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Fix system_clock overflowing in 2038

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libstdc++ implemented system_clock::now() with gettimeofday, whose
timeval has a long tv_sec, and long is 32 bits on Windows. Both
system_clock and steady_clock (derived from it on this target) would
overflow in January 2038 on x64 as well as x86, no matter how time_t is
configured. A new patch reads the system time directly, preferring
GetSystemTimePreciseAsFileTime as mingw-w64's gettimeofday does.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
This commit is contained in:
Christopher Wellons
2026-10-18 10:08:23 -04:00
parent 58dca7f70d
commit cfd8597704
2 changed files with 55 additions and 0 deletions
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1
src/gcc/series
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@@ -2,3 +2,4 @@ avx-misaligned.patch
dynamic-load-tzdb.patch
libgfortran-mapview.patch
trap-terminate.patch
system-clock-y2038.patch

54
src/gcc/system-clock-y2038.patch Normal file
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@@ -0,0 +1,54 @@
libstdc++: read system_clock directly from Windows
On mingw-w64 libstdc++ implements system_clock::now() with gettimeofday,
whose timeval has a long tv_sec. long is 32 bits on Windows, including
x86_64, so system_clock overflows in January 2038 no matter how time_t is
configured. steady_clock is derived from system_clock on this target and
fails along with it.
Read the time with GetSystemTimePreciseAsFileTime when available,
falling back to GetSystemTimeAsFileTime on older systems, the same
choice mingw-w64 makes for gettimeofday.
--- a/libstdc++-v3/src/c++11/chrono.cc
+++ b/libstdc++-v3/src/c++11/chrono.cc
@@ -37,6 +37,11 @@
#include <sys/syscall.h>
#endif
+#if defined(_WIN32) && !defined(__CYGWIN__)
+#define WIN32_LEAN_AND_MEAN
+#include <windows.h>
+#endif
+
namespace std _GLIBCXX_VISIBILITY(default)
{
_GLIBCXX_BEGIN_NAMESPACE_VERSION
@@ -50,7 +55,26 @@
system_clock::time_point
system_clock::now() noexcept
{
-#ifdef _GLIBCXX_USE_CLOCK_REALTIME
+#if defined(_WIN32) && !defined(__CYGWIN__)
+ // gettimeofday's timeval has a 32-bit tv_sec (long) on Windows,
+ // which overflows in 2038, so read the system time directly.
+ // GetSystemTimePreciseAsFileTime requires Windows 8.
+ using get_time_t = void (WINAPI *)(LPFILETIME);
+ static const get_time_t get_time = [] {
+ FARPROC proc = GetProcAddress(GetModuleHandleW(L"kernel32.dll"),
+ "GetSystemTimePreciseAsFileTime");
+ return proc
+ ? reinterpret_cast<get_time_t>(reinterpret_cast<void (*)()>(proc))
+ : GetSystemTimeAsFileTime;
+ }();
+ FILETIME ft;
+ get_time(&ft);
+ // 100ns intervals since 1601-01-01, the Unix epoch being
+ // 11644473600 seconds later.
+ long long ticks = (long long)ft.dwHighDateTime << 32 | ft.dwLowDateTime;
+ ticks -= 116444736000000000LL;
+ return time_point(duration(chrono::nanoseconds(ticks * 100)));
+#elif defined(_GLIBCXX_USE_CLOCK_REALTIME)
timespec tp;
// -EINVAL, -EFAULT
#ifdef _GLIBCXX_USE_CLOCK_GETTIME_SYSCALL