Clang-Tidy ¶

Contents

Clang-Tidy
- Using clang-tidy
- Getting Involved

Using clang-tidy ¶

clang-tidy is a LibTooling-based tool, and it’s easier to work with if you set up a compile command database for your project (for an example of how to do this see How To Setup Tooling For LLVM). You can also specify compilation options on the command line after --:

$ clang-tidy test.cpp -- -Imy_project/include -DMY_DEFINES ...

clang-tidy has its own checks and can also run Clang static analyzer checks. Each check has a name and the checks to run can be chosen using the -checks= option, which specifies a comma-separated list of positive and negative (prefixed with -) globs. Positive globs add subsets of checks, negative globs remove them. For example,

$ clang-tidy test.cpp -checks=-*,clang-analyzer-*,-clang-analyzer-cplusplus*

will disable all default checks (-*) and enable all clang-analyzer-* checks except for clang-analyzer-cplusplus* ones.

The -list-checks option lists all the enabled checks. When used without -checks=, it shows checks enabled by default. Use -checks=* to see all available checks or with any other value of -checks= to see which checks are enabled by this value.

There are currently the following groups of checks:

Name prefix	Description
`android-`	Checks related to Android.
`boost-`	Checks related to Boost library.
`bugprone-`	Checks that target bugprone code constructs.
`cert-`	Checks related to CERT Secure Coding Guidelines.
`cppcoreguidelines-`	Checks related to C++ Core Guidelines.
`clang-analyzer-`	Clang Static Analyzer checks.
`fuchsia-`	Checks related to Fuchsia coding conventions.
`google-`	Checks related to Google coding conventions.
`hicpp-`	Checks related to High Integrity C++ Coding Standard.
`llvm-`	Checks related to the LLVM coding conventions.
`misc-`	Checks that we didn’t have a better category for.
`modernize-`	Checks that advocate usage of modern (currently “modern” means “C++11”) language constructs.
`mpi-`	Checks related to MPI (Message Passing Interface).
`objc-`	Checks related to Objective-C coding conventions.
`performance-`	Checks that target performance-related issues.
`readability-`	Checks that target readability-related issues that don’t relate to any particular coding style.

Clang diagnostics are treated in a similar way as check diagnostics. Clang diagnostics are displayed by clang-tidy and can be filtered out using -checks= option. However, the -checks= option does not affect compilation arguments, so it can not turn on Clang warnings which are not already turned on in build configuration. The -warnings-as-errors= option upgrades any warnings emitted under the -checks= flag to errors (but it does not enable any checks itself).

Clang diagnostics have check names starting with clang-diagnostic-. Diagnostics which have a corresponding warning option, are named clang-diagnostic-<warning-option>, e.g. Clang warning controlled by -Wliteral-conversion will be reported with check name clang-diagnostic-literal-conversion.

The -fix flag instructs clang-tidy to fix found errors if supported by corresponding checks.

An overview of all the command-line options:

$ clang-tidy -help
USAGE: clang-tidy [options] <source0> [... <sourceN>]

OPTIONS:

Generic Options:

  -help                        - Display available options (-help-hidden for more)
  -help-list                   - Display list of available options (-help-list-hidden for more)
  -version                     - Display the version of this program

clang-tidy options:

  -analyze-temporary-dtors     -
                                 Enable temporary destructor-aware analysis in
                                 clang-analyzer- checks.
                                 This option overrides the value read from a
                                 .clang-tidy file.
  -checks=<string>             -
                                 Comma-separated list of globs with optional '-'
                                 prefix. Globs are processed in order of
                                 appearance in the list. Globs without '-'
                                 prefix add checks with matching names to the
                                 set, globs with the '-' prefix remove checks
                                 with matching names from the set of enabled
                                 checks. This option's value is appended to the
                                 value of the 'Checks' option in .clang-tidy
                                 file, if any.
  -config=<string>             -
                                 Specifies a configuration in YAML/JSON format:
                                   -config="{Checks: '*',
                                             CheckOptions: [{key: x,
                                                             value: y}]}"
                                 When the value is empty, clang-tidy will
                                 attempt to find a file named .clang-tidy for
                                 each source file in its parent directories.
  -dump-config                 -
                                 Dumps configuration in the YAML format to
                                 stdout. This option can be used along with a
                                 file name (and '--' if the file is outside of a
                                 project with configured compilation database).
                                 The configuration used for this file will be
                                 printed.
                                 Use along with -checks=* to include
                                 configuration of all checks.
  -enable-check-profile        -
                                 Enable per-check timing profiles, and print a
                                 report to stderr.
  -explain-config              -
                                 For each enabled check explains, where it is
                                 enabled, i.e. in clang-tidy binary, command
                                 line or a specific configuration file.
  -export-fixes=<filename>     -
                                 YAML file to store suggested fixes in. The
                                 stored fixes can be applied to the input source
                                 code with clang-apply-replacements.
  -extra-arg=<string>          - Additional argument to append to the compiler command line
  -extra-arg-before=<string>   - Additional argument to prepend to the compiler command line
  -fix                         -
                                 Apply suggested fixes. Without -fix-errors
                                 clang-tidy will bail out if any compilation
                                 errors were found.
  -fix-errors                  -
                                 Apply suggested fixes even if compilation
                                 errors were found. If compiler errors have
                                 attached fix-its, clang-tidy will apply them as
                                 well.
  -format-style=<string>       -
                                 Style for formatting code around applied fixes:
                                   - 'none' (default) turns off formatting
                                   - 'file' (literally 'file', not a placeholder)
                                     uses .clang-format file in the closest parent
                                     directory
                                   - '{ <json> }' specifies options inline, e.g.
                                     -format-style='{BasedOnStyle: llvm, IndentWidth: 8}'
                                   - 'llvm', 'google', 'webkit', 'mozilla'
                                 See clang-format documentation for the up-to-date
                                 information about formatting styles and options.
                                 This option overrides the 'FormatStyle` option in
                                 .clang-tidy file, if any.
  -header-filter=<string>      -
                                 Regular expression matching the names of the
                                 headers to output diagnostics from. Diagnostics
                                 from the main file of each translation unit are
                                 always displayed.
                                 Can be used together with -line-filter.
                                 This option overrides the 'HeaderFilter' option
                                 in .clang-tidy file, if any.
  -line-filter=<string>        -
                                 List of files with line ranges to filter the
                                 warnings. Can be used together with
                                 -header-filter. The format of the list is a
                                 JSON array of objects:
                                   [
                                     {"name":"file1.cpp","lines":[[1,3],[5,7]]},
                                     {"name":"file2.h"}
                                   ]
  -list-checks                 -
                                 List all enabled checks and exit. Use with
                                 -checks=* to list all available checks.
  -p=<string>                  - Build path
  -quiet                       -
                                 Run clang-tidy in quiet mode. This suppresses
                                 printing statistics about ignored warnings and
                                 warnings treated as errors if the respective
                                 options are specified.
  -system-headers              - Display the errors from system headers.
  -warnings-as-errors=<string> -
                                 Upgrades warnings to errors. Same format as
                                 '-checks'.
                                 This option's value is appended to the value of
                                 the 'WarningsAsErrors' option in .clang-tidy
                                 file, if any.

-p <build-path> is used to read a compile command database.

        For example, it can be a CMake build directory in which a file named
        compile_commands.json exists (use -DCMAKE_EXPORT_COMPILE_COMMANDS=ON
        CMake option to get this output). When no build path is specified,
        a search for compile_commands.json will be attempted through all
        parent paths of the first input file . See:
        http://clang.llvm.org/docs/HowToSetupToolingForLLVM.html for an
        example of setting up Clang Tooling on a source tree.

<source0> ... specify the paths of source files. These paths are
        looked up in the compile command database. If the path of a file is
        absolute, it needs to point into CMake's source tree. If the path is
        relative, the current working directory needs to be in the CMake
        source tree and the file must be in a subdirectory of the current
        working directory. "./" prefixes in the relative files will be
        automatically removed, but the rest of a relative path must be a
        suffix of a path in the compile command database.


Configuration files:
  clang-tidy attempts to read configuration for each source file from a
  .clang-tidy file located in the closest parent directory of the source
  file. If any configuration options have a corresponding command-line
  option, command-line option takes precedence. The effective
  configuration can be inspected using -dump-config:

    $ clang-tidy -dump-config
    ---
    Checks:          '-*,some-check'
    WarningsAsErrors: ''
    HeaderFilterRegex: ''
    AnalyzeTemporaryDtors: false
    FormatStyle:     none
    User:            user
    CheckOptions:
      - key:             some-check.SomeOption
        value:           'some value'
    ...

clang-tidy diagnostics are intended to call out code that does not adhere to a coding standard, or is otherwise problematic in some way. However, if it is known that the code is correct, the check-specific ways to silence the diagnostics could be used, if they are available (e.g. bugprone-use-after-move can be silenced by re-initializing the variable after it has been moved out, misc-string-integer-assignment can be suppressed by explicitly casting the integer to char, readability-implicit-bool-conversion can also be suppressed by using explicit casts, etc.). If they are not available or if changing the semantics of the code is not desired, the NOLINT or NOLINTNEXTLINE comments can be used instead. For example:

class Foo
{
  // Silent all the diagnostics for the line
  Foo(int param); // NOLINT

  // Silent only the specified checks for the line
  Foo(double param); // NOLINT(google-explicit-constructor, google-runtime-int)

  // Silent only the specified diagnostics for the next line
  // NOLINTNEXTLINE(google-explicit-constructor, google-runtime-int)
  Foo(bool param);
};

The formal syntax of NOLINT/NOLINTNEXTLINE is the following:

lint-comment:
  lint-command
  lint-command lint-args

lint-args:
  ( check-name-list )

check-name-list:
  check-name
  check-name-list , check-name

lint-command:
  NOLINT
  NOLINTNEXTLINE

Note that whitespaces between NOLINT/NOLINTNEXTLINE and the opening parenthesis are not allowed (in this case the comment will be treated just as NOLINT/NOLINTNEXTLINE), whereas in check names list (inside the parenthesis) whitespaces can be used and will be ignored.

Getting Involved ¶

clang-tidy has several own checks and can run Clang static analyzer checks, but its power is in the ability to easily write custom checks.

Checks are organized in modules, which can be linked into clang-tidy with minimal or no code changes in clang-tidy.

Checks can plug into the analysis on the preprocessor level using PPCallbacks or on the AST level using AST Matchers. When an error is found, checks can report them in a way similar to how Clang diagnostics work. A fix-it hint can be attached to a diagnostic message.

The interface provided by clang-tidy makes it easy to write useful and precise checks in just a few lines of code. If you have an idea for a good check, the rest of this document explains how to do this.

There are a few tools particularly useful when developing clang-tidy checks:

add_new_check.py is a script to automate the process of adding a new check, it will create the check, update the CMake file and create a test;
rename_check.py does what the script name suggests, renames an existing check;
clang-query is invaluable for interactive prototyping of AST matchers and exploration of the Clang AST;
clang-check with the -ast-dump (and optionally -ast-dump-filter) provides a convenient way to dump AST of a C++ program.

Choosing the Right Place for your Check ¶

If you have an idea of a check, you should decide whether it should be implemented as a:

Clang diagnostic: if the check is generic enough, targets code patterns that most probably are bugs (rather than style or readability issues), can be implemented effectively and with extremely low false positive rate, it may make a good Clang diagnostic.
Clang static analyzer check: if the check requires some sort of control flow analysis, it should probably be implemented as a static analyzer check.
clang-tidy check is a good choice for linter-style checks, checks that are related to a certain coding style, checks that address code readability, etc.

Preparing your Workspace ¶

If you are new to LLVM development, you should read the Getting Started with the LLVM System, Using Clang Tools and How To Setup Tooling For LLVM documents to check out and build LLVM, Clang and Clang Extra Tools with CMake.

Once you are done, change to the llvm/tools/clang/tools/extra directory, and let’s start!

The Directory Structure ¶

clang-tidy source code resides in the llvm/tools/clang/tools/extra directory and is structured as follows:

clang-tidy/                       # Clang-tidy core.
|-- ClangTidy.h                   # Interfaces for users and checks.
|-- ClangTidyModule.h             # Interface for clang-tidy modules.
|-- ClangTidyModuleRegistry.h     # Interface for registering of modules.
   ...
|-- google/                       # Google clang-tidy module.
|-+
  |-- GoogleTidyModule.cpp
  |-- GoogleTidyModule.h
        ...
|-- llvm/                         # LLVM clang-tidy module.
|-+
  |-- LLVMTidyModule.cpp
  |-- LLVMTidyModule.h
        ...
|-- objc/                         # Objective-C clang-tidy module.
|-+
  |-- ObjCTidyModule.cpp
  |-- ObjCTidyModule.h
        ...
|-- tool/                         # Sources of the clang-tidy binary.
        ...
test/clang-tidy/                  # Integration tests.
    ...
unittests/clang-tidy/             # Unit tests.
|-- ClangTidyTest.h
|-- GoogleModuleTest.cpp
|-- LLVMModuleTest.cpp
|-- ObjCModuleTest.cpp
    ...

Writing a clang-tidy Check ¶

So you have an idea of a useful check for clang-tidy.

First, if you’re not familiar with LLVM development, read through the Getting Started with LLVM document for instructions on setting up your workflow and the LLVM Coding Standards document to familiarize yourself with the coding style used in the project. For code reviews we mostly use LLVM Phabricator.

Next, you need to decide which module the check belongs to. Modules are located in subdirectories of clang-tidy/ and contain checks targeting a certain aspect of code quality (performance, readability, etc.), certain coding style or standard (Google, LLVM, CERT, etc.) or a widely used API (e.g. MPI). Their names are same as user-facing check groups names described above.

After choosing the module and the name for the check, run the clang-tidy/add_new_check.py script to create the skeleton of the check and plug it to clang-tidy. It’s the recommended way of adding new checks.

If we want to create a readability-awesome-function-names, we would run:

$ clang-tidy/add_new_check.py readability awesome-function-names

The add_new_check.py script will:

create the class for your check inside the specified module’s directory and register it in the module and in the build system;
create a lit test file in the test/clang-tidy/ directory;
create a documentation file and include it into the docs/clang-tidy/checks/list.rst.

Let’s see in more detail at the check class definition:

...

#include "../ClangTidy.h"

namespace clang {
namespace tidy {
namespace readability {

...
class AwesomeFunctionNamesCheck : public ClangTidyCheck {
public:
  AwesomeFunctionNamesCheck(StringRef Name, ClangTidyContext *Context)
      : ClangTidyCheck(Name, Context) {}
  void registerMatchers(ast_matchers::MatchFinder *Finder) override;
  void check(const ast_matchers::MatchFinder::MatchResult &Result) override;
};

} // namespace readability
} // namespace tidy
} // namespace clang

...

Constructor of the check receives the Name and Context parameters, and must forward them to the ClangTidyCheck constructor.

In our case the check needs to operate on the AST level and it overrides the registerMatchers and check methods. If we wanted to analyze code on the preprocessor level, we’d need instead to override the registerPPCallbacks method.

In the registerMatchers method we create an AST Matcher (see AST Matchers for more information) that will find the pattern in the AST that we want to inspect. The results of the matching are passed to the check method, which can further inspect them and report diagnostics.

using namespace ast_matchers;

void AwesomeFunctionNamesCheck::registerMatchers(MatchFinder *Finder) {
  Finder->addMatcher(functionDecl().bind("x"), this);
}

void AwesomeFunctionNamesCheck::check(const MatchFinder::MatchResult &Result) {
  const auto *MatchedDecl = Result.Nodes.getNodeAs<FunctionDecl>("x");
  if (MatchedDecl->getName().startswith("awesome_"))
    return;
  diag(MatchedDecl->getLocation(), "function %0 is insufficiently awesome")
      << MatchedDecl
      << FixItHint::CreateInsertion(MatchedDecl->getLocation(), "awesome_");
}

(If you want to see an example of a useful check, look at clang-tidy/google/ExplicitConstructorCheck.h and clang-tidy/google/ExplicitConstructorCheck.cpp).

Registering your Check ¶

(The add_new_check.py takes care of registering the check in an existing module. If you want to create a new module or know the details, read on.)

The check should be registered in the corresponding module with a distinct name:

class MyModule : public ClangTidyModule {
 public:
  void addCheckFactories(ClangTidyCheckFactories &CheckFactories) override {
    CheckFactories.registerCheck<ExplicitConstructorCheck>(
        "my-explicit-constructor");
  }
};

Now we need to register the module in the ClangTidyModuleRegistry using a statically initialized variable:

static ClangTidyModuleRegistry::Add<MyModule> X("my-module",
                                                "Adds my lint checks.");

When using LLVM build system, we need to use the following hack to ensure the module is linked into the clang-tidy binary:

Add this near the ClangTidyModuleRegistry::Add<MyModule> variable:

// This anchor is used to force the linker to link in the generated object file
// and thus register the MyModule.
volatile int MyModuleAnchorSource = 0;

And this to the main translation unit of the clang-tidy binary (or the binary you link the clang-tidy library in) clang-tidy/tool/ClangTidyMain.cpp:

// This anchor is used to force the linker to link the MyModule.
extern volatile int MyModuleAnchorSource;
static int MyModuleAnchorDestination = MyModuleAnchorSource;

Configuring Checks ¶

If a check needs configuration options, it can access check-specific options using the Options.get<Type>("SomeOption", DefaultValue) call in the check constructor. In this case the check should also override the ClangTidyCheck::storeOptions method to make the options provided by the check discoverable. This method lets clang-tidy know which options the check implements and what the current values are (e.g. for the -dump-config command line option).

class MyCheck : public ClangTidyCheck {
  const unsigned SomeOption1;
  const std::string SomeOption2;

public:
  MyCheck(StringRef Name, ClangTidyContext *Context)
    : ClangTidyCheck(Name, Context),
      SomeOption(Options.get("SomeOption1", -1U)),
      SomeOption(Options.get("SomeOption2", "some default")) {}

  void storeOptions(ClangTidyOptions::OptionMap &Opts) override {
    Options.store(Opts, "SomeOption1", SomeOption1);
    Options.store(Opts, "SomeOption2", SomeOption2);
  }
  ...

Assuming the check is registered with the name “my-check”, the option can then be set in a .clang-tidy file in the following way:

CheckOptions:
  - key: my-check.SomeOption1
    value: 123
  - key: my-check.SomeOption2
    value: 'some other value'

If you need to specify check options on a command line, you can use the inline YAML format:

$ clang-tidy -config="{CheckOptions: [{key: a, value: b}, {key: x, value: y}]}" ...

Testing Checks ¶

To run tests for clang-tidy use the command:

$ ninja check-clang-tools

clang-tidy checks can be tested using either unit tests or lit tests. Unit tests may be more convenient to test complex replacements with strict checks. Lit tests allow using partial text matching and regular expressions which makes them more suitable for writing compact tests for diagnostic messages.

The check_clang_tidy.py script provides an easy way to test both diagnostic messages and fix-its. It filters out CHECK lines from the test file, runs clang-tidy and verifies messages and fixes with two separate FileCheck invocations: once with FileCheck’s directive prefix set to CHECK-MESSAGES, validating the diagnostic messages, and once with the directive prefix set to CHECK-FIXES, running against the fixed code (i.e., the code after generated fix-its are applied). In particular, CHECK-FIXES: can be used to check that code was not modified by fix-its, by checking that it is present unchanged in the fixed code. The full set of FileCheck directives is available (e.g., CHECK-MESSAGES-SAME:, CHECK-MESSAGES-NOT:), though typically the basic CHECK forms (CHECK-MESSAGES and CHECK-FIXES) are sufficient for clang-tidy tests. Note that the FileCheck documentation mostly assumes the default prefix (CHECK), and hence describes the directive as CHECK:, CHECK-SAME:, CHECK-NOT:, etc. Replace CHECK by either CHECK-FIXES or CHECK-MESSAGES for clang-tidy tests.

An additional check enabled by check_clang_tidy.py ensures that if CHECK-MESSAGES: is used in a file then every warning or error must have an associated CHECK in that file.

To use the check_clang_tidy.py script, put a .cpp file with the appropriate RUN line in the test/clang-tidy directory. Use CHECK-MESSAGES: and CHECK-FIXES: lines to write checks against diagnostic messages and fixed code.

It’s advised to make the checks as specific as possible to avoid checks matching to incorrect parts of the input. Use [[@LINE+X]]/[[@LINE-X]] substitutions and distinct function and variable names in the test code.

Here’s an example of a test using the check_clang_tidy.py script (the full source code is at test/clang-tidy/google-readability-casting.cpp):

// RUN: %check_clang_tidy %s google-readability-casting %t

void f(int a) {
  int b = (int)a;
  // CHECK-MESSAGES: :[[@LINE-1]]:11: warning: redundant cast to the same type [google-readability-casting]
  // CHECK-FIXES: int b = a;
}

There are many dark corners in the C++ language, and it may be difficult to make your check work perfectly in all cases, especially if it issues fix-it hints. The most frequent pitfalls are macros and templates:

code written in a macro body/template definition may have a different meaning depending on the macro expansion/template instantiation;
multiple macro expansions/template instantiations may result in the same code being inspected by the check multiple times (possibly, with different meanings, see 1), and the same warning (or a slightly different one) may be issued by the check multiple times; clang-tidy will deduplicate _identical_ warnings, but if the warnings are slightly different, all of them will be shown to the user (and used for applying fixes, if any);
making replacements to a macro body/template definition may be fine for some macro expansions/template instantiations, but easily break some other expansions/instantiations.

Running clang-tidy on LLVM ¶

To test a check it’s best to try it out on a larger code base. LLVM and Clang are the natural targets as you already have the source code around. The most convenient way to run clang-tidy is with a compile command database; CMake can automatically generate one, for a description of how to enable it see How To Setup Tooling For LLVM. Once compile_commands.json is in place and a working version of clang-tidy is in PATH the entire code base can be analyzed with clang-tidy/tool/run-clang-tidy.py. The script executes clang-tidy with the default set of checks on every translation unit in the compile command database and displays the resulting warnings and errors. The script provides multiple configuration flags.

The default set of checks can be overridden using the -checks argument, taking the identical format as clang-tidy does. For example -checks=-*,modernize-use-override will run the modernize-use-override check only.
To restrict the files examined you can provide one or more regex arguments that the file names are matched against. run-clang-tidy.py clang-tidy/.*Check\.cpp will only analyze clang-tidy checks. It may also be necessary to restrict the header files warnings are displayed from using the -header-filter flag. It has the same behavior as the corresponding clang-tidy flag.
To apply suggested fixes -fix can be passed as an argument. This gathers all changes in a temporary directory and applies them. Passing -format will run clang-format over changed lines.