How to integrate clang-tidy to CMake and GCC?

Question

I want to integrate clang-tidy to our C and C++, CMake based project which is compiled using a custom GCC toolchain.

I've tried following this tutorial, setting CMAKE_CXX_CLANG_TIDY. I've also tried generating a compilation database by setting CMAKE_EXPORT_COMPILE_COMMANDS to ON and pointing run-clang-tidy.py to its directory.

In both cases, I've encountered (the same) few errors that are probably related to differences between Clang and GCC:

1. Some warning flags that are enabled in the CMake files are not supported in Clang but are supported in GCC (like -Wlogical-op). As the compiler is GCC, the file builds correctly, and the flag is written to the compilation database, but clang-tidy complains about it.
2. clang-tidy complains some defines and functions are unavailable, even though the code compiles just fine. As an example, the android-cloexec-open check suggested using O_CLOEXEC to improve security and force the closing of files, but trying to use this define leads to an undefined identifier error (even though our GCC compiles the code). As an example to a function that is not found, there is clock_gettime.

Our code compiles with the C11 standard and C++14 standard, without GNU extensions:

set(CMAKE_C_STANDARD 11)
set(CMAKE_CXX_STANDARD 14)

set(CMAKE_C_EXTENSIONS OFF)
set(CMAKE_CXX_EXTENSIONS OFF)

The custom toolchain is a cross-compilation toolchain which runs on Linux and compiles to FreeBSD.

1. Is there a way to disable the passing of some flags by CMake to clang-tidy? Am I using clang-tidy wrong?
2. I suspect this issue is related to disabling GNU extensions, using a cross-compilation toolchain, and some feature-test-macro which is not defined by default in Clang but is defined with GCC (e.g. _GNU_SOURCE/_POSIX_SOURCE). If this is the case, how can I check it? If not, should I use clang-tidy differently?

EDIT

As @pablo285 asked, here are 2 warnings I get for a single file, and then as I added --warnings-as-errors=*, the build stops:

error: unknown warning option '-Wlogical-op' ; did you mean '-Wlong-long'? [clang-diagnostic-error]

<file path>: error: use of undeclared identifier 'O_CLOEXEC' [clang-diagnostic-error]
O_WRONLY | O_CLOEXEC
           ^

I decided to write a python script that will replace clang-tidy, receive the commandline from CMake and edit it to fix various errors. Here are the modification to the commandline I tried:

1. Remove none clang compile flags
   • This helps with things like the first warning, as now I don't pass flags that clang doesn't know. It seems like I can't configure CMake to pass different set of flags to GCC and to clang-tidy, so if anyone is familiar with some solution to this problem, I'll be happy to hear!
2. I changed the include directories that are passed to clang-tidy
   • As mentioned in the post, I use a custom toolchain (which cross-compiles). I used this post and Python to extract the list of standard include directories, and added them to the flag list as a list of -isystem <dir>. I also added -nostdinc so that clang-tidy won't try to look on his own headers instead of mine
     • This helped with the issue above, as now various defines such as O_CLOEXEC is defined in the toolchain's headers, but as my toolchain is based on GCC, clang couldn't parse the <type_traits> header which includes calls to many compiler intrinsics
   • I'm not sure what's the best approach in this case

@shycha: Thanks for the tip, I'll try disabling this specific check and I'll edit this post again

Answer 1

Ok, I think that I have a solution. After a couple of evenings I was able to make it work.

In general I compile like this

rm -rf build
mkdir build
cd build
cmake -C ../cmake-scripts/clang-tidy-all.cmake .. && make

Where cmake-scripts directory contains:

clang-tidy-all.cmake
toolchain_arm_clang.cmake

The two important files are listed below. But what is more important, is how you need to compile this.

First, toolchain_arm_clang.cmake is referenced directly from clang-tidy-all.cmake via set(CMAKE_TOOLCHAIN_FILE ...). It must be, however, referenced from the point of view of the building directory, so if you use multiple levels of build-dirs, e.g.: build/x86, build/arm, build/darwin, etc., then you must modify that path accordingly.

Second, the purpose of set(CONFIG_SCRIPT_PRELOADED ...) is to be sure that the config script was pre-loaded, i.e., cmake -C ../cmake-scripts/clang-tidy-all.cmake .. Typically, you would want to have something like this somewhere in your CMakeLists.txt file:

message(STATUS "CONFIG_SCRIPT_PRELOADED: ${CONFIG_SCRIPT_PRELOADED}")
if(NOT CONFIG_SCRIPT_PRELOADED)
    message(FATAL_ERROR "Run cmake -C /path/to/cmake.script to preload a config script!")
endif()

Third, there is /lib/ld-musl-armhf.so.1 hard-coded in set(CMAKE_LINKER_ARM_COMPAT_STATIC ...); on the development box that I use, it points to /lib/libc.so, so it might by OK to use /lib/libc.sh instead. I've never tried.

Fourth, using set(CMAKE_C_LINK_EXECUTABLE ...) and set(CMAKE_LINKER_ARM_COMPAT_STATIC ...) was because CMake was complaining about some linking problems during checking the compiler, i.e., before even running make.

Fifth, I was only compiling C++ code, so if you need to compile some C, then it might be required to also properly configure set(CMAKE_C_CREATE_SHARED_LIBRARY ...), but I have no idea whether there is such a config option.

General Advice

Do not integrate it immediately. First test some simple CMake project with one library (preferably a C++ one) and make it work, then add the second library, but in C, tweak it again. And only after that incorporate it into the code base.

Toolchain

I used a custom toolchain with GCC 8.3.0 and musl C library, so locations of some files might be different for other toolchains.

Custom CMake

Some variables, like (already mentioned) CONFIG_SCRIPT_PRELOADED, EXPORT_PACKAGE_TO_GLOBAL_REGISTRY, DO_NOT_BUILD_TESTS, or DO_NOT_BUILD_BENCHMARKS are not generic CMake options, i.e., I use them only in my CMakeLists.txt, so you can safely ignore them.

Variables that are unset at the end of each *.cmake file, e.g., build_test, extra_clang_tidy_unchecks_for_tests_only, don't need to be present in the project's main CMakeLists.txt.

Clang

$ clang --version
clang version 10.0.0 (https://github.com/llvm/llvm-project.git 4650b2f36949407ef25686440e3d65ac47709deb)
Target: x86_64-unknown-linux-gnu
Thread model: posix
InstalledDir: /opt/local/bin

Files

clang-tidy-all.cmake:

set(ALL_CXX_WARNING_FLAGS --all-warnings -Weverything -Wno-c++98-compat -Wno-c++98-c++11-compat -Wno-c++98-c++11-c++14-compat -Wno-padded -Wno-c++98-compat-pedantic)
set(CXX_COMPILE_OPTIONS "-std=c++17;-O3;${ALL_CXX_WARNING_FLAGS}" CACHE INTERNAL "description")


set(CMAKE_CROSSCOMPILING True)
set(CMAKE_TOOLCHAIN_FILE "../cmake-scripts/toolchain_arm_clang.cmake" CACHE FILEPATH "CMake toolchain file")

set(CONFIG_SCRIPT_PRELOADED true CACHE BOOL "Ensures that config script was preloaded")


set(build_test False)

if(build_test)
    message(STATUS "Using test mode clang-tidy checks!")
    set(extra_clang_tidy_unchecks_for_tests_only ",-google-readability-avoid-underscore-in-googletest-name,-cppcoreguidelines-avoid-magic-numbers,-cppcoreguidelines-special-member-functions")
endif()

set(CMAKE_CXX_CLANG_TIDY "clang-tidy;--enable-check-profile;--checks=-*,abseil-string-find-startswith,bugprone-*,cert-*,clang-analyzer-*,cppcoreguidelines-*,google-*,hicpp-*,llvm-*,misc-*,modernize-*,-modernize-use-trailing-return-type,performance-*,readability-*,-readability-static-definition-in-anonymous-namespace,-readability-simplify-boolean-expr,portability-*${extra_clang_tidy_unchecks_for_tests_only}" CACHE INTERNAL "clang-tidy")

message(STATUS "build_test: ${build_test}")
message(STATUS "extra_clang_tidy_unchecks_for_tests_only: ${extra_clang_tidy_unchecks_for_tests_only}")
message(STATUS "CMAKE_CXX_CLANG_TIDY: ${CMAKE_CXX_CLANG_TIDY}")

# We want to skip building tests when clang-tidy is run (it takes too much time and serves nothing)
if(DEFINED CMAKE_CXX_CLANG_TIDY AND NOT build_test)
    set(DO_NOT_BUILD_TESTS true CACHE BOOL "Turns OFF building tests")
    set(DO_NOT_BUILD_BENCHMARKS true CACHE BOOL "Turns OFF building benchmarks")
endif()



unset(build_test)
unset(extra_clang_tidy_unchecks_for_tests_only)
set(EXPORT_PACKAGE_TO_GLOBAL_REGISTRY "OFF" CACHE INTERNAL "We don't export clang-tidy-all version to global register")

toolchain_arm_clang.cmake:

set(CMAKE_SYSTEM_NAME Linux)
set(CMAKE_SYSTEM_VERSION 4.14.0)
set(CMAKE_SYSTEM_PROCESSOR arm)


set(gcc_version 8.3.0)
set(x_tools "/opt/zynq/xtl")

set(CMAKE_C_COMPILER "clang" CACHE INTERNAL STRING)
set(CMAKE_CXX_COMPILER "clang++" CACHE INTERNAL STRING)
set(CMAKE_RANLIB "llvm-ranlib" CACHE INTERNAL STRING)
set(CMAKE_AR "llvm-ar" CACHE INTERNAL STRING)
set(CMAKE_AS "llvm-as" CACHE INTERNAL STRING)
set(CMAKE_LINKER "ld.lld" CACHE INTERNAL STRING)

execute_process(
    COMMAND bash -c "dirname `whereis ${CMAKE_LINKER} | tr -s ' ' '\n' | grep ${CMAKE_LINKER}`"
    OUTPUT_VARIABLE cmake_linker_dir
)
string(REGEX REPLACE "\n$" "" cmake_linker_dir "${cmake_linker_dir}")
set(cmake_linker_with_dir "${cmake_linker_dir}/${CMAKE_LINKER}" CACHE INTERNAL STRING)

set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -iwithsysroot /include/c++/${gcc_version} -iwithsysroot /include/c++/${gcc_version}/arm-linux-musleabihf" CACHE INTERNAL STRING)


set(CMAKE_SYSROOT ${x_tools}/arm-linux-musleabihf)
set(CMAKE_FIND_ROOT_PATH ${x_tools}/arm-linux-musleabihf)
set(CMAKE_INSTALL_PREFIX ${x_tools}/arm-linux-musleabihf)

set(CMAKE_FIND_ROOT_PATH_MODE_PROGRAM NEVER)
set(CMAKE_FIND_ROOT_PATH_MODE_LIBRARY BOTH)
set(CMAKE_FIND_ROOT_PATH_MODE_INCLUDE BOTH)
set(CMAKE_FIND_ROOT_PATH_MODE_PACKAGE NEVER)



set(triple arm-linux-musleabihf)
set(CMAKE_LIBRARY_ARCHITECTURE ${triple})
set(CMAKE_C_COMPILER_TARGET ${triple})
set(CMAKE_CXX_COMPILER_TARGET ${triple})


set(lib_path_arm ${x_tools}/arm-linux-musleabihf/lib)

## Bootstrap library stuff:
set(Scrt1_o ${lib_path_arm}/Scrt1.o)
set(crti_o ${lib_path_arm}/crti.o)
set(crtn_o ${lib_path_arm}/crtn.o)

set(lib_path_gcc ${x_tools}/lib/gcc/${triple}/${gcc_version})
set(crtbeginS_o ${lib_path_gcc}/crtbeginS.o)
set(crtendS_o ${lib_path_gcc}/crtendS.o)


# Clang as linker
# --no-pie disable position independent executable, which is required when building
# statically linked executables.
set(CMAKE_CXX_LINK_EXECUTABLE "clang++ --target=${triple} -Wl,--no-pie --sysroot=${CMAKE_SYSROOT} ${CMAKE_CXX_FLAGS} -fuse-ld=${cmake_linker_with_dir} <CMAKE_CXX_LINK_FLAGS> <LINK_FLAGS> <LINK_LIBRARIES> <OBJECTS> -o  <TARGET> ")
set(CMAKE_CXX_CREATE_SHARED_LIBRARY "clang++ -Wl, --target=${triple} --sysroot=${CMAKE_SYSROOT} ${CMAKE_CXX_FLAGS} -fuse-ld=${cmake_linker_with_dir} -shared <CMAKE_CXX_LINK_FLAGS> <LINK_FLAGS> <LINK_LIBRARIES> <OBJECTS> -o  <TARGET> ")
#
# Do not use CMAKE_CXX_CREATE_STATIC_LIBRARY -- it is created automatically
# by cmake using ar and ranlib
#
#set(CMAKE_CXX_CREATE_STATIC_LIBRARY "clang++ -Wl,--no-pie,--no-export-dynamic,-v -v --target=${triple} --sysroot=${CMAKE_SYSROOT} ${CMAKE_CXX_FLAGS} -fuse-ld=ld.lld <CMAKE_CXX_LINK_FLAGS> <LINK_FLAGS> <LINK_LIBRARIES> <OBJECTS> -o  <TARGET> ")


## Linker as linker
set(CMAKE_LINKER_ARM_COMPAT_STATIC "-pie -EL -z relro -X --hash-style=gnu --eh-frame-hdr -m armelf_linux_eabi -dynamic-linker /lib/ld-musl-armhf.so.1 ${Scrt1_o} ${crti_o} ${crtbeginS_o} -lstdc++ -lm -lgcc_s -lgcc -lc ${crtendS_o} ${crtn_o}")
set(CMAKE_C_LINK_EXECUTABLE "${CMAKE_LINKER} ${CMAKE_LINKER_ARM_COMPAT_STATIC} <CMAKE_C_LINK_FLAGS> <LINK_FLAGS> <LINK_LIBRARIES> <OBJECTS> -o  <TARGET>")


# Debian bug 708744(?)
#include_directories("${CMAKE_SYSROOT}/usr/include/")
#include_directories("${CMAKE_SYSROOT}/usr/include/c++/${gcc_version}")
#include_directories("${CMAKE_SYSROOT}/usr/include/c++/${gcc_version}/${triple}")

## Clang workarounds:
set(toolchain_lib_dir_0 "${CMAKE_SYSROOT}/lib")
set(toolchain_lib_dir_1 "${CMAKE_SYSROOT}/../lib")
set(toolchain_lib_dir_2 "${CMAKE_SYSROOT}/../lib/gcc/${triple}/${gcc_version}")
set(CMAKE_TOOLCHAIN_LINK_FLAGS "-L${toolchain_lib_dir_0} -L${toolchain_lib_dir_1} -L${toolchain_lib_dir_2}")

## CMake workarounds
set(CMAKE_EXE_LINKER_FLAGS ${CMAKE_TOOLCHAIN_LINK_FLAGS} CACHE INTERNAL "exe link flags")
set(CMAKE_MODULE_LINKER_FLAGS ${CMAKE_TOOLCHAIN_LINK_FLAGS} CACHE INTERNAL "module link flags")
set(CMAKE_SHARED_LINKER_FLAGS ${CMAKE_TOOLCHAIN_LINK_FLAGS} CACHE INTERNAL "shared link flags")



unset(cmake_linker_with_dir)
unset(cmake_linker_dir)