MATLAB: Do I see strange behavior when I modify the inputs to the MEX file

MATLAB

I am writing a MEX-file and in the MEX-file I am modifying my input to the MEX-file. However, I see strange results, such as the contents of other variables changing that I am not modifying. Why does this happen?

Best Answer

In general, you should not modify the inputs of your MEX-file. The data that is associated with the inputs to the MEX file is cleaned up by the MATLAB memory manager, so touching already allocated memory is not a good idea.

If you would like to use the data that is contained in your input variables, we recommend making a copy of the input with the mxDuplicateArray function. For example:

plhs[0] = mxDuplicateArray(prhs[0]);

Note that the default mexFunction() function header is:

void mexFunction(int nlhs, mxArray *plhs[], int nrhs, const mxArray *prhs[]);

The prhs pointer is declared with the "const" keyword, so modifying the data pointed by it is not a good idea.

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MATLAB: How to input and output variable in mex function

Some issues:

    b       = mxGetPr(plhs[1]);
    d       = mxGetPr(plhs[2]);

The above lines crash MATLAB when you try to use b and d downstream because plhs[1] and plhs[2] haven't been created. The plhs variables are output variables that you the programmer need to create before you access them. So, create them first. E.g.,

    if( nlhs < 3 ) {
        mexErrMsgTxt("Need to specify three output variables");
    }
    plhs[0] = mxCreateDoubleMatrix(ynum,1,mxREAL);
    plhs[1] = mxDuplicateArray(prhs[1]);
    plhs[2] = mxDuplicateArray(prhs[3]);
    x       = mxGetPr(plhs[0]);
    b       = mxGetPr(plhs[1]);
    d       = mxGetPr(plhs[2]);

Then of course you don't need these earlier lines at all:

    b       = mxGetPr(prhs[1]);
        :
    d       = mxGetPr(prhs[3]);

And, to make your mex routine more robust, you should be checking the number and class and size of all of your inputs before using them downstream in your code.

MATLAB: Mex compile error caused by different position of mexPrintf()

In C (C89 and older I believe) variables must be declared at the beginning of a scope before any other statements are made in the code. So having that function call before declaring variables makes the compiler unhappy.

You could rename the file to .cpp and that should pacify it or just leave the declarations first.

Best Answer

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MATLAB: How to input and output variable in mex function

MATLAB: Mex compile error caused by different position of mexPrintf()

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