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

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;

Thanks,

Ahmad

Answer 1

[James Tursa]

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.

[AP]

Very clear and useful explanation. Thank you. Where can I find such information? Is it in the documentation?

[James Tursa]

You can find this detailed information in the JUG MEX document (James's Ultimate Guide to MEX programming) that I have written. You can find the document on the FEX ... umm ... wait a minute ... I haven't actually published it to the FEX yet ... well, when I finally get around to finishing this document and posting it to the FEX then you can find all these gory details (along with undocumented API function details, etc.). I will try to finish that up soon and post it.

[AP]

James,

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

Ahmad

[AP]

James,

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

[James Tursa]

You can use the Contact Author link:

http://www.mathworks.com/matlabcentral/fileexchange/authors/29734

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.