Switching between GCC and Clang LLVM using CMake

Question

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

User · Answer

If the default compiler chosen by cmake is gcc and you have installed clang  you can use the easy way to compile your project with clang     mkdir build  amp  amp  cd build   CXX clang   CC clang cmake      make -j2

User · Answer

System wide C   change on Ubuntu   sudo apt-get install clang sudo update-alternatives --config c     Will print something like this     Selection    Path              Priority   Status ------------------------------------------------------------   0             usr bin g         20        auto mode   1             usr bin clang     10        manual mode   2             usr bin g         20        manual mode   Then just select clang

User · Answer

You can use the option command   option USE CLANG  build application with clang  OFF    OFF is the default   and then wrap the clang-compiler settings in if  s   if USE CLANG      SET                endif USE CLANG    This way it is displayed as an cmake option in the gui-configuration tools   To make it systemwide you can of course use an environment variable as the default value or stay with Ferruccio s answer

User · Answer

You can use the syntax   ENV environment-variable  in your CMakeLists txt to access environment variables  You could create scripts which initialize a set of environment variables appropriately and just have references to those variables in your CMakeLists txt files

User · Answer

You definitely don t need to use the various different llvm-ar etc programs    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      These are made to work on the llvm internal format and as such aren t useful to the build of your application   As a note -O4 will invoke LTO on your program which you may not want  it will increase compile time greatly  and clang defaults to c99 mode so that flag isn t necessarily needed either

User · Answer

You can use the toolchain file mechanism of cmake  for this purpose  see e g  here  You write a toolchain file for each compiler containing the corresponding definitions  At config time  you run e g   cmake -DCMAKE TOOLCHAIN FILE  path to clang-toolchain cmake      and all the compiler information will be set during the project   call from the toolchain file  Though in the documentation is mentionend only in the context of cross-compiling  it works as well for different compilers on the same system

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CMake honors the environment variables CC and CXX upon detecting the C and C   compiler to use    export CC  usr bin clang   export CXX  usr bin clang     cmake    -- The C compiler identification is Clang -- The CXX compiler identification is Clang  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  SET  CMAKE C FLAGS INIT                 quot -Wall -std c99 quot   SET  CMAKE C FLAGS DEBUG INIT           quot -g quot   SET  CMAKE C FLAGS MINSIZEREL INIT      quot -Os -DNDEBUG quot   SET  CMAKE C FLAGS RELEASE INIT         quot -O3 -DNDEBUG quot   SET  CMAKE C FLAGS RELWITHDEBINFO INIT  quot -O2 -g quot    SET  CMAKE CXX FLAGS INIT                 quot -Wall quot   SET  CMAKE CXX FLAGS DEBUG INIT           quot -g quot   SET  CMAKE CXX FLAGS MINSIZEREL INIT      quot -Os -DNDEBUG quot   SET  CMAKE CXX FLAGS RELEASE INIT         quot -O3 -DNDEBUG quot   SET  CMAKE CXX FLAGS RELWITHDEBINFO INIT  quot -O2 -g quot    The suffix  INIT will make CMake initialize the corresponding   FLAGS variable with the given value  Then invoke cmake in the following way    cmake -DCMAKE USER MAKE RULES OVERRIDE   ClangOverrides txt     Finally to force the use of the LLVM binutils  set the internal variable  CMAKE TOOLCHAIN PREFIX  This variable is honored by the CMakeFindBinUtils module    cmake -D CMAKE TOOLCHAIN PREFIX llvm-     Putting this all together you can write a shell wrapper which sets up the environment variables CC and CXX and then invokes cmake with the mentioned variable overrides  Also see this CMake FAQ on make override files

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System wide C change on Ubuntu   sudo update-alternatives --config cc  System wide C   change on Ubuntu   sudo update-alternatives --config c    For each of the above  press Selection number  1  and Enter to select Clang     Selection    Path            Priority   Status ------------------------------------------------------------   0             usr bin gcc     20        auto mode   1             usr bin clang   10        manual mode   2             usr bin gcc     20        manual mode Press enter to keep the current choice     or type selection number

User · Answer

According to the help of cmake   -C  lt initial-cache gt       Pre-load a script to populate the cache        When cmake is first run in an empty build tree  it creates a CMakeCache txt file and populates it with customizable settings for the project   This option may be used to specify a  file  from      which to load cache entries before the first pass through the project s cmake listfiles   The loaded entries take priority over the project s default values   The given file should be a CMake      script containing SET commands that use the CACHE option  not a cache-format file    You make be able to create files like gcc compiler txt and clang compiler txt to includes all relative configuration in CMake syntax    Clang Example  clang compiler txt     set CMAKE C COMPILER   usr bin clang  CACHE string  clang compiler  FORCE    Then run it as  GCC   cmake -C gcc compiler txt XXXXXXXX   Clang   cmake -C clang compiler txt XXXXXXXX

[cmake] Switching between GCC and Clang/LLVM using CMake

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