CMake 3.9对 OpenMP 的支持有了很大的提升，但是根据Modern CMake中的教程，如下配置可能不会生效：

find_package(OpenMP)
if(OpenMP_CXX_FOUND)
    target_link_libraries(MyTarget PUBLIC OpenMP::OpenMP_CXX)
endif()

而是可能出现如下提示：

-- Could NOT find OpenMP_C (missing: OpenMP_C_FLAGS OpenMP_C_LIB_NAMES) 
-- Could NOT find OpenMP_CXX (missing: OpenMP_CXX_FLAGS OpenMP_CXX_LIB_NAMES) 
-- Could NOT find OpenMP (missing: OpenMP_C_FOUND OpenMP_CXX_FOUND)

解决办法为：

1. 先安装 libomp：

brew install libomp

2. 如果安装后再运行CMake无错误提示，那就直接可以用。如果还是报同样的错误，则需要继续设置环境变量：

export OpenMP_ROOT=$(brew --prefix)/opt/libomp

参考

  1. How to find OpenMP with clang on macOS

2. OpenMP simple C example with CMake

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