=============
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 :doc:`FrontendAction
tutorial <RAVFrontendAction>` 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.

.. code-block:: c++

  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<>``:

.. code-block:: c++

  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<>``:

.. code-block:: c++

  // 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
<https://llvm.org/viewvc/llvm-project/cfe/trunk/examples/PrintFunctionNames/PrintFunctionNames.cpp?view=markup>`_.

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:

.. code-block:: console

  $ 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 <https://llvm.org/viewvc/llvm-project/cfe/trunk/examples/PrintFunctionNames/README.txt?view=markup>`_


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`):

.. code-block:: c++

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