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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

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ed2c4e4
fix libc++ std modules compilation on Windows using clang-cl
siradam7th Jul 15, 2025
b6bf097
fix python file encoding error on Windows
siradam7th Jul 20, 2025
4e7a4c9
add missing functions defined as inline, remove '_BUILD_STD_MODULE'
siradam7th Jul 20, 2025
bf7ca14
remove 'FIXME' for 'get_new_handler()' test
siradam7th Jul 20, 2025
b64ac1e
fix missing symbols for 'get_new_handler()' and 'set_new_handler()'
siradam7th Jul 22, 2025
78a0e71
Revert "fix python file encoding error on Windows"
siradam7th Sep 8, 2025
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5 changes: 5 additions & 0 deletions libcxx/include/__new/new_handler.h
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Original file line number Diff line number Diff line change
Expand Up @@ -17,6 +17,11 @@

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

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

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@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.

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@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.

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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.

// <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)();
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6 changes: 4 additions & 2 deletions libcxx/modules/std.compat/ctime.inc
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Expand Up @@ -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
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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))

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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...

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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++

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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.

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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?

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@frederick-vs-ja frederick-vs-ja Jul 19, 2025 •
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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.

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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?

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I guess we can try this:

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

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

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
3 changes: 3 additions & 0 deletions libcxx/modules/std.cppm.in
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Expand Up @@ -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>
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6 changes: 4 additions & 2 deletions libcxx/modules/std/ctime.inc
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Expand Up @@ -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
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