Getting started with openMP. install on windows

Question

I want to write parallel program in C++ using OpenMP, so I am getting started with OpenMP. On the other words I am a beginner and I need good OpenMP guide telling how to install

Answer 1

OpenMP is not something that you install. It comes with your compiler. You just need a decent compiler that supports OpenMP and you need to know how to enable OpenMP support since it is usually disabled by default.

The standard compiler for Windows comes from Microsoft and it is the Microsoft Visual C/C++ compiler from Visual Studio. Unfortunately its OpenMP support is a bit outdated - even the latest and greatest Visual Studio only supports OpenMP 2.0 (an outdated standard version from 2002). See here for more information on how to use OpenMP in Visual Studio. There are other compilers available as well - both Intel C/C++ Compiler (commercial license required) and GCC (freely available) support newer OpenMP versions and other compilers are available too.

You can start learning OpenMP by visiting the OpenMP web site here. Also there is a great tutorial on OpenMP from Lawrence Livermore National Laboratory available here.

2020 Update: Microsoft now ships Clang for Windows with Visual Studio. Although it is a bit convoluted, one can (ab)use the Clang-cl toolset to produce working 32-bit OpenMP programs. A number of steps are necessary:

1. If not already installed, add Clang and Clang-cl using the Visual Studio 2019 Installer.
2. Set the project's platform toolset (project Properties -> General -> Platform Toolset) to "LLVM (clang-cl)".
3. Enable Clang OpenMP support by adding -Xclang -fopenmp to the compiler options in project Properties -> C/C++ -> All Options -> Additional Options.
   Important: make sure that OpenMP support is disabled before switching the platform toolset (this is the default for new C++ projects). It seems that VS remembers the setting and still passes /openmp even though the language configuration for Clang has no option for OpenMP. If clang-cl.exe throws error MSB8055 (unsupported /openmp option) during build, set the platform toolset back to "Visual Studio 2019 (vXXX)" and disable the OpenMP support in Properties -> C/C++ -> Language -> Open MP Support, then switch the platform toolset again to "LLVM (Clang-cl)".
4. Add libomp.lib to the additional libraries in project Properties -> Linker -> Input -> Additional Dependencies.
5. Add the path to libomp.lib to the linker search path by adding a new entry with value $(LLVMInstallDir)\lib in project Properties -> Linker -> General -> Additional Library Directories.

6. Add a post-build action that copies LLVM's libomp.dll to the project output directory (without this step, running the executable will fail unless libomp.dll is in the DLL search path). In project Properties -> Build Events -> Post-Build Event -> Command Line:

   xcopy /y "$(LLVMInstallDir)\bin\libomp.dll" "$(SolutionDir)$(Configuration)"

7. Build and run the project.

Note: this is very much likely still unsupported by Microsoft and it only works for x86 projects since the LLVM libraries shipped with VS are 32-bit only.

Answer 2

So here is what I did to finally get OpenMP working on my Windows 10 PC:

1. Get MinGW - Download and grab what you need to get the basic gcc compiler and the g++ pakage (its really easy to do). You can always run g++ -v to make sure it is up and running
2. Run mingw-get upgrade --recursive "gcc<4.7.*" "gcc-g++<4.7.*" This is the "Fun" part. Because at this time there was no libgomp library supported in their 4.9.* version my gcc wasn't able to recognize <omp.h> the last support version was 4.7.2 so with this I finally was able to run my openMP

To compile run g++ -fopenmp myOpenMPFile.cpp -o myOpenMP and it will all work from there

gcc -fopenmp myOpenMPFile.cpp -o myOpenMP will also work for C code

Answer 3

I would like to share what I did to get OpenMP working on my Windows 10 PC (things have got even simpler in 2019)

1. I installed MinGW distribution from here with GCC 8.2.0 compiler. The maintainer of the distribution has already added winpthreads and OpenMP support to GCC.

2. I compiled my code with -fopenmp flag as follows: g++ -fopenmp main.cpp -o exec

Note: the MinGW distribution provides support for many useful libraries (such as Boost 1.69.0) and other utilities. I found it to be very useful.