==========
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:
.. code-block:: c++
#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.
.. code-block:: c++
#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:
.. code-block:: c++
// 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``.
.. code-block:: c++
// 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:
.. code-block:: bash
$ 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:
.. code-block:: bash
# 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:
.. code-block:: bash
$ 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
<https://github.com/llvm/llvm-project/blob/main/clang/tools/clang-check/CMakeLists.txt>`_).
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