https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100164
--- Comment #9 from David Edelsohn <dje at gcc dot gnu.org> --- The previous semaphore_base.h implementation had a fallback that hid a bug in the macros: #if defined _GLIBCXX_HAVE_LINUX_FUTEX && !_GLIBCXX_REQUIRE_POSIX_SEMAPHORE // Use futex if available and didn't force use of POSIX using __fast_semaphore = __atomic_semaphore<__detail::__platform_wait_t>; #elif _GLIBCXX_HAVE_POSIX_SEMAPHORE using __fast_semaphore = __platform_semaphore; #else using __fast_semaphore = __atomic_semaphore<ptrdiff_t>; #endif The problem is that libstdc++ configure defines _GLIBCXX_HAVE_POSIX_SEMAPHORE in config.h. libstdc++ uses sed to rewrite config.h to c++config.h and prepends _GLIBCXX_, so c++config.h contains #define _GLIBCXX__GLIBCXX_HAVE_POSIX_SEMAPHORE 1 And bits/semaphore_base.h is not testing that corrupted macro. Either semaphore_base.h needs to test for the corrupted macro, or libtsdc++ configure needs to define HAVE_POSIX_SEMAPHORE without itself prepending _GLIBCXX_ so that the c++config.h rewriting works correctly and defines the correct macro for semaphore_base.h.
