# Extra Clang Tools 22.1.2 documentation

## Clang-Tidy

See also:

- List of Clang-Tidy Checks
- Query Based Custom Clang-Tidy Checks
- Clang-tidy IDE/Editor Integrations
- Getting Involved
- External Clang-Tidy Examples

clang-tidy is a clang-based C++ “linter” tool. Its purpose is to provide an extensible framework for diagnosing and fixing typical programming errors, like style violations, interface misuse, or bugs that can be deduced via static analysis. clang-tidy is modular and provides a convenient interface for writing new checks.

## 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 ...

If there are too many options or source files to specify on the command line, you can store them in a parameter file, and use clang-tidy with that parameters file:

    $ clang-tidy @parameters_file

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, and 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
--- | ---
`abseil-` | Checks related to Abseil library.
`altera-` | Checks related to OpenCL programming for FPGAs.
`android-` | Checks related to Android.
`boost-` | Checks related to Boost library.
`bugprone-` | Checks that target bug-prone code constructs.
`cert-` | Checks related to CERT Secure Coding Guidelines.
`clang-analyzer-` | Clang Static Analyzer checks.
`concurrency-` | Checks related to concurrent programming (including threads, fibers, coroutines, etc.).
`cppcoreguidelines-` | Checks related to C++ Core Guidelines.
`darwin-` | Checks related to Darwin coding conventions.
`fuchsia-` | Checks related to Fuchsia coding conventions.
`google-` | Checks related to Google coding conventions.
`hicpp-` | Checks related to High Integrity C++ Coding Standard.
`linuxkernel-` | Checks related to the Linux Kernel coding conventions.
`llvm-` | Checks related to the LLVM coding conventions.
`llvmlibc-` | Checks related to the LLVM-libc coding standards.
`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.
`openmp-` | Checks related to OpenMP API.
`performance-` | Checks that target performance-related issues.
`portability-` | Checks that target portability-related issues that don’t relate to any particular coding style.
`readability-` | Checks that target readability-related issues that don’t relate to any particular coding style.
`zircon-` | Checks related to Zircon kernel coding conventions.

Clang diagnostics are treated in a similar way as check diagnostics. Clang diagnostics are displayed by clang-tidy and can be filtered out using the `-checks=` option. However, the `-checks=` option does not affect compilation arguments, so it cannot turn on Clang warnings which are not already turned on in the 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`.

Clang compiler errors (such as syntax errors, semantic errors, or other failures that prevent Clang from compiling the code) are reported with the check name `clang-diagnostic-error`. These represent fundamental compilation failures that must be fixed before clang-tidy can perform its analysis. Unlike other diagnostics, `clang-diagnostic-error` cannot be disabled, as clang-tidy requires valid code to function.

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:

      --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: {x: 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.
      --config-file=<string>           - Specify the path of .clang-tidy or custom config file:
                                          e.g. --config-file=/some/path/myTidyConfigFile
                                         This option internally works exactly the same way as
                                          --config option after reading specified config file.
                                         Use either --config-file or --config, not both.
      --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.
      --enable-module-headers-parsing  - Enables preprocessor-level module header parsing
                                         for C++20 and above, empowering specific checks
                                         to detect macro definitions within modules. This
                                         feature may cause performance and parsing issues
                                         and is therefore considered experimental.
      --exclude-header-filter=<string> - Regular expression matching the names of the
                                         headers to exclude diagnostics from. Diagnostics
                                         from the main file of each translation unit are
                                         always displayed.
                                         Must be used together with --header-filter.
                                         Can be used together with -line-filter.
                                         This option overrides the 'ExcludeHeaderFilterRegex'
                                         option in .clang-tidy file, if any.
      --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.
      --fix-notes                      - If a warning has no fix, but a single fix can
                                         be found through an associated diagnostic note,
                                         apply the fix.
                                         Specifying this flag will implicitly enable the
                                         '--fix' flag.
      --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. The default
                                         value is '.*', i.e. diagnostics from all non-system
                                         headers are displayed by default. Diagnostics
                                         from the main file of each translation unit are
                                         always displayed.
                                         Can be used together with -line-filter.
                                         This option overrides the 'HeaderFilterRegex'
                                         option in .clang-tidy file, if any.
      --line-filter=<string>           - List of files and line ranges to output diagnostics from.
                                         The range is inclusive on both ends. Can be used together
                                         with -header-filter. The format of the list is a JSON
                                         array of objects. For example:

                                           [
                                             {"name":"file1.cpp","lines":[[1,3],[5,7]]},
                                             {"name":"file2.h"}
                                           ]

                                         This will output diagnostics from 'file1.cpp' only for
                                         the line ranges [1,3] and [5,7], as well as all from the
                                         entire 'file2.h'.
      --list-checks                    - List all enabled checks and exit. Use with
                                         -checks=* to list all available checks.
      --load=<pluginfilename>          - Load the specified plugin
      -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.
      --store-check-profile=<prefix>   - By default reports are printed in tabulated
                                         format to stderr. When this option is passed,
                                         these per-TU profiles are instead stored as JSON.
      --system-headers                 - Display the errors from system headers.
                                         This option overrides the 'SystemHeaders' option
                                         in .clang-tidy file, if any.
      --use-color                      - Use colors in diagnostics. If not set, colors
                                         will be used if the terminal connected to
                                         standard output supports colors.
                                         This option overrides the 'UseColor' option in
                                         .clang-tidy file, if any.
      --verify-config                  - Check the config files to ensure each check and
                                         option is recognized.
      --vfsoverlay=<filename>          - Overlay the virtual filesystem described by file
                                         over the real file system.
      --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.
      --allow-no-checks                - Allow empty enabled checks. This suppresses
                                         the "no checks enabled" error when disabling
                                         all of the checks.

    -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:
      https://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.

    Parameters files:
      A large number of options or source files can be passed as parameter files
      by use '@parameter-file' in the command line.

    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. The .clang-tidy file is specified in YAML format. If any configuration
      options have a corresponding command-line option, command-line option takes
      precedence.

      The following configuration options may be used in a .clang-tidy file:

      CheckOptions                 - List of key-value pairs defining check-specific
                                     options. Example:
                                       CheckOptions:
                                         some-check.SomeOption: 'some value'
      Checks                       - Same as '--checks'. Additionally, the list of
                                     globs can be specified as a list instead of a
                                     string.
      CustomChecks                 - List of user defined checks based on
                                     Clang-Query syntax.
      ExcludeHeaderFilterRegex     - Same as '--exclude-header-filter'.
      ExtraArgs                    - Same as '--extra-arg'.
      ExtraArgsBefore              - Same as '--extra-arg-before'.
      FormatStyle                  - Same as '--format-style'.
      HeaderFileExtensions         - File extensions to consider to determine if a
                                     given diagnostic is located in a header file.
      HeaderFilterRegex            - Same as '--header-filter'.
      ImplementationFileExtensions - File extensions to consider to determine if a
                                     given diagnostic is located in an
                                     implementation file.
      InheritParentConfig          - If this option is true in a config file, the
                                     configuration file in the parent directory
                                     (if any exists) will be taken and the current
                                     config file will be applied on top of the
                                     parent one.
      SystemHeaders                - Same as '--system-headers'.
      UseColor                     - Same as '--use-color'.
      User                         - Specifies the name or e-mail of the user
                                     running clang-tidy. This option is used, for
                                     example, to place the correct user name in
                                     TODO() comments in the relevant check.
      WarningsAsErrors             - Same as '--warnings-as-errors'.
      RemovedArgs                  - Same as '--removed-arg'

      The effective configuration can be inspected using --dump-config:

        $ clang-tidy --dump-config
        ---
        Checks:              '-*,some-check'
        WarningsAsErrors:    ''
        HeaderFileExtensions:         ['', 'h','hh','hpp','hxx']
        ImplementationFileExtensions: ['c','cc','cpp','cxx']
        HeaderFilterRegex:   '.*'
        FormatStyle:         none
        InheritParentConfig: true
        User:                user
        CheckOptions:
          some-check.SomeOption: 'some value'
        ...

## Clang-Tidy Automation

clang-tidy can analyze multiple source files by specifying them on the command line. For larger projects, automation scripts provide additional functionality like parallel execution and integration with version control systems.

### Running Clang-Tidy in Parallel

clang-tidy can process multiple files sequentially, but for projects with many source files, the run-clang-tidy.py script provides parallel execution to significantly reduce analysis time. This script is included with clang-tidy and runs clang-tidy over all files in a compilation database or a specified path concurrently.

The script requires a compilation database (`compile_commands.json`) which can be generated by build systems like CMake (using `-DCMAKE_EXPORT_COMPILE_COMMANDS=ON`) or by tools like Bear.

The script supports most of the same options as clang-tidy itself, including `-checks=`, `-fix`, `-header-filter=`, and configuration options. Run `run-clang-tidy.py --help` for a complete list of available options.

Example invocations:

    # Run clang-tidy on all files in the compilation database in parallel
    $ run-clang-tidy.py -p=build/

    # Run with specific checks and apply fixes
    $ run-clang-tidy.py -p=build/ -fix -checks=-*,readability-*

    # Run on specific files/directories with header filtering
    $ run-clang-tidy.py -p=build/ -header-filter=src/ src/

    # Run with parallel execution (uses all CPU cores by default)
    $ run-clang-tidy.py -p=build/ -j 4

### Running Clang-Tidy on Diff

The clang-tidy-diff.py script allows you to run clang-tidy on the lines that have been modified in your working directory or in a specific diff. Importantly, clang-tidy-diff.py only reports diagnostics for changed lines; clang-tidy still analyzes the entire file and filters out unchanged lines after analysis, so this does not improve performance. This is particularly useful for code reviews and continuous integration, as it focuses analysis on the changed code rather than the entire codebase.

The script can work with various diff sources:

- Git working directory changes

- Output from `git diff`

- Output from `svn diff`

- Patch files

Example invocations:

    # Run clang-tidy on all changes in the working directory
    $ git diff -U0 --no-color HEAD^ | clang-tidy-diff.py -p1

    # Run with specific checks and apply fixes
    $ git diff -U0 --no-color HEAD^ | clang-tidy-diff.py -p1 -fix \
      -checks=-*,readability-*

    # Run on staged changes
    $ git diff -U0 --no-color --cached | clang-tidy-diff.py -p1

    # Run on changes between two commits
    $ git diff -U0 --no-color HEAD~2 HEAD | clang-tidy-diff.py -p1

    # Run on a patch file
    $ clang-tidy-diff.py -p1 < changes.patch

The `-p1` option tells the script to strip one level of path prefix from the diff, which is typically needed for Git diffs. The script supports most of the same options as clang-tidy itself, including `-checks=`, `-fix`, `-header-filter=`, and configuration options.

While clang-tidy-diff.py is useful for focusing on recent changes, relying solely on it may lead to incomplete analysis. Since the script only reports warnings from the modified lines, it may miss issues that are caused by the changes but manifest elsewhere in the code. For example, changes that only add lines to a function may cause it to violate size limits (e.g., readability-function-size), but the diagnostic will be reported at the function declaration, which may not be in the diff and thus filtered out. Modifications to header files may also affect many implementation files, but only warnings in the modified header lines will be reported.

For comprehensive analysis, especially before merging significant changes, consider running clang-tidy on the entire affected files or the whole project using run-clang-tidy.py.

## Suppressing Undesired Diagnostics

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 the code is known to be correct, it may be useful to silence the warning. Some clang-tidy checks provide a check-specific way to silence the diagnostics, e.g. bugprone-use-after-move can be silenced by re-initializing the variable after it has been moved out, bugprone-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 a specific suppression mechanism is not available for a certain warning, or its use is not desired for some reason, clang-tidy has a generic mechanism to suppress diagnostics using `NOLINT`, `NOLINTNEXTLINE`, and `NOLINTBEGIN` … `NOLINTEND` comments.

The `NOLINT` comment instructs clang-tidy to ignore warnings on the same line (it doesn’t apply to a function, a block of code or any other language construct; it applies to the line of code it is on). If introducing the comment on the same line would change the formatting in an undesired way, the `NOLINTNEXTLINE` comment allows suppressing clang-tidy warnings on the next line. The `NOLINTBEGIN` and `NOLINTEND` comments allow suppressing clang-tidy warnings on multiple lines (affecting all lines between the two comments).

All comments can be followed by an optional list of check names in parentheses (see below for the formal syntax). The list of check names supports globbing, with the same format and semantics as for enabling checks. Note: negative globs are ignored here, as they would effectively re-activate the warning.

For example:

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

      // Consider explaining the motivation to suppress the warning
      Foo(char param); // NOLINT: Allow implicit conversion from `char`, because <some valid reason>

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

      // Silence all checks from the `google` module
      Foo(bool param); // NOLINT(google*)

      // Silence all checks ending with `-avoid-c-arrays`
      int array[10]; // NOLINT(*-avoid-c-arrays)

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

      // Silence all checks from the `google` module for the next line
      // NOLINTNEXTLINE(google*)
      Foo(bool param);

      // Silence all checks ending with `-avoid-c-arrays` for the next line
      // NOLINTNEXTLINE(*-avoid-c-arrays)
      int array[10];

      // Silence only the specified checks for all lines between the BEGIN and END
      // NOLINTBEGIN(google-explicit-constructor, google-runtime-int)
      Foo(short param);
      Foo(long param);
      // NOLINTEND(google-explicit-constructor, google-runtime-int)

      // Silence all checks from the `google` module for all lines between the BEGIN and END
      // NOLINTBEGIN(google*)
      Foo(bool param);
      // NOLINTEND(google*)

      // Silence all checks ending with `-avoid-c-arrays` for all lines between the BEGIN and END
      // NOLINTBEGIN(*-avoid-c-arrays)
      int array[10];
      // NOLINTEND(*-avoid-c-arrays)
    };

The formal syntax of `NOLINT`, `NOLINTNEXTLINE`, and `NOLINTBEGIN` … `NOLINTEND` 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
      NOLINTBEGIN
      NOLINTEND

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

All `NOLINTBEGIN` comments must be paired by an equal number of `NOLINTEND` comments. Moreover, a pair of comments must have matching arguments – for example, `NOLINTBEGIN(check-name)` can be paired with `NOLINTEND(check-name)` but not with `NOLINTEND` (zero arguments). clang-tidy will generate a `clang-tidy-nolint` error diagnostic if any `NOLINTBEGIN`/`NOLINTEND` comment violates these requirements.