MATLAB Answers


MEX: Problem creating a cell matrix and assign it to the output

Asked by AP
on 24 Oct 2012

Dear All,

By using a MEX file, I am trying to loop through array "group_ptr" with size 1000×1 to make a cell with size 4×1. group_ptr(i) contains the group ID which "i" belongs to. The 4 groups have different number of elements. My MEX file does other things beside to creating this cell. It takes one input and generates 4 outputs. When it gets to making the cell, MATLAB crashes.

However, when I comment the part for making the cell and reduce the number of outputs to 3, MEX file works fine. I would be grateful if someone could help me what the problem could be.

Below is the part of my code which creates this cell. The MEX file has 4 output which the last one is this cell.

    #define CELLGROUP plhs[3]    
    mxArray *CellArray_ptr;
    CellArray_ptr = mxCreateCellMatrix( 4, 1 );    
    double *a = new double[ 1000 ];
    for( mwIndex i=0; i<4; i++ )
         int counter = 0;
         for( int j=0; j < 1000; j++ )
             if( (mwIndex)group_ptr[ j ]== (i+1) )
                 a[ counter++ ] = j;            
 	 mxArray *A;    
         A = mxCreateDoubleMatrix( counter, 1, mxREAL );        
         memcpy( mxGetPr(A), a, sizeof(double)*counter );
         mxSetCell( CellArray_ptr, i, A );		               
 	 mxDestroyArray( A );
     CELLGROUP = CellArray_ptr;
     delete[] a;






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1 Answer

Answer by James Tursa
on 24 Oct 2012
Edited by James Tursa
on 24 Oct 2012
 Accepted answer

Your code:

      mxSetCell( CellArray_ptr, i, A );		               
      mxDestroyArray( A );  // get rid of this line

Get rid of the mxDestroyArray call. When you call the mxSetCell function, three things happen:

1) The address of A gets put into the i'th cell location in the CellArray_ptr variable

2) The type of A gets changed from "temporary" to "sub-element"

3) The address of A gets removed from the garbage collection list.

After the mxSetCell function call, A is literally now part of the CellArray_ptr array. Its ultimate clearing will depend entirely on what happens to CellArray_ptr. When you call mxDestroyArray on A, you are invalidating part of a legitimate variable. Hence when MATLAB tries to access or clear CellArray_ptr later on it crashes since it accesses invalid memory.


on 24 Oct 2012


This is great news. Can't wait to see it published.


on 24 Oct 2012


How can I privately send you my email address so you could let me know when it is published?

You can use the Contact Author link:

I can probably send you a preliminary incomplete doc if you are interested, with all the usual caveats about potential errors in the doc and using undocumented functions. Just e-mail me through the link and let me know.

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