OpenMP CMake Apple - macos

Question

I am currently trying to compile a project on Mac using cmake, but running into trouble. I have already looked at this article [1], but am still running into some trouble. My CMakeLists.txt looks as follows.
project(tpch_framework)
# enable c++11
#set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++11")
set(CMAKE_CXX_STANDARD 14)
set(CMAKE_CXX_STANDARD_REQUIRED ON)
set(CMAKE_BUILD_TYPE "Release")
set(CMAKE_C_COMPILER "/usr/local/Cellar/llvm/10.0.0_3/bin/clang")
set(CMAKE_CXX_COMPILER "/usr/local/Cellar/llvm/10.0.0_3/bin/clang++")
set(OPENMP_LIBRARIES "/usr/local/Cellar/llvm/10.0.0_3/lib")
set(OPENMP_INCLUDES "/usr/local/Cellar/llvm/10.0.0_3/include")
OPTION (USE_OpenMP "Use OpenMP to enamble <omp.h>" ON)
# Find OpenMP
if(APPLE AND USE_OpenMP)
if(CMAKE_C_COMPILER_ID MATCHES "Clang")
set(OpenMP_C_FLAGS "-Xpreprocessor -fopenmp")
set(OpenMP_C_LIB_NAMES "omp")
set(OpenMP_omp_LIBRARY omp)
endif()
if(CMAKE_CXX_COMPILER_ID MATCHES "Clang")
set(OpenMP_CXX_FLAGS "-Xpreprocessor -fopenmp")
set(OpenMP_CXX_LIB_NAMES "omp")
set(OpenMP_omp_LIBRARY omp)
endif()
endif()
if(USE_OpenMP)
find_package(OpenMP REQUIRED)
endif(USE_OpenMP)
if (OPENMP_FOUND)
include_directories("${OPENMP_INCLUDES}")
link_directories("${OPENMP_LIBRARIES}")
set (CMAKE_C_FLAGS "${CMAKE_C_FLAGS} ${OpenMP_C_FLAGS} -libomp")
set (CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} ${OpenMP_CXX_FLAGS} -libomp")
set (CMAKE_EXE_LINKER_FLAGS "${CMAKE_EXE_LINKER_FLAGS} ${OpenMP_EXE_LINKER_FLAGS}")
endif(OPENMP_FOUND)
# Configure required Boost libraries
set(BOOST_ROOT "" CACHE PATH "Boost build root (useful on Windows)")
option(Boost_USE_STATIC_LIBS
"Search for static boost libs" OFF)
option(Boost_USE_MULTITHREADED
"Search for multithreaded boost libs" ON)
option(Boost_USE_STATIC_RUNTIME
"Search for boost libs linked against static C++ runtime" OFF)
find_package(Boost 1.47.0 REQUIRED filesystem system)
# ensure that dependant libraries not explicitly specified here
# are found by the linker:
link_directories(${Boost_LIBRARY_DIRS})
include_directories(${Boost_INCLUDE_DIRS})
set(LIBS ${LIBS} ${Boost_LIBRARIES})
#Bring the headers into the project
include_directories(include)
FILE(GLOB_RECURSE INC_ALL "include/*.hpp")
#However, the file(GLOB...) allows for wildcard additions:
file(GLOB SOURCES "src/*.cpp")
add_library(tpch_framework ${SOURCES})
add_executable(framework main.cpp ${INC_ALL})
target_link_libraries(framework tpch_framework)
#target_link_libraries(framework stdc++fs)
target_link_libraries(framework ${LIBS})
When I execute this, I get the following output.
-- The C compiler identification is AppleClang 11.0.0.11000033
-- The CXX compiler identification is AppleClang 11.0.0.11000033
-- Check for working C compiler: /Library/Developer/CommandLineTools/usr/bin/cc
-- Check for working C compiler: /Library/Developer/CommandLineTools/usr/bin/cc - works
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Detecting C compile features
-- Detecting C compile features - done
-- Check for working CXX compiler: /Library/Developer/CommandLineTools/usr/bin/c++
-- Check for working CXX compiler: /Library/Developer/CommandLineTools/usr/bin/c++ - works
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Detecting CXX compile features
-- Detecting CXX compile features - done
-- Found OpenMP_C: -Xpreprocessor -fopenmp
-- Found OpenMP_CXX: -Xpreprocessor -fopenmp
-- Found OpenMP: TRUE
-- Found Boost: /usr/local/lib/cmake/Boost-1.72.0/BoostConfig.cmake (found suitable version "1.72.0", minimum required is "1.47.0") found components: filesystem system
-- Configuring done
-- Generating done
-- Build files have been written to: /Users/...
However, when I then use make, I get the following error.
/Users/myname/Desktop/Uni/MHD/tpch_framework_challenge_mhd_2020/src/task4.cpp:48:5: error: use of undeclared identifier 'omp_set_num_threads'
omp_set_num_threads(4);
Which I am guessing comes, as I get this warning: clang-10: warning: -libomp: 'linker' input unused [-Wunused-command-line-argument].
Does anybody have any ideas? I have been stuck with this for way too long and would appreciate any tips.
Kind regards,
Moritz

Answer 1

If you can use a recent version of CMake, this build should work.
# NOTE: Every top-level CMakeLists.txt must start with these two lines
cmake_minimum_required(VERSION 3.16)
project(tpch_framework)
## Enable C++14
set(CMAKE_CXX_STANDARD 14)
set(CMAKE_CXX_STANDARD_REQUIRED ON)
## Find dependencies
# OpenMP
find_package(OpenMP REQUIRED)
# Boost
set(BOOST_ROOT "" CACHE PATH "Boost build root (useful on Windows)")
option(Boost_USE_STATIC_LIBS
"Search for static boost libs" OFF)
option(Boost_USE_MULTITHREADED
"Search for multithreaded boost libs" ON)
option(Boost_USE_STATIC_RUNTIME
"Search for boost libs linked against static C++ runtime" OFF)
find_package(Boost 1.47.0 REQUIRED COMPONENTS filesystem system)
## Add main project targets
# NOTE: This is completely wrong and will break incremental builds after
# doing a "git pull" or similar. You should never glob for source
# files, but instead list them explicitly.
file(GLOB SOURCES "src/*.cpp")
add_library(tpch_framework ${SOURCES})
target_include_directories(tpch_framework PUBLIC BUILD_INTERFACE:include)
target_link_libraries(tpch_framework
PUBLIC
OpenMP::OpenMP_CXX
Boost::boost
Boost::filesystem
Boost::system)
add_executable(framework main.cpp)
target_link_libraries(framework PRIVATE tpch_framework)
There is so much bad CMake advice here on SO and in tutorials, it's a real shame.