Mac 使用 OpenMP/Clang

新建 hello.cpp 文件：

#include <omp.h>
#include <stdio.h>
int main() {
    #pragma omp parallel
    printf("Hello from thread %d, nthreads %d
", omp_get_thread_num(), omp_get_num_threads());
    return 0;
}

编译会遇到如下错误：

hello.cpp:1:10: fatal error: 'omp.h' file not found
#include <omp.h>
          ^~~~~~
1 error generated.

因为默认的 g++ 编译器不支持 openmp，我们可以设置 LLVM/Clang 编译器来编译 openmp。
执行以下命令：

brew install llvm # 安装 LLVM 编译器
brew install libomp # 安装 OpenMP 库
echo 'export PATH="/usr/local/opt/llvm/bin:$PATH"' >> ~/.bash_profile # 将 llvm 的可执行文件添加到 PATH 目录

然后执行

clang -fopenmp hello.cpp -o hello
./hello

我的CPP文件中用到了STL 中的 vector，然后就遇到了新的错误：

Undefined symbols for architecture x86_64:
  "std::__1::__vector_base_common<true>::__throw_length_error() const", referenced from:
      std::__1::vector<int, std::__1::allocator<int> >::assign(unsigned long, int const&) in seq-3efcea.o
      std::__1::vector<std::__1::vector<int, std::__1::allocator<int> >, std::__1::allocator<std::__1::vector<int, std::__1::allocator<int> > > >::allocate(unsigned long) in seq-3efcea.o
      std::__1::vector<int, std::__1::allocator<int> >::allocate(unsigned long) in seq-3efcea.o
      void std::__1::vector<int, std::__1::allocator<int> >::__push_back_slow_path<int>(int&&) in seq-3efcea.o
  "std::logic_error::logic_error(char const*)", referenced from:
      std::__1::vector<std::__1::vector<int, std::__1::allocator<int> >, std::__1::allocator<std::__1::vector<int, std::__1::allocator<int> > > >::allocate(unsigned long) in seq-3efcea.o
      std::__1::vector<int, std::__1::allocator<int> >::allocate(unsigned long) in seq-3efcea.o
      std::__1::__split_buffer<int, std::__1::allocator<int>&>::__split_buffer(unsigned long, unsigned long, std::__1::allocator<int>&) in seq-3efcea.o
      std::__1::deque<int, std::__1::allocator<int> >::__add_back_capacity() in seq-3efcea.o
      std::__1::__split_buffer<int*, std::__1::allocator<int*>&>::__split_buffer(unsigned long, unsigned long, std::__1::allocator<int*>&) in seq-3efcea.o
  "std::length_error::~length_error()", referenced from:
      std::__1::vector<std::__1::vector<int, std::__1::allocator<int> >, std::__1::allocator<std::__1::vector<int, std::__1::allocator<int> > > >::allocate(unsigned long) in seq-3efcea.o
      std::__1::vector<int, std::__1::allocator<int> >::allocate(unsigned long) in seq-3efcea.o
      std::__1::__split_buffer<int, std::__1::allocator<int>&>::__split_buffer(unsigned long, unsigned long, std::__1::allocator<int>&) in seq-3efcea.o
      std::__1::deque<int, std::__1::allocator<int> >::__add_back_capacity() in seq-3efcea.o
      std::__1::__split_buffer<int*, std::__1::allocator<int*>&>::__split_buffer(unsigned long, unsigned long, std::__1::allocator<int*>&) in seq-3efcea.o
  "std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >::__init(char const*, unsigned long)", referenced from:
      _main in seq-3efcea.o
  "std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >::insert(unsigned long, char const*)", referenced from:
      _main in seq-3efcea.o
  "std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >::~basic_string()", referenced from:
      _main in seq-3efcea.o
  "std::terminate()", referenced from:
      ___clang_call_terminate in seq-3efcea.o
  "typeinfo for std::length_error", referenced from:
      std::__1::vector<std::__1::vector<int, std::__1::allocator<int> >, std::__1::allocator<std::__1::vector<int, std::__1::allocator<int> > > >::allocate(unsigned long) in seq-3efcea.o
      std::__1::vector<int, std::__1::allocator<int> >::allocate(unsigned long) in seq-3efcea.o
      std::__1::__split_buffer<int, std::__1::allocator<int>&>::__split_buffer(unsigned long, unsigned long, std::__1::allocator<int>&) in seq-3efcea.o
      std::__1::deque<int, std::__1::allocator<int> >::__add_back_capacity() in seq-3efcea.o
      std::__1::__split_buffer<int*, std::__1::allocator<int*>&>::__split_buffer(unsigned long, unsigned long, std::__1::allocator<int*>&) in seq-3efcea.o
  "vtable for std::length_error", referenced from:
      std::__1::vector<std::__1::vector<int, std::__1::allocator<int> >, std::__1::allocator<std::__1::vector<int, std::__1::allocator<int> > > >::allocate(unsigned long) in seq-3efcea.o
      std::__1::vector<int, std::__1::allocator<int> >::allocate(unsigned long) in seq-3efcea.o
      std::__1::__split_buffer<int, std::__1::allocator<int>&>::__split_buffer(unsigned long, unsigned long, std::__1::allocator<int>&) in seq-3efcea.o
      std::__1::deque<int, std::__1::allocator<int> >::__add_back_capacity() in seq-3efcea.o
      std::__1::__split_buffer<int*, std::__1::allocator<int*>&>::__split_buffer(unsigned long, unsigned long, std::__1::allocator<int*>&) in seq-3efcea.o
  NOTE: a missing vtable usually means the first non-inline virtual member function has no definition.
  "operator delete(void*)", referenced from:
      std::__1::__vector_base<int, std::__1::allocator<int> >::~__vector_base() in seq-3efcea.o
      std::__1::__deque_base<int, std::__1::allocator<int> >::~__deque_base() in seq-3efcea.o
      std::__1::__deque_base<int, std::__1::allocator<int> >::clear() in seq-3efcea.o
      std::__1::__split_buffer<int*, std::__1::allocator<int*> >::~__split_buffer() in seq-3efcea.o
      std::__1::__vector_base<std::__1::vector<int, std::__1::allocator<int> >, std::__1::allocator<std::__1::vector<int, std::__1::allocator<int> > > >::~__vector_base() in seq-3efcea.o
      std::__1::__split_buffer<int, std::__1::allocator<int>&>::~__split_buffer() in seq-3efcea.o
      std::__1::deque<int, std::__1::allocator<int> >::__add_back_capacity() in seq-3efcea.o
      ...
  "operator new(unsigned long)", referenced from:
      std::__1::vector<std::__1::vector<int, std::__1::allocator<int> >, std::__1::allocator<std::__1::vector<int, std::__1::allocator<int> > > >::allocate(unsigned long) in seq-3efcea.o
      std::__1::vector<int, std::__1::allocator<int> >::allocate(unsigned long) in seq-3efcea.o
      std::__1::__split_buffer<int, std::__1::allocator<int>&>::__split_buffer(unsigned long, unsigned long, std::__1::allocator<int>&) in seq-3efcea.o
      std::__1::deque<int, std::__1::allocator<int> >::__add_back_capacity() in seq-3efcea.o
      std::__1::__split_buffer<int*, std::__1::allocator<int*>&>::__split_buffer(unsigned long, unsigned long, std::__1::allocator<int*>&) in seq-3efcea.o
  "___cxa_allocate_exception", referenced from:
      std::__1::vector<std::__1::vector<int, std::__1::allocator<int> >, std::__1::allocator<std::__1::vector<int, std::__1::allocator<int> > > >::allocate(unsigned long) in seq-3efcea.o
      std::__1::vector<int, std::__1::allocator<int> >::allocate(unsigned long) in seq-3efcea.o
      std::__1::__split_buffer<int, std::__1::allocator<int>&>::__split_buffer(unsigned long, unsigned long, std::__1::allocator<int>&) in seq-3efcea.o
      std::__1::deque<int, std::__1::allocator<int> >::__add_back_capacity() in seq-3efcea.o
      std::__1::__split_buffer<int*, std::__1::allocator<int*>&>::__split_buffer(unsigned long, unsigned long, std::__1::allocator<int*>&) in seq-3efcea.o
  "___cxa_begin_catch", referenced from:
      ___clang_call_terminate in seq-3efcea.o
      std::__1::__split_buffer<int*, std::__1::allocator<int*> >::shrink_to_fit() in seq-3efcea.o
  "___cxa_end_catch", referenced from:
      std::__1::__split_buffer<int*, std::__1::allocator<int*> >::shrink_to_fit() in seq-3efcea.o
  "___cxa_free_exception", referenced from:
      std::__1::vector<std::__1::vector<int, std::__1::allocator<int> >, std::__1::allocator<std::__1::vector<int, std::__1::allocator<int> > > >::allocate(unsigned long) in seq-3efcea.o
      std::__1::vector<int, std::__1::allocator<int> >::allocate(unsigned long) in seq-3efcea.o
      std::__1::__split_buffer<int, std::__1::allocator<int>&>::__split_buffer(unsigned long, unsigned long, std::__1::allocator<int>&) in seq-3efcea.o
      std::__1::deque<int, std::__1::allocator<int> >::__add_back_capacity() in seq-3efcea.o
      std::__1::__split_buffer<int*, std::__1::allocator<int*>&>::__split_buffer(unsigned long, unsigned long, std::__1::allocator<int*>&) in seq-3efcea.o
  "___cxa_throw", referenced from:
      std::__1::vector<std::__1::vector<int, std::__1::allocator<int> >, std::__1::allocator<std::__1::vector<int, std::__1::allocator<int> > > >::allocate(unsigned long) in seq-3efcea.o
      std::__1::vector<int, std::__1::allocator<int> >::allocate(unsigned long) in seq-3efcea.o
      std::__1::__split_buffer<int, std::__1::allocator<int>&>::__split_buffer(unsigned long, unsigned long, std::__1::allocator<int>&) in seq-3efcea.o
      std::__1::deque<int, std::__1::allocator<int> >::__add_back_capacity() in seq-3efcea.o
      std::__1::__split_buffer<int*, std::__1::allocator<int*>&>::__split_buffer(unsigned long, unsigned long, std::__1::allocator<int*>&) in seq-3efcea.o
  "___gxx_personality_v0", referenced from:
      _main in seq-3efcea.o
      std::__1::vector<std::__1::vector<int, std::__1::allocator<int> >, std::__1::allocator<std::__1::vector<int, std::__1::allocator<int> > > >::vector(unsigned long, std::__1::vector<int, std::__1::allocator<int> > const&) in seq-3efcea.o
      std::__1::vector<std::__1::vector<int, std::__1::allocator<int> >, std::__1::allocator<std::__1::vector<int, std::__1::allocator<int> > > >::allocate(unsigned long) in seq-3efcea.o
      std::__1::vector<int, std::__1::allocator<int> >::vector(std::__1::vector<int, std::__1::allocator<int> > const&) in seq-3efcea.o
      std::__1::vector<int, std::__1::allocator<int> >::allocate(unsigned long) in seq-3efcea.o
      std::__1::vector<int, std::__1::allocator<int> >::vector(unsigned long, int const&) in seq-3efcea.o
      void std::__1::vector<int, std::__1::allocator<int> >::__push_back_slow_path<int>(int&&) in seq-3efcea.o
      ...
ld: symbol(s) not found for architecture x86_64
clang-6.0: error: linker command failed with exit code 1 (use -v to see invocation)

是因为我们编译的是 C++文件，clang 没有链接 STL 库所以出错了，我们可以显式地链接标准库：

clang -fopenmp hello.cpp -o hello -lstdc++
#或者 clang -fopenmp hello.cpp -o hello -lc++
./hello 

也可以直接用 clang++：

clang++ -fopenmp hello.cpp -o hello
./hello