CMake's find_package(OpenMP) cannot find OpenMP when using clang-12

Question

I'm currently trying to find OpenMP using cmake.

Below is my CMakeLists.txt:

cmake_minimum_required(VERSION 3.20)

set(CMAKE_C_STANDARD 11)

set(GCC_COMPILE_FLAGS -Wall -Werror -Wvla -O0)

SET(CMAKE_BUILD_TYPE Debug)

# Find OpenMP
find_package(OpenMP)
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()

I'm using Ubuntu 20.04 and I have libomp-dev installed as shown below:

libomp-dev is already the newest version (1:10.0-50~exp1).

When I set gcc as my compiler, cmake has no issues finding OpenMP.

However, when I use clang-12 as my compiler by specifying the following cmake flags:

-DCMAKE_C_COMPILER=/usr/bin/clang-12 -DCMAKE_CXX_COMPILER=/usr/bin/clang++-12

I get the following "cannot find OpenMP error" from cmake:

CMake Error at cmake/share/cmake-3.20/Modules/FindPackageHandleStandardArgs.cmake:230 (message):
  Could NOT find OpenMP_C

How should I set up my CMakeLists.txt file so that OpenMP can be found when using clang as my compiler?

Answer 1

As @Tsyvarev pointed out, since I had libomp-dev version 10 installed, CMake would be able to find OpenMP if I used clang-10.

So in order for me to use OpenMP with clang-12, I had to make sure that libomp-dev version 12 was installed, which I did using the following shell command:

sudo apt -y install libomp-12-dev

Now CMake has no issues finding OpenMP when I use clang-12 as my compiler.