset<N> a(0b100110110011010101101010), b;
> + std::wstringstream ss;
> + ss << a;
> + ss >> b;
> + VERIFY( b == a );
> +}
> +
> +int main()
> +{
> + test_pr124370();
> + test_round_trip<24>();
> + test_round_trip<1024>();
> +}
> --
> 2.55.0
>
>
