https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108331
--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> ---
That would be more flexible, but I'm not sure this is a problem that will
happen again. This is a particular case where we have a completely unused
member variable that just needs a specific layout.
It occurs to me libstdc++ could just check the GCC_GTHR_WIN32_H include guard,
although an explicitly-defined macro with a documented purpose (even if just
via a comment) would be less fragile.
How's this?
diff --git a/libgcc/config/i386/gthr-win32.h b/libgcc/config/i386/gthr-win32.h
index 146357fa436..050c7a21fcc 100644
--- a/libgcc/config/i386/gthr-win32.h
+++ b/libgcc/config/i386/gthr-win32.h
@@ -381,6 +381,14 @@ typedef struct timespec __gthread_time_t;
#define __GTHREAD_COND_INIT_FUNCTION __gthread_cond_init_function
#define __GTHREAD_TIME_INIT {0, 0}
+// Libstdc++ std::basic_filebuf needs the old definition of __gthread_mutex_t
+// for layout purposes, but doesn't actually use it.
+typedef struct {
+ long __unused1;
+ void *__unused2;
+} __gthr_win32_legacy_mutex_t;
+#define __GTHREAD_LEGACY_MUTEX_T __gthr_win32_legacy_mutex_t
+
#if defined (_WIN32) && !defined(__CYGWIN__)
#define MINGW32_SUPPORTS_MT_EH 1
/* Mingw runtime >= v0.3 provides a magic variable that is set to nonzero
diff --git a/libstdc++-v3/config/io/c_io_stdio.h
b/libstdc++-v3/config/io/c_io_stdio.h
index 1a5e05a844a..e9e6e3ef4d8 100644
--- a/libstdc++-v3/config/io/c_io_stdio.h
+++ b/libstdc++-v3/config/io/c_io_stdio.h
@@ -39,7 +39,14 @@ namespace std _GLIBCXX_VISIBILITY(default)
{
_GLIBCXX_BEGIN_NAMESPACE_VERSION
+#ifdef __GTHREAD_LEGACY_MUTEX_T
+ // The layout of __gthread_mutex_t changed in GCC 13, but libstdc++ doesn't
+ // actually use the basic_filebuf::_M_lock member, so define it consistently
+ // with the old __gthread_mutex_t to avoid an unnecessary layout change:
+ typedef __GTHREAD_LEGACY_MUTEX_T __c_lock;
+#else
typedef __gthread_mutex_t __c_lock;
+#endif
// for basic_file.h
typedef FILE __c_file;
