Segmentation Violation in a MEX file with Structure array as output.

Hello all!

Well, I've been trying to run a MEX file where I really need the output to be a struct array, but it seems there are some issues considering memory allocation. So, let me present a simplified part of the code that probably causes the problem:

 mxArray * mxp1, *mxp2, *mxp3;  
 int ** pr;  

 plhs[0]  =  mxCreateStructArray (ndim, dims, nfields, fieldnames);

 mxp1     =  mxCreateNumericArray (ndim, dims1 , mxINT32_CLASS, mxREAL);
 mxp2     =  mxCreateNumericArray (ndim, dims2 , mxINT32_CLASS, mxREAL);  
 mxp3     =  mxCreateNumericArray (ndim, dims3 , mxINT32_CLASS, mxREAL);

 mxSetField (plhs[0], index, fieldnames[0], mxp1);

 mxSetField (plhs[0], index, fieldnames[1], mxp2);

 mxSetField (plhs[0], index, fieldnames[2], mxp3);

 pr = (int**) mxMalloc( 3 * sizeof(int*) );

 pr [0] =  (int*) mxGetData(mxp1);
 pr [1] =  (int*) mxGetData(mxp2);
 pr [2] =  (int*) mxGetData(mxp3);

As you can see, pr is an array of pointers. Later, each of its elements ( pr[0], pr[1], pr[2] ) will be the input arguments of a function that looks like:

int function (int* x1, int* x2, int* x3)

Most of the times when I run the above mex-file, I get a MATLAB system error indicating Segmentation Violation.

These are the last three steps from the Slack Trace report that seem to cause the problem:

 [56] 0x7520ee1c      C:\Windows\system32\kernel32.dll+00323100 (BaseThreadInitThunk+000018 )
 [57] 0x777b37eb      C:\Windows\SYSTEM32\ntdll.dll+00407531   (RtlInitializeExceptionChain+000239 )
 [58] 0x777b37be      C:\Windows\SYSTEM32\ntdll.dll+00407486  (RtlInitializeExceptionChain+000194 )

Let me note that there are also a few times that my mex-file returns a struct array, but with NULL fields.

I would be grateful if someone had some hint on this.

[Jan]

In C all details matter. There could be a bug in the values of ndim, dims, nfields and index. The program might fail because you use an int pointer for a mxINT32_CLASS array, which leads to errors on 64 bit systems. So, without seeing the complete relevant code it is impossible to guess the problem(s).