# Clang 14 documentation

## 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 How To Setup Clang Tooling For LLVM

## Introduction

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

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;

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

    int main(int argc, const char **argv) {
      // CommonOptionsParser constructor will parse arguments and create a
      // CompilationDatabase.  In case of error it will terminate the program.
      CommonOptionsParser OptionsParser(argc, argv, MyToolCategory);

      // 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) {
      CommonOptionsParser OptionsParser(argc, argv, MyToolCategory);
      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 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

### 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).