Qt oriented code checker based on clang framework. Krazy's little brother.
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artwork Add a logo and an icon Jul 28, 2018
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.cmake_has_regex_test.cpp Add option to build clazy into libclang Jan 3, 2018
.gitignore Support CPack Jan 21, 2018
CMakeLists.txt Make it easier to link to READMEs by removing the level name. May 15, 2018
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ClazySources.cmake generate.py now also generates CheckSources.cmake Mar 2, 2018
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clazy-cl.bat msvc: Silence -Wmicrosoft-enum-value on Windows, it's very noisy Feb 12, 2017
clazy.bat Fix installation of clazy.bat Feb 12, 2017
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readmes.cmake unneeded-cast: Move to level 3 Aug 18, 2018


WARNING: master is the development branch. Please use the v1.3 tag.

clazy v1.3

clazy is a compiler plugin which allows clang to understand Qt semantics. You get more than 50 Qt related compiler warnings, ranging from unneeded memory allocations to misusage of API, including fix-its for automatic refactoring.

Table of contents

Source Code

You can get clazy from:

Build Instructions


Install dependencies

  • OpenSUSE tumbleweed: zypper install cmake git-core llvm llvm-devel llvm-clang llvm-clang-devel
  • Ubuntu-16.04: apt install g++ cmake clang llvm-dev git-core libclang-3.8-dev qtbase5-dev
  • Archlinux: pacman -S make llvm clang python2 cmake qt5-base git gcc
  • Fedora: be sure to remove the llvm-static package and only install the one with dynamic libraries
  • Other distros: Check llvm/clang build docs.

Build and install clang

clang and LLVM >= 3.9 are required. Use clazy v1.1 if you need 3.7 support, or v1.3 for 3.8 support.

If your distro provides clang then you can skip this step.

  $ git clone https://github.com/llvm-mirror/llvm.git <some_directory>
  $ cd <some_directory>/tools/ && git clone https://github.com/llvm-mirror/clang.git
  $ cd <some_directory>/projects && git clone https://github.com/llvm-mirror/compiler-rt.git
  $ mkdir <some_directory>/build && cd <some_directory>/build
  $ make -jX && make install

Build clazy

  $ cd clazy/
  $ make && make install

See troubleshooting section if you have problems.


3rdparty pre-built msvc2015 clang and clazy binaries

Building for Windows is a lengthy and tedious task, therefore the maintainer won't be creating them anymore on his free/KDE time. KDAB however has offered to produce these binaries and they will appear on their website.

If you really want to build clang and clazy yourself then read on, otherwise skip the building topic.

Build and install clang

These instructions assume your terminal is suitable for development (msvc2015). jom, nmake, git, cmake and cl should be in your PATH.

clang and LLVM >= 4.0 are required.

Be sure to pass -DLLVM_EXPORT_SYMBOLS_FOR_PLUGINS=ON to CMake when building LLVM, otherwise clazy won't work.

  > git clone https://github.com/llvm-mirror/llvm.git <some_directory>
  > cd <some_directory>\tools\ && git clone https://github.com/llvm-mirror/clang.git
  > git checkout release_40
  > cd clang
  > git checkout release_40
  > mkdir <some_directory>\build && cd <some_directory>\build
  > jom
  > nmake install
  > Add c:\my_install_folder\llvm\bin\ to PATH

Build clazy

Be sure to point CLANG_LIBRARY_IMPORT to clang.lib. It's probably inside your LLVM build dir since it doesn't get installed.

  > cd clazy\
  > cmake -DCMAKE_INSTALL_PREFIX=c:\my_install_folder\llvm\ -DCLANG_LIBRARY_IMPORT=C:\path\to\llvm-build\lib\clang.lib -DCMAKE_BUILD_TYPE=Release -G "NMake Makefiles JOM"
  > jom && nmake install

macOS with MacPorts

Install clang

$ sudo port install clang-3.9 llvm-3.9
$ sudo ln -sf /opt/local/bin/llvm-config-mp-3.9 /opt/local/bin/llvm-config
$ sudo port select --set clang mp-clang-3.9

Build clazy

  $ export CXX=clang++
  $ cmake
  $ make
  $ make install

macOS with Homebrew

The recommended way is to build clazy yourself, but alternatively you can try user recipes, such as:

$ brew install kde-mac/kde/clazy

for stable branch, or for master:

$ brew install kde-mac/kde/clazy --HEAD

As these are not verified or tested by the clazy developers please don't report bugs to us.

For building yourself, read on. You'll have to install clang and build clazy from source.

Install clang

$ brew install --with-clang llvm

Build clazy

  $ export CXX=clang++
  $ export LLVM_ROOT=/usr/local/opt/llvm
  $ cmake
  $ make
  $ make install

Setting up your project to build with clazy

Note: Wherever clazy it mentioned, replace with clazy-cl.bat if you're on Windows. Note: If you prefer running clazy over a JSON compilation database instead of using it as a plugin, jump to clazy-standalone.

You should now have the clazy command available to you, in <prefix>/bin/. Compile your programs with it instead of clang++/g++.

Note that this command is just a convenience wrapper which calls: clang++ -Xclang -load -Xclang ClangLazy.so -Xclang -add-plugin -Xclang clang-lazy

If you have multiple versions of clang installed (say clang++-3.8 and clang++-3.9) you can choose which one to use by setting the CLANGXX environment variable, like so: export CLANGXX=clang++-3.8; clazy

To build a CMake project use: cmake . -DCMAKE_CXX_COMPILER=clazy and rebuild.

To make it the compiler for qmake projects, just run qmake like: qmake -spec linux-clang QMAKE_CXX="clazy"

Alternatively, if you want to use clang directly, without the wrapper: qmake -spec linux-clang QMAKE_CXXFLAGS="-Xclang -load -Xclang ClangLazy.so -Xclang -add-plugin -Xclang clang-lazy"

You can also edit mkspecs/common/clang.conf and change QMAKE_CXX to clazy instead of clang++ and run qmake -spec linux-clang

It's recommended that you disable pre-compiled headers and don't use ccache.

You're all set, clazy will now run some checks on your project, but not all of them. Read on if you want to enable/disable which checks are run.

List of checks

There are many checks and they are divided in levels:

  • level0: Very stable checks, 99.99% safe, mostly no false-positives, very desirable
  • level1: The default level. Very similar to level 0, slightly more false-positives but very few.
  • level2: Also very few false-positives, but contains noisy checks which not everyone agree should be default.
  • level3: Contains checks with high rate of false-positives.
  • manual: Checks here need to be enabled explicitly, as they don't belong to any level. Checks here are very stable and have very few false-positives.

clazy runs all checks from level1 by default.

Selecting which checks to enable

You may want to choose which checks to enable before starting to compile. If you don't specify anything then all checks from level0 and level1 will run. To specify a list of checks to run, or to choose a level, you can use the CLAZY_CHECKS env variable or pass arguments to the compiler. You can disable checks by prefixing with no-, in case you don't want all checks from a given level.

Example via env variable

export CLAZY_CHECKS="unneeded-cast,qmap-with-key-pointer,virtual-call-ctor" # Enables only these 3 checks
export CLAZY_CHECKS="level0,no-qenums" # Enables all checks from level0, except for qenums
export CLAZY_CHECKS="level0,detaching-temporary" # Enables all from level0 and also detaching-temporary

Example via compiler argument

clazy -Xclang -plugin-arg-clang-lazy -Xclang level0,detaching-temporary Don't forget to re-run cmake/qmake/etc if you altered the c++ flags to specify flags.

clazy-standalone and JSON database support

The clazy-standalone binary allows you to run clazy over a compilation database JSON file, in the same way you would use clang-tidy or other clang tooling. This way you don't need to build your application, only the static analysis is performed.

clazy-standalone supports the same env variables as the clazy plugin. You can also specify a list of checks via the -checks argument.

Running on one cpp file: clazy-standalone -checks=install-event-filter,qmap-with-pointer-key,level0 -p compile_commands.json my.file.cpp

Running on all cpp files: find . -name "*cpp" | xargs clazy-standalone -checks=level2 -p default/compile_commands.json

See https://clang.llvm.org/docs/JSONCompilationDatabase.html for how to generate the compile_commands.json file. Basically it's generated by passing -DCMAKE_EXPORT_COMPILE_COMMANDS to CMake, or using Bear to intercept compiler commands, or, if you're using qbs:

qbs generate --generator clangdb

Note: Be sure the clazy-standalone binary is located in the same folder as the clang binary, otherwise it will have trouble finding builtin headers, like stddef.h. Alternatively, you can symlink to the folder containing the builtin headers:

(Assuming clazy was built with -DCMAKE_INSTALL_PREFIX=/myprefix/)

$ touch foo.c && clang++ '-###' -c foo.c 2>&1 | tr ' ' '\n' | grep -A1 resource
  "/usr/bin/../lib/clang/4.0.1" # this is the interesting path (without the version)
$ ln -sf /usr/bin/../lib/clang/ /myprefix/lib/clang
$ ln -sf /usr/bin/../include/c++/ /myprefix/include/c++ # Required on macOS

If that doesn't work, run clang -v and check what's the InstalledDir. Move clazy-standalone to that folder.

clang-tidy support will be added after https://bugs.llvm.org//show_bug.cgi?id=32739 is fixed.

Enabling Fixits

Some checks support fixits, in which clazy will re-write your source files whenever it can fix something. You can enable a fixit through the env variable, for example: export CLAZY_FIXIT="fix-qlatin1string-allocations"

Only one fixit can be enabled each time. WARNING: Backup your code, don't blame me if a fixit is not applied correctly. For better results don't use parallel builds, otherwise a fixit being applied in an header file might be done twice.


  • clang: symbol lookup error: /usr/lib/x86_64-linux-gnu/ClangLazy.so: undefined symbol: _ZNK5clang15DeclarationName11getAsStringEv. This is due to mixing ABIs. Your clang/llvm was compiled with the new gcc c++ ABI but you compiled the clazy plugin with clang (which uses the old ABI).

    The solution is to build the clazy plugin with gcc or use a distro which hasn't migrated to gcc5 ABI yet, such as archlinux.

  • [Fedora] cmake can't find LLVM ? Try building llvm/clang yourself (There are reports that /usr/share/llvm/cmake/LLVM-Config.cmake is buggy).

  • [Fedora] CommandLine Error: Option 'opt-bisect-limit' registered more than once! Remove the llvm-static package and use the dynamically linked libraries instead

  • Some checks are mysteriously not producing warnings or not applying fixits ? Check if you have ccache interfering and turn it off.

  • fatal error: 'stddef.h' file not found, while using clazy-standalone Be sure the clazy-standalone binary is located in the same folder as the clang binary.

  • Be sure to disble pch.

  • macOS: Be sure you're not using Apple Clang

  • Windows: fatal error LNK1112: module machine type ‘X86’ conflicts with target machine type ‘x64’ If you're building in 32-bit, open clazy-cl.bat and insert a -m32 argument. Should read: %~dp0\clang\clang.exe –driver-mode=cl -m32 (...)

Qt4 compatibility mode

When running on codebases that must still compile with Qt4, you can pass --qt4compat (a convenience option equivalent to passing -Xclang -plugin-arg-clang-lazy -Xclang qt4-compat) to disable checks that only make sense with Qt5.

For example, to build a CMake project with Qt4 compatibility use: CXX="clazy --qt4compat"; cmake . and rebuild.

Reducing warning noise

If you think you found a false-positive, file a bug report. But do make sure to test first without icecc/distcc enabled.

If you want to suppress warnings from headers of Qt or 3rd party code, include them with -isystem instead of -I.

Alternatively you can set the CLAZY_HEADER_FILTER env variable to a regexp matching the path where you want warnings.

You can also suppress individual warnings by file or by line by inserting comments:

  • To disable clazy in a specific source file, insert this comment, anywhere in the file: // clazy:skip

  • To disable specific checks in a source file, insert a comment such as // clazy:excludeall=check1,check2

  • To disable specific checks in specific source lines, insert a comment in the same line as the warning: (...) // clazy:exclude=check1,check2

Don't include the clazy- prefix. If, for example, you want to disable qstring-allocations you would write: // clazy:exclude=qstring-allocations not clazy-qstring-allocations.

Reporting bugs and wishes


  • Sérgio Martins

with contributions from:

  • Allen Winter
  • Kevin Funk
  • Mathias Hasselmann
  • Laurent Montel
  • Albert Astals Cid
  • Aurélien Gâteau
  • Hannah von Reth
  • Volker Krause
  • Christian Ehrlicher

and thanks to:

  • Klarälvdalens Datakonsult AB (http://www.kdab.com), for letting me work on clazy as a research project

Contributing patches

New features go to master and bug fixes go to 1.3 branch. The prefered way to contributing is by using KDE's phabricator, see:

If you rather just create a pull request in https://github.com/KDE/clazy for a drive-by change, it's also fine, but beware that the maintainer might forget to check on github and the KDE bot will close the PR. In that case just send a reminder to the maintainer (smartins at kde.org).