I have a number of projects built using CMake and I'd like to be able to easily switch between using GCC or Clang/LLVM to compile them. I believe (please correct me if I'm mistaken!) that to use Clang I need to set the following:
SET (CMAKE_C_COMPILER "/usr/bin/clang")
SET (CMAKE_C_FLAGS "-Wall -std=c99")
SET (CMAKE_C_FLAGS_DEBUG "-g")
SET (CMAKE_C_FLAGS_MINSIZEREL "-Os -DNDEBUG")
SET (CMAKE_C_FLAGS_RELEASE "-O4 -DNDEBUG")
SET (CMAKE_C_FLAGS_RELWITHDEBINFO "-O2 -g")
SET (CMAKE_CXX_COMPILER "/usr/bin/clang++")
SET (CMAKE_CXX_FLAGS "-Wall")
SET (CMAKE_CXX_FLAGS_DEBUG "-g")
SET (CMAKE_CXX_FLAGS_MINSIZEREL "-Os -DNDEBUG")
SET (CMAKE_CXX_FLAGS_RELEASE "-O4 -DNDEBUG")
SET (CMAKE_CXX_FLAGS_RELWITHDEBINFO "-O2 -g")
SET (CMAKE_AR "/usr/bin/llvm-ar")
SET (CMAKE_LINKER "/usr/bin/llvm-ld")
SET (CMAKE_NM "/usr/bin/llvm-nm")
SET (CMAKE_OBJDUMP "/usr/bin/llvm-objdump")
SET (CMAKE_RANLIB "/usr/bin/llvm-ranlib")
Is there an easy way of switching between these and the default GCC variables, preferably as a system-wide change rather than project specific (i.e. not just adding them into a project's CMakeLists.txt)?
Also, is it necessary to use the llvm-*
programs rather than the system defaults when compiling using clang instead of gcc? What's the difference?
Best Answer
CMake honors the environment variables
CC
andCXX
upon detecting the C and C++ compiler to use:The compiler specific flags can be overridden by putting them into a make override file and pointing the
CMAKE_USER_MAKE_RULES_OVERRIDE
variable to it. Create a file~/ClangOverrides.txt
with the following contents:The suffix
_INIT
will make CMake initialize the corresponding*_FLAGS
variable with the given value. Then invokecmake
in the following way:Finally to force the use of the LLVM binutils, set the internal variable
_CMAKE_TOOLCHAIN_PREFIX
. This variable is honored by theCMakeFindBinUtils
module:Putting this all together you can write a shell wrapper which sets up the environment variables
CC
andCXX
and then invokescmake
with the mentioned variable overrides.Also see this CMake FAQ on make override files.