Simple OpenMP-enabled code does not compile: "clang: error: linker command failed with exit code 1"

Question

我尝试使用 Xcode 附带的 Apple Clang 编译器在 MacOS 上测试 OpenMP。

代码：

#include <iostream>
#include <cmath>
#include </usr/local/opt/libomp/include/omp.h>

int main() {
    std::cout << "Hello, World!" << std::endl;
    #pragma omp parallel
    {
        printf("Hello World! \n");
    }
    return 0;
}

这是我的 CMakeLists.txt:

cmake_minimum_required(VERSION 3.19)
project(untitled)

set(CMAKE_CXX_STANDARD 14)
include (FindOpenMP)
if (OPENMP_FOUND)
    set (CMAKE_C_FLAGS "${CMAKE_C_FLAGS} ${OpenMP_C_FLAGS}")
    set (CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} ${OpenMP_CXX_FLAGS}")
    set (CMAKE_EXE_LINKER_FLAGS "${CMAKE_EXE_LINKER_FLAGS} ${OpenMP_EXE_LINKER_FLAGS}")
    message (VERBOSE "OpenML library found")
else()
    message (FATAL_ERROR "OpenML library not found (required for multithreading)")
endif ()
add_executable(untitled main.cpp)

这是编译器的错误：

Scanning dependencies of target untitled
[ 50%] Building CXX object CMakeFiles/untitled.dir/main.cpp.o
[100%] Linking CXX executable untitled
Undefined symbols for architecture x86_64:
  "___kmpc_fork_call", referenced from:
      _main in main.cpp.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)
make[3]: *** [untitled] Error 1
make[2]: *** [CMakeFiles/untitled.dir/all] Error 2
make[1]: *** [CMakeFiles/untitled.dir/rule] Error 2
make: *** [untitled] Error 2

OpenMP是用hombrew安装的libomp我也试过添加环境变量LDFLAGS=-L/usr/local/opt/libomp/lib/不影响结果

---

更新：使用 VERBOSE=1 进行编译：

...
[100%] Linking CXX executable untitled
/Applications/CLion.app/Contents/bin/cmake/mac/bin/cmake -E cmake_link_script CMakeFiles/untitled.dir/link.txt --verbose=1
/Library/Developer/CommandLineTools/usr/bin/c++  -Xclang -fopenmp -g -isysroot /Library/Developer/CommandLineTools/SDKs/MacOSX11.3.sdk -Wl,-search_paths_first -Wl,-headerpad_max_install_names -L/usr/local/opt/libomp/lib/  CMakeFiles/untitled.dir/main.cpp.o -o untitled 
Undefined symbols for architecture x86_64:
  "___kmpc_fork_call", referenced from:
      _main in main.cpp.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)
make[3]: *** [untitled] Error 1
make[2]: *** [CMakeFiles/untitled.dir/all] Error 2
make[1]: *** [CMakeFiles/untitled.dir/rule] Error 2
make: *** [untitled] Error 2

Answer 1

通过在其他地方使用 set (CMAKE_C_FLAGS "${CMAKE_C_FLAGS} ${OpenMP_C_FLAGS}") 和使用 set，您将替换 CMAKE 放置的标志。在这种情况下最好附加而不是替换。您可以使用 add_compile_options(-Wall) .

如@Tsyvarev 所述，使用导入的库可能更好。 Here 是您可以使用的其他几种方式 target_link_libraries。

对于您的具体情况，最好更换

find_package(OpenMP REQUIRED)

您目前 include (FindOpenMP)。此外，还要删除 if 条件。最后，将以下内容添加到 CMakeLists.txt 文件的末尾。

target_link_libraries(
  untitled
  PRIVATE
  OpenMP::OpenMP_CXX
)

Here很好地解释了PRIVATE和PUBLIC之间的区别。

您的最终 CMakeLists.txt 文件应如下所示：

cmake_minimum_required(VERSION 3.19)
project(untitled)

set(CMAKE_CXX_STANDARD 14)
find_package(OpenMP REQUIRED)

add_executable(
  untitled 
  main.cpp
)

target_link_libraries(
  untitled
  PRIVATE
  OpenMP::OpenMP_CXX
)

感谢 Tsyvarev，几个月前帮助我使用 CMake。此外，感谢 Tsyvarev 指出的答案，我的答案基于这些答案（连同我自己的测试）。