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 available in two places in Chromium:
Clang-tidy automatically runs on any CL that Chromium committers upload to Gerrit, and will leave code review comments there. This is the recommended way of using clang-tidy.
Chromium globally enables a subset of all of clang-tidy's checks (see ${chromium}/src/.clang-tidy
). We want these checks to cover as much as we reasonably can, but we also strive to strike a reasonable balance between signal and noise on code reviews. Hence, a large number of clang-tidy checks are disabled.
New checks require review from [email protected]. If you propose a check and it gets approved, you may turn it on, though please note that this is only provisional approval: we get signal from users clicking “Not Useful” on comments. If feedback is overwhelmingly “users don't find this useful,” the check may be removed.
Traditionally, petitions to add checks include an evaluation of the check under review. Crucially, this includes two things:
It‘s expected that the person proposing the check has manually surveyed every clang-tidy diagnostic in the sample, noting any bugs, odd behaviors, or interesting patterns they’ve noticed. If clang-tidy emits FixIts, these are expected to be considered by the evaluation, too.
An example of a previous proposal email thread is here.
Running clang-tidy requires some setup. First, you'll need to sync clang-tidy, which requires adding checkout_clang_tidy
to your .gclient
file:
solutions = [ { 'custom_vars': { 'checkout_clang_tidy': True, }, } ]
Your next run of gclient runhooks
should cause clang-tidy to be synced.
To run clang-tidy across all of Chromium, you‘ll need a checkout of Chromium’s tools/build/ repository. Once you have that and a Chromium out/
dir with an args.gn
, running clang-tidy across all of Chromium is a single command:
$ cd ${chromium}/src $ ${chromium_tools_build}/recipes/recipe_modules/tricium_clang_tidy/resources/tricium_clang_tidy_script.py \ --base_path $PWD \ --out_dir out/Linux \ --findings_file all_findings.json \ --clang_tidy_binary $PWD/third_party/llvm-build/Release+Asserts/bin/clang-tidy \ --all
To only run clang-tidy against certain files, replace the --all
parameter with the individual file paths.
All clang-tidy checks are run on Linux builds of Chromium, so please set up your args.gn
to build Linux.
all_findings.json
is where all of clang-tidy's findings will be dumped. The format of this file is detailed in tricium_clang_tidy_script.py
.
Note that the above command will use Chromium‘s top-level .clang-tidy
file (or .clang-tidy
files scattered throughout third_party/
, depending on the files we lint. In order to test a new check, it’s recommended that you use tricium_clang_tidy_script.py
's --tidy_checks
flag. Usage of this looks like:
$ cd ${chromium}/src $ ${chromium_build}/recipes/recipe_modules/tricium_clang_tidy/resources/tricium_clang_tidy_script.py \ --base_path $PWD \ --out_dir out/Linux \ --findings_file all_findings.json \ --clang_tidy_binary $PWD/third_party/llvm-build/Release+Asserts/bin/clang-tidy \ --tidy_checks='-*,YOUR-NEW-CHECK-NAME-HERE' --all
If a check is invalid on a particular piece of code, clang-tidy supports // NOLINT
and // NOLINTNEXTLINE
for ignoring all lint checks in the current and next lines, respectively. To suppress a specific lint, you can put it in parenthesis, e.g., // NOLINTNEXTLINE(modernize-use-nullptr)
. For more, please see the documentation.
Please note that adding comments that exist only to silence clang-tidy is actively discouraged. These comments clutter code, can easily get out-of-date, and don‘t provide much value to readers. Moreover, clang-tidy only complains on Gerrit when lines are touched, and making Chromium clang-tidy clean is an explicit non-goal; making code less readable in order to silence a rarely-surfaced complaint isn’t a good trade-off.
If clang-tidy emits a diagnostic that's incorrect due to a subtlety in the code, adding an explanantion of what the code is doing with a trailing NOLINT
may be fine. Put differently, the comment should be able to stand on its own even if we removed the NOLINT
. The fact that the comment also silences clang-tidy is a convenient side-effect.
For example:
Not OK; comment exists just to silence clang-tidy:
// NOLINTNEXTLINE for (int i = 0; i < arr.size(); i++) { // ... }
Not OK; comment exists just to verbosely silence clang-tidy:
// Clang-tidy doesn't get that we can't range-for-ize this loop. NOLINTNEXTLINE for (int i = 0; i < arr.size(); i++) { // ... }
Not OK; it‘s obvious that this loop modifies arr
, so the comment doesn’t actually clarify anything:
// It'd be invalid to make this into a range-for loop, since the body might add // elements to `arr`. NOLINTNEXTLINE for (int i = 0; i < arr.size(); i++) { if (i % 4) { arr.push_back(4); arr.push_back(2); } }
OK; comment calls out a non-obvious property of this loop's body. As an afterthought, it silences clang-tidy:
// It'd be invalid to make this into a range-for loop, since the call to `foo` // here might add elements to `arr`. NOLINTNEXTLINE for (int i = 0; i < arr.size(); i++) { foo(); bar(); }
In the end, as always, what is and isn't obvious at some point is highly context-dependent. Please use your best judgement.
If you want to sync the officially-supported clang-tidy
to your workstation, add the following to your .gclient file:
solutions = [ { 'custom_vars': { 'checkout_clang_tidy': True, }, }, ]
If you already have solutions
and custom_vars
, just add checkout_clang_tidy
to the existing custom_vars
map.
Once the above update has been made, run gclient runhooks
, and clang-tidy should appear at src/third_party/llvm-build/Release+Asserts/bin/clang-tidy
if your Chromium tree is sufficiently up-to-date.
Note that the local flows with clang-tidy are experimental, and require an LLVM checkout. Tricium is happy to run on WIP CLs, and we strongly encourage its use.
That said, assuming you have the LLVM sources available, you'll need to bring your own clang-apply-replacements
binary if you want to use the -fix
option noted below.
Note: If you‘re on a system that offers a clang tools through its package manager (e.g., on Debian/Ubuntu, sudo apt-get install clang-tidy clang-tools
), you might not need an LLVM checkout to make the required binaries and scripts (clang-tidy
, run-clang-tidy
and clang-apply-replacements
) available in your $PATH
. However, the system packaged binaries might be several versions behind Chromium’s toolchain, so not all flags are guaranteed to work. If this is a problem, consider building clang-tidy from the same revision the current toolchain is using, rather than filing a bug against the toolchain component. This can be done as follows:
tools/clang/scripts/build_clang_tools_extra.py \ --fetch out/Release clang-tidy clang-apply-replacements
Running clang-tidy is then (hopefully) simple.
ninja -C out/Release chrome
gn gen out/Release --export-compile-commands
cd out/Release
<PATH_TO_LLVM_SRC>/clang-tools-extra/clang-tidy/tool/run-clang-tidy.py \ -p . \# Set the root project directory, where compile_commands.json is. # Set the clang-tidy binary path, if it's not in your $PATH. -clang-tidy-binary <PATH_TO_LLVM_BUILD>/bin/clang-tidy \ # Set the clang-apply-replacements binary path, if it's not in your $PATH # and you are using the `fix` behavior of clang-tidy. -clang-apply-replacements-binary \ <PATH_TO_LLVM_BUILD>/bin/clang-apply-replacements \ # The checks to employ in the build. Use `-*,...` to omit default checks. -checks=<CHECKS> \ -header-filter=<FILTER> \# Optional, limit results to only certain files. -fix \# Optional, used if you want to have clang-tidy auto-fix errors. 'chrome/browser/.*' # A regex of the files you want to check. Copy-Paste Friendly (though you'll still need to stub in the variables): <PATH_TO_LLVM_SRC>/clang-tools-extra/clang-tidy/tool/run-clang-tidy.py \ -p . \ -clang-tidy-binary <PATH_TO_LLVM_BUILD>/bin/clang-tidy \ -clang-apply-replacements-binary \ <PATH_TO_LLVM_BUILD>/bin/clang-apply-replacements \ -checks=<CHECKS> \ -header-filter=<FILTER> \ -fix \ 'chrome/browser/.*'
Note that the source file regex must match how the build specified the file. This means that on Windows, you must use (escaped) backslashes even from a bash shell.
Questions about the local flow? Reach out to [email protected], [email protected], or [email protected].
Questions about the Gerrit flow? Email [email protected] or [email protected], or file a bug against Infra>LUCI>BuildService>PreSubmit>Tricium
. Please CC [email protected] and [email protected] on any of these.
Discoveries? Update the doc!