How to use clang 17 and CMake 3.28 to compile a simple hello world with c++20 modules in Linux?

Question

I followed a CMake official blog of import CMake; the Experiment is Over! and created the simple hello world program with modules in the blog. An error of /usr/include/c++/v1/stddef.h:17:15: fatal error: 'stddef.h' file not found appeared when compiling. Thanks in advance.

CMakeLists.txt file

cmake_minimum_required(VERSION 3.28)
project(std_module_example CXX)

# Turning off extensions avoids and issue with the clang 16 compiler
# clang 17 and greater can avoid this setting
set(CMAKE_CXX_EXTENSIONS OFF)
# Set the version of C++ for the project
set(CMAKE_CXX_STANDARD 20)

# Create a library
add_library(foo)
# Add the module file to the library
target_sources(foo
  PUBLIC
    FILE_SET CXX_MODULES FILES
      foo.cxx
)
# Create an executable
add_executable(hello main.cxx)
# Link to the library foo
target_link_libraries(hello foo)

foo.cxx file

// Global module fragment where #includes can happen
module;
#include <iostream>

// first thing after the Global module fragment must be a module command
export module foo;

export class foo {
    public:
        foo();
        ~foo();
        void helloworld();
};

foo::foo() = default;
foo::~foo() = default;
void foo::helloworld() { std::cout << "hello world\n"; }

main.cxx file

import foo;

int main() {
    foo f;
    f.helloworld();
    return 0;
}

build:

❯ CXX=clang++ /opt/cmake/bin/cmake . -B build -GNinja -DCMAKE_CXX_FLAGS='-stdlib=libc++' -DCMAKE_EXE_LINKER_FLAGS='-stdlib=libc++' && /opt/cmake/bin/cmake --build build -v
-- The CXX compiler identification is Clang 17.0.3
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Check for working CXX compiler: /usr/lib64/ccache/clang++ - skipped
-- Detecting CXX compile features
-- Detecting CXX compile features - done
-- Configuring done (0.2s)
-- Generating done (0.0s)
-- Build files have been written to: /home/ruby/fedora-src/modules-in-clang/t2/build
Change Dir: '/home/ruby/fedora-src/modules-in-clang/t2/build'

Run Build Command(s): /usr/bin/ninja-build -v
[1/8] "/usr/bin/clang-scan-deps" -format=p1689 -- /usr/lib64/ccache/clang++   -stdlib=libc++ -std=c++20 -x c++ /home/ruby/fedora-src/modules-in-clang/t2/main.cxx -c -o CMakeFiles/hello.dir/main.cxx.o -MT CMakeFiles/hello.dir/main.cxx.o.ddi -MD -MF CMakeFiles/hello.dir/main.cxx.o.ddi.d > CMakeFiles/hello.dir/main.cxx.o.ddi
[2/8] "/usr/bin/clang-scan-deps" -format=p1689 -- /usr/lib64/ccache/clang++   -stdlib=libc++ -std=c++20 -x c++ /home/ruby/fedora-src/modules-in-clang/t2/foo.cxx -c -o CMakeFiles/foo.dir/foo.cxx.o -MT CMakeFiles/foo.dir/foo.cxx.o.ddi -MD -MF CMakeFiles/foo.dir/foo.cxx.o.ddi.d > CMakeFiles/foo.dir/foo.cxx.o.ddi
FAILED: CMakeFiles/foo.dir/foo.cxx.o.ddi 
"/usr/bin/clang-scan-deps" -format=p1689 -- /usr/lib64/ccache/clang++   -stdlib=libc++ -std=c++20 -x c++ /home/ruby/fedora-src/modules-in-clang/t2/foo.cxx -c -o CMakeFiles/foo.dir/foo.cxx.o -MT CMakeFiles/foo.dir/foo.cxx.o.ddi -MD -MF CMakeFiles/foo.dir/foo.cxx.o.ddi.d > CMakeFiles/foo.dir/foo.cxx.o.ddi
Error while scanning dependencies for /home/ruby/fedora-src/modules-in-clang/t2/foo.cxx:
In file included from /home/ruby/fedora-src/modules-in-clang/t2/foo.cxx:3:
In file included from /usr/include/c++/v1/iostream:43:
In file included from /usr/include/c++/v1/ios:222:
In file included from /usr/include/c++/v1/__locale:15:
In file included from /usr/include/c++/v1/__memory/shared_ptr.h:22:
In file included from /usr/include/c++/v1/__memory/allocation_guard.h:15:
In file included from /usr/include/c++/v1/__memory/allocator_traits.h:14:
In file included from /usr/include/c++/v1/__memory/construct_at.h:23:
In file included from /usr/include/c++/v1/new:99:
In file included from /usr/include/c++/v1/cstdlib:87:
In file included from /usr/include/c++/v1/stdlib.h:94:
In file included from /usr/include/stdlib.h:32:
/usr/include/c++/v1/stddef.h:17:15: fatal error: 'stddef.h' file not found
ninja: build stopped: subcommand failed.

❯ /opt/cmake/bin/cmake --version 
cmake version 3.28.0-rc3

CMake suite maintained and supported by Kitware (kitware.com/cmake).

Solution

The issue can be solved by adding -I/usr/lib/clang/17/include to clang++ compiler option if setting C++ compiler with CXX=clang++, which it will not point to the real location of C++ compiler. It will also can be solved by setting the real binary compiler, like -DCMAKE_CXX_COMPILER=/usr/bin/clang++.

Ref: https://github.com/llvm/llvm-project/issues/61006 https://gitlab.kitware.com/cmake/cmake/-/issues/25180

Answer 1

It seems like some headers are missing. After confirming clang is installed I'd try running

apt-get install libc6-dev 

or your distro's equivalent.

Answer 2

I put these lines at the top of my CMakeLists.txt

cmake_minimum_required(VERSION 3.28)
set(CMAKE_CXX_EXTENSIONS OFF)
set(CMAKE_CXX_STANDARD 20)
set(CMAKE_CXX_COMPILER clang++)
set(CMAKE_SCAN_FOR_MODULES)

This ensures clang++ is used instead of g++