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
• Build Instructions
  • Linux
    • Install dependencies
    • Build and install clang
    • Build clazy
  • Windows
    • Build and install clang
    • Build clazy
  • macOS with MacPorts
    • Install clang
    • Build clazy
  • macOS with Homebrew
    • Install clang
    • Build clazy
• Setting up your project to build with clazy
• List of checks
• Selecting which checks to enable
  • Example via env variable
  • Example via compiler argument
• clazy-standalone and JSON database support
• Enabling Fixits
• Troubleshooting
• Reducing warning noise
• Reporting bugs and wishes
• Authors
• Contributing patches

Source Code

You can get clazy from:

• https://github.com/KDE/clazy
• https://invent.kde.org/sdk/clazy

Supported platforms

Clazy has been tested on Linux, macOS and Windows/MSVC. Other platforms are not supported but we'll gladly accept patches.

Build Instructions

Linux

Install dependencies

• OpenSUSE tumbleweed: zypper install cmake git-core llvm llvm-devel llvm-clang llvm-clang-devel
• Ubuntu: apt install g++ cmake clang llvm-dev git-core libclang-dev
• Archlinux: pacman -S make llvm clang python2 cmake 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 >= 11.0 are required.

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
  $ cmake -DCMAKE_INSTALL_PREFIX=<prefix> -DLLVM_TARGETS_TO_BUILD=X86 -DCMAKE_BUILD_TYPE=Release -G Ninja ..
  $ cmake --build .
  $ cmake --build . --target install

Build clazy

  $ cd clazy/
  $ cmake -DCMAKE_INSTALL_PREFIX=<prefix> -DCMAKE_BUILD_TYPE=Release -G Ninja
  $ cmake --build .
  $ cmake --build . --target install

See troubleshooting section if you have problems.

Windows

Build and install clang

These instructions assume your terminal is suitable for development. Ninja (or equivalent), git, cmake, and cl (msvc2022) should be in your PATH.

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/llvm-project.git -b llvmorg-11.0.0 <some_directory>
  > mkdir build # Important that this is outside of the source directory
  > cd build
  > cmake -DCMAKE_INSTALL_PREFIX=c:\my_install_folder\llvm\ -DLLVM_EXPORT_SYMBOLS_FOR_PLUGINS=ON -DLLVM_INCLUDE_EXAMPLES=OFF -DLLVM_TARGETS_TO_BUILD="X86" -DLLVM_ENABLE_PROJECTS="clang;clang-tools-extra" -DCMAKE_BUILD_TYPE=Release -G "Ninja" ../<some_directory>/llvm
  > cmake --build .
  > cmake --build . --target install
  > Add c:\my_install_folder\llvm\bin\ to PATH

Also be sure to copy the generated C:\path\to\llvm-build\lib\clang.lib to your installation folder somewhere. It contains the exported symbols of clang.exe, which the plugins need. Unfortunately LLVM doesn't install it. You can put it anywhere, just save it so you can delete the build directory.

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 "Ninja"
  > cmake --build .
  > cmake --build . --target install

macOS with MacPorts

Install clang

$ sudo port install llvm-11 clang-11 cmake ninja coreutils
$ sudo ln -sf /opt/local/bin/llvm-config-mp-11 /opt/local/bin/llvm-config
$ sudo port select --set clang mp-clang-11

Build clazy

  $ export CXX=clang++
  $ cmake -G Ninja
  $ cmake --build .
  $ cmake --build . --target 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 -G Ninja
  $ cmake --build .
  $ cmake --build . --target install

Setting up your project to build with clazy

Note: Wherever clazy is mentioned, replace with clazy-cl.bat if you're on Windows, or replace with Clazy-x86_64.AppImage if you're using AppImage. 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 ClazyPlugin.so -Xclang -add-plugin -Xclang clazy

If you have multiple versions of clang installed (say clang++-10 and clang++-11) you can choose which one to use by setting the CLANGXX environment variable, like so: export CLANGXX=clang++-11; 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"

On Windows with MSVC it's simply: qmake QMAKE_CXX="clazy-cl.bat"

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

On Windows it's similar, just inspect the contents of clazy-cl.bat.

To use clazy from a preset in CMakePresets.json or CMakeUserPresets.json:

"cacheVariables": {
    "CMAKE_CXX_FLAGS": "-Xclang -plugin-arg-clazy -Xclang export-fixes -Xclang -plugin-arg-clazy -Xclang  level0,detaching-temporary",
    "CMAKE_CXX_COMPILER": "clazy"
}

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.
• manual: Checks here need to be enabled explicitly, as they don't belong to any level. They can be very stable or very unstable.

clazy runs all checks from level1 by default.

• Checks from Manual Level:

  • assert-with-side-effects
  • compare-member-check
  • container-inside-loop
  • detaching-member
  • heap-allocated-small-trivial-type
  • ifndef-define-typo
  • isempty-vs-count
  • jni-signatures
  • modernize-list-initialization
  • qbytearray-conversion-to-c-style (fix-qbytearray-conversion-to-c-style)
  • qhash-with-char-pointer-key
  • qproperty-type-mismatch
  • qrequiredresult-candidates
  • qstring-ref (fix-missing-qstringref)
  • qstring-varargs
  • qt-keyword-emit (fix-qt-keyword-emit)
  • qt-keywords (fix-qt-keywords)
  • qvariant-template-instantiation
  • raw-environment-function
  • reserve-candidates
  • sanitize-inline-keyword (fix-sanitize-inline-keyword)
  • signal-with-return-value
  • thread-with-slots
  • tr-non-literal
  • unexpected-flag-enumerator-value
  • unneeded-cast
  • unused-result-check
  • use-arrow-operator-instead-of-data
  • use-chrono-in-qtimer
  • used-qunused-variable

• Checks from Level 0:

  • connect-by-name
  • connect-non-signal
  • connect-not-normalized
  • container-anti-pattern
  • empty-qstringliteral
  • fully-qualified-moc-types (fix-fully-qualified-moc-types)
  • lambda-in-connect
  • lambda-unique-connection
  • lowercase-qml-type-name
  • mutable-container-key
  • no-module-include
  • overloaded-signal
  • qcolor-from-literal (fix-qcolor-from-literal)
  • qdatetime-utc (fix-qdatetime-utc)
  • qenums
  • qfileinfo-exists (fix-qfileinfo-exists)
  • qgetenv (fix-qgetenv)
  • qmap-with-pointer-key
  • qstring-arg
  • qstring-comparison-to-implicit-char
  • qstring-insensitive-allocation
  • qt-macros
  • strict-iterators
  • temporary-iterator
  • unused-non-trivial-variable
  • use-static-qregularexpression
  • writing-to-temporary
  • wrong-qevent-cast
  • wrong-qglobalstatic

• Checks from Level 1:

  • auto-unexpected-qstringbuilder (fix-auto-unexpected-qstringbuilder)
  • child-event-qobject-cast
  • connect-3arg-lambda
  • const-signal-or-slot
  • detaching-temporary
  • foreach
  • incorrect-emit
  • install-event-filter
  • modernize-overloaded-connects
  • non-pod-global-static
  • overridden-signal
  • post-event
  • qdeleteall
  • qhash-namespace
  • qlatin1string-non-ascii
  • qproperty-without-notify
  • qstring-left
  • range-loop-detach (fix-range-loop-add-qasconst)
  • range-loop-reference (fix-range-loop-add-ref)
  • readlock-detaching
  • returning-data-from-temporary
  • rule-of-two-soft
  • skipped-base-method
  • virtual-signal

• Checks from Level 2:

  • base-class-event
  • copyable-polymorphic
  • ctor-missing-parent-argument
  • function-args-by-ref (fix-function-args-by-ref)
  • function-args-by-value
  • global-const-char-pointer
  • implicit-casts
  • missing-qobject-macro (fix-missing-qobject-macro)
  • missing-typeinfo
  • old-style-connect (fix-old-style-connect)
  • qstring-allocations (fix-qlatin1string-allocations,fix-fromLatin1_fromUtf8-allocations,fix-fromCharPtrAllocations)
  • returning-void-expression
  • rule-of-three
  • static-pmf
  • virtual-call-ctor

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-pointer-key,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-clazy -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.

Note: If you're using the AppImage, use Clazy-x86_64.AppImage --standalone instead of clazy-standalone.

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 # Make sure this clang here is not Apple clang. Use for example clang++-mp-8.0 if on macOS and haven't run `port select` yet.
  "-resource-dir"
  "/opt/local/libexec/llvm-8.0/lib/clang/8.0.1" # The interesting part is /opt/local/libexec/llvm-8.0
$ ln -sf /opt/local/libexec/llvm-8.0/lib/clang/ /myprefix/lib/clang
$ mkdir /myprefix/include/
$ ln -sf /opt/local/libexec/llvm-8.0/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.

Filtering warnings by line

clazy-standalone accepts a -line-filter argument that uses the same JSON format as clang-tidy. Only warnings reported on the listed line ranges are emitted, which is useful for only reporting on the lines touched by a commit or diff:

clazy-standalone -checks=level1 \
  -line-filter='[{"name":"widget.cpp","lines":[[40,44]]}]' \
  -p compile_commands.json widget.cpp

An entry without a "lines" member matches the whole file. File names are matched by suffix, so relative paths from a diff work as expected. Note that this filters on the location a warning is reported at, so a check that reports at a declaration rather than the changed statement (for example, function-size style checks) may be filtered out even though an edit inside the body triggered it.

Clang-Tidy

Clazy optionally builds a plugin for integrating its checks into clang-tidy. This needs to be explicitly loaded using -load=ClazyClangTidy.so. In case the plugin is not in a standard location or in the LD_LIBRARY_PATH environment variable, an absolute path should be specified.

Checks need to be enabled explicitly. Due to clang-tidy only reporting warnings from enabled checks, one can not specify levels. For example -checks=clazy-qdatetime-utc,clazy-qgetenv"

To enable all checks from level0 use: clazy-overloaded-signal,clazy-connect-by-name,clazy-connect-non-signal,clazy-qstring-comparison-to-implicit-char,clazy-wrong-qevent-cast,clazy-lambda-in-connect,clazy-lambda-unique-connection,clazy-qdatetime-utc,clazy-qgetenv,clazy-qstring-insensitive-allocation,clazy-fully-qualified-moc-types,clazy-unused-non-trivial-variable,clazy-connect-not-normalized,clazy-mutable-container-key,clazy-qenums,clazy-qmap-with-pointer-key,clazy-qstring-ref,clazy-strict-iterators,clazy-writing-to-temporary,clazy-container-anti-pattern,clazy-qcolor-from-literal,clazy-qfileinfo-exists,clazy-qstring-arg,clazy-empty-qstringliteral,clazy-qt-macros,clazy-temporary-iterator,clazy-wrong-qglobalstatic,clazy-lowercase-qml-type-name,clazy-no-module-include,clazy-use-static-qregularexpression
To enable all checks from level1 use: clazy-auto-unexpected-qstringbuilder,clazy-connect-3arg-lambda,clazy-const-signal-or-slot,clazy-detaching-temporary,clazy-foreach,clazy-incorrect-emit,clazy-install-event-filter,clazy-non-pod-global-static,clazy-post-event,clazy-qdeleteall,clazy-qlatin1string-non-ascii,clazy-qproperty-without-notify,clazy-qstring-left,clazy-range-loop-detach,clazy-range-loop-reference,clazy-returning-data-from-temporary,clazy-rule-of-two-soft,clazy-child-event-qobject-cast,clazy-virtual-signal,clazy-overridden-signal,clazy-qhash-namespace,clazy-skipped-base-method,clazy-readlock-detaching
To enable all checks from level2 use: clazy-ctor-missing-parent-argument,clazy-base-class-event,clazy-copyable-polymorphic,clazy-function-args-by-ref,clazy-function-args-by-value,clazy-global-const-char-pointer,clazy-implicit-casts,clazy-missing-qobject-macro,clazy-missing-typeinfo,clazy-old-style-connect,clazy-qstring-allocations,clazy-returning-void-expression,clazy-rule-of-three,clazy-virtual-call-ctor,clazy-static-pmf

Enabling Fixits

Some checks support fixits, in which clazy will help re-write your source files whenever it can fix something. Simply pass -Xclang -plugin-arg-clazy -Xclang export-fixes to clang, or -export-fixes=somefile.yaml for clazy-standalone. Alternatively, set the CLAZY_EXPORT_FIXES env variable (works only with the plugin, not with standalone). Then run clang-apply-replacements <folder_with_yaml_files>, which will modify your code.

When using fixits, prefer to run only a single check each time, so they don't conflict with each other modifying the same source lines.

WARNING: Backup your code and make sure all changes done by clazy are correct.

Troubleshooting

• CommandLine Error: Option 'foo' registered more than once! Means you're building against a static version of LLVM (*.a files instead of *.so). Try passing to cmake -DLINK_CLAZY_TO_LLVM=OFF when building clazy, this was tested successfully against a static LLVM 7.0, and might work with other versions. In case you build LLVM from source, try adding -DLLVM_BUILD_LLVM_DYLIB=ON -DLLVM_LINK_LLVM_DYLIB=ON to your CMake options

• 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 disable pch.

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

• macOS: System Integrity Protection blocks the use of DYLD_LIBRARY_PATH. With SIP enabled you need to pass the full path to ClazyPlugin.dylib, otherwise you'll get image not found error.

• 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 (...)

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 (gcc/clang only). For MSVC use /external, which is available since VS 15.6.

Alternatively you can set the CLAZY_HEADER_FILTER env variable to a regexp matching the path where you want warnings, for example CLAZY_HEADER_FILTER=.*myapplication.*.

You can also exclude paths using a regexp by setting CLAZY_IGNORE_DIRS, for example CLAZY_IGNORE_DIRS=.*my_qt_folder.*.

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

• To disable specific checks in source files for the following line, insert a comment: // 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

• bug tracker: https://bugs.kde.org/enter_bug.cgi?product=clazy
• GitLab issues: https://invent.kde.org/sdk/clazy/-/issues

When reporting a bug please include a minimal compilable testcase. No matter how simple it is, it saves me time from deciphering a bug report. Time spent doing triaging is time not spent writing fixes.

A minimal testcase is also something I can copy to the test suite.

Make sure you can reproduce with clazy (outside of QtCreator), otherwise it can be a QtCreator bug instead, which you can report at https://bugreports.qt.io/.

Authors

• Sérgio Martins, author and long time maintainer

Current maintainer

• Alexander Lohnau

with contributions from:

• Allen Winter
• Kevin Funk
• Mathias Hasselmann
• Laurent Montel
• Albert Astals Cid
• Aurélien Gâteau
• Hannah von Reth
• Volker Krause
• Christian Ehrlicher
• Christian Gagneraud
• Nikolai Kosjar
• Jesper K. Pedersen
• Lucie Gerard
• Christian Schärf
• Waqar Ahmed
• Shivam Kunwar
• Ivan Čukić

and thanks to:

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

Contributing patches

New features go to master and bug fixes go to the last 1.X branch. Please contribute by using KDE's GitLab instance, see https://community.kde.org/Infrastructure/GitLab.