OMP Error: /usr/include/eigen3/Eigen/src/Core/products/GeneralMatrixMatrix.h:93: undefined reference to `omp_get_num_threads' ---> Solution to this error

Error:

  /usr/include/eigen3/Eigen/src/Core/products/GeneralMatrixMatrix.h:93: undefined reference to `omp_get_num_threads'
  CMakeFiles/ndt_feature_eval.dir/src/ndt_feature_eval.o: In function `void Eigen::internal::parallelize_gemm, Eigen::Transpose const>, Eigen::Matrix, Eigen::Matrix, Eigen::internal::gemm_blocking_space<0 -1="" double="" false=""> >, int>(Eigen::internal::gemm_functor, Eigen::Transpose const>, Eigen::Matrix, Eigen::Matrix, Eigen::internal::gemm_blocking_space<0 -1="" double="" false=""> > const&, int, int, bool)':
  /usr/include/eigen3/Eigen/src/Core/products/Parallelizer.h:105: undefined reference to `omp_get_num_threads'
  /usr/include/eigen3/Eigen/src/Core/products/Parallelizer.h:145: undefined reference to `GOMP_parallel_start'
  /usr/include/eigen3/Eigen/src/Core/products/Parallelizer.h:145: undefined reference to `GOMP_parallel_end'
  CMakeFiles/ndt_feature_eval.dir/src/ndt_feature_eval.o: In function `nbThreads':
  /usr/include/eigen3/Eigen/src/Core/products/Parallelizer.h:47: undefined reference to `omp_get_max_threads'
  collect2: ld returned 1 exit status
  make[3]: *** [../bin/ndt_feature_eval] Error 1
  make[3]: Leaving directory `/home/sai/fuerte_workspace/oru-ros-pkg-read-only/perception_oru/ndt_feature_reg/build'
  make[2]: *** [CMakeFiles/ndt_feature_eval.dir/all] Error 2
  make[2]: *** Waiting for unfinished jobs....
  Linking CXX executable ../bin/ndt_feature_reg_node
  CMakeFiles/ndt_feature_reg_node.dir/src/ndt_feature_reg_node.o: In function `lslgeneric::NDTMatcherD2D::derivativesNDT(std::vector*, std::allocator*> > const&, lslgeneric::NDTMap const&, Eigen::Matrix&, Eigen::Matrix&, bool) [clone ._omp_fn.0]':
  /home/sai/fuerte_workspace/oru-ros-pkg-read-only/perception_oru/ndt_registration/include/impl/ndt_matcher_d2d.hpp:1476: undefined reference to `omp_get_num_threads'
  /home/sai/fuerte_workspace/oru-ros-pkg-read-only/perception_oru/ndt_registration/include/impl/ndt_matcher_d2d.hpp:1476: undefined reference to `omp_get_thread_num'
  /home/sai/fuerte_workspace/oru-ros-pkg-read-only/perception_oru/ndt_registration/include/impl/ndt_matcher_d2d.hpp:1476: undefined reference to `GOMP_barrier'
  CMakeFiles/ndt_feature_reg_node.dir/src/ndt_feature_reg_node.o: In function `void Eigen::internal::parallelize_gemm, Eigen::Transpose const>, Eigen::Matrix, Eigen::Matrix, Eigen::internal::gemm_blocking_space<0 -1="" double="" false=""> >, int>(Eigen::internal::gemm_functor, Eigen::Transpose const>, Eigen::Matrix, Eigen::Matrix, Eigen::internal::gemm_blocking_space<0 -1="" double="" false=""> > const&, int, int, bool) [clone ._omp_fn.1]':
  /usr/include/eigen3/Eigen/src/Core/products/Parallelizer.h:131: undefined reference to `omp_get_num_threads'
  /usr/include/eigen3/Eigen/src/Core/products/Parallelizer.h:131: undefined reference to `omp_get_thread_num'
  CMakeFiles/ndt_feature_reg_node.dir/src/ndt_feature_reg_node.o: In function `lslgeneric::NDTMatcherD2D::derivativesNDT(std::vector*, std::allocator*> > const&, lslgeneric::NDTMap const&, Eigen::Matrix&, Eigen::Matrix&, bool)':
  /home/sai/fuerte_workspace/oru-ros-pkg-read-only/perception_oru/ndt_registration/include/impl/ndt_matcher_d2d.hpp:1550: undefined reference to `GOMP_parallel_start'
  /home/sai/fuerte_workspace/oru-ros-pkg-read-only/perception_oru/ndt_registration/include/impl/ndt_matcher_d2d.hpp:1550: undefined reference to `GOMP_parallel_end'
  CMakeFiles/ndt_feature_reg_node.dir/src/ndt_feature_reg_node.o: In function `Eigen::internal::general_matrix_matrix_product::run(int, int, int, double const*, int, double const*, int, double*, int, double, Eigen::internal::level3_blocking&, Eigen::internal::GemmParallelInfo*)':
  /usr/include/eigen3/Eigen/src/Core/products/GeneralMatrixMatrix.h:92: undefined reference to `omp_get_thread_num'
  /usr/include/eigen3/Eigen/src/Core/products/GeneralMatrixMatrix.h:93: undefined reference to `omp_get_num_threads'
  CMakeFiles/ndt_feature_reg_node.dir/src/ndt_feature_reg_node.o: In function `void Eigen::internal::parallelize_gemm, Eigen::Transpose const>, Eigen::Matrix, Eigen::Matrix, Eigen::internal::gemm_blocking_space<0 -1="" double="" false=""> >, int>(Eigen::internal::gemm_functor, Eigen::Transpose const>, Eigen::Matrix, Eigen::Matrix, Eigen::internal::gemm_blocking_space<0 -1="" double="" false=""> > const&, int, int, bool)':
  /usr/include/eigen3/Eigen/src/Core/products/Parallelizer.h:105: undefined reference to `omp_get_num_threads'
  /usr/include/eigen3/Eigen/src/Core/products/Parallelizer.h:145: undefined reference to `GOMP_parallel_start'
  /usr/include/eigen3/Eigen/src/Core/products/Parallelizer.h:145: undefined reference to `GOMP_parallel_end'
  CMakeFiles/ndt_feature_reg_node.dir/src/ndt_feature_reg_node.o: In function `nbThreads':
  /usr/include/eigen3/Eigen/src/Core/products/Parallelizer.h:47: undefined reference to `omp_get_max_threads'
  collect2: ld returned 1 exit status

Solution:

Add these lines in your CMakeLists.txt

#check for OpenMP
find_package(OpenMP)
if(OPENMP_FOUND)
  set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} ${OpenMP_CXX_FLAGS}")
  set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} ${OpenMP_C_FLAGS}")
  if(MSVC90 OR MSVC10)
    if(MSVC90)
      set(OPENMP_DLL VCOMP90)
    elseif(MSVC10)
      set(OPENMP_DLL VCOMP100)
    endif(MSVC90)
    set(CMAKE_SHARED_LINKER_FLAGS_DEBUG "${CMAKE_SHARED_LINKER_FLAGS_DEBUG} /DELAYLOAD:${OPENMP_DLL}D.dll")
    set(CMAKE_SHARED_LINKER_FLAGS_RELEASE "${CMAKE_SHARED_LINKER_FLAGS_RELEASE} /DELAYLOAD:${OPENMP_DLL}.dll")
  endif(MSVC)
else(OPENMP_FOUND)
  message (STATUS "OpenMP not found")
endif()

if (MSVC)
  Set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} /EHsc /O2 ${SSE_FLAGS}")
else (MSVC)
  set(CMAKE_CXX_FLAGS "-O3 ${CMAKE_CXX_FLAGS} ${SSE_FLAGS}")
  set(CMAKE_CXX_FLAGS_DEBUG "${CMAKE_CXX_FLAGS} -O0 -g ${SSE_FLAGS}")
  set(CMAKE_CXX_FLAGS_RELEASE " ${CMAKE_CXX_FLAGS} -O3 ${SSE_FLAGS}")
endif (MSVC)