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<h1>Clang Plugins</h1>
<p>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.</p>

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<h2 id="intro">Introduction</h2>
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<p>Clang Plugins run FrontendActions over code. See the
<a href="RAVFrontendAction.html">FrontendAction tutorial</a> on how to write a
FrontendAction using the RecursiveASTVisitor. In this tutorial, we'll
demonstrate how to write a simple clang plugin.
</p>

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<h2 id="pluginactions">Writing a PluginASTAction</h2>
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<p>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.
</p>
<pre>
  bool ParseArgs(const CompilerInstance &amp;CI,
                 const std::vector&lt;std::string>&amp; args) {
    for (unsigned i = 0, e = args.size(); i != e; ++i) {
      if (args[i] == "-some-arg") {
        // Handle the command line argument.
      }
    }
    return true;
  }
</pre>

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<h2 id="registerplugin">Registering a plugin</h2>
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<p>A plugin is loaded from a dynamic library at runtime by the compiler. To register
a plugin in a library, use FrontendPluginRegistry::Add:</p>
<pre>
  static FrontendPluginRegistry::Add&lt;MyPlugin> X("my-plugin-name", "my plugin description");
</pre>

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<h2 id="example">Putting it all together</h2>
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<p>Let's look at an example plugin that prints top-level function names.
This example is also checked into the clang repository; please also take a look
at the latest <a href="http://llvm.org/viewvc/llvm-project/cfe/trunk/examples/PrintFunctionNames/PrintFunctionNames.cpp?view=markup">checked in version of PrintFunctionNames.cpp</a>.</p>
<pre>
#include "clang/Frontend/FrontendPluginRegistry.h"
#include "clang/AST/ASTConsumer.h"
#include "clang/AST/AST.h"
#include "clang/Frontend/CompilerInstance.h"
#include "llvm/Support/raw_ostream.h"
using namespace clang;

namespace {

class PrintFunctionsConsumer : public ASTConsumer {
public:
  virtual bool HandleTopLevelDecl(DeclGroupRef DG) {
    for (DeclGroupRef::iterator i = DG.begin(), e = DG.end(); i != e; ++i) {
      const Decl *D = *i;
      if (const NamedDecl *ND = dyn_cast&lt;NamedDecl>(D))
        llvm::errs() &lt;&lt; "top-level-decl: \"" &lt;&lt; ND->getNameAsString() &lt;&lt; "\"\n";
    }

    return true;
  }
};

class PrintFunctionNamesAction : public PluginASTAction {
protected:
  ASTConsumer *CreateASTConsumer(CompilerInstance &amp;CI, llvm::StringRef) {
    return new PrintFunctionsConsumer();
  }

  bool ParseArgs(const CompilerInstance &amp;CI,
                 const std::vector&lt;std::string>&amp; args) {
    for (unsigned i = 0, e = args.size(); i != e; ++i) {
      llvm::errs() &lt;&lt; "PrintFunctionNames arg = " &lt;&lt; args[i] &lt;&lt; "\n";

      // Example error handling.
      if (args[i] == "-an-error") {
        DiagnosticsEngine &amp;D = CI.getDiagnostics();
        unsigned DiagID = D.getCustomDiagID(
          DiagnosticsEngine::Error, "invalid argument '" + args[i] + "'");
        D.Report(DiagID);
        return false;
      }
    }
    if (args.size() &amp;&amp; args[0] == "help")
      PrintHelp(llvm::errs());

    return true;
  }
  void PrintHelp(llvm::raw_ostream&amp; ros) {
    ros &lt;&lt; "Help for PrintFunctionNames plugin goes here\n";
  }

};

}

static FrontendPluginRegistry::Add&lt;PrintFunctionNamesAction>
X("print-fns", "print function names");
</pre>

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<h2 id="running">Running the plugin</h2>
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<p>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-&lt;plugin-name>.</p>

<p>Note that those options must reach clang's cc1 process. There are two
ways to do so:</p>
<ul>
<li>
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.
</li>
<li>
Use clang as usual, but prefix all arguments to the cc1 process with -Xclang.
</li>
</ul>
<p>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:</p>
<pre>
  $ export BD=/path/to/build/directory
  $ (cd $BD &amp;&amp; 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
</pre>

<p>Also see the print-function-name plugin example's
<a href="http://llvm.org/viewvc/llvm-project/cfe/trunk/examples/PrintFunctionNames/README.txt?view=markup">README</a></p>


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