https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77704
--- Comment #11 from GCC Commits <cvs-commit at gcc dot gnu.org> --- The master branch has been updated by Jonathan Wakely <r...@gcc.gnu.org>: https://gcc.gnu.org/g:23ef0f68ad5fca1fd7027caaaa5f6cb9f6d27b28 commit r15-510-g23ef0f68ad5fca1fd7027caaaa5f6cb9f6d27b28 Author: Jonathan Wakely <jwak...@redhat.com> Date: Fri May 3 20:00:08 2024 +0100 libstdc++: Fix data race in std::basic_ios::fill() [PR77704] The lazy caching in std::basic_ios::fill() updates a mutable member without synchronization, which can cause a data race if two threads both call fill() on the same stream object when _M_fill_init is false. To avoid this we can just cache the _M_fill member and set _M_fill_init early in std::basic_ios::init, instead of doing it lazily. As explained by the comment in init, there's a good reason for doing it lazily. When char_type is neither char nor wchar_t, the locale might not have a std::ctype<char_type> facet, so getting the fill character would throw an exception. The current lazy init allows using unformatted I/O with such a stream, because the fill character is never needed and so it doesn't matter if the locale doesn't have a ctype<char_type> facet. We can maintain this property by only setting the fill character in std::basic_ios::init if the ctype facet is present at that time. If fill() is called later and the fill character wasn't set by init, we can get it from the stream's current locale at the point when fill() is called (and not try to cache it without synchronization). If the stream hasn't been imbued with a locale that includes the facet when we need the fill() character, then throw bad_cast at that point. This causes a change in behaviour for the following program: std::ostringstream out; out.imbue(loc); auto fill = out.fill(); Previously the fill character would have been set when fill() is called, and so would have used the new locale. This commit changes it so that the fill character is set on construction and isn't affected by the new locale being imbued later. This new behaviour seems to be what the standard requires, and matches MSVC. The new 27_io/basic_ios/fill/char/fill.cc test verifies that it's still possible to use a std::basic_ios without the ctype<char_type> facet being present at construction. libstdc++-v3/ChangeLog: PR libstdc++/77704 * include/bits/basic_ios.h (basic_ios::fill()): Do not modify _M_fill and _M_fill_init in a const member function. (basic_ios::fill(char_type)): Use _M_fill directly instead of calling fill(). Set _M_fill_init to true. * include/bits/basic_ios.tcc (basic_ios::init): Set _M_fill and _M_fill_init here instead. * testsuite/27_io/basic_ios/fill/char/1.cc: New test. * testsuite/27_io/basic_ios/fill/wchar_t/1.cc: New test.
