mexAtExit and static variables

I have a quick question regarding static variables in mex files. Basically it is not behaving as I expected. However I'm not a programmer so perhaps this is simply a silly error. I have the problem in a large mex file but the following snippit is enough to demonstrate the issue.

---------------------

#include stuff

static double *X  = NULL;

static void clearX(void)
{
  mexPrintf("Clearing X\n");
  mxFree(X);
}

void mexFunction(int nlhs, mxArray *plhs[], int nrhs, const mxArray *prhs[])

{

    double *X2;

    ptrdiff_t stride = 1, m;

    m = (ptrdiff_t)mxGetM(prhs[0]);

    if( X  == NULL ){

      mexPrintf("\tAllocating Memory for X.\n");

      X  = (double *) mxMalloc((size_t) m*sizeof(double));

      mexAtExit(clearX);

    }

    X2 = mxGetPr(prhs[0]);

    mexPrintf("\tCopying %f to X.\n",X2[0]);

    mexPrintf("\tX was %f.\n",X[0]);

    dcopy(&m,X2,&stride,X,&stride);

    mexPrintf("\tX is now %f.\n",X[0]);

}

--------------------------------------

This is then compiled

mex -g -O -largeArrayDims testMex.c -lmwblas

I then test it:

>> a=randn(1);testMex(a)

          Allocating Memory for X.

          Copying -0.271685 to X.

          X was 0.000000.

          X is now -0.271685.

>> a=randn(1);testMex(a)

          Copying 0.604548 to X.

          X was 0.000000.

          X is now 0.604548.

So the question is, if X is not being freed (clearX is not being called) and its not being reallocated, why is X reset to 0.0?

Answer 1

[Kaustubha Govind]

You should consider declaring your persistent variable X as an mxArray, since you are using MATLAB-managed memory anyway. mxArrays can then be declared as persistent using mexMakeArrayPersistent, which ensures that MATLAB doesn't release that memory (I don't know if you can do that with built-in pointers allocated using mxMalloc). Please see Persistent Arrays for more information.

[Jason]

I need to use BLAS and LAPACK on the arrays. Using an mxArray seems to add more complexity as one needs the mxArray and a separate double pointer for each static variable, but I just swiched it over and it seems to work as expected.

Thank you

[Kaustubha Govind]

You just need to use mxGetPr to get the internal double* representation to pass it to native libraries.

Answer 2

[Walter Roberson]

You are printing out double precision numbers in the C code. You should be using %lf rather than %f .

Using a stride of 1 is something I would cross-check, but I am having difficulty finding a reference for what dcopy() does.

[Jason]

dcopy is in BLAS and copies one vector to another.

stride = 1 is correct I think.

%lf does not change the behavior.