# Clang 4 documentation

## 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.

## Introduction

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");

## Defining pragmas

Plugins can also define pragmas by declaring a `PragmaHandler` and registering it using `PragmaHandlerRegistry::Add<>`:

    // Define a pragma handler for #pragma example_pragma
    class ExamplePragmaHandler : public PragmaHandler {
    public:
      ExamplePragmaHandler() : PragmaHandler("example_pragma") { }
      void HandlePragma(Preprocessor &PP, PragmaIntroducerKind Introducer,
                        Token &PragmaTok) {
        // Handle the pragma
      }
    };

    static PragmaHandlerRegistry::Add<ExamplePragmaHandler> Y("example_pragma","example pragma 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

### Using the cc1 command line

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-<plugin-name>.

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 )
    $ clang++ -D_GNU_SOURCE -D_DEBUG -D__STDC_CONSTANT_MACROS \
              -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS -D_GNU_SOURCE \
              -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/PrintFunctionNames.so -Xclang \
              -plugin -Xclang print-fns

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

### Using the clang command line

Using -fplugin=plugin on the clang command line passes the plugin through as an argument to -load on the cc1 command line. If the plugin class implements the `getActionType` method then the plugin is run automatically. For example, to run the plugin automatically after the main AST action (i.e. the same as using -add-plugin):

    // Automatically run the plugin after the main AST action
    PluginASTAction::ActionType getActionType() override {
      return AddAfterMainAction;
    }