Building libc++

The instructions on this page are aimed at vendors who ship libc++ as part of an operating system distribution, a toolchain or similar shipping vehicules. If you are a user merely trying to use libc++ in your program, you most likely want to refer to your vendor’s documentation, or to the general documentation for using libc++ here.


If your operating system already provides libc++, it is important to be careful not to replace it. Replacing your system’s libc++ installation could render it non-functional. Use the CMake option CMAKE_INSTALL_PREFIX to select a safe place to install libc++.

The default build

The default way of building libc++, libc++abi and libunwind is to root the CMake invocation at <monorepo>/runtimes. While those projects are under the LLVM umbrella, they are different in nature from other build tools, so it makes sense to treat them as a separate set of entities. The default build can be achieved with the following CMake invocation:

$ git clone
$ cd llvm-project
$ mkdir build
$ cmake -G Ninja -S runtimes -B build -DLLVM_ENABLE_RUNTIMES="libcxx;libcxxabi;libunwind" # Configure
$ ninja -C build cxx cxxabi unwind                                                        # Build
$ ninja -C build check-cxx check-cxxabi check-unwind                                      # Test
$ ninja -C build install-cxx install-cxxabi install-unwind                                # Install


See CMake Options below for more configuration options.

After building the various install-XXX targets, shared libraries for libc++, libc++abi and libunwind should now be present in <CMAKE_INSTALL_PREFIX>/lib, and headers in <CMAKE_INSTALL_PREFIX>/include/c++/v1. See using an alternate libc++ installation for information on how to use this libc++ over the default one.

In the default configuration, the runtimes will be built using the compiler available by default on your system. Of course, you can change what compiler is being used with the usual CMake variables. If you wish to build the runtimes from a just-built Clang, the bootstrapping build explained below makes this task easy.

Bootstrapping build

It is possible to build Clang and then build the runtimes using that just-built compiler in a single CMake invocation. This is usually the correct way to build the runtimes when putting together a toolchain, or when the system compiler is not adequate to build them (too old, unsupported, etc.). To do this, use the following CMake invocation, and in particular notice how we’re now rooting the CMake invocation at <monorepo>/llvm:

$ mkdir build
$ cmake -G Ninja -S llvm -B build -DLLVM_ENABLE_PROJECTS="clang"                      \  # Configure
                                  -DLLVM_ENABLE_RUNTIMES="libcxx;libcxxabi;libunwind" \
$ ninja -C build runtimes                                                                # Build
$ ninja -C build check-runtimes                                                          # Test
$ ninja -C build install-runtimes                                                        # Install


This type of build is also commonly called a “Runtimes build”, but we would like to move away from that terminology, which is too confusing.

Support for Windows

libcxx supports being built with clang-cl, but not with MSVC’s cl.exe, as cl doesn’t support the #include_next extension. Furthermore, VS 2017 or newer (19.14) is required.

libcxx also supports being built with clang targeting MinGW environments.

CMake + Visual Studio

Building with Visual Studio currently does not permit running tests. However, it is the simplest way to build.

> cmake -G "Visual Studio 16 2019" -S libcxx -B build ^
        -T "ClangCL"                                  ^
        -DLIBCXX_ENABLE_SHARED=YES                    ^
        -DLIBCXX_ENABLE_STATIC=NO                     ^
> cmake --build build

CMake + ninja (MSVC)

Building with ninja is required for development to enable tests. A couple of tests require Bash to be available, and a couple dozens of tests require other posix tools (cp, grep and similar - LLVM’s tests require the same). Without those tools the vast majority of tests can still be ran successfully.

If Git for Windows is available, that can be used to provide the bash shell by adding the right bin directory to the path, e.g. set PATH=%PATH%;C:\Program Files\Git\usr\bin.

Alternatively, one can also choose to run the whole build in a MSYS2 shell. That can be set up e.g. by starting a Visual Studio Tools Command Prompt (for getting the environment variables pointing to the headers and import libraries), and making sure that clang-cl is available in the path. From there, launch an MSYS2 shell via e.g. C:\msys64\msys2_shell.cmd -full-path -mingw64 (preserving the earlier environment, allowing the MSVC headers/libraries and clang-cl to be found).

In either case, then run:

> cmake -G Ninja -S libcxx -B build                                                 ^
        -DCMAKE_C_COMPILER=clang-cl                                                 ^
        -DCMAKE_CXX_COMPILER=clang-cl                                               ^
> ninja -C build cxx
> ninja -C build check-cxx

If you are running in an MSYS2 shell and you have installed the MSYS2-provided clang package (which defaults to a non-MSVC target), you should add e.g. -DLIBCXX_TARGET_TRIPLE=x86_64-windows-msvc (replacing x86_64 with the architecture you’re targeting) to the cmake command line above. This will instruct check-cxx to use the right target triple when invoking clang++.

Also note that if not building in Release mode, a failed assert in the tests pops up a blocking dialog box, making it hard to run a larger number of tests.

CMake + ninja (MinGW)

libcxx can also be built in MinGW environments, e.g. with the MinGW compilers in MSYS2. This requires clang to be available (installed with e.g. the mingw-w64-x86_64-clang package), together with CMake and ninja.

> cmake -G Ninja -S libcxx -B build                                                 \
        -DCMAKE_C_COMPILER=clang                                                    \
        -DCMAKE_CXX_COMPILER=clang++                                                \
        -DLIBCXX_HAS_WIN32_THREAD_API=ON                                            \
        -DLIBCXX_CXX_ABI=libstdc++                                                  \
> ninja -C build cxx
> cp /mingw64/bin/{libstdc++-6,libgcc_s_seh-1,libwinpthread-1}.dll lib
> ninja -C build check-cxx

As this build configuration ends up depending on a couple other DLLs that aren’t available in path while running tests, copy them into the same directory as the tested libc++ DLL.

(Building a libc++ that depends on libstdc++ isn’t necessarily a config one would want to deploy, but it simplifies the config for testing purposes.)

CMake Options

Here are some of the CMake variables that are used often, along with a brief explanation and LLVM-specific notes. For full documentation, check the CMake docs or execute cmake --help-variable VARIABLE_NAME.


Sets the build type for make based generators. Possible values are Release, Debug, RelWithDebInfo and MinSizeRel. On systems like Visual Studio the user sets the build type with the IDE settings.


Path where LLVM will be installed if “make install” is invoked or the “INSTALL” target is built.


The C++ compiler to use when building and testing libc++.

libc++ specific options


Default: ON

Toggle the installation of the library portion of libc++.


Default: ON

Toggle the installation of the libc++ headers.


Default: OFF

Build libc++ with assertions enabled.


Default: ON

Build libc++ as a shared library. Either LIBCXX_ENABLE_SHARED or LIBCXX_ENABLE_STATIC has to be enabled.


Default: ON

Build libc++ as a static library. Either LIBCXX_ENABLE_SHARED or LIBCXX_ENABLE_STATIC has to be enabled.


Extra suffix to append to the directory where libraries are to be installed. This option overrides LLVM_LIBDIR_SUFFIX.


Default: OFF

Do not export any symbols from the static libc++ library. This is useful when the static libc++ library is being linked into shared libraries that may be used in with other shared libraries that use different C++ library. We want to avoid exporting any libc++ symbols in that case.


Default: ON except on Windows when using MSVC.

This option can be used to enable or disable the filesystem components on platforms that may not support them. For example on Windows when using MSVC.


Default: ON

This option can be used to disable support for wchar_t in the library. It also allows the library to work on top of a C Standard Library that does not provide support for wchar_t. This is especially useful in embedded settings where C Standard Libraries don’t always provide all the usual bells and whistles.


Default: ON

Whether to enable support for incomplete library features. Incomplete features are new library features under development. These features don’t guarantee ABI stability nor the quality of completed library features. Vendors shipping the library may want to disable this option.



Path where built libc++ libraries should be installed. If a relative path, relative to CMAKE_INSTALL_PREFIX.


Default: include/c++/v1

Path where target-agnostic libc++ headers should be installed. If a relative path, relative to CMAKE_INSTALL_PREFIX.


Default: include/c++/v1 or include/${LLVM_DEFAULT_TARGET_TRIPLE}/c++/v1

Path where target-specific libc++ headers should be installed. If a relative path, relative to CMAKE_INSTALL_PREFIX.

libc++experimental Specific Options


Default: ON

Build and test libc++experimental.a.



Install libc++experimental.a alongside libc++.

ABI Library Specific Options


Values: none, libcxxabi, system-libcxxabi, libcxxrt, libstdc++, libsupc++.

Select the ABI library to build libc++ against.


Provide additional search paths for the ABI library headers.


Provide the path to the ABI library that libc++ should link against.


Default: OFF

If this option is enabled, libc++ will try and link the selected ABI library statically.


Default: ON by default on UNIX platforms other than Apple unless ‘LIBCXX_ENABLE_STATIC_ABI_LIBRARY’ is ON. Otherwise the default value is OFF.

This option generate and installs a linker script as which links the correct ABI library.


Default: OFF

Build and use the LLVM unwinder. Note: This option can only be used when libc++abi is the C++ ABI library used.

libc++ Feature Options


Default: ON

Build libc++ with exception support.


Default: ON

Build libc++ with run time type information.


Default: ON (or value of LLVM_INCLUDE_TESTS)

Build the libc++ tests.


Default: ON

Build the libc++ benchmark tests and the Google Benchmark library needed to support them.


Default: --benchmark_min_time=0.01

A semicolon list of arguments to pass when running the libc++ benchmarks using the check-cxx-benchmarks rule. By default we run the benchmarks for a very short amount of time, since the primary use of check-cxx-benchmarks is to get test and sanitizer coverage, not to get accurate measurements.


Default:: ""

Values:: libc++, libstdc++

Build the libc++ benchmark tests and Google Benchmark library against the specified standard library on the platform. On Linux this can be used to compare libc++ to libstdc++ by building the benchmark tests against both standard libraries.


Use the specified GCC toolchain and standard library when building the native stdlib benchmark tests.


Default: OFF

Pick the default for whether to constrain ABI-unstable symbols to each individual translation unit. This setting controls whether _LIBCPP_HIDE_FROM_ABI_PER_TU_BY_DEFAULT is defined by default – see the documentation of that macro for details.

libc++ ABI Feature Options

The following options allow building libc++ for a different ABI version.


Default: 1

Defines the target ABI version of libc++.


Default: OFF

Build the “unstable” ABI version of libc++. Includes all ABI changing features on top of the current stable version.


Default: __n where n is the current ABI version.

This option defines the name of the inline ABI versioning namespace. It can be used for building custom versions of libc++ with unique symbol names in order to prevent conflicts or ODR issues with other libc++ versions.


When providing a custom namespace, it’s the users responsibility to ensure the name won’t cause conflicts with other names defined by libc++, both now and in the future. In particular, inline namespaces of the form __[0-9]+ are strictly reserved by libc++ and may not be used by users. Doing otherwise could cause conflicts and hinder libc++ ABI evolution.


Default: ""

A semicolon-separated list of ABI macros to persist in the site config header. See include/__config for the list of ABI macros.

LLVM-specific options


Extra suffix to append to the directory where libraries are to be installed. On a 64-bit architecture, one could use -DLLVM_LIBDIR_SUFFIX=64 to install libraries to /usr/lib64.


Build 32-bits executables and libraries on 64-bits systems. This option is available only on some 64-bits Unix systems. Defaults to OFF.


Arguments given to lit. make check and make clang-test are affected. By default, '-sv --no-progress-bar' on Visual C++ and Xcode, '-sv' on others.

Using Alternate ABI libraries

In order to implement various features like exceptions, RTTI, dynamic_cast and more, libc++ requires what we refer to as an ABI library. Typically, that library implements the Itanium C++ ABI.

By default, libc++ uses libc++abi as an ABI library. However, it is possible to use other ABI libraries too.

Using libsupc++ on Linux

You will need libstdc++ in order to provide libsupc++.

Figure out where the libsupc++ headers are on your system. On Ubuntu this is /usr/include/c++/<version> and /usr/include/c++/<version>/<target-triple>

You can also figure this out by running

$ echo | g++ -Wp,-v -x c++ - -fsyntax-only
ignoring nonexistent directory "/usr/local/include/x86_64-linux-gnu"
ignoring nonexistent directory "/usr/lib/gcc/x86_64-linux-gnu/4.7/../../../../x86_64-linux-gnu/include"
#include "..." search starts here:
#include &lt;...&gt; search starts here:
End of search list.

Note that the first two entries happen to be what we are looking for. This may not be correct on all platforms.

We can now run CMake:

$ cmake -G Ninja -S runtimes -B build       \
  -DLLVM_ENABLE_RUNTIMES="libcxx"           \
  -DLIBCXX_CXX_ABI=libstdc++                \
$ ninja -C build install-cxx

You can also substitute -DLIBCXX_CXX_ABI=libsupc++ above, which will cause the library to be linked to libsupc++ instead of libstdc++, but this is only recommended if you know that you will never need to link against libstdc++ in the same executable as libc++. GCC ships libsupc++ separately but only as a static library. If a program also needs to link against libstdc++, it will provide its own copy of libsupc++ and this can lead to subtle problems.

Using libcxxrt on Linux

You will need to keep the source tree of libcxxrt available on your build machine and your copy of the libcxxrt shared library must be placed where your linker will find it.

We can now run CMake like:

$ cmake -G Ninja -S runtimes -B build                               \
        -DLLVM_ENABLE_RUNTIMES="libcxx"                             \
        -DLIBCXX_CXX_ABI=libcxxrt                                   \
$ ninja -C build install-cxx

Unfortunately you can’t simply run clang with “-stdlib=libc++” at this point, as clang is set up to link for libc++ linked to libsupc++. To get around this you’ll have to set up your linker yourself (or patch clang). For example,

$ clang++ -stdlib=libc++ helloworld.cpp \
          -nodefaultlibs -lc++ -lcxxrt -lm -lc -lgcc_s -lgcc

Alternately, you could just add libcxxrt to your libraries list, which in most situations will give the same result:

$ clang++ -stdlib=libc++ helloworld.cpp -lcxxrt