Testing libc++

Getting Started

libc++ uses LIT to configure and run its tests.

The primary way to run the libc++ tests is by using make check-cxx.

However since libc++ can be used in any number of possible configurations it is important to customize the way LIT builds and runs the tests. This guide provides information on how to use LIT directly to test libc++.

Please see the Lit Command Guide for more information about LIT.


After building libc++, you can run parts of the libc++ test suite by simply running llvm-lit on a specified test or directory. If you’re unsure whether the required libraries have been built, you can use the cxx-test-depends target. For example:

$ cd <monorepo-root>
$ make -C <build> cxx-test-depends # If you want to make sure the targets get rebuilt
$ <build>/bin/llvm-lit -sv libcxx/test/std/re # Run all of the std::regex tests
$ <build>/bin/llvm-lit -sv libcxx/test/std/depr/depr.c.headers/stdlib_h.pass.cpp # Run a single test
$ <build>/bin/llvm-lit -sv libcxx/test/std/atomics libcxx/test/std/threads # Test std::thread and std::atomic

In the default configuration, the tests are built against headers that form a fake installation root of libc++. This installation root has to be updated when changes are made to the headers, so you should re-run the cxx-test-depends target before running the tests manually with lit when you make any sort of change, including to the headers.

Sometimes you’ll want to change the way LIT is running the tests. Custom options can be specified using the –param=<name>=<val> flag. The most common option you’ll want to change is the standard dialect (ie -std=c++XX). By default the test suite will select the newest C++ dialect supported by the compiler and use that. However if you want to manually specify the option like so:

$ <build>/bin/llvm-lit -sv libcxx/test/std/containers # Run the tests with the newest -std
$ <build>/bin/llvm-lit -sv libcxx/test/std/containers --param=std=c++03 # Run the tests in C++03

Occasionally you’ll want to add extra compile or link flags when testing. You can do this as follows:

$ <build>/bin/llvm-lit -sv libcxx/test --param=compile_flags='-Wcustom-warning'
$ <build>/bin/llvm-lit -sv libcxx/test --param=link_flags='-L/custom/library/path'

Some other common examples include:

# Specify a custom compiler.
$ <build>/bin/llvm-lit -sv libcxx/test/std --param=cxx_under_test=/opt/bin/g++

# Disable warnings in the test suite
$ <build>/bin/llvm-lit -sv libcxx/test --param=enable_warnings=False

# Use UBSAN when running the tests.
$ <build>/bin/llvm-lit -sv libcxx/test --param=use_sanitizer=Undefined

Using a custom site configuration

By default, the libc++ test suite will use a site configuration that matches the current CMake configuration. It does so by generating a lit.site.cfg file in the build directory from one of the configuration file templates in libcxx/test/configs/, and pointing llvm-lit (which is a wrapper around llvm/utils/lit/lit.py) to that file. So when you’re running <build>/bin/llvm-lit, the generated lit.site.cfg file is always loaded instead of libcxx/test/lit.cfg.py. If you want to use a custom site configuration, simply point the CMake build to it using -DLIBCXX_TEST_CONFIG=<path-to-site-config>, and that site configuration will be used instead. That file can use CMake variables inside it to make configuration easier.

$ cmake <options> -DLIBCXX_TEST_CONFIG=<path-to-site-config>
$ make -C <build> cxx-test-depends
$ <build>/bin/llvm-lit -sv libcxx/test # will use your custom config file

LIT Options

lit [options…] [filenames…]

Command Line Options

To use these options you pass them on the LIT command line as --param NAME or --param NAME=VALUE. Some options have default values specified during CMake’s configuration. Passing the option on the command line will override the default.


Specify the compiler used to build the tests.

stdlib=<stdlib name>

Values: libc++, libstdc++, msvc

Specify the C++ standard library being tested. The default is libc++ if this option is not provided. This option is intended to allow running the libc++ test suite against other standard library implementations.

std=<standard version>

Values: c++03, c++11, c++14, c++17, c++20, c++2b

Change the standard version used when building the tests.


Specify the c++ standard library headers that are tested. By default the headers in the source tree are used.


Specify the directory of the libc++ library to be tested. By default the library folder of the build directory is used.


Specify the directory of the libc++ library to use at runtime. This directory is not added to the linkers search path. This can be used to compile tests against one version of libc++ and run them using another. The default value for this option is cxx_library_root.


Default: False

Enable or disable testing against the installed version of libc++ library. This impacts whether the use_system_cxx_lib Lit feature is defined or not. The cxx_library_root and cxx_runtime_root parameters should still be used to specify the path of the library to link to and run against, respectively.


Values: 0, 1

Enable the use of debug mode. Level 0 enables assertions and level 1 enables assertions and debugging of iterator misuse.

use_sanitizer=<sanitizer name>

Values: Memory, MemoryWithOrigins, Address, Undefined

Run the tests using the given sanitizer. If LLVM_USE_SANITIZER was given when building libc++ then that sanitizer will be used by default.


Enable the use of LLVM unwinder instead of libgcc.


Path to the builtins library to use instead of libgcc.

Writing Tests

When writing tests for the libc++ test suite, you should follow a few guidelines. This will ensure that your tests can run on a wide variety of hardware and under a wide variety of configurations. We have several unusual configurations such as building the tests on one host but running them on a different host, which add a few requirements to the test suite. Here’s some stuff you should know:

  • All tests are run in a temporary directory that is unique to that test and cleaned up after the test is done.

  • When a test needs data files as inputs, these data files can be saved in the repository (when reasonable) and referenced by the test as // FILE_DEPENDENCIES: <path-to-dependencies>. Copies of these files or directories will be made available to the test in the temporary directory where it is run.

  • You should never hardcode a path from the build-host in a test, because that path will not necessarily be available on the host where the tests are run.

  • You should try to reduce the runtime dependencies of each test to the minimum. For example, requiring Python to run a test is bad, since Python is not necessarily available on all devices we may want to run the tests on (even though supporting Python is probably trivial for the build-host).


Libc++ contains benchmark tests separately from the test of the test suite. The benchmarks are written using the Google Benchmark library, a copy of which is stored in the libc++ repository.

For more information about using the Google Benchmark library see the official documentation.

Building Benchmarks

The benchmark tests are not built by default. The benchmarks can be built using the cxx-benchmarks target.

An example build would look like:

$ cd build
$ ninja cxx-benchmarks

This will build all of the benchmarks under <libcxx-src>/benchmarks to be built against the just-built libc++. The compiled tests are output into build/projects/libcxx/benchmarks.

The benchmarks can also be built against the platforms native standard library using the -DLIBCXX_BUILD_BENCHMARKS_NATIVE_STDLIB=ON CMake option. This is useful for comparing the performance of libc++ to other standard libraries. The compiled benchmarks are named <test>.libcxx.out if they test libc++ and <test>.native.out otherwise.

Also See:

Running Benchmarks

The benchmarks must be run manually by the user. Currently there is no way to run them as part of the build.

For example:

$ cd build/projects/libcxx/benchmarks
$ ./algorithms.libcxx.out # Runs all the benchmarks
$ ./algorithms.libcxx.out --benchmark_filter=BM_Sort.* # Only runs the sort benchmarks

For more information about running benchmarks see Google Benchmark.