How to compile an MPI included c program using cmake

Question

I am trying to apply openmp and mpi techniques to an open source C program which requires "cmake . && make" to be built. I already found at How to set linker flags for OpenMP in CMake's try_compile function how to include the -fopenmp flags but still confused about mpi. What can I do about that?

Solution 1

OpenMP

Is this a question about OpenMP? Then all you have to do is compile with -fopenmp which you can do by appending it to CMAKE_C_FLAGS, for example:

SET(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -fopenmp)

MPI

For MPI, you have to find mpi first

find_package(MPI) #make it REQUIRED, if you want

then add it's header files to your search path

include_directories(SYSTEM ${MPI_INCLUDE_PATH})

and finally link your program(s) (which is my_mpi_target in my case)

 target_link_libraries(my_mpi_target ${MPI_C_LIBRARIES})

Solution 2

In modern CMake 3.X which is target based, the CMakeLists.txt should look like this:

cmake_minimum_required(VERSION 3.0)

project(main)

find_package(MPI REQUIRED)
# add this line only when you are using openmpi which has a different c++ bindings
add_definitions(-DOMPI_SKIP_MPICXX)

# Use imported targets would make things much eazier. Thanks Levi for pointing it out.
add_executable(main main.cpp)
target_link_libraries(main
  PRIVATE
  MPI_C)

# Old way.
#target_link_libraries(main
# PRIVATE
# ${MPI_C_LIBRARIES})

#target_include_directories(main
# PRIVATE
# ${MPI_C_INCLUDE_PATH}) 

Solution 3

OpenMP and MPI together

For those who would like to use both OpenMP and MPI within a single CMake file:

cmake_minimum_required(VERSION 3.9.1)
project(parallel_task)

set(CMAKE_CXX_STANDARD 14)
set(GCC_COVERAGE_COMPILE_FLAGS "-Wall -pedantic -lm -O3 -funroll-loops")
set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} ${GCC_COVERAGE_COMPILE_FLAGS}")

add_executable(parallel_task example.cpp example.h)

# OpenMP
find_package(OpenMP)
if(OpenMP_CXX_FOUND)
    target_link_libraries(parallel_task PUBLIC OpenMP::OpenMP_CXX)
endif()

# MPI
find_package(MPI REQUIRED)
target_link_libraries(parallel_task PUBLIC MPI::MPI_CXX)

or even simpler:

cmake_minimum_required(VERSION 3.9.1)
project(parallel_task)

set(CMAKE_CXX_STANDARD 14)

# -fopenmp flag (enables OpenMP)
set(GCC_COVERAGE_COMPILE_FLAGS "-Wall -pedantic -lm -O3 -funroll-loops -fopenmp")
set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} ${GCC_COVERAGE_COMPILE_FLAGS}")

add_executable(parallel_task example.cpp example.h)

# MPI
find_package(MPI REQUIRED)
target_link_libraries(parallel_task PUBLIC MPI::MPI_CXX)

If you want to run your program directly from CLion (with a specified number of processes), check also this answer: https://stackoverflow.com/a/66992251/9723204