How to use OpenMP properly from C++ static library?

Hi,

I am trying to incooperate a static library written in C++ in Matlab. The C++ static library uses OpenMP for parallel processing. ;

I have built the library interface following https://uk.mathworks.com/help/matlab/build-matlab-interface-to-c-library.html. I am using Windows.

When I call the C++ library from Matlab, it works, but parallel processing is not working and it only runs in serial. In Matlab, the compiler has not been changed. MEX configured to use 'Microsoft Visual C++ 2019 (C)' for C language compilation

How do I setup Matlab or generate library interface properly to use OpenMP of the C++ library?

Thanks,

Hein

[James Tursa]

How does your library code run in parallel? Does it query the system for number of threads available, and then try to run with that many threads? Can you show the setup code you use for this?

[hH]

Thanks for your reply.

Yes, I check the number of threads avaiable from the system and try to run with that many thread. I use the following set up.

The nThreads value printed out by fprintf is multiple (e.g. 4).

The library also seems to work with OpenMP properly if I am calling the library from another C++ application.

Thanks.

int nThreads = omp_get_max_threads() - 1;
fprintf(stderr, "Num. threads = %d\n", nThreads); 
#pragma omp parallel for num_threads(nThreads)

[James Tursa]

Can you show the code for how you are calling the library functions?

Answer 1

[Infinite_king]

Hi,

As per my understanding you are able to use the library from MATLAB but the operations are not being performed in parallel.

To enable OpenMP support for your C++ library when using it in MATLAB through MEX, you need to ensure that the OpenMP flags are correctly set during compilation. Since you're using Microsoft Visual C++ 2019, you need to include the OpenMP flag (/openmp) while compiling.

Follow the below troubleshooting steps to possibly resolve the issue.

1. Make sure to compile the library with the same compiler that you are using with MATLAB.
2. Pass the OpenMP flag (/openmp) while compiling Interface.
3. Change the Mex configuration file or pass the OpenMP flag while compiling mex. Refer the following link for more information, https://www.mathworks.com/help/matlab/ref/mex.html#btw193g-1
4. Checkout the following answer, you may find it useful, https://www.mathworks.com/matlabcentral/answers/237411-can-i-make-use-of-openmp-in-my-matlab-mex-files

Hope this is helpful.