Building libc++

Getting Started

On Mac OS 10.7 (Lion) and later, the easiest way to get this library is to install Xcode 4.2 or later. However if you want to install tip-of-trunk from here (getting the bleeding edge), read on.

The following instructions describe how to checkout, build, test and (optionally) install libc++ and libc++abi.

If your system already provides a libc++ installation it is important to be careful not to replace it. Remember Use the CMake option CMAKE_INSTALL_PREFIX to select a safe place to install libc++.


  • Replacing your systems libc++ installation could render the system non-functional.

  • macOS will not boot without a valid copy of libc++.1.dylib in /usr/lib.

$ git clone
$ cd llvm-project
$ mkdir build && cd build
$ cmake -DCMAKE_C_COMPILER=clang \
        -DCMAKE_CXX_COMPILER=clang++ \
        -DLLVM_ENABLE_PROJECTS="libcxx;libcxxabi" \
$ make # Build
$ make check-cxx # Test
$ make install-cxx install-cxxabi # Install

For more information about configuring libc++ see CMake Options. You may also want to read the LLVM getting started documentation.

Shared libraries for libc++ and libc++ abi should now be present in build/lib. See using an alternate libc++ installation for information on how to use this libc++.

The instructions are for building libc++ on FreeBSD, Linux, or Mac using libc++abi as the C++ ABI library. On Linux, it is also possible to use libsupc++ or libcxxrt.

It is possible to build libc++ standalone (i.e. without building other LLVM projects). A standalone build would look like this:

$ git clone llvm-project
$ cd llvm-project
$ mkdir build && cd build
$ cmake -DCMAKE_C_COMPILER=clang \
        -DCMAKE_CXX_COMPILER=clang++ \
        -DLIBCXX_CXX_ABI=libcxxabi \
        -DLIBCXX_CXX_ABI_INCLUDE_PATHS=path/to/separate/libcxxabi/include \
$ make
$ make check-cxx # optional

Experimental Support for Windows

The Windows support requires building with clang-cl as cl does not support one required extension: #include_next. Furthermore, VS 2015 or newer (19.00) is required. In the case of clang-cl, we need to specify the “MS Compatibility Version” as it defaults to 2014 (18.00).

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 14 2015"              ^
        -T "LLVM-vs2014"                        ^
        -DLIBCXX_ENABLE_SHARED=YES              ^
        -DLIBCXX_ENABLE_STATIC=NO               ^
> cmake --build .

CMake + ninja

Building with ninja is required for development to enable tests. Unfortunately, doing so requires additional configuration as we cannot just specify a toolset.

> cmake -G Ninja                                                                    ^
        -DCMAKE_MAKE_PROGRAM=/path/to/ninja                                         ^
        -DCMAKE_SYSTEM_NAME=Windows                                                 ^
        -DCMAKE_C_COMPILER=clang-cl                                                 ^
        -DCMAKE_C_FLAGS="-fms-compatibility-version=19.00 --target=i686--windows"   ^
        -DCMAKE_CXX_COMPILER=clang-cl                                                ^
        -DCMAKE_CXX_FLAGS="-fms-compatibility-version=19.00 --target=i686--windows" ^
        -DLLVM_PATH=/path/to/llvm/tree                                              ^
        -DLIBCXX_ENABLE_SHARED=YES                                                  ^
        -DLIBCXX_ENABLE_STATIC=NO                                                   ^
        -DLIBCXX_ENABLE_EXPERIMENTAL_LIBRARY=NO                                     ^
> /path/to/ninja cxx
> /path/to/ninja check-cxx

Note that the paths specified with backward slashes must use the \ as the directory separator as clang-cl may otherwise parse the path as an argument.

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: OFF

Build libc++ as a 32 bit library. Also see LLVM_BUILD_32_BITS.


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: ""

Define libc++ destination prefix.


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.

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

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, 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_DIR)

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

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 other platforms.

We can now run CMake:

$ CC=clang CXX=clang++ cmake -G "Unix Makefiles" \
  -DLIBCXX_CXX_ABI=libstdc++ \
  -DLIBCXX_CXX_ABI_INCLUDE_PATHS="/usr/include/c++/4.7/;/usr/include/c++/4.7/x86_64-linux-gnu/" \

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.

$ make cxx
$ make install

You can now run clang with -stdlib=libc++.

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:

$ CC=clang CXX=clang++ cmake -G "Unix Makefiles" \
        -DLIBCXX_CXX_ABI=libcxxrt \
        -DLIBCXX_CXX_ABI_INCLUDE_PATHS=path/to/libcxxrt-sources/src \
              -DCMAKE_BUILD_TYPE=Release \
              -DCMAKE_INSTALL_PREFIX=/usr \
$ make cxx
$ make install

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

Using a local ABI library installation


This is not recommended in almost all cases.

These instructions should only be used when you can’t install your ABI library.

Normally you must link libc++ against a ABI shared library that the linker can find. If you want to build and test libc++ against an ABI library not in the linker’s path you need to set -DLIBCXX_CXX_ABI_LIBRARY_PATH=/path/to/abi/lib when configuring CMake.

An example build using libc++abi would look like:

$ CC=clang CXX=clang++ cmake \
            -DLIBCXX_CXX_ABI=libc++abi  \
            -DLIBCXX_CXX_ABI_INCLUDE_PATHS="/path/to/libcxxabi/include" \
            -DLIBCXX_CXX_ABI_LIBRARY_PATH="/path/to/libcxxabi-build/lib" \
$ make

When testing libc++ LIT will automatically link against the proper ABI library.