I'm trying to generate Ninja makefiles to cross compile a C++ project with Clang for an ARM Cortex A5 CPU. I created a toolchain file for CMake but it seems like there is an error or something missing which I'm unable to find. When invoke CMake with the toolchain file below I get the following error.
CMake command:
cmake -DCMAKE_TOOLCHAIN_FILE="..\Src\Build\Toolchain-clang-arm.cmake"
-GNinja ..\Src\
Output:
— The C compiler identification is Clang 7.0.0 CMake Error at C:/Users/user/scoop/apps/cmake/3.13.4/share/cmake-3.13/Modules/CMakeDetermineCompilerId.cmake:802
(message): The Clang compiler tool"C:/Program Files/LLVM/bin/clang.exe"
targets the MSVC ABI but has a GNU-like command-line interface.
This is not supported. Use 'clang-cl' instead, e.g. by setting
'CC=clang-cl' in the environment. Furthermore, use the MSVC
command-line environment. Call Stack (most recent call first):
C:/Users/user/scoop/apps/cmake/3.13.4/share/cmake-3.13/Modules/CMakeDetermineCCompiler.cmake:113
(CMAKE_DIAGNOSE_UNSUPPORTED_CLANG) CMakeLists.txt:2 (PROJECT)— Configuring incomplete, errors occurred!
CMake toolchain file (Toolchain-clang-arm.cmake):
set(CMAKE_CROSSCOMPILING TRUE)
SET(CMAKE_SYSTEM_NAME Generic)
set(CMAKE_SYSTEM_PROCESSOR arm)
# Clang target triple
SET(TARGET armv7-none-eabi)
# specify the cross compiler
SET(CMAKE_C_COMPILER_TARGET ${TARGET})
SET(CMAKE_C_COMPILER clang)
SET(CMAKE_CXX_COMPILER_TARGET ${TARGET})
SET(CMAKE_CXX_COMPILER clang++)
SET(CMAKE_ASM_COMPILER_TARGET ${TARGET})
SET(CMAKE_ASM_COMPILER clang)
# C/C++ toolchain
SET(TOOLCHAIN "C:/Program Files (x86)/GNU Tools ARM Embedded/7 2018-q2-update")
SET(CMAKE_C_COMPILER_EXTERNAL_TOOLCHAIN ${TOOLCHAIN})
SET(CMAKE_CXX_COMPILER_EXTERNAL_TOOLCHAIN ${TOOLCHAIN})
# specify compiler flags
SET(ARCH_FLAGS "-target armv7-none-eabi -mcpu=cortex-a5")
SET(CMAKE_C_FLAGS "-Wall -Wextra ${ARCH_FLAGS}" CACHE STRING "Common flags for C compiler")
SET(CMAKE_CXX_FLAGS "-Wall -Wextra -std=c++11 -fno-exceptions -fno-threadsafe-statics ${ARCH_FLAGS}" CACHE STRING "Common flags for C++ compiler")
I used the CMake and Clang documentation and some random link from the net to create the toolchain file. The whole project compiles fine with ARM GCC for Windows, so the toolchain file seems to be the only missing piece of the puzzle.
EDIT
I tried to work around the CMake compiler checks by forcing the compiler. I replace the lines with SET(CMAKE_C_COMPILER clang), SET(CMAKE_CXX_COMPILER clang++) etc. with:
CMAKE_FORCE_C_COMPILER(clang Clang)
CMAKE_FORCE_CXX_COMPILER(clang++ Clang)
The error stays the same.
EDIT
I can successfully compile a hello world example with clang -target arm-none-eabi. So the issue seems to be in CMake. I created a bug in the CMake issue tracker.
Tool versions:
- clang version 7.0.0 (tags/RELEASE_700/final)
- cmake version 3.13.4
Best Answer
My question was answered through the reply to my bug report, but I add the answer here to have all information in one place for future reference.
In short: CMake currently does not support to use the clang/clang++ command line interface if you install Clang from llvm.org. If you want to use the clang/clang++ interface (which is necessary to cross-compile for ARM) you have to install Clang via msys2.
In detail
Clang on Windows has two different command line interfaces:
In order to cross-compile for ARM you need the clang/clang++ interface. The Problem is CMake supports different interfaces depending on how you installed Clang (see the bug in the CMake issue tracker for more details):
So here is what I did:
Toolchain file
There were two problems with my original toolchain file that took my a long time to fix. So I hope this will save others some time:
Here is the final toolchain file I used (you can also find a good example for a toolchain file in this GitHub repo):