Clang 3.8 documentation

Clang Plugins

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Clang Plugins

Clang Plugins make it possible to run extra user defined actions during a compilation. This document will provide a basic walkthrough of how to write and run a Clang Plugin.


Clang Plugins run FrontendActions over code. See the FrontendAction tutorial on how to write a FrontendAction using the RecursiveASTVisitor. In this tutorial, we’ll demonstrate how to write a simple clang plugin.

Writing a PluginASTAction

The main difference from writing normal FrontendActions is that you can handle plugin command line options. The PluginASTAction base class declares a ParseArgs method which you have to implement in your plugin.

bool ParseArgs(const CompilerInstance &CI,
               const std::vector<std::string>& args) {
  for (unsigned i = 0, e = args.size(); i != e; ++i) {
    if (args[i] == "-some-arg") {
      // Handle the command line argument.
  return true;

Registering a plugin

A plugin is loaded from a dynamic library at runtime by the compiler. To register a plugin in a library, use FrontendPluginRegistry::Add<>:

static FrontendPluginRegistry::Add<MyPlugin> X("my-plugin-name", "my plugin description");

Putting it all together

Let’s look at an example plugin that prints top-level function names. This example is checked into the clang repository; please take a look at the latest version of PrintFunctionNames.cpp.

Running the plugin

To run a plugin, the dynamic library containing the plugin registry must be loaded via the -load command line option. This will load all plugins that are registered, and you can select the plugins to run by specifying the -plugin option. Additional parameters for the plugins can be passed with -plugin-arg-.

Note that those options must reach clang’s cc1 process. There are two ways to do so:

  • Directly call the parsing process by using the -cc1 option; this has the downside of not configuring the default header search paths, so you’ll need to specify the full system path configuration on the command line.
  • Use clang as usual, but prefix all arguments to the cc1 process with -Xclang.

For example, to run the print-function-names plugin over a source file in clang, first build the plugin, and then call clang with the plugin from the source tree:

$ export BD=/path/to/build/directory
$ (cd $BD && make PrintFunctionNames )
          -I$BD/tools/clang/include -Itools/clang/include -I$BD/include -Iinclude \
          tools/clang/tools/clang-check/ClangCheck.cpp -fsyntax-only \
          -Xclang -load -Xclang $BD/lib/ -Xclang \
          -plugin -Xclang print-fns

Also see the print-function-name plugin example’s README

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