missing: OpenMP_C_FLAGS OpenMP_C_LIB_NAMES

Question

I'm struggling in compiling a project using OpenMP on Mac OSX. The error is:

CMake Error at /usr/local/Cellar/cmake/3.10.2/share/cmake/Modules/FindPackageHandleStandardArgs.cmake:137 (message):
Could NOT find OpenMP_C (missing: OpenMP_C_FLAGS OpenMP_C_LIB_NAMES)
Call stack most recent call first) 
/usr/local/Cellar/cmake/3.10.2/share/cmake/Modules/FindPackageHandleStandardArgs.cmake:378 (_FPHSA_FAILURE_MESSAGE)
/usr/local/Cellar/cmake/3.10.2/share/cmake/Modules/FindOpenMP.cmake:447 (find_package_handle_standard_args)
libRORPO/CMakeLists.txt:7 (find_package)

The CMakeLists file associated to the search of OpenMP in the project is the following:

# RORPO Lib

project(libRORPO)
cmake_minimum_required(VERSION 2.8)

# FIND OPENMP
find_package( OpenMP REQUIRED)
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}")
endif()

# ADD FILES
file(GLOB_RECURSE RORPO_HEADERS *.hpp *.h)
file(GLOB_RECURSE RORPO_SOURCES *.c)

add_library(RORPO ${LIB_TYPE} ${RORPO_SOURCES} ${RORPO_HEADERS})

install( FILES ${RORPO_HEADERS} DESTINATION include/libRORPO)
install( TARGETS RORPO DESTINATION lib)

I verified if the OpenMP installation runs well using either gcc-7 and Clang-omp. I've searched for similar bugs. Responses pointed that there is a recent package in cmake that allows the finding of OpenMp. I reinstalled CMAKE to have a recent version but it still doesn't work. I'm not expert in cmake configurations.., I would appreciate if you can guide me.

Answer 1

This is because the Xcode clang not support, use system clang instead.

method 1: set env variables, then run cmake https://gist.github.com/svenevs/f00ed3898d2af6248921b63254aa8cc1

export CC=/usr/local/opt/llvm/bin/clang
export CXX=/usr/local/opt/llvm/bin/clang++
export LDFLAGS="-L/usr/local/opt/llvm/lib"
export CPPFLAGS="-I/usr/local/opt/llvm/include"

method2: set env variables in CMakeLists.txt

message(STATUS "set env var")
set(ENV{CC} "/usr/local/opt/llvm/bin/clang")
set(ENV{CXX} "/usr/local/opt/llvm/bin/clang++")
set(ENV{LDFLAGS} "-L/usr/local/opt/llvm/lib")
set(ENV{CPPFLAGS} "-I/usr/local/opt/llvm/include")

method3: change var in CMakeLists.txt

message(STATUS "set compipler")
set(CMAKE_C_COMPILER "/usr/local/Cellar/llvm/5.0.1/bin/clang")
set(CMAKE_CXX_COMPILER "/usr/local/Cellar/llvm/5.0.1/bin/clang++")
set(OPENMP_LIBRARIES "/usr/local/Cellar/llvm/5.0.1/lib")
set(OPENMP_INCLUDES "/usr/local/Cellar/llvm/5.0.1/include")

set(OPENMP_LIBRARIES "/usr/local/opt/llvm/lib")
link_directories(${OPENMP_LIBRARIES})

method4. change env variable when run cmake

cmake .. -DCC=xxx -DCXX=xxx -DLDFLAGS="" -DCPPFLAGS=""

Answer 2

I found a solution porposed in : http://stechschulte.net/2016/03/20/openmp-osx-cmake.html

The trick is that cmake must be run with the CC and CXX environment variables pointing to clang-omp:

CC=clang-omp CXX=clang-omp++ cmake

Note that clang-omp and clang-omp++ have to be defined after installing llvm via:

brew install llvm

and create two symlink:

ln -s /usr/local/opt/llvm/bin/clang /usr/local/bin/clang-omp
ln -s /usr/local/opt/llvm/bin/clang++ /usr/local/bin/clang-omp++

Answer 3

Find OpenMP

Despite the other answers so far, one can use the system compiler.

if(APPLE)
if(CMAKE_C_COMPILER_ID MATCHES "Clang")
    set(OpenMP_C "${CMAKE_C_COMPILER}" CACHE STRING "" FORCE)
    set(OpenMP_C_FLAGS "-fopenmp=libomp -Wno-unused-command-line-argument" CACHE STRING "" FORCE)
    set(OpenMP_C_LIB_NAMES "libomp" "libgomp" "libiomp5" CACHE STRING "" FORCE)
    set(OpenMP_libomp_LIBRARY ${OpenMP_C_LIB_NAMES} CACHE STRING "" FORCE)
    set(OpenMP_libgomp_LIBRARY ${OpenMP_C_LIB_NAMES} CACHE STRING "" FORCE)
    set(OpenMP_libiomp5_LIBRARY ${OpenMP_C_LIB_NAMES} CACHE STRING "" FORCE)
endif()
if(CMAKE_CXX_COMPILER_ID MATCHES "Clang")
  set(OpenMP_CXX "${CMAKE_CXX_COMPILER}" CACHE STRING "" FORCE)
  set(OpenMP_CXX_FLAGS "-fopenmp=libomp -Wno-unused-command-line-argument" CACHE STRING "" FORCE)
  set(OpenMP_CXX_LIB_NAMES "libomp" "libgomp" "libiomp5" CACHE STRING "" FORCE)
  set(OpenMP_libomp_LIBRARY ${OpenMP_CXX_LIB_NAMES} CACHE STRING "" FORCE)
  set(OpenMP_libgomp_LIBRARY ${OpenMP_CXX_LIB_NAMES} CACHE STRING "" FORCE)
  set(OpenMP_libiomp5_LIBRARY ${OpenMP_CXX_LIB_NAMES} CACHE STRING "" FORCE)
endif()
endif()

Answer 4

I ran into this issue on an ARM-based MacOS machine and installing libomp fixed it:

arch -x86_64 brew install libomp