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

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AP
AP 2012 年 10 月 24 日
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

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James Tursa
James Tursa 2012 年 10 月 24 日
編集済み: James Tursa 2012 年 10 月 24 日
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.
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James Tursa
James Tursa 2012 年 10 月 24 日
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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