`libc++.modules.json` resource does not exist

[h-2]

I am trying this C++ Modules PoC:

CMakeLists.txt:

cmake_minimum_required(VERSION 3.30)

set(CMAKE_CXX_STANDARD 23)
set(CMAKE_CXX_STANDARD_REQUIRED YES)

set(CMAKE_EXPERIMENTAL_CXX_IMPORT_STD "0e5b6991-d74f-4b3d-a41c-cf096e0b2508")
set(CMAKE_CXX_MODULE_STD ON)

project(example LANGUAGES CXX)
add_executable(main main.cpp)

main.cpp:

import std;

int main()
{
        std::print("foobar\n");
}

I have the following relevant packages installed:

ii  clang-19                                                 1:19.1.5~++20241203083347+ab4b5a2db582-1~exp1~20241203083401.69 amd64        C, C++ and Objective-C compiler
ii  cmake                                                    3.30.2-0kitware1ubuntu24.04.1                                   amd64        cross-platform, open-source make system
ii  cmake-data                                               3.30.2-0kitware1ubuntu24.04.1                                   all          CMake data files (modules, templates and documentation
ii  libc++-19-dev:amd64                                      1:19.1.5~++20241203083347+ab4b5a2db582-1~exp1~20241203083401.69 amd64        LLVM C++ Standard library (development files)
ii  libc++1-19:amd64                                         1:19.1.5~++20241203083347+ab4b5a2db582-1~exp1~20241203083401.69 amd64        LLVM C++ Standard library
ii  libc++abi-19-dev:amd64                                   1:19.1.5~++20241203083347+ab4b5a2db582-1~exp1~20241203083401.69 amd64        LLVM low level support for a standard C++ library (development files)
ii  libc++abi1-19:amd64

I build the project with:
cmake ../ -DCMAKE_CXX_COMPILER=clang++-19 -DCMAKE_CXX_FLAGS="-stdlib=libc++" -DCMAKE_BUILD_TYPE=Debug -G Ninja

It outputs:

-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Check for working CXX compiler: /usr/bin/clang++-19 - skipped
-- Detecting CXX compile features
CMake Warning (dev) at /usr/share/cmake-3.30/Modules/Compiler/CMakeCommonCompilerMacros.cmake:248 (cmake_language):
  CMake's support for `import std;` in C++23 and newer is experimental.  It
  is meant only for experimentation and feedback to CMake developers.
Call Stack (most recent call first):
  /usr/share/cmake-3.30/Modules/CMakeDetermineCompilerSupport.cmake:113 (cmake_create_cxx_import_std)
  /usr/share/cmake-3.30/Modules/CMakeTestCXXCompiler.cmake:83 (CMAKE_DETERMINE_COMPILER_SUPPORT)
  CMakeLists.txt:10 (project)
This warning is for project developers.  Use -Wno-dev to suppress it.

-- Detecting CXX compile features - done
-- Configuring done (0.4s)
CMake Error in CMakeLists.txt:
  The "CXX_MODULE_STD" property on the target "main" requires that the
  "__CMAKE::CXX23" target exist, but it was not provided by the toolchain.
  Reason:

    `libc++.modules.json` resource does not exist


-- Generating done (0.0s)
CMake Generate step failed.  Build files cannot be regenerated correctly.

The requested file is installed by the official apt-package; it is located at: /usr/lib/llvm-19/lib/libc++.modules.json

But clang doesn't seem to be able to find it there.

[tsurumi-yizhou]

The same to me.

[ChuanqiXu9]

What's your environment? If it is mac m1, it, this is duplicated with #109895

[h-2]

What's your environment?

Linux HeDeTh 6.8.0-49-generic #49-Ubuntu SMP PREEMPT_DYNAMIC Mon Nov  4 02:06:24 UTC 2024 x86_64 x86_64 x86_64 GNU/Linux

Distributor ID: Ubuntu
Description:    Ubuntu 24.04.1 LTS
Release:        24.04
Codename:       noble

[llvmbot]

@llvm/issue-subscribers-clang-modules

Author: Hannes Hauswedell (h-2)

I am trying this C++ Modules PoC:

CMakeLists.txt:

cmake_minimum_required(VERSION 3.30)

set(CMAKE_CXX_STANDARD 23)
set(CMAKE_CXX_STANDARD_REQUIRED YES)

set(CMAKE_EXPERIMENTAL_CXX_IMPORT_STD "0e5b6991-d74f-4b3d-a41c-cf096e0b2508")
set(CMAKE_CXX_MODULE_STD ON)

project(example LANGUAGES CXX)
add_executable(main main.cpp)

main.cpp:

import std;

int main()
{
        std::print("foobar\n");
}

I have the following relevant packages installed:

ii  clang-19                                                 1:19.1.5~++20241203083347+ab4b5a2db582-1~exp1~20241203083401.69 amd64        C, C++ and Objective-C compiler
ii  cmake                                                    3.30.2-0kitware1ubuntu24.04.1                                   amd64        cross-platform, open-source make system
ii  cmake-data                                               3.30.2-0kitware1ubuntu24.04.1                                   all          CMake data files (modules, templates and documentation
ii  libc++-19-dev:amd64                                      1:19.1.5~++20241203083347+ab4b5a2db582-1~exp1~20241203083401.69 amd64        LLVM C++ Standard library (development files)
ii  libc++1-19:amd64                                         1:19.1.5~++20241203083347+ab4b5a2db582-1~exp1~20241203083401.69 amd64        LLVM C++ Standard library
ii  libc++abi-19-dev:amd64                                   1:19.1.5~++20241203083347+ab4b5a2db582-1~exp1~20241203083401.69 amd64        LLVM low level support for a standard C++ library (development files)
ii  libc++abi1-19:amd64

I build the project with:
cmake ../ -DCMAKE_CXX_COMPILER=clang++-19 -DCMAKE_CXX_FLAGS="-stdlib=libc++" -DCMAKE_BUILD_TYPE=Debug -G Ninja

It outputs:

-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Check for working CXX compiler: /usr/bin/clang++-19 - skipped
-- Detecting CXX compile features
CMake Warning (dev) at /usr/share/cmake-3.30/Modules/Compiler/CMakeCommonCompilerMacros.cmake:248 (cmake_language):
  CMake's support for `import std;` in C++23 and newer is experimental.  It
  is meant only for experimentation and feedback to CMake developers.
Call Stack (most recent call first):
  /usr/share/cmake-3.30/Modules/CMakeDetermineCompilerSupport.cmake:113 (cmake_create_cxx_import_std)
  /usr/share/cmake-3.30/Modules/CMakeTestCXXCompiler.cmake:83 (CMAKE_DETERMINE_COMPILER_SUPPORT)
  CMakeLists.txt:10 (project)
This warning is for project developers.  Use -Wno-dev to suppress it.

-- Detecting CXX compile features - done
-- Configuring done (0.4s)
CMake Error in CMakeLists.txt:
  The "CXX_MODULE_STD" property on the target "main" requires that the
  "__CMAKE::CXX23" target exist, but it was not provided by the toolchain.
  Reason:

    `libc++.modules.json` resource does not exist


-- Generating done (0.0s)
CMake Generate step failed.  Build files cannot be regenerated correctly.

The requested file is installed by the official apt-package; it is located at: /usr/lib/llvm-19/lib/libc++.modules.json

But clang doesn't seem to be able to find it there.

[llvmbot]

@llvm/issue-subscribers-clang-driver

Author: Hannes Hauswedell (h-2)

I am trying this C++ Modules PoC:

CMakeLists.txt:

cmake_minimum_required(VERSION 3.30)

set(CMAKE_CXX_STANDARD 23)
set(CMAKE_CXX_STANDARD_REQUIRED YES)

set(CMAKE_EXPERIMENTAL_CXX_IMPORT_STD "0e5b6991-d74f-4b3d-a41c-cf096e0b2508")
set(CMAKE_CXX_MODULE_STD ON)

project(example LANGUAGES CXX)
add_executable(main main.cpp)

main.cpp:

import std;

int main()
{
        std::print("foobar\n");
}

I have the following relevant packages installed:

ii  clang-19                                                 1:19.1.5~++20241203083347+ab4b5a2db582-1~exp1~20241203083401.69 amd64        C, C++ and Objective-C compiler
ii  cmake                                                    3.30.2-0kitware1ubuntu24.04.1                                   amd64        cross-platform, open-source make system
ii  cmake-data                                               3.30.2-0kitware1ubuntu24.04.1                                   all          CMake data files (modules, templates and documentation
ii  libc++-19-dev:amd64                                      1:19.1.5~++20241203083347+ab4b5a2db582-1~exp1~20241203083401.69 amd64        LLVM C++ Standard library (development files)
ii  libc++1-19:amd64                                         1:19.1.5~++20241203083347+ab4b5a2db582-1~exp1~20241203083401.69 amd64        LLVM C++ Standard library
ii  libc++abi-19-dev:amd64                                   1:19.1.5~++20241203083347+ab4b5a2db582-1~exp1~20241203083401.69 amd64        LLVM low level support for a standard C++ library (development files)
ii  libc++abi1-19:amd64

I build the project with:
cmake ../ -DCMAKE_CXX_COMPILER=clang++-19 -DCMAKE_CXX_FLAGS="-stdlib=libc++" -DCMAKE_BUILD_TYPE=Debug -G Ninja

It outputs:

-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Check for working CXX compiler: /usr/bin/clang++-19 - skipped
-- Detecting CXX compile features
CMake Warning (dev) at /usr/share/cmake-3.30/Modules/Compiler/CMakeCommonCompilerMacros.cmake:248 (cmake_language):
  CMake's support for `import std;` in C++23 and newer is experimental.  It
  is meant only for experimentation and feedback to CMake developers.
Call Stack (most recent call first):
  /usr/share/cmake-3.30/Modules/CMakeDetermineCompilerSupport.cmake:113 (cmake_create_cxx_import_std)
  /usr/share/cmake-3.30/Modules/CMakeTestCXXCompiler.cmake:83 (CMAKE_DETERMINE_COMPILER_SUPPORT)
  CMakeLists.txt:10 (project)
This warning is for project developers.  Use -Wno-dev to suppress it.

-- Detecting CXX compile features - done
-- Configuring done (0.4s)
CMake Error in CMakeLists.txt:
  The "CXX_MODULE_STD" property on the target "main" requires that the
  "__CMAKE::CXX23" target exist, but it was not provided by the toolchain.
  Reason:

    `libc++.modules.json` resource does not exist


-- Generating done (0.0s)
CMake Generate step failed.  Build files cannot be regenerated correctly.

The requested file is installed by the official apt-package; it is located at: /usr/lib/llvm-19/lib/libc++.modules.json

But clang doesn't seem to be able to find it there.

[ChuanqiXu9]

For such environment sensitive problem, I may not be able to fix it actually.

@mathstuf I am wondering if CMake can offer an option as STD_MODULE_JSON_PATH so that users can workaround when the compiler has problems in different environments.

[ChuanqiXu9]

If anyone on Ubuntu is willing to look into this, I'll be appreciated. This should majorly be a driver problem with different path pattern in different platforms.

[tsunho12]

This is because clang++ -print-file-name=libc++.modules.json does not return the full path of “libc++.modules.json” (I don’t know why).

But the workaround is here. On Intel/apple silicon Mac, seem Linux haven't Clang-CXX-CXXImportStd.cmake, this file just for Mac, and other file for Windows, Linux now have't like this configuration file.

This is being continued in this issue: https://gitlab.kitware.com/cmake/cmake/-/issues/25965#note_1523575

– Open the file “some path/cmake/some version/share/cmake/Modules/Compiler/Clang-CXX-CXXImportStd.cmake”
– Change line 13
– ” -print-file-name=libc++.modules.json” to ” -print-file-name=../../c++/libc++.modules.json”

[mathstuf]

But the workaround is here. On Intel/apple silicon Mac, seem Linux haven't Clang-CXX-CXXImportStd.cmake, this file just for Mac, and other file for Windows, Linux now have't like this configuration file.

It affects Linux too. The core issue here is that the installation layout and the runtime expectations for the file are mismatched on macOS. CMake's code is fine, but it can host such a workaround in the meantime.

[h-2]

Yes, as I pointed out, this issue is not about macOS, it is about the official LLVM deb packages from apt.llvm.org on the most recent LTS version of Ubuntu.

As such, I don't think "Ubuntu" can or should fix this. It's about how LLVM is configured in the packages provided by LLVM.