Can I pass a pointer to a matrix created in MATLAB to C via a MEX file and change the data in it?

In MATLAB I would like to create an 1xN array of zeros, pass it to a C routine as an input argument, have the C routine update the (type double) values in the array (matrix in MATLAB). I would like to do this instead of having the MEX routine return a matrix (as is done in the example arrayProduct.c)

Is this possible, and if so, does anyone have any examples they would be willing to share?

Answer 1

[Tommy Thomasson]

Make the second to last line in step 2 this and it will work:

workOnExistingMatrix(mxGetPr(prhs[0]));

Here's the corrected MEX file:

//This is the MEX file (written in C)
#include "mex.h"
void workOnExistingMatrix(double* aMatrixCreatedInMatlab) {
    aMatrixCreatedInMatlab[0] = 0.1234;
}
//The gateway function
void mexFunction( int nlhs, mxArray *plhs[],
                  int nrhs, const mxArray *prhs[]) {
    //This works =)
    workOnExistingMatrix(mxGetPr(prhs[0]));
}

[Jan]

Be careful.

a = 1:10;
b = a;
yourMexFcn(b);
disp(a)
disp(b)

Both arrays are modified! A more evil example:

function main
a = zeros(1, 10);
subfunction(a);
disp(a)
end

function subfunction(b)
yourMexFcn(b)
disp(b);
end

There is no chance to see in the main function, why a(1) is modified. Do not underestimate the effects of shared data copies.

Answer 2

[Jan]

This works directly, but it can have side-effects, because Matlab uses a copy-on-write strategy and shared data copies:

a = rand(100,100);
b = a;

Now the data pointer of a and b point to the same block of memory. A deep data copy is performed only, if b is modified:

b(1) = 0;

Now b has its own data.

Now to your question: If you provide a shared data copy to a C-Mex function and modify it, you touch both variables:

a = rand(100,100);
b = a;
YourInplaceOperation(b);

If you set e.g. b(1) to -1, a(1) is -1 also, because both variables share the data.

Therefore you have to check at first, if the input to the C-Mex function is a shared data copy. See James Tursa's FEX submissions:

• https://www.mathworks.com/matlabcentral/fileexchange/42212-mexputvariableshareddatacopy
• https://www.mathworks.com/matlabcentral/fileexchange/65842-sharedchild-creates-a-shared-data-copy-of-a-contiguous-subsection-of-an-existing-variable
• https://www.mathworks.com/matlabcentral/fileexchange/65861-isshared-determines-data-sharing-status-among-workspace-variables

Note that the method of shared data copies is not documented and the documentation of Matlab states, that it is not supported to modify the input data directly.

[EDITED] Keep in mind, that the input of your MEX can be a temporary variable:

a = rand(100,100);
YourInplaceOperation(a + 1);

Now modifying the input would be meaningless.

[Tommy Thomasson]

Thanks Jan - that's helpful information, but it's still not clear how to pass a matrix pointer to a MEX function. Below is a simplified code example of what I want to do. For now, assume I'm not concerned about error checking the input arguments in the MEX gateway function.

Step 1: In MATLAB, I would do something like this:

%this is MATLAB code
myMatlabMatrix = zeros(1,5)

%workOnExistingMatrix is a MEX function defined in Step 2
workOnExistingMatrix(myMatlabMatrix)

Step 2: And here's the MEX file:

//This is the MEX file (written in C)
#include "mex.h"

void workOnExistingMatrix(double* aMatrixCreatedInMatlab) {
    aMatrixCreatedInMatlab[0] = 0.1234;
}

//The gateway function
void mexFunction( int nlhs, mxArray *plhs[],
                  int nrhs, mxArray *prhs[]) {
    //This doesn't work =(
    //I assume I need a cast or a conversion of some sort
    workOnExistingMatrix(prhs[0]);
}

Step 3: Back in MATLAB, I would then be able to do this:

>> %now we're back in MATLAB
>> myMatlabMatrix 

myMatlabMatrix =

     0.1234           0           0           0           0

>>% =)

I need help with the second-to-last line of the code in Step 2. Something else needs to happen here, but I haven't been able to figure out what.

[James Tursa]

What is missing is you need to pass the data pointer, not the mxArray pointer. E.g.,

workOnExistingMatrix(mxGetPr(prhs[0]));