LibTooling

LibTooling is a library to support writing standalone tools based on Clang. This document will provide a basic walkthrough of how to write a tool using LibTooling.

For the information on how to setup Clang Tooling for LLVM see {doc}HowToSetupToolingForLLVM

Introduction

Tools built with LibTooling, like Clang Plugins, run FrontendActions over code.

% See FIXME for a tutorial on how to write FrontendActions.

In this tutorial, we‘ll demonstrate the different ways of running Clang’s SyntaxOnlyAction, which runs a quick syntax check, over a bunch of code.

Parsing a code snippet in memory

If you ever wanted to run a FrontendAction over some sample code, for example to unit test parts of the Clang AST, runToolOnCode is what you looked for. Let me give you an example:

#include "clang/Tooling/Tooling.h"

TEST(runToolOnCode, CanSyntaxCheckCode) {
  // runToolOnCode returns whether the action was correctly run over the
  // given code.
  EXPECT_TRUE(runToolOnCode(std::make_unique<clang::SyntaxOnlyAction>(), "class X {};"));
}

Writing a standalone tool

Once you unit tested your FrontendAction to the point where it cannot possibly break, it‘s time to create a standalone tool. For a standalone tool to run clang, it first needs to figure out what command line arguments to use for a specified file. To that end we create a CompilationDatabase. There are different ways to create a compilation database, and we need to support all of them depending on command-line options. There’s the CommonOptionsParser class that takes the responsibility to parse command-line parameters related to compilation databases and inputs, so that all tools share the implementation.

Parsing common tools options

CompilationDatabase can be read from a build directory or the command line. Using CommonOptionsParser allows for explicit specification of a compile command line, specification of build path using the -p command-line option, and automatic location of the compilation database using source files paths.

#include "clang/Tooling/CommonOptionsParser.h"
#include "llvm/Support/CommandLine.h"

using namespace clang::tooling;
using namespace llvm;

// Apply a custom category to all command-line options so that they are the
// only ones displayed.
static cl::OptionCategory MyToolCategory("my-tool options");

int main(int argc, const char **argv) {
  // CommonOptionsParser::create will parse arguments and create a
  // CompilationDatabase.
  auto ExpectedParser = CommonOptionsParser::create(argc, argv, MyToolCategory);
  if (!ExpectedParser) {
    // Fail gracefully for unsupported options.
    llvm::errs() << toString(ExpectedParser.takeError());
    return 1;
  }
  CommonOptionsParser& OptionsParser = ExpectedParser.get();

  // Use OptionsParser.getCompilations() and OptionsParser.getSourcePathList()
  // to retrieve CompilationDatabase and the list of input file paths.
}

Creating and running a ClangTool

Once we have a CompilationDatabase, we can create a ClangTool and run our FrontendAction over some code. For example, to run the SyntaxOnlyAction over the files “a.cc” and “b.cc” one would write:

// A clang tool can run over a number of sources in the same process...
std::vector<std::string> Sources;
Sources.push_back("a.cc");
Sources.push_back("b.cc");

// We hand the CompilationDatabase we created and the sources to run over into
// the tool constructor.
ClangTool Tool(OptionsParser.getCompilations(), Sources);

// The ClangTool needs a new FrontendAction for each translation unit we run
// on.  Thus, it takes a FrontendActionFactory as parameter.  To create a
// FrontendActionFactory from a given FrontendAction type, we call
// newFrontendActionFactory<clang::SyntaxOnlyAction>().
int result = Tool.run(newFrontendActionFactory<clang::SyntaxOnlyAction>().get());

Putting it together --- the first tool

Now we combine the two previous steps into our first real tool. A more advanced version of this example tool is also checked into the clang tree at tools/clang-check/ClangCheck.cpp.

// Declares clang::SyntaxOnlyAction.
#include "clang/Frontend/FrontendActions.h"
#include "clang/Tooling/CommonOptionsParser.h"
#include "clang/Tooling/Tooling.h"
// Declares llvm::cl::extrahelp.
#include "llvm/Support/CommandLine.h"

using namespace clang::tooling;
using namespace llvm;

// Apply a custom category to all command-line options so that they are the
// only ones displayed.
static cl::OptionCategory MyToolCategory("my-tool options");

// CommonOptionsParser declares HelpMessage with a description of the common
// command-line options related to the compilation database and input files.
// It's nice to have this help message in all tools.
static cl::extrahelp CommonHelp(CommonOptionsParser::HelpMessage);

// A help message for this specific tool can be added afterwards.
static cl::extrahelp MoreHelp("\nMore help text...\n");

int main(int argc, const char **argv) {
  auto ExpectedParser = CommonOptionsParser::create(argc, argv, MyToolCategory);
  if (!ExpectedParser) {
    llvm::errs() << toString(ExpectedParser.takeError());
    return 1;
  }
  CommonOptionsParser& OptionsParser = ExpectedParser.get();
  ClangTool Tool(OptionsParser.getCompilations(),
                 OptionsParser.getSourcePathList());
  return Tool.run(newFrontendActionFactory<clang::SyntaxOnlyAction>().get());
}

Running the tool on some code

When you check out and build clang, clang-check is already built and available to you in bin/clang-check inside your build directory.

You can run clang-check on a file in the llvm repository by specifying all the needed parameters after a “--” separator:

$ cd /path/to/source/llvm
$ export BD=/path/to/build/llvm
$ $BD/bin/clang-check tools/clang/tools/clang-check/ClangCheck.cpp -- \
      clang++ -D__STDC_CONSTANT_MACROS -D__STDC_LIMIT_MACROS \
      -Itools/clang/include -I$BD/include -Iinclude \
      -Itools/clang/lib/Headers -c

As an alternative, you can also configure cmake to output a compile command database into its build directory:

# Alternatively to calling cmake, use ccmake, toggle to advanced mode and
# set the parameter CMAKE_EXPORT_COMPILE_COMMANDS from the UI.
$ cmake -DCMAKE_EXPORT_COMPILE_COMMANDS=ON .

This creates a file called compile_commands.json in the build directory. Now you can run {program}clang-check over files in the project by specifying the build path as first argument and some source files as further positional arguments:

$ cd /path/to/source/llvm
$ export BD=/path/to/build/llvm
$ $BD/bin/clang-check -p $BD tools/clang/tools/clang-check/ClangCheck.cpp

(libtooling_builtin_includes)=

Builtin includes

Clang tools need their builtin headers and search for them the same way Clang does. Thus, the default location to look for builtin headers is in a path $(dirname /path/to/tool)/../lib/clang/3.3/include relative to the tool binary. This works out-of-the-box for tools running from llvm's toplevel binary directory after building clang-resource-headers, or if the tool is running from the binary directory of a clang install next to the clang binary.

Tips: if your tool fails to find stddef.h or similar headers, call the tool with -v and look at the search paths it looks through.

Linking

For a list of libraries to link, look at one of the tools' CMake files (for example clang-check/CMakeList.txt).