[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.

github-actions · 2025-07-16T07:22:54Z

⚠️ Python code formatter, darker found issues in your code. ⚠️

You can test this locally with the following command:

darker --check --diff -r HEAD~1...HEAD libcxx/utils/generate_libcxx_cppm_in.py

View the diff from darker here.

--- generate_libcxx_cppm_in.py	2025-07-16 02:27:04.000000 +0000
+++ generate_libcxx_cppm_in.py	2025-07-16 07:22:36.872831 +0000
@@ -11,11 +11,13 @@
 
 from libcxx.header_information import module_c_headers, module_headers, header_restrictions, headers_not_available, libcxx_root
 
 
 def write_file(module):
-    with open(libcxx_root / "modules" / f"{module}.cppm.in", "w", encoding='utf-8') as module_cpp_in:
+    with open(
+        libcxx_root / "modules" / f"{module}.cppm.in", "w", encoding="utf-8"
+    ) as module_cpp_in:
         module_cpp_in.write(
             """\
 // -*- C++ -*-
 //===----------------------------------------------------------------------===//
 //

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

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siradam7th added 2 commits July 16, 2025 03:24

fix python file encoding error on Windows

74d49fc

move define '_BUILD_STD_MODULE' to 'generate_libcxx_cppm_in.py'

bec4906

frederick-vs-ja reviewed Jul 16, 2025

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

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	// 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++

[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