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-libcxx. 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.

Setting up the Environment

After building libc++ you must setup your environment to test libc++ using LIT.

  1. Create a shortcut to the actual lit executable so that you can invoke it easily from the command line.

    $ alias lit='python path/to/llvm/utils/lit/lit.py'
  2. Tell LIT where to find your build configuration.

    $ export LIBCXX_SITE_CONFIG=path/to/build-libcxx/test/lit.site.cfg

Example Usage

Once you have your environment set up and you have built libc++ you can run parts of the libc++ test suite by simply running lit on a specified test or directory. For example:

$ cd path/to/src/libcxx
$ lit -sv test/std/re # Run all of the std::regex tests
$ lit -sv test/std/depr/depr.c.headers/stdlib_h.pass.cpp # Run a single test
$ lit -sv test/std/atomics test/std/threads # Test std::thread and std::atomic

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:

$ lit -sv test/std/containers # Run the tests with the newest -std
$ lit -sv --param=std=c++03 test/std/containers # 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:

$ lit -sv --param=compile_flags='-Wcustom-warning'
$ lit -sv --param=link_flags='-L/custom/library/path'

Some other common examples include:

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

# Enable warnings in the test suite
$ lit -sv --param=enable_warnings=true test/std

# Use UBSAN when running the tests.
$ lit -sv --param=use_sanitizer=Undefined

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.

cxx_stdlib_under_test=<stdlib name>

Values: libc++, libstdc++

Specify the C++ standard library being tested. Unless otherwise specified libc++ is used. This option is intended to allow running the libc++ test suite against other standard library implementations.

std=<standard version>

Values: c++98, c++03, c++11, c++14, c++17, c++2a

Change the standard version used when building the tests.


Specify the site configuration to use when running the tests. This option overrides the environment variable LIBCXX_SITE_CONFIG.


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. This option cannot be used when use_system_cxx_lib is provided.


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. Note: This does not use the installed headers.


Enable or disable the use of LIT’s internal shell in ShTests. If the environment variable LIT_USE_INTERNAL_SHELL is present then that is used as the default value. Otherwise the default value is True on Windows and False on every other platform.


Specify additional compile flags as a space delimited string. Note: This options should not be used to change the standard version used.

Specify additional link flags as a space delimited string.


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 colorized compile diagnostics. If the color_diagnostics option is specified or the environment variable LIBCXX_COLOR_DIAGNOSTICS is present then color diagnostics will be enabled.

Environment Variables


Specify the site configuration to use when running the tests. Also see libcxx_site_config.


If LIBCXX_COLOR_DIAGNOSTICS is defined then the test suite will attempt to use color diagnostic outputs from the compiler. Also see color_diagnostics.


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
$ cmake [options] <path to libcxx sources>
$ make 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/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/benchmarks
$ make cxx-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.