How to pass as input of a mex function an integer value?

Question

I am trying to pass as argument of a mexfunction, an integer representing the number of columns for mxCreateDoubleMatrix. This integer is not supposed to be used anywhere else than in the main mexFunction.

Somehow, this doesn't seem to work.


// mex function for calling c++ code .
void mexFunction(int nlhs, mxArray *plhs[], int nrhs, const mxArray *prhs[])
{
    char *input_buf;
    size_t buflen;
    double ncols;
    double *result_final;
    double *result_second;

    /* get the length of the input string */
    buflen = (mxGetM(prhs[0]) * mxGetN(prhs[0])) + 1;

    /* copy the string data from prhs[0] into a C string input_ buf.    */
    input_buf = mxArrayToString(prhs[0]);

    /* copy the int from prhs[0] to decide on length of the results.    */
    ncols = (int) (size_t) mxGetPr(prhs[1]);

    plhs[0] = mxCreateDoubleMatrix(1, ncols, mxREAL);
    plhs[1] = mxCreateDoubleMatrix(1, ncols, mxREAL);
    result_final = mxGetPr(plhs[0]);
    result_second = mxGetPr(plhs[1]);

    /* Do the actual computations in a subroutine */
    subroutine(input_buf, buflen, result_final, result_second);
}

If I take out the ncols line, all the rest works as expected. I did not include ncols as input to subroutine because it is not actually used there but only in the main, to define the size of the output array.

I am expected to have a matrix of 1x100 for the output array if I call myfun('examplefile.txt',100) instead the matrix that is shown at the end of the call has an infinite/very long number of columns.

Someone can help with this?

You access prhs[0] and prhs[1] without knowing if they exist. You should always test for input variables first. If you accidentally call your MEX-function with too few input arguments, your MATLAB will crash, instead of just giving an error message. — Cris Luengo

Ander Biguri Ander Biguri · Accepted Answer · 2019-03-27T09:47:04

You are converting the pointer to the value into a size_t and then an int. But its a pointer, an adress in RAM of the location of the value, not the value itself.

 ncols = (int) (size_t) mxGetPr(prhs[1]); %mex Get Pointer!!

Get the value instead.

 ncols = (int)(mxGetScalar(prhs[1]));

How to pass as input of a mex function an integer value?

1 Answers