Modules in libc++¶

Warning

Modules are an experimental feature. It has additional build requirements and not all libc++ configurations are supported yet.

The work is still in an early developement state and not considered stable nor complete

This page contains information regarding C++23 module support in libc++. There are two kinds of modules available in Clang

Clang specific modules

C++ modules

This page mainly discusses the C++ modules. In C++20 there are also header units, these are not part of this document.

Overview¶

The module sources are stored in .cppm files. Modules need to be available as BMIs, which are .pcm files for Clang. BMIs are not portable, they depend on the compiler used and its compilation flags. Therefore there needs to be a way to distribute the .cppm files to the user and offer a way for them to build and use the .pcm files. It is expected this will be done by build systems in the future. To aid early adaptor and build system vendors libc++ currently ships a CMake project to aid building modules.

Note

This CMake file is intended to be a temporary solution and will be removed in the future. The timeline for the removal depends on the availability of build systems with proper module support.

What works¶

Building BMIs

Running tests using the std module

Using the std module in external projects

The following “parts disabled” configuration options are supported

LIBCXX_ENABLE_LOCALIZATION

LIBCXX_ENABLE_WIDE_CHARACTERS

Some of the current limitations¶

There is no official build system support, libc++ has experimental CMake support

Requires CMake 3.26

Requires Ninja 1.11

Requires a recent Clang 17

The path to the compiler may not be a symlink, clang-scan-deps does not handle that case properly

Only C++23 is tested

Libc++ is not tested with modules instead of headers

The module .cppm files are not installed

The experimental PSTL library is not supported

Clang supports modules using GNU extensions, but libc++ does not work using GNU extensions.

Clang:

Including headers after importing the std module may fail. This is hard to solve and there is a work-around by first including all headers bug report.

Blockers¶

libc++

Currently the tests only test with modules enabled, but do not import modules instead of headers. When converting tests to using modules there are still failures. These are under investigation.

It has not been determined how to fully test libc++ with modules instead of headers.

Clang

Some concepts do not work properly bug report.

Using in external projects¶

Users need to be able to build their own BMI files.

Note

The requirements for users to build their own BMI files will remain true for the forseeable future. For now this needs to be done manually. Once libc++’s implementation is more mature we will reach out to build system vendors, with the goal that building the BMI files is done by the build system.

Currently this requires a local build of libc++ with modules enabled. Since modules are not part of the installation yet, they are used from the build directory. First libc++ needs to be build with module support enabled.

$ git clone https://github.com/llvm/llvm-project.git
$ cd llvm-project
$ mkdir build
$ cmake -G Ninja -S runtimes -B build -DLIBCXX_ENABLE_STD_MODULES=ON -DLLVM_ENABLE_RUNTIMES="libcxx;libcxxabi;libunwind"
$ ninja -C build

The above build directory will be referred to as <build> in the rest of these instructions.

This is a small sample program that uses the module std. It consists of a CMakeLists.txt and a main.cpp file.

import std;

int main() { std::cout << "Hello modular world\n"; }

cmake_minimum_required(VERSION 3.26.0 FATAL_ERROR)
project("module"
  LANGUAGES CXX
)

#
# Set language version used
#

# At the moment only C++23 is tested.
set(CMAKE_CXX_STANDARD 23)
set(CMAKE_CXX_STANDARD_REQUIRED YES)
# Libc++ doesn't support compiler extensions for modules.
set(CMAKE_CXX_EXTENSIONS OFF)

#
# Enable modules in CMake
#

# This is required to write your own modules in your project.
if(CMAKE_VERSION VERSION_LESS "3.27.0")
  set(CMAKE_EXPERIMENTAL_CXX_MODULE_CMAKE_API "2182bf5c-ef0d-489a-91da-49dbc3090d2a")
else()
  set(CMAKE_EXPERIMENTAL_CXX_MODULE_CMAKE_API "aa1f7df0-828a-4fcd-9afc-2dc80491aca7")
endif()
set(CMAKE_EXPERIMENTAL_CXX_MODULE_DYNDEP 1)

#
# Import the modules from libc++
#

include(FetchContent)
FetchContent_Declare(
  std
  URL "file://${LIBCXX_BUILD}/modules/c++/v1/"
  DOWNLOAD_EXTRACT_TIMESTAMP TRUE
)
FetchContent_GetProperties(std)
if(NOT std_POPULATED)
  FetchContent_Populate(std)
  add_subdirectory(${std_SOURCE_DIR} ${std_BINARY_DIR} EXCLUDE_FROM_ALL)
endif()

#
# Adjust project compiler flags
#

add_compile_options($<$<COMPILE_LANGUAGE:CXX>:-fprebuilt-module-path=${CMAKE_BINARY_DIR}/_deps/std-build/CMakeFiles/std.dir/>)
add_compile_options($<$<COMPILE_LANGUAGE:CXX>:-nostdinc++>)
# The include path needs to be set to be able to use macros from headers.
# For example from, the headers <cassert> and <version>.
add_compile_options($<$<COMPILE_LANGUAGE:CXX>:-isystem>)
add_compile_options($<$<COMPILE_LANGUAGE:CXX>:${LIBCXX_BUILD}/include/c++/v1>)

#
# Adjust project linker flags
#

add_link_options($<$<COMPILE_LANGUAGE:CXX>:-nostdlib++>)
add_link_options($<$<COMPILE_LANGUAGE:CXX>:-L${LIBCXX_BUILD}/lib>)
add_link_options($<$<COMPILE_LANGUAGE:CXX>:-Wl,-rpath,${LIBCXX_BUILD}/lib>)
# Linking against std is required for CMake to get the proper dependencies
link_libraries(std c++)

#
# Add the project
#

add_executable(main)
target_sources(main
  PRIVATE
    main.cpp
)

Building this project is done with the following steps, assuming the files main.cpp and CMakeLists.txt are copied in the current directory.

$ mkdir build
$ cmake -G Ninja -S . -B build -DCMAKE_CXX_COMPILER=<path-to-compiler> -DLIBCXX_BUILD=<build>
$ ninja -C build
$ build/main

Warning

<path-to-compiler> should point point to the real binary and not to a symlink.

Warning

When using these examples in your own projects make sure the compilation flags are the same for the std module and your project. Some flags will affect the generated code, when these are different the module cannot be used. For example using -pthread in your project and not in the module will give errors like

error: POSIX thread support was disabled in PCH file but is currently enabled

error: module file _deps/std-build/CMakeFiles/std.dir/std.pcm cannot be loaded due to a configuration mismatch with the current compilation [-Wmodule-file-config-mismatch]

If you have questions about modules feel free to ask them in the #libcxx channel on LLVM’s Discord server.

If you think you’ve found a bug please it using the LLVM bug tracker. Please make sure the issue you found is not one of the known bugs or limitations on this page.