[libc++] Fix C++23 standard modules when using with `clang-cl` on Windows #148992

siradam7th · 2025-07-15T23:49:17Z

The following PR fixes a couple of issues and compilation errors when using libcxx with clang-cl on Windows.

Issues:

compilation error because of missing definition of get_new_handler() in the ucrt/new.h that is included by libcxx/include/__new
compilation errors due to static inline in functions definition for ctime inside std.cppm.in and std.compat.cppm.in (std.cppm, std.compat.cppm).

Fixes:

defining get_new_handler() function under the std namespace when using _LIBCPP_ABI_VCRUNTIME.
defining _BUILD_STD_MODULE before including <ctime> inside std.cppm.in because MSVC's headers already have a fix in place.
both std.cppm and std.compat.cppm are fixed.

This pull request fixes the following issues:

[libc++] <ctime> std module on windows with ucrt #64812

github-actions · 2025-07-15T23:49:36Z

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llvmbot · 2025-07-15T23:50:06Z

@llvm/pr-subscribers-libcxx

Author: Adam (siradam7th)

Changes

The following PR fixes a couple of issues and compilation errors when using libcxx with clang-cl on Windows.

Issues:

compilation error because of missing definition of get_new_handler() in the ucrt/new.h that is included by libcxx/include/__new
compilation errors due to static inline in functions definition for ctime inside std.cppm.in and std.compat.cppm.in (std.cppm, std.compat.cppm).

Fixes:

defining get_new_handler() function under the std namespace when using _LIBCPP_ABI_VCRUNTIME.
defining _BUILD_STD_MODULE before including <ctime> inside std.cppm.in because MSVC's headers already have a fix in place.
both std.cppm and std.compat.cppm are fixed.

This pull request fixes the following issues:

[libc++] <ctime> std module on windows with ucrt #64812

Full diff: https://github.com/llvm/llvm-project/pull/148992.diff

4 Files Affected:

(modified) libcxx/include/__new/new_handler.h (+5)
(modified) libcxx/modules/std.compat/ctime.inc (+4-2)
(modified) libcxx/modules/std.cppm.in (+3)
(modified) libcxx/modules/std/ctime.inc (+4-2)

diff --git a/libcxx/include/__new/new_handler.h b/libcxx/include/__new/new_handler.h
index 05f4e846c3ef9..2350f9ae9bc19 100644
--- a/libcxx/include/__new/new_handler.h
+++ b/libcxx/include/__new/new_handler.h
@@ -17,6 +17,11 @@
 
 #if defined(_LIBCPP_ABI_VCRUNTIME)
 #  include <new.h>
+// <new.h> does not define 'get_new_handler'
+// which makes the 'std' module build fail, this fixes it
+namespace std {
+_LIBCPP_EXPORTED_FROM_ABI new_handler get_new_handler() _NOEXCEPT;
+}
 #else
 _LIBCPP_BEGIN_UNVERSIONED_NAMESPACE_STD
 typedef void (*new_handler)();
diff --git a/libcxx/modules/std.compat/ctime.inc b/libcxx/modules/std.compat/ctime.inc
index eba8234a08969..7547b055b6807 100644
--- a/libcxx/modules/std.compat/ctime.inc
+++ b/libcxx/modules/std.compat/ctime.inc
@@ -14,15 +14,17 @@ export {
 
   using ::timespec _LIBCPP_USING_IF_EXISTS;
   using ::tm _LIBCPP_USING_IF_EXISTS;
-
   using ::asctime _LIBCPP_USING_IF_EXISTS;
   using ::clock _LIBCPP_USING_IF_EXISTS;
+  using ::strftime _LIBCPP_USING_IF_EXISTS;
+
+#ifndef _LIBCPP_ABI_VCRUNTIME
   using ::ctime _LIBCPP_USING_IF_EXISTS;
   using ::difftime _LIBCPP_USING_IF_EXISTS;
   using ::gmtime _LIBCPP_USING_IF_EXISTS;
   using ::localtime _LIBCPP_USING_IF_EXISTS;
   using ::mktime _LIBCPP_USING_IF_EXISTS;
-  using ::strftime _LIBCPP_USING_IF_EXISTS;
   using ::time _LIBCPP_USING_IF_EXISTS;
   using ::timespec_get _LIBCPP_USING_IF_EXISTS;
+#endif
 } // export
diff --git a/libcxx/modules/std.cppm.in b/libcxx/modules/std.cppm.in
index 984b18321923c..f19dc07169fda 100644
--- a/libcxx/modules/std.cppm.in
+++ b/libcxx/modules/std.cppm.in
@@ -51,6 +51,9 @@ module;
 #include <cstdio>
 #include <cstdlib>
 #include <cstring>
+#ifdef _LIBCPP_ABI_VCRUNTIME
+#define _BUILD_STD_MODULE
+#endif
 #include <ctime>
 #include <cuchar>
 #include <cwchar>
diff --git a/libcxx/modules/std/ctime.inc b/libcxx/modules/std/ctime.inc
index 5bfa61917e5f2..e9f8ecb6ec63e 100644
--- a/libcxx/modules/std/ctime.inc
+++ b/libcxx/modules/std/ctime.inc
@@ -14,15 +14,17 @@ export namespace std {
 
   using std::timespec _LIBCPP_USING_IF_EXISTS;
   using std::tm _LIBCPP_USING_IF_EXISTS;
-
   using std::asctime _LIBCPP_USING_IF_EXISTS;
   using std::clock _LIBCPP_USING_IF_EXISTS;
+  using std::strftime _LIBCPP_USING_IF_EXISTS;
+
+#ifndef _LIBCPP_ABI_VCRUNTIME
   using std::ctime _LIBCPP_USING_IF_EXISTS;
   using std::difftime _LIBCPP_USING_IF_EXISTS;
   using std::gmtime _LIBCPP_USING_IF_EXISTS;
   using std::localtime _LIBCPP_USING_IF_EXISTS;
   using std::mktime _LIBCPP_USING_IF_EXISTS;
-  using std::strftime _LIBCPP_USING_IF_EXISTS;
   using std::time _LIBCPP_USING_IF_EXISTS;
   using std::timespec_get _LIBCPP_USING_IF_EXISTS;
+#endif
 } // namespace std

libcxx/modules/std.cppm.in

github-actions · 2025-07-16T00:51:22Z

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siradam7th · 2025-07-16T02:36:27Z

I have moved the macro definition to the file generate_libcxx_cppm_in.py.
I also fixed an encoding problem with that same file that stops it from running on Windows.

frederick-vs-ja · 2025-07-16T03:03:54Z

libcxx/modules/std.compat/ctime.inc

  using ::asctime _LIBCPP_USING_IF_EXISTS;
  using ::clock _LIBCPP_USING_IF_EXISTS;
+  using ::strftime _LIBCPP_USING_IF_EXISTS;
+
+#ifndef _LIBCPP_ABI_VCRUNTIME


Are you simply leaving these functions missing when using UCRT?

Also, it seem that we should modify this line libcxx/utils/libcxx/test/features.py to enable the tests:

llvm-project/libcxx/utils/libcxx/test/features.py

Line 330 in 968d38d

or ("_WIN32" in compilerMacros(cfg) and not _mingwSupportsModules(cfg))

I'm looking into it, however it seems that moving the definition '_BUILD_STD_MODULE' triggered some other weird errors, I'll do more testing tomorrow and see...

Aha, these lines should be removed. Now get_new_handler.pass.cpp should always pass.

llvm-project/libcxx/test/std/language.support/support.dynamic/alloc.errors/set.new.handler/get_new_handler.pass.cpp

Lines 11 to 17 in 653872f

// FIXME: When libc++ is linked against vcruntime (i.e. the default config in

// MSVC mode), the declarations of std::set_new_handler and std::get_new_handler

// are provided by vcruntime/UCRT's new.h. However, that header actually only

// declares set_new_handler - it's missing a declaration of get_new_handler.

// XFAIL: msvc && stdlib=libc++

It looks like just defining the macro is not enough, I have resorted to defining the actual functions as inline, I'll push the changes once I made sure that everything works with both std and std.compact.

The _LIBCPP_USING_IF_EXISTS attribute is intended to make things work when the declarations are not available, and avoid precisely these #ifdef blocks. What's the issue here?

The issue on these <ctime> functions is that UCRT incorrectly adds static to them, so we can't directly export them. At this time we might simply skip export for them to make the standard modules buildable. MSVC STL chooses to reimplement them in the std namespace as a workaround.

Also, it seem that we should modify this line libcxx/utils/libcxx/test/features.py to enable the tests:

llvm-project/libcxx/utils/libcxx/test/features.py

Line 330 in 968d38d

or ("_WIN32" in compilerMacros(cfg) and not _mingwSupportsModules(cfg))

Is just removing that line enough?

I guess we can try this:

or ("_WIN32" in compilerMacros(cfg) and "__MINGW32__" in compilerMacros(cfg) and not _mingwSupportsModules(cfg))

I'm not familiar with how the tests are run/generated yet, I'll see...

github-actions · 2025-07-16T07:22:54Z

✅ With the latest revision this PR passed the Python code formatter.

ldionne · 2025-07-18T15:28:48Z

libcxx/include/__new/new_handler.h

@@ -17,6 +17,11 @@

 #if defined(_LIBCPP_ABI_VCRUNTIME)
 #  include <new.h>


How does the MSVC STL get their own definition for get_new_handler()? It seems really brittle to include <new.h> but have our definition for get_new_handler(). We should either declare all of them (set_new_handler(), the new_handler alias, etc) or rely on <new.h> to provide them, but not a mixture of both.

MSVC STL just (additionally) declares it in <new>. The declaration is not available from <new.h> or <vcruntime_new.h>.

@siradam7th I think it would be better to follow what MSVC STL does. We can just avoid special-casing _LIBCPP_ABI_VCRUNTIME, and consistently declare these 3 things ourselves.

@siradam7th I think it would be better to follow what MSVC STL does. We can just avoid special-casing _LIBCPP_ABI_VCRUNTIME, and consistently declare these 3 things ourselves.

I'll give it a try.

I have removed the _LIBCPP_ABI_VCRUNTIME macro conditional from libcxx/include/__new/new_handler.h.
and fixed the missing symbols which are now defined by libcxx/src/new_handler.cpp, and everything is working correctly.
apart from a couple of warnings of redeclaration since some of the cpp files include <new>:

llvm-project\build\include\c++\v1\__new/new_handler.h(20,39): warning: redeclaration of 'std::set_new_handler' should not add 'dllexport' attribute [-Wdll-attribute-on-redeclaration] 20 | _LIBCPP_EXPORTED_FROM_ABI new_handler set_new_handler(new_handler) _NOEXCEPT; C:\Program Files (x86)\Windows Kits\10\Include\10.0.26100.0\ucrt\new.h(32,42): note: previous declaration is here 32 | _CRTIMP2 new_handler __cdecl set_new_handler(_In_opt_ new_handler _NewHandler) throw(); | ^ 1 warning generated.

ldionne · 2025-07-18T15:31:49Z

libcxx/modules/std.compat/ctime.inc

  using ::asctime _LIBCPP_USING_IF_EXISTS;
  using ::clock _LIBCPP_USING_IF_EXISTS;
+  using ::strftime _LIBCPP_USING_IF_EXISTS;
+
+#ifndef _LIBCPP_ABI_VCRUNTIME


The _LIBCPP_USING_IF_EXISTS attribute is intended to make things work when the declarations are not available, and avoid precisely these #ifdef blocks. What's the issue here?

ldionne · 2025-07-18T15:32:01Z

libcxx/modules/std.cppm.in

@@ -7,6 +7,10 @@
 //
 //===----------------------------------------------------------------------===//

+#ifdef _LIBCPP_ABI_VCRUNTIME


I assume this change is temporary?

github-actions · 2025-07-21T01:47:40Z

⚠️ C/C++ code formatter, clang-format found issues in your code. ⚠️

You can test this locally with the following command:

git-clang-format --diff HEAD~1 HEAD --extensions cpp,inc,h -- libcxx/include/__new/new_handler.h libcxx/modules/std.compat/ctime.inc libcxx/modules/std/ctime.inc libcxx/test/std/language.support/support.dynamic/alloc.errors/set.new.handler/get_new_handler.pass.cpp

View the diff from clang-format here.

diff --git a/libcxx/test/std/language.support/support.dynamic/alloc.errors/set.new.handler/get_new_handler.pass.cpp b/libcxx/test/std/language.support/support.dynamic/alloc.errors/set.new.handler/get_new_handler.pass.cpp
index 2c0081703..d20a65d8b 100644
--- a/libcxx/test/std/language.support/support.dynamic/alloc.errors/set.new.handler/get_new_handler.pass.cpp
+++ b/libcxx/test/std/language.support/support.dynamic/alloc.errors/set.new.handler/get_new_handler.pass.cpp
@@ -8,7 +8,6 @@
 
 // test get_new_handler
 
-
 #include <new>
 #include <cassert>

philnik777 · 2025-07-22T05:01:34Z

libcxx/modules/std.compat/ctime.inc

+// A workaround for UCRT because it defines these functions
+// as static and that causes the error "internal linkage cannot be exported"
+#ifdef _LIBCPP_ABI_VCRUNTIME
+
+  template <int = 0>
+  inline char* __CRTDECL ctime(_In_ time_t const* const _Time) noexcept {
+    return _ctime64(_Time);
+  }
+
+  template <int = 0>
+  inline double __CRTDECL difftime(_In_ time_t const _Time1, _In_ time_t const _Time2) noexcept {
+    return _difftime64(_Time1, _Time2);
+  }
+
+  template <int = 0>
+  inline tm* __CRTDECL gmtime(_In_ time_t const* const _Time) noexcept {
+    return _gmtime64(_Time);
+  }
+
+  template <int = 0>
+  inline tm* __CRTDECL localtime(_In_ time_t const* const _Time) noexcept {
+    return _localtime64(_Time);
+  }
+
+  template <int = 0>
+  inline time_t __CRTDECL mktime(_Inout_ tm* const _Tm) noexcept {
+    return _mktime64(_Tm);
+  }
+
+  template <int = 0>
+  inline time_t __CRTDECL time(_Out_opt_ time_t* const _Time) noexcept {
+    return _time64(_Time);
+  }
+
+  template <int = 0>
+  inline int __CRTDECL timespec_get(_Out_ timespec* const _Ts, _In_ int const _Base) noexcept {
+    return _timespec64_get(reinterpret_cast<_timespec64*>(_Ts), _Base);
+  }
+#endif


I really don't like this. It makes us clearly non-conforming and working around an issue that is trivial to fix.

I don't see how it is trivial? even MSVC's STL has the same workaround because there has been no changes in UCRT in the past years, because of those few functions modules cannot be compiled.

I don't see how it is trivial?

The fix in UCRT would be very trivial. UCRT team can just remove static on these functions in less than one minute, as UCRT expects the __inline extension to behave same as inline in C++.

The crux is that nobody in the UCRT team seems to be able to work on this, and contribution from open source community is still impossible.

The crux is that nobody in the UCRT team seems to be able to work on this, and contribution from open source community is still impossible.

This is one of the main reasons for the existance of this PR, if they would've done it, they at least would have done it for MSVC's sake, but here we are...

That is not a good reason to add a comparatively complex and fragile workaround, but a reason to complain to them to get their asses moving. I don't know what Microsoft's reasons are, but so far this is a very clear "we don't care" and not "we can't" to me. I have no interest in supporting anybodys "we don't care".

Hmm, but IMO we should at least to skip export of these functions to build standard modules.

That is definitely more palatable. I'm not sure I'd approve it; I'll have to think about that a bit more I think.

philnik777 · 2025-07-22T05:02:20Z

libcxx/include/__new/new_handler.h

-#if defined(_LIBCPP_ABI_VCRUNTIME)
-#  include <new.h>
-#else


Can we split this into a separate patch? This doesn't seem to be directly related to modules.

it is, because <new> does not define get_new_handler() and that stops standard modules from building, the change was requested by @frederick-vs-ja to simplify the previous solution.

This also fixes other tests IIUC, so this isn't specific to modules. It just happens to surface with modules as well.

fix libc++ std modules compilation on Windows using clang-cl

ed2c4e4

siradam7th requested a review from a team as a code owner July 15, 2025 23:49

llvmbot added the libc++ libc++ C++ Standard Library. Not GNU libstdc++. Not libc++abi. label Jul 15, 2025

frederick-vs-ja reviewed Jul 16, 2025

View reviewed changes

libcxx/modules/std.cppm.in Outdated Show resolved Hide resolved

frederick-vs-ja reviewed Jul 16, 2025

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ldionne reviewed Jul 18, 2025

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siradam7th added 2 commits July 20, 2025 02:49

fix python file encoding error on Windows

b6bf097

add missing functions defined as inline, remove '_BUILD_STD_MODULE'

4e7a4c9

siradam7th force-pushed the libcxx-std-module-windows-fix branch from bec4906 to 4e7a4c9 Compare July 20, 2025 02:01

remove 'FIXME' for 'get_new_handler()' test

bf7ca14

fix missing symbols for 'get_new_handler()' and 'set_new_handler()'

b64ac1e

philnik777 requested changes Jul 22, 2025

View reviewed changes

	// FIXME: When libc++ is linked against vcruntime (i.e. the default config in
	// MSVC mode), the declarations of std::set_new_handler and std::get_new_handler
	// are provided by vcruntime/UCRT's new.h. However, that header actually only
	// declares set_new_handler - it's missing a declaration of get_new_handler.

	// XFAIL: msvc && stdlib=libc++

		@@ -17,6 +17,11 @@

		#if defined(_LIBCPP_ABI_VCRUNTIME)
		# include <new.h>

[libc++] Fix C++23 standard modules when using with clang-cl on Windows #148992

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[libc++] Fix C++23 standard modules when using with clang-cl on Windows #148992

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[libc++] Fix C++23 standard modules when using with `clang-cl` on Windows #148992

[libc++] Fix C++23 standard modules when using with `clang-cl` on Windows #148992

frederick-vs-ja Jul 19, 2025 •

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