MATLAB: Wrong result when using BLAS dot product routine (DDOT) from a MEX file

blasmex

Hello,
I am trying to call some BLAS routines from MEX code. First, I am trying to compute a scalar product with the fuction DDOT.
Here is my MEX file:
#include "mex.h"
#include "blas.h"
void mexFunction(int nlhs, mxArray *plhs[], int nrhs, const mxArray *prhs[])
{
  double *A, *B, C;
  int m, one=1;
  A = mxGetPr(prhs[0]);
  B = mxGetPr(prhs[1]);
  m = mxGetM(prhs[0]); /* number of rows */
    /* Pass all arguments to Fortran by reference */
    C = (double) ddot(&m,A,&one,B,&one);
    /* create output */
    plhs[0] = mxCreateDoubleScalar(C);
    return;
  }
The compilation returns no error. When using it, though, I do not get the expected result:
>> a = ones(2,1);
>> b = ones(2,1);
>> mydot(a,b)
ans =
       0
Am I calling DDOT inappropriately?
— H. G.

Best Answer

Hi,
you need to change the variable definition of m and one and use two variables for "one":
ptrdiff_t m, one1 = 1, one2 = 1;
C = (double) ddot(&m,A,&one1,B,&one2);
Then it works.
Titus
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