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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 run clang's SyntaxOnlyAction, which runs a quick syntax
check, as part of a clang plugin.
</p>
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<h2 id="pluginactions">Writing a PluginASTAction</h2>
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<p>The main difference when writing PluginASTActions 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 &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;
}
</pre>
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<h2 id="registerplugin">Registering a plugin</h2>
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<p>A plugin is loaded as dynamic library at runtime by the compiler. To register
a plugin in a library, use FrontendPluginRegistry::Add:</p>
<pre>
static FrontendPluginRegistry::Add<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?revision=155629&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<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");
</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-<plugin-name>.</p>
<p>Note that those options must be 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 && 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?revision=146845&view=markup">README</a></p>
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