Memory sharing between Matlab and C++ in MEX

Question

I'm currently trying to write a program that processes a fairly large file (~16GB) and then performs analysis upon it. Ideally, I would do the data processing in C/C++ (I already have an efficient implementation written) and then do the analysis in Matlab to make use of its efficient algorithms and ease of use.

My natural inclination is to use MEX to call routines written in C at the beginning of the program and then continue in Matlab. What I want to know (and what I for whatever reason can't seem to find online) is the way in which memory would be shared if I were to use this method:

Say that I were to make a single large heap-allocated array in C to pass to Matlab. Would this array need to be copied in memory before my Matlab functions could work on it, or would Matlab be able to access the original array directly, with no extra copying? I assume and hope that this will work in the second way, but I would rather make sure before spending my time and effort.

Answer 1

Memory can indeed be shared if you used the functions provided by Matlab for this purpose. For example, to create a matrix that is passed back to matlab you can use something like this:

plhs[0] = mxCreateNumericArray(2, out_dims, mxDOUBLE_CLASS, mxREAL);
double *result = mxGetPr(plhs[0]);

That would create an array in place, that matlab will later use. You fill it in using *result, and since the memory was allocated by use of the mx functions, then matlab will delete it when appropriate.