MEX file crashing for an array of cell

Hello!

I have written a MEX file that takes in a cell array of double matrices, each of dimension 250x1000. The cell array dimensions are 29x1. The MEX file is assigning the value 3 to all the entries of the matrices. However, whenever I call the MEX file, MATLAB crashes. I have no idea why this is happening as the index of my matrices are in the allowed range.

To reproduce the bug/error, please run the following code on the attached files:

    load test;
    test(pp);

The files are large in size and are available here.

For a quick review, the test.c file is as follows:

#include <math.h>
#include <matrix.h>
#include <mex.h>
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
void mexFunction(int nlhs, mxArray *plhs[],
        int nrhs, const mxArray *prhs[])
{
    const mwSize *dims;
    const mxArray *cell;
    const mxArray *cellArray;
    double *inMatrix;
    int mom, cellSize, jcell;
    mwIndex i, j, count;
    mwSize ncol, nrow;

      // Read the cell
      cell = prhs[0];
      dims = mxGetDimensions(prhs[0]);

      for (jcell=1; jcell<dims[0]; jcell++) {
          cellArray = mxGetCell(cell,jcell);
          inMatrix = mxGetPr(cellArray);
          nrow = mxGetM(cellArray);
          ncol = mxGetN(cellArray);

          printf("%d, %d, %d\n", nrow, ncol, cellSize);
          count = 0;
          for(i=0;i<nrow;i++){
              for(j=0;j<ncol;j++){
                  inMatrix[count] = 3;
                  count++;
              }
          }
      }
  }

Thanks

Answer 1

[Jan]

If you want to assign all elements of the cell array, start at jcell=0 (as in the example http://www.mathworks.com/matlabcentral/answers/84135-getting-the-contents-from-cells-in-a-cell-array-using-mex).

mxGetCell replies a NULL pointer if the input is not a cell or if the cell element is not initialized. Check this in every case:

cellArray = mxGetCell(cell,jcell);
if (cellArray == NULL) {
  ... perform an error or an exception
}

You are writing directly into the input array. This is dangerous, because you do not check if this is a shared array. Never modify the inputs in a C-Mex function, if you are not really sure about what you are doing. But here this can lead to bad values in Matlab, but not to a crash.

jcell is an int and dims an mwSize. Are you sure that jcell<dims[0] does, what you expect? The same happens for i, j and nrow, ncol. Better rely on the same integer types.

[Ubaid Ullah]

Thanks .. actually some of the matrices in the array were of type SPARSE and some were of type FULL. The SPARSE matrices were creating problem as I was treating them full matrices, assuming they have nrowxncol size.