[PATCH v2] libstdc++: Fix std::system_category().message(int) on mingw32 target

[Jonathan Wakely]

On Sat, 10 Jan 2026 at 10:45, Wang Jinghao <zheng.xianyuwang@gmail.com> wrote:
>
> On the mingw32 target, std::system_category().message(int) uses
> FormatMessage api to format error messages. When the error message contains
> insert sequences, it is unsafe not to use the FORMAT_MESSAGE_OGNORE_INSERTS
> flag, as seen at:
> https://devblogs.microsoft.com/oldnewthing/20071128-00/?p=24353
>
> The output of FormatMessage ends with "\r\n" and includes a Full stop character
> used by the current thread's UI language. Now, we will remove "\r\n" and
> any trailing '.' from the output in any language environment.
>
> In the testsuite for std::system_category().message(int), we first switch the
> thread UI language to en-US tomeet expectations in any language environment.

Thanks, the patch looks good now.

> libstdc++-v3/ChangeLog:
>
>         * src/c++11/system_error.cc (system_error_category) [_WIN32]:
>         Fix typo in __MINGW32__ macro name.
>         Adjust behavior on the mingw32 target.

The two lines above relate to the error_category/system_category.cc
test, not the src/c++11/system_category.cc source file, right?

>         Use FormatMessageA function instead of FormatMessage macro.
>         * testsuite/19_diagnostics/error_category/system_category.cc:
>         Adjust behavior on the mingw32 target.

I've just noticed that there's no Signed-off-by trailer in the patch.
Do you have a copyright assignment filed with the FSF?

As documented at https://gcc.gnu.org/contribute.html#legal we need
either a copyright assignment, or DCO sign-off.

If you don't have a copyrigh assignment, could you please add the
Signed-off-by trailer if you are willing and able to agree to the
terms of https://gcc.gnu.org/dco.html ?

Thanks




> ---
>  libstdc++-v3/src/c++11/system_error.cc        | 21 ++++++++++---------
>  .../error_category/system_category.cc         | 18 +++++++++++++---
>  2 files changed, 26 insertions(+), 13 deletions(-)
>
> diff --git a/libstdc++-v3/src/c++11/system_error.cc b/libstdc++-v3/src/c++11/system_error.cc
> index 548c88fcfde..7e3a3a0c904 100644
> --- a/libstdc++-v3/src/c++11/system_error.cc
> +++ b/libstdc++-v3/src/c++11/system_error.cc
> @@ -161,22 +161,23 @@ namespace
>  #if defined(_WIN32) && !defined(__CYGWIN__)
>        char* buf = nullptr;
>        auto len
> -       = FormatMessage(FORMAT_MESSAGE_FROM_SYSTEM
> -                       | FORMAT_MESSAGE_ALLOCATE_BUFFER,
> -                       nullptr,
> -                       i,
> -                       LANG_USER_DEFAULT,
> -                       reinterpret_cast<LPTSTR>(&buf),
> -                       0,
> -                       nullptr);
> +       = FormatMessageA (FORMAT_MESSAGE_FROM_SYSTEM
> +                         | FORMAT_MESSAGE_ALLOCATE_BUFFER
> +                         | FORMAT_MESSAGE_IGNORE_INSERTS,
> +                         nullptr,
> +                         i,
> +                         LANG_USER_DEFAULT,
> +                         reinterpret_cast<LPTSTR>(&buf),
> +                         0,
> +                         nullptr);
>        if (len > 0)
>        {
>         struct deleter {
>           void operator()(void* p) const { ::LocalFree(p); }
>         };
>         std::unique_ptr<char[], deleter> guard(buf);
> -       if (len > 3 && !__builtin_memcmp(buf + len - 3, ".\r\n", 3)) [[likely]]
> -         len -= 3;
> +       if (len > 2 && !__builtin_memcmp (buf + len - 2, "\r\n", 2)) [[likely]]
> +         len -= 2 + (buf[len - 3] == '.');
>         return string(buf, len);
>        }
>        return string("Unknown error code");
> diff --git a/libstdc++-v3/testsuite/19_diagnostics/error_category/system_category.cc b/libstdc++-v3/testsuite/19_diagnostics/error_category/system_category.cc
> index 250d34f9bb5..03577234c43 100644
> --- a/libstdc++-v3/testsuite/19_diagnostics/error_category/system_category.cc
> +++ b/libstdc++-v3/testsuite/19_diagnostics/error_category/system_category.cc
> @@ -21,6 +21,11 @@
>  #include <locale>
>  #include <testsuite_hooks.h>
>
> +#if defined __MINGW32__ || defined __MINGW64__
> +#define WIN32_LEAN_AND_MEAN
> +#include <windows.h>
> +#endif
> +
>  void
>  test01()
>  {
> @@ -34,7 +39,7 @@ test02()
>    const std::error_category& cat = std::system_category();
>    std::error_condition cond;
>
> -#if defined __MING32__ || defined __MINGW64__
> +#if defined __MINGW32__ || defined __MINGW64__
>    cond = cat.default_error_condition(8); // ERROR_NOT_ENOUGH_MEMORY
>    VERIFY( cond.value() == ENOMEM );
>    VERIFY( cond.category() == std::generic_category() );
> @@ -112,9 +117,16 @@ test03()
>    // set "C" locale to get expected message
>    auto loc = std::locale::global(std::locale::classic());
>
> -#if defined __MING32__ || defined __MINGW64__
> +#if defined __MINGW32__ || defined __MINGW64__
> +  // On Windows, set thread preferred UI languages to "en-US"
> +  // to get expected message
> +  ULONG num_langs = 1;
> +  SetThreadPreferredUILanguages(MUI_LANGUAGE_NAME, L"en-US\0", &num_langs);
> +
>    std::string msg = std::system_category().message(5); // ERROR_ACCESS_DENIED
> -  VERIFY(msg == "Access denied");
> +  VERIFY(msg == "Access is denied");
> +
> +  SetThreadPreferredUILanguages(MUI_RESET_FILTERS, nullptr, nullptr);
>  #else
>    std::string msg = std::system_category().message(EBADF);
>    VERIFY( msg.find("file") != std::string::npos );
> --
> 2.52.0
>