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In the parent commit, I forgot to update the build so that include-what-you-use
links with all targets, which led to link failures for builds against LLVM build
trees (as opposed to the Debian packages).

While troubleshooting this, I found a patch by @Romain-Geissler-1A that I had
previously misunderstood/overlooked:
#854 (comment)

Borrowing the link dependencies from that patch to complete my accidental
plagiarism fixes the build again.

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Include What You Use


For more in-depth documentation, see docs.

Instructions for Users

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


This is alpha quality software -- at best (as of July 2018). It was originally 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.

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 test 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.

The IWYU master branch follows Clang main branch.

We also have convenience tags and branches for released versions of Clang (called clang_<version>, e.g. clang_5.0). To build against a Clang release, check out the corresponding branch in IWYU before configuring the build. More details in the instructions below.

We assume you already have compiled LLVM and Clang libraries on your system, either via packages for your platform or built from source. You can use this mapping table to combine Clang and IWYU versions correctly:

Clang IWYU version IWYU branch
3.6 0.4 clang_3.6
3.7 0.5 clang_3.7
3.8 0.6 clang_3.8
3.9 0.7 clang_3.9
4.0 0.8 clang_4.0-r2
5.0 0.9 clang_5.0
6 0.10 clang_6.0
7 0.11 clang_7.0
8 0.12 clang_8.0
9 0.13 clang_9.0
10 0.14 clang_10
11 0.15 clang_11
12 0.16 clang_12
... ... ...
main master

NOTE: If you use the Debian/Ubuntu packaging available from, you'll need the following packages installed:

  • llvm-<version>-dev
  • libclang-<version>-dev
  • clang-<version>

Packaging for other platforms will likely be subtly different.

To set up an environment for building:

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

  • Clone the IWYU Git repo:

    iwyu$ git clone
  • 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 6.0 installed, use the clang_6.0 branch. IWYU master tracks LLVM & Clang main:

    iwyu$ cd include-what-you-use
    iwyu/include-what-you-use$ git checkout clang_6.0
  • Create a build root and use CMake to generate a build system linked with LLVM/Clang prebuilts:

    # This example uses the Makefile generator, but anything should work.
    iwyu/include-what-you-use$ cd ..
    iwyu$ mkdir build && cd build
    # For IWYU 0.10/Clang 6 and earlier
    iwyu/build$ cmake -G "Unix Makefiles" -DIWYU_LLVM_ROOT_PATH=/usr/lib/llvm-6.0 ../include-what-you-use
    # For IWYU 0.11/Clang 7 and later
    iwyu/build$ cmake -G "Unix Makefiles" -DCMAKE_PREFIX_PATH=/usr/lib/llvm-7 ../include-what-you-use

    (substitute the llvm-6.0 or llvm-7 suffixes with the actual version compatible with your IWYU branch)

    or, if you have a local LLVM and Clang build tree, you can specify that as CMAKE_PREFIX_PATH for IWYU 0.11 and later:

    iwyu/build$ cmake -G "Unix Makefiles" -DCMAKE_PREFIX_PATH=~/llvm-project/build ../include-what-you-use
  • Once CMake has generated a build system, you can invoke it directly from build, e.g.

    iwyu/build$ make

Instructions for building Clang are available at

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.

Note that some distributions/packages may have different defaults, you can use clang -print-resource-dir to find the base path of the built-in headers on your system.

So for IWYU to function correctly, you need to copy the Clang include directory to the expected location before running (similarly, use include-what-you-use -print-resource-dir to learn exactly where IWYU wants the headers).

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


  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;-Xiwyu;any;-Xiwyu;iwyu;-Xiwyu;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;-Xiwyu;any;-Xiwyu;iwyu;-Xiwyu;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 script pre-dates 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 ... -p .

or, on Windows systems:

  mkdir build && cd build
  python -p .

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

See --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 < /tmp/iwyu.out

If you don't like the way munges your #include lines, you can control its behavior via flags. --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 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 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 has wrongly added or removed a forward-declare, just fix it up manually.
  • If 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.