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 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 experimental software, as of June 2024. 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)!

Clang compatibility

Include-what-you-use makes heavy use of Clang internals, and will occasionally break when Clang is updated. We build IWYU regularly against Clang mainline to detect and fix such compatibility breaks as soon as possible.

NOTE: 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. 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`
13	0.17	`clang_13`
14	0.18	`clang_14`
15	0.19	`clang_15`
16	0.20	`clang_16`
17	0.21	`clang_17`
18	0.22	`clang_18`
19	0.23	`clang_19`
20	0.24	`clang_20`
...	...	...
main		`master`

NOTE: If you use the Debian/Ubuntu packaging available from https://apt.llvm.org, you'll need the following packages installed:

llvm-<version>-dev

libclang-<version>-dev

clang-<version>

Packaging for other platforms will likely be subtly different.

How to build standalone

This build mode assumes you already have compiled LLVM and Clang libraries on your system, either via packages for your platform or built from source. To set up an environment for building IWYU:

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

Clone the IWYU Git repo:

iwyu$ 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 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
```

How to build as part of LLVM

Instructions for building LLVM and Clang are available at https://clang.llvm.org/get_started.html.

To include IWYU in the LLVM build, use the LLVM_EXTERNAL_PROJECTS and LLVM_EXTERNAL_*_SOURCE_DIR CMake variables when configuring LLVM:

llvm-project/build$ cmake -G "Unix Makefiles" -DLLVM_ENABLE_PROJECTS=clang \
    -DLLVM_EXTERNAL_PROJECTS=iwyu -DLLVM_EXTERNAL_IWYU_SOURCE_DIR=/path/to/iwyu\
    /path/to/llvm-project/llvm
llvm-project/build$ make

This builds all of LLVM, Clang and IWYU in a single tree.

How to install

To install and use a pre-built IWYU, besides any dynamic library dependencies, you need to make sure it can find the Clang built-in headers (stdarg.h and friends).

This is a surprisingly complex problem, so it helps to first understand how Clang locates the built-in headers.

The built-in headers live in what Clang calls the resource directory, which contains various runtime resources for the compiler. The resource dir is configurable at Clang build time, using the CLANG_RESOURCE_DIR CMake variable. CLANG_RESOURCE_DIR is always a relative path, so the effective absolute path can be computed at runtime relative to the clang executable.

The Clang build sets up a resource dir in the build tree and copies the relevant resources there (the built-in headers among them), so it's possible to run clang directly from the build tree. Furthermore, the Clang install target will copy the resource dir to the install tree.

The Clang Driver library is responsible for computing the effective path at runtime. It will look up the current executable path (typically clang), strip off the filename and append CLANG_RESOURCE_DIR to form the resource dir path. You can use clang -print-resource-dir to print the effective resource dir for a particular Clang tree.

Phew! What does this mean for IWYU?

IWYU links to the Clang Driver library, and so would nominally get the exact same policy by default: CLANG_RESOURCE_DIR relative to the include-what-you-use executable. This means the IWYU build would have to create the resource dir in its build tree, and also make sure it's available in the install tree, using a custom install target. But CLANG_RESOURCE_DIR is not exported from the Clang CMake system, so it's not possible to know at build-time where the resources need to be.

Since Clang has all the knowledge about which resources need to go into the resource dir, and also decides under the covers where it has to be, it's difficult for IWYU to make any principled decisions. We side-step this conflict by exposing our own set of CMake variables for the resource dir:

IWYU_RESOURCE_DIR: same semantics as Clang's CLANG_RESOURCE_DIR
IWYU_RESOURCE_RELATIVE_TO: which executable to serve as the anchor path for the resource directory (clang or iwyu)

First, IWYU_RESOURCE_DIR exists to supplement CLANG_RESOURCE_DIR. Packagers for a platform where Clang has a custom CLANG_RESOURCE_DIR can repeat the same customization for IWYU. By default it will use the same default pattern as Clang, i.e. ../lib/clang/<clang-version>.

Second, we can use IWYU_RESOURCE_RELATIVE_TO to decide which executable path to use as the anchor for the relative IWYU_RESOURCE_DIR.

If it is clang, we resolve the path to the clang executable at configure- time, and bake that absolute path into include-what-you-use.

If it is iwyu, the include-what-you-use executable resolves its own path at runtime.

That means packagers can easily build:

an include-what-you-use that has a package dependency on Clang, and relies entirely on its resource dir (-DIWYU_RESOURCE_RELATIVE_TO=clang)
an include-what-you-use that has a package dependency on a Clang with a custom resource dir (-DIWYU_RESOURCE_RELATIVE_TO=clang -DIWYU_RESOURCE_DIR=../what/clang/said)
an include-what-you-use that can be installed separate from Clang in its own prefix (-DIWYU_RESOURCE_RELATIVE_TO=iwyu -DCMAKE_INSTALL_PREFIX=/usr/local), assuming a custom install step to also copy the built-in headers to the default IWYU_RESOURCE_DIR in the same prefix.
an include-what-you-use that can be installed separate from Clang in an arbitrary prefix with a custom resource dir (-DIWYU_RESOURCE_RELATIVE_TO=iwyu -DIWYU_RESOURCE_DIR=../share/include-what-you-use), assuming a custom install step to also copy the built-in headers to the custom IWYU_RESOURCE_DIR.

IWYU uses IWYU_RESOURCE_RELATIVE_TO=clang by default, because that produces a runnable include-what-you-use in the build tree, which depends directly on the Clang package it was configured for with -DCMAKE_PREFIX_PATH. It's also suitable for packaging, in the sense that the IWYU package can be made to depend on the Clang package, and will then automatically use the Clang resource dir on the target system.

IWYU_RESOURCE_RELATIVE_TO=iwyu is more suitable to build a fully independent IWYU package, but also requires some custom logic outside the IWYU build to package and install relevant parts of the resource dir from Clang in a suitable location.

Use include-what-you-use -print-resource-dir to learn exactly where IWYU expects the resource dir to be installed.

How to run

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

Running on single source file

The simplest way to use IWYU is to run it against a single source file:

include-what-you-use $CXXFLAGS myfile.cc

where $CXXFLAGS are the flags you would normally pass to the compiler.

Plugging into existing build system

Typically there is already a build system containing the relevant compiler flags for all source files. Replace your compiler with include-what-you-use to generate a large batch of IWYU advice. Depending on your build system/build tools, this can take many forms, but for a simple GNU Make system it might look like this:

make -k CXX=include-what-you-use CXXFLAGS="-Xiwyu --error_always"

(The additional -Xiwyu --error_always switch makes include-what-you-use always exit with an error code, so the build system knows it didn't build a .o file. Hence the need for -k.)

In this mode include-what-you-use only analyzes the .cc (or .cpp) files known to your build system, 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. It is possible to run IWYU against individual header files, provided the compiler flags are carefully constructed to match all includers.

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=include-what-you-use ...

or, on Windows systems:

mkdir build && cd build
cmake -DCMAKE_CXX_COMPILER="%VCINSTALLDIR%/bin/cl.exe" -DCMAKE_CXX_INCLUDE_WHAT_YOU_USE=include-what-you-use -G Ninja ...

These examples assume that include-what-you-use is in the PATH. If it isn't, consider changing the value to an absolute path. Arguments to IWYU can be added using CMake's semicolon-separated list syntax, e.g.:

... cmake -DCMAKE_CXX_INCLUDE_WHAT_YOU_USE="include-what-you-use;-w;-Xiwyu;--verbose=7" ...

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 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 ...
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 ...
python3 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 python3):

make -k CXX=include-what-you-use CXXFLAGS="-Xiwyu --error_always" 2> /tmp/iwyu.out
python3 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.

More questions?

See our FAQ for longer-form Q&A.

License

EpicGames/include-what-you-use

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