=============
Clang Plugins
=============

NOTE: this document applies to the original Clang project, not the DirectX
Compiler. It's made available for informational purposes only.

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");

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

Running the plugin
==================

To run a plugin, the dynamic library containing the plugin registry must be
loaded via the :option:`-load` command line option. This will load all plugins
that are registered, and you can select the plugins to run by specifying the
:option:`-plugin` option. Additional parameters for the plugins can be passed with
:option:`-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 :option:`-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
  :option:`-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 <http://llvm.org/viewvc/llvm-project/cfe/trunk/examples/PrintFunctionNames/README.txt?view=markup>`_