Include What You Use

For more in-depth documentation, see http://github.com/include-what-you-use/include-what-you-use/tree/master/docs.

Instructions for Users

"Include what you use" means this: for every symbol (type, function, variable, or macro) that you use in foo.cc (or foo.cpp), either foo.cc or foo.h should include a .h file that exports the declaration of that symbol. (Similarly, for foo_test.cc, either foo_test.cc or foo.h should do the including.) Obviously symbols defined in foo.cc itself are excluded from this requirement.

This puts us in a state where every file includes the headers it needs to declare the symbols that it uses. When every file includes what it uses, then it is possible to edit any file and remove unused headers, without fear of accidentally breaking the upwards dependencies of that file. It also becomes easy to automatically track and update dependencies in the source code.

CAVEAT

This is alpha quality software -- at best (as of February 2011). It was written to work specifically in the Google source tree, and may make assumptions, or have gaps, that are immediately and embarrassingly evident in other types of code. For instance, we only run this on C++ code, not C or Objective C. Even for Google code, the tool still makes a lot of mistakes.

While we work to get IWYU quality up, we will be stinting new features, and will prioritize reported bugs along with the many existing, known bugs. The best chance of getting a problem fixed is to submit a patch that fixes it (along with a unittest case that verifies the fix)!

How to Build

Include-what-you-use makes heavy use of Clang internals, and will occasionally break when Clang is updated. Usually such discrepancies are detected by build bot and fixed promptly.

We support two build configurations: out-of-tree and in-tree.

Building out-of-tree

In an out-of-tree configuration, we assume you already have compiled LLVM and Clang headers and libs somewhere on your filesystem, such as via the libclang-dev package.

• Create a directory for IWYU development, e.g. iwyu-trunk

• Clone the IWYU Git repo:

  iwyu-trunk$ git clone https://github.com/include-what-you-use/include-what-you-use.git
  

• Presumably, you'll be building IWYU with a released version of LLVM and Clang, so check out the corresponding branch. For example if you have Clang 3.2 installed, use the clang_3.2 branch. IWYU master tracks LLVM & Clang trunk:

  iwyu-trunk$ cd include-what-you-use
  iwyu-trunk/include-what-you-use$ git checkout clang_3.2
  iwyu-trunk/include-what-you-use$ cd ..
  

• Create a build root and use CMake to generate a build system linked with LLVM/Clang prebuilts. Note that CMake settings need to match exactly, so you may need to add -DCMAKE_BUILD_TYPE=Release or more to the command-line below:

  # This example uses the Makefile generator, but anything should work.
  iwyu-trunk$ mkdir build && cd build
  iwyu-trunk/build$ cmake -G "Unix Makefiles" -DIWYU_LLVM_ROOT_PATH=/usr/lib/llvm-3.4 ../include-what-you-use
  

• Once CMake has generated a build system, you can invoke it directly from build, e.g.

  iwyu-trunk/build$ make
  

This configuration is more useful if you want to get IWYU up and running quickly without building Clang and LLVM from scratch.

Building in-tree

You will need the Clang and LLVM trees on your system, such as by checking out their SVN trees (but don't configure or build before you've done the following.)

• Clone the IWYU Git repo into the Clang source tree:

  llvm/tools/clang/tools$ git clone https://github.com/include-what-you-use/include-what-you-use.git
  

• Edit tools/clang/tools/CMakeLists.txt and put in add_subdirectory(include-what-you-use)

• Once this is done, IWYU is recognized and picked up by CMake workflow as described in the Clang Getting Started guide

This configuration is more useful if you're actively developing IWYU against Clang trunk. It's easier to set up correctly, but it requires that you build all of LLVM and Clang.

How to Install

If you're building IWYU out-of-tree or installing pre-built binaries, you need to make sure it can find Clang built-in headers (stdarg.h and friends.)

Clang's default policy is to look in path/to/clang-executable/../lib/clang/<clang ver>/include. So if Clang 3.5.0 is installed in /usr/bin, it will search for built-ins in /usr/lib/clang/3.5.0/include.

Clang tools have the same policy by default, so in order for IWYU to analyze any non-trivial code, it needs to find Clang's built-ins in path/to/iwyu/../lib/clang/3.5.0/include where 3.5.0 is a stand-in for the version of Clang your IWYU was built against.

So for IWYU to function correctly, you need to copy in the Clang headers at a good location before running.

This weirdness is tracked in issue 100, hopefully we can make this more transparent over time.

How to Run

The original design was built for Make, but a number of alternative run modes have come up over the years.

Plugging into Make

The easiest way to run IWYU over your codebase is to run

  make -k CXX=/path/to/llvm/Debug+Asserts/bin/include-what-you-use

or

  make -k CXX=/path/to/llvm/Release/bin/include-what-you-use

(include-what-you-use always exits with an error code, so the build system knows it didn't build a .o file. Hence the need for -k.)

Include-what-you-use only analyzes .cc (or .cpp) files built by make, along with their corresponding .h files. If your project has a .h file with no corresponding .cc file, IWYU will ignore it unless you use the --check_also switch to add it for analysis together with a .cc file.

Using with CMake

CMake has grown native support for IWYU as of version 3.3. See their documentation for CMake-side details.

The CMAKE_CXX_INCLUDE_WHAT_YOU_USE option enables a mode where CMake first compiles a source file, and then runs IWYU on it.

Use it like this:

  mkdir build && cd build
  CC="clang" CXX="clang++" cmake -DCMAKE_CXX_INCLUDE_WHAT_YOU_USE="path/to/iwyu any iwyu args" ...

or, on Windows systems:

  mkdir build && cd build
  cmake -DCMAKE_CXX_COMPILER="%VCINSTALLDIR%/bin/cl.exe" -DCMAKE_CXX_INCLUDE_WHAT_YOU_USE="path/to/iwyu any iwyu args" -G Ninja ...

The option appears to be separately supported for both C and C++, so use CMAKE_C_INCLUDE_WHAT_YOU_USE for C code.

Note that with Microsoft's Visual C++ compiler, IWYU needs the --driver-mode=cl argument to understand the MSVC options from CMake.

Using with a compilation database

The iwyu_tool.py script predates the native CMake support, and works off the compilation database format. For example, CMake generates such a database named compile_commands.json with the CMAKE_EXPORT_COMPILE_COMMANDS option enabled.

The script's command-line syntax is designed to mimic Clang's LibTooling, but they are otherwise unrelated. It can be used like this:

  mkdir build && cd build
  CC="clang" CXX="clang++" cmake -DCMAKE_EXPORT_COMPILE_COMMANDS=ON ...
  iwyu_tool.py -p .

or, on Windows systems:

  mkdir build && cd build
  cmake -DCMAKE_CXX_COMPILER="%VCINSTALLDIR%/bin/cl.exe" -DCMAKE_C_COMPILER="%VCINSTALLDIR%/VC/bin/cl.exe" -DCMAKE_EXPORT_COMPILE_COMMANDS=ON -G Ninja ...
  python iwyu_tool.py -p .

Unless a source filename is provided, all files in the project will be analyzed.

See iwyu_tool.py --help for more options.

Applying fixes

We also include a tool that automatically fixes up your source files based on the IWYU recommendations. This is also alpha-quality software! Here's how to use it (requires python):

  make -k CXX=/path/to/llvm/Debug+Asserts/bin/include-what-you-use 2> /tmp/iwyu.out
  python fix_includes.py < /tmp/iwyu.out

If you don't like the way fix_includes.py munges your #include lines, you can control its behavior via flags. fix_includes.py --help will give a full list, but these are some common ones:

• -b: Put blank lines between system and Google includes
• --nocomments: Don't add the 'why' comments next to includes

How to Correct IWYU Mistakes

• If fix_includes.py has removed an #include you actually need, add it back in with the comment '// IWYU pragma: keep' at the end of the #include line. Note that the comment is case-sensitive.
• If fix_includes.py has added an #include you don't need, just take it out. We hope to come up with a more permanent way of fixing later.
• If fix_includes.py has wrongly added or removed a forward-declare, just fix it up manually.
• If fix_includes.py has suggested a private header file (such as <bits/stl_vector.h>) instead of the proper public header file (<vector>), you can fix this by inserting a specially crafted comment near top of the private file (assuming you can write to it): '// IWYU pragma: private, include "the/public/file.h"'.

Current IWYU pragmas are described in IWYUPragmas.