MacOS, CMake and OpenMP

Question

I am using the newest CMake (3.9.3) from Homebrew along with LLVM 5.0.0 also from Brew, because Clang here has OpenMP support.

This worked in CMake 3.8.2 with LLVM 5

Answer 1

Maybe it's a CMake version thing, I come up with a slightly different solution with Franzi's.

I also use brew install libomp on my machine. It seems like OpenMP_CXX_FLAGS is used for compiling project source code instead of compiling the omp (the flag is stored in the omp target and will be populated by command target_link_libraries).

Besides that, OpenMP_CXX_LIB_NAMES shouldn't have prefix lib because it will cause an error like -llibomp not found, where -lomp should be used instead.

I also noticed that CMAKE_C_COMPILER_ID is AppleClang instead of Clang if I put project(playground) after cmake_minimum_required. In reverse, it's Clang, quite annoying and I don't know why.

Xpreprocessor used here is because apple clang doesn't ship with OpenMP and this flag tells the compiler to look for pragma (preprocessor expansion) elsewhere. In our case, it's the header files in the include path where the libomp is installed.

cmake_minimum_required(VERSION 3.12)

project(playground)

if(APPLE)
    set(CMAKE_C_COMPILER clang)
    set(CMAKE_CXX_COMPILER clang++)

    if(CMAKE_C_COMPILER_ID MATCHES "Clang\$")
        set(OpenMP_C_FLAGS "-Xpreprocessor -fopenmp")
        set(OpenMP_C_LIB_NAMES "omp")
        set(OpenMP_omp_LIBRARY omp)
    endif()

    if(CMAKE_CXX_COMPILER_ID MATCHES "Clang\$")
        set(OpenMP_CXX_FLAGS "-Xpreprocessor -fopenmp")
        set(OpenMP_CXX_LIB_NAMES "omp")
        set(OpenMP_omp_LIBRARY omp)
    endif()

endif()

find_package(OpenMP REQUIRED)

add_executable(helloworld helloworld.cxx)
target_link_libraries(helloworld PRIVATE OpenMP::OpenMP_CXX)

Here's my helloworld

#include 
#include 
#include 

int main(void)
{
    #pragma omp parallel
    {
      std::stringstream ss;
      ss << std::this_thread::get_id();
      printf("%s, Hello, world.\n", ss.str().c_str());
    }

  return 0;
}

output is,

0x700002dc8000, Hello, world.
0x10a17d5c0, Hello, world.
0x7000045d1000, Hello, world.
0x7000055d7000, Hello, world.
0x700005dda000, Hello, world.
0x7000035cb000, Hello, world.
0x7000065dd000, Hello, world.
0x700003dce000, Hello, world.
0x700007de6000, Hello, world.
0x700004dd4000, Hello, world.
0x7000075e3000, Hello, world.
0x700006de0000, Hello, world.