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<h1>LibTooling</h1>
<p>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.</p>

<!-- ======================================================================= -->
<h2 id="intro">Introduction</h2>
<!-- ======================================================================= -->

<p>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.</p>

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<h2 id="runoncode">Parsing a code snippet in memory.</h2>
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<p>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:
<pre>
  #include "clang/Tooling/Tooling.h"

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

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<h2 id="standalonetool">Writing a standalone tool.</h2>
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<p>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.</p>

<h3 id="compilationdb">Creating a compilation database.</h3>
<p>CompilationDatabase provides static factory functions to help with parsing
compile commands from a build directory or the command line. The following code
allows for both explicit specification of a compile command line, as well as
retrieving the compile commands lines from a database.
<pre>
int main(int argc, const char **argv) {
  // First, try to create a fixed compile command database from the command line
  // arguments.
  llvm::OwningPtr&lt;CompilationDatabase> Compilations(
    FixedCompilationDatabase::loadFromCommandLine(argc, argv));

  // Next, use normal llvm command line parsing to get the tool specific
  // parameters.
  cl::ParseCommandLineOptions(argc, argv);

  if (!Compilations) {
    // In case the user did not specify the compile command line via positional
    // command line arguments after "--", try to load the compile commands from
    // a database in the specified build directory.
    std::string ErrorMessage;
    Compilations.reset(CompilationDatabase::loadFromDirectory(BuildPath,
                                                              ErrorMessage));

    // If there is still no valid compile command database, we don't know how
    // to run the tool.
    if (!Compilations)
      llvm::report_fatal_error(ErrorMessage);
  }
...
}
</pre>
</p>

<h3 id="tool">Creating and running a ClangTool.</h3>
<p>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:
<pre>
  // A clang tool can run over a number of sources in the same process...
  std::vector&lt;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(*Compilations, 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&lt;clang::SyntaxOnlyAction>().
  int result = Tool.run(newFrontendActionFactory&lt;clang::SyntaxOnlyAction>());
</pre>
</p>

<h3 id="main">Putting it together - the first tool.</h3>
<p>Now we combine the two previous steps into our first real tool. This example
tool is also checked into the clang tree at tools/clang-check/ClangCheck.cpp.
<pre>
  #include "llvm/Support/CommandLine.h"
  #include "clang/Frontend/FrontendActions.h"
  #include "clang/Tooling/CompilationDatabase.h"
  #include "clang/Tooling/Tooling.h"

  using namespace clang::tooling;
  using namespace llvm;

  cl::opt&lt;std::string> BuildPath(
    cl::Positional,
    cl::desc("&lt;build-path>"));

  cl::list&lt;std::string> SourcePaths(
    cl::Positional,
    cl::desc("&lt;source0> [... &lt;sourceN>]"),
    cl::OneOrMore);

  int main(int argc, const char **argv) {
    llvm::OwningPtr&lt;CompilationDatabase> Compilations(
      FixedCompilationDatabase::loadFromCommandLine(argc, argv));
    cl::ParseCommandLineOptions(argc, argv);
    if (!Compilations) {
      std::string ErrorMessage;
      Compilations.reset(CompilationDatabase::loadFromDirectory(BuildPath,
                                                                ErrorMessage));
      if (!Compilations)
        llvm::report_fatal_error(ErrorMessage);
    }
    ClangTool Tool(*Compilations, SourcePaths);
    return Tool.run(newFrontendActionFactory&lt;clang::SyntaxOnlyAction>());
  }
</pre>
</p>

<h3 id="running">Running the tool on some code.</h3>
<p>When you check out and build clang, clang-check is already built and
available to you in bin/clang-check inside your build directory.</p>
<p>You can run clang-check on a file in the llvm repository by specifying
all the needed parameters after a "--" separator:
<pre>
  $ 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
</pre>
</p>

<p>As an alternative, you can also configure cmake to output a compile command
database into its build directory:
<pre>
  # 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 .
</pre>
</p>
<p>
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:
<pre>
  $ cd /path/to/source/llvm
  $ export BD=/path/to/build/llvm
  $ $BD/bin/clang-check $BD tools/clang/tools/clang-check/ClangCheck.cpp
</pre>
</p>

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