Mex, how to copy an array to output

Question

Xingwang Yong 2020-12-26

0
链接

此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/702877-mex-how-to-copy-an-array-to-output

回答： James Tursa 2020-12-26

In my mex function my_expm.c, I calculated matrix expoential for the input matrix,

void mexFunction(int nlhs,mxArray *plhs[], int nrhs, const mxArray *prhs[])
{
	double *inputMat;
    double *outputMat;
    int numRows;
    
    inputMat = mxGetDoubles(prhs[0]); 
    numRows = mxGetM(prhs[0]);
    
    plhs[0] = mxCreateDoubleMatrix(numRows, numRows, mxREAL);
    outputMat = mxGetDoubles(plhs[0]);
       
    outputMat = r8mat_expm1(numRows, inputMat); // matrix exponential
    return;
}

Calling it by

c = my_expm([1,0;0,1])

will return all zeros, as explained in this post.

I tried the two suggestions in that post to fix this problem, both failed.

1,

by using

r8mat_expm1(numRows, inputMat, outputMat)

instead of

outputMat = r8mat_expm1(numRows, inputMat);

this still returns all zeros. I used gdb to debug, it turns out when using outputMat = r8mat_expm1(numRows, inputMat)，the outputMat in mex is right and when it returns to matlab, they all become zero. But when using r8mat_expm1(numRows, inputMat, outputMat), the outputMat in mex are already all zeros. I do not know why they are zeros since I only simply changed the signature of r8mat_expm1().

2,

by using mxSetDoubles(), as in this post. This call get the work done. However, this mex function can run only once, if I call it again, matlab will crash. I can not see whether there is a problem with mxSetDoubles().

So, is there another way that I can simply copy outputMat to plhs[0]?