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");

Putting it all together

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 checked in version of PrintFunctionNames.cpp.

#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<NamedDecl>(D))
        llvm::errs() << "top-level-decl: \"" << ND->getNameAsString() << "\"\n";
    }

    return true;
  }
};

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

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

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

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

};

}

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

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-<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