Notes about Using IWYU on macOS

I have recently found IWYU, a very useful tool to identify whether you have included header files correctly. It can be cleanly installed in Ubuntu by apt, though some configuration is needed to make it identify problems more correctly, i.e., let it know that a header file is private and we should use a public header file that includes it, or a symbol should be defined in a certain header file and we should not care where it is really defined in the implementation.

I did encounter some problems in macOS. I installed it, but it had problems with Xcode header files, causing something like “fatal error: ‘stdarg.h’ file not found” when used. A quick search showed that it was a known problem, and people mentioned that it seemed a problem that deteriorated with more recent versions of LLVM, which IWYU used internally.

I happened to have LLVM 7.0 installed from Homebrew, so I had a try. Here are the simple steps:

  1. Make sure you have llvm@7. If not, ‘brew install llvm@7’ would do.
  2. Check out IWYU to a directory.
  3. Execute ‘git checkout clang_7.0’ inside the IWYU directory to choose the Clang 7 branch.
  4. Execute ‘mkdir build && cd build’ to use a build directory.
  5. Execute ‘CC=/usr/local/opt/llvm@7/bin/clang CXX=/usr/local/opt/llvm@7/bin/clang++ cmake -DCMAKE_PREFIX_PATH=/usr/local/opt/llvm@7 ..’ to configure IWYU to use the Homebrew LLVM 7.0.
  6. Execute ‘make’ to build IWYU.
  7. Execute ‘mkdir -k lib/clang/7.1.0 && ln -s /usr/local/opt/llvm@7/lib/clang/7.1.0/include lib/clang/7.1.0/’ to symlink the Clang 7 include directory inside IWYU. This step is critical to solve the “file not found” problem, but regretfully it does not work with a more recent LLVM version like LLVM 11.
  8. Symlink executables to your bin directory for quick access. Something like:
cd ~/bin
ln -s ~/code/include-what-you-use/build/bin/include-what-you-use .
ln -s ~/code/include-what-you-use/iwyu_tool.py iwyu_tool
ln -s include-what-you-use iwyu

Now IWYU is ready to use.

Please be aware that IWYU does not often work out of the box, and some configuration is needed. The key document to read is IWYU Mappings, and the bundled mapping files (.imp) can be good examples. You probably want to use libcxx.imp as a start. Some mappings are already included by default, and you can find them in the file iwyu_include_picker.cc.

While it is not perfect, it did help me identify many inclusion issues. This commit is a result of using IWYU.

Happy hacking!

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