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.
Warning
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 https://github.com/llvm/llvm-project.git
$ 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
Note
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" \
-DLLVM_RUNTIME_TARGETS="<target-triple>"
$ ninja -C build runtimes # Build
$ ninja -C build check-runtimes # Test
$ ninja -C build install-runtimes # Install
Note
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 runtimes -B build ^
-T "ClangCL" ^
-DLLVM_ENABLE_RUNTIMES=libcxx ^
-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 runtimes -B build ^
-DCMAKE_C_COMPILER=clang-cl ^
-DCMAKE_CXX_COMPILER=clang-cl ^
-DLLVM_ENABLE_RUNTIMES=libcxx
> 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. -DCMAKE_CXX_COMPILER_TARGET=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 runtimes -B build \
-DCMAKE_C_COMPILER=clang \
-DCMAKE_CXX_COMPILER=clang++ \
-DLLVM_ENABLE_RUNTIMES=libcxx \
-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
.
- CMAKE_BUILD_TYPE:STRING
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.- CMAKE_INSTALL_PREFIX:PATH
Path where LLVM will be installed if “make install” is invoked or the “INSTALL” target is built.
- CMAKE_CXX_COMPILER:STRING
The C++ compiler to use when building and testing libc++.
libc++ specific options¶
- LIBCXX_INSTALL_LIBRARY:BOOL¶
Default:
ON
Toggle the installation of the library portion of libc++.
- LIBCXX_INSTALL_HEADERS:BOOL¶
Default:
ON
Toggle the installation of the libc++ headers.
- LIBCXX_ENABLE_ASSERTIONS:BOOL¶
Default:
OFF
Build libc++ with assertions enabled in the compiled library, and enable assertions by default when building user code as well. Assertions can be turned off by users by defining
_LIBCPP_ENABLE_ASSERTIONS=0
. For details, see the documentation.
- LIBCXX_ENABLE_SHARED:BOOL¶
Default:
ON
Build libc++ as a shared library. Either LIBCXX_ENABLE_SHARED or LIBCXX_ENABLE_STATIC has to be enabled.
- LIBCXX_ENABLE_STATIC:BOOL¶
Default:
ON
Build libc++ as a static library. Either LIBCXX_ENABLE_SHARED or LIBCXX_ENABLE_STATIC has to be enabled.
- LIBCXX_LIBDIR_SUFFIX:STRING¶
Extra suffix to append to the directory where libraries are to be installed. This option overrides LLVM_LIBDIR_SUFFIX.
- LIBCXX_HERMETIC_STATIC_LIBRARY:BOOL¶
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.
- LIBCXX_ENABLE_FILESYSTEM:BOOL¶
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.
- LIBCXX_ENABLE_WIDE_CHARACTERS:BOOL¶
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 forwchar_t
. This is especially useful in embedded settings where C Standard Libraries don’t always provide all the usual bells and whistles.
- LIBCXX_INSTALL_LIBRARY_DIR:PATH¶
Default:
lib${LIBCXX_LIBDIR_SUFFIX}
Path where built libc++ libraries should be installed. If a relative path, relative to
CMAKE_INSTALL_PREFIX
.
- LIBCXX_INSTALL_INCLUDE_DIR:PATH¶
Default:
include/c++/v1
Path where target-agnostic libc++ headers should be installed. If a relative path, relative to
CMAKE_INSTALL_PREFIX
.
- LIBCXX_INSTALL_INCLUDE_TARGET_DIR:PATH¶
Default:
include/c++/v1
orinclude/${LLVM_DEFAULT_TARGET_TRIPLE}/c++/v1
Path where target-specific libc++ headers should be installed. If a relative path, relative to
CMAKE_INSTALL_PREFIX
.
- LIBCXX_SHARED_OUTPUT_NAME:STRING¶
Default:
c++
Output name for the shared libc++ runtime library.
- LIBCXX_ADDITIONAL_COMPILE_FLAGS:STRING¶
Default:
""
Additional Compile only flags which can be provided in cache.
- LIBCXX_ADDITIONAL_LIBRARIES:STRING¶
Default:
""
Additional libraries libc++ is linked to which can be provided in cache.
ABI Library Specific Options¶
- LIBCXX_CXX_ABI:STRING¶
Values:
none
,libcxxabi
,system-libcxxabi
,libcxxrt
,libstdc++
,libsupc++
,vcruntime
.Select the ABI library to build libc++ against.
- LIBCXX_CXX_ABI_INCLUDE_PATHS:PATHS¶
Provide additional search paths for the ABI library headers.
- LIBCXX_CXX_ABI_LIBRARY_PATH:PATH¶
Provide the path to the ABI library that libc++ should link against. This is only useful when linking against an out-of-tree ABI library.
- LIBCXX_ENABLE_STATIC_ABI_LIBRARY:BOOL¶
Default:
OFF
If this option is enabled, libc++ will try and link the selected ABI library statically.
- LIBCXX_ENABLE_ABI_LINKER_SCRIPT:BOOL¶
Default:
ON
by default on UNIX platforms other than Apple unless ‘LIBCXX_ENABLE_STATIC_ABI_LIBRARY’ is ON. Otherwise the default value isOFF
.This option generate and installs a linker script as
libc++.so
which links the correct ABI library.
- LIBCXXABI_USE_LLVM_UNWINDER:BOOL¶
Default:
OFF
Build and use the LLVM unwinder. Note: This option can only be used when libc++abi is the C++ ABI library used.
- LIBCXXABI_ADDITIONAL_COMPILE_FLAGS:STRING¶
Default:
""
Additional Compile only flags which can be provided in cache.
- LIBCXXABI_ADDITIONAL_LIBRARIES:STRING¶
Default:
""
Additional libraries libc++abi is linked to which can be provided in cache.
libc++ Feature Options¶
- LIBCXX_ENABLE_EXCEPTIONS:BOOL¶
Default:
ON
Build libc++ with exception support.
- LIBCXX_ENABLE_RTTI:BOOL¶
Default:
ON
Build libc++ with run time type information.
- LIBCXX_INCLUDE_TESTS:BOOL¶
Default:
ON
(or value ofLLVM_INCLUDE_TESTS
)Build the libc++ tests.
- LIBCXX_INCLUDE_BENCHMARKS:BOOL¶
Default:
ON
Build the libc++ benchmark tests and the Google Benchmark library needed to support them.
- LIBCXX_BENCHMARK_TEST_ARGS:STRING¶
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 ofcheck-cxx-benchmarks
is to get test and sanitizer coverage, not to get accurate measurements.
- LIBCXX_BENCHMARK_NATIVE_STDLIB:STRING¶
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.
- LIBCXX_BENCHMARK_NATIVE_GCC_TOOLCHAIN:STRING¶
Use the specified GCC toolchain and standard library when building the native stdlib benchmark tests.
libc++ ABI Feature Options¶
The following options allow building libc++ for a different ABI version.
- LIBCXX_ABI_VERSION:STRING¶
Default:
1
Defines the target ABI version of libc++.
- LIBCXX_ABI_UNSTABLE:BOOL¶
Default:
OFF
Build the “unstable” ABI version of libc++. Includes all ABI changing features on top of the current stable version.
- LIBCXX_ABI_NAMESPACE:STRING¶
Default:
__n
wheren
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.
Warning
When providing a custom namespace, it’s the user’s 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]+
could cause conflicts with future versions of the library, and so should be avoided.
- LIBCXX_ABI_DEFINES:STRING¶
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¶
- LLVM_LIBDIR_SUFFIX:STRING¶
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
.
- LLVM_BUILD_32_BITS:BOOL¶
Build 32-bits executables and libraries on 64-bits systems. This option is available only on some 64-bits Unix systems. Defaults to OFF.
- LLVM_LIT_ARGS:STRING¶
Arguments given to lit.
make check
andmake 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 <...> search starts here:
/usr/include/c++/4.7
/usr/include/c++/4.7/x86_64-linux-gnu
/usr/include/c++/4.7/backward
/usr/lib/gcc/x86_64-linux-gnu/4.7/include
/usr/local/include
/usr/lib/gcc/x86_64-linux-gnu/4.7/include-fixed
/usr/include/x86_64-linux-gnu
/usr/include
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++ \
-DLIBCXX_CXX_ABI_INCLUDE_PATHS="/usr/include/c++/4.7/;/usr/include/c++/4.7/x86_64-linux-gnu/"
$ 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 \
-DLIBCXX_CXX_ABI_INCLUDE_PATHS=path/to/libcxxrt-sources/src
$ 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