Use BLAS library in C Mex-File

Hi,

I am trying to use the BLAS library for my C-Mex-Code to analyze performance differences with and without BLAS. However, I couldn't figure out how to use BLAS correctly without getting compile errors/warnings (and eventually Matlab Crashes).

Below is a sample code where I would like to calculate a vector product using the ddot()-function of Blas ( https://software.intel.com/de-de/node/468398#D4E53C70-D8FA-4095-A800-4203CAFE64FE ):

#include <mex.h>
#include <math.h>
#include <blas.h>

void TestBlas(double *L, double *R, mwSize n)
{

      int one = 1;
      double sum = ddot(&n,L,&one,L,&one);

      //Output Matrix R not defined, just for test purposes...

} 

void mexFunction( int nlhs, mxArray *plhs[],
                  int nrhs, const mxArray *prhs[])
{
    double *inMatrix;               /* 1xN input matrix */
    size_t ncols;                   /* size of matrix */
    long *ncolsPr;                  /* output matrix */
    double *outMatrix;              /* output matrix */

      inMatrix = mxGetPr(prhs[0]);
      ncols = mxGetN(prhs[0]);

      /* create the output matrix */
      plhs[0] = mxCreateDoubleMatrix(1,(mwSize)ncols,mxREAL);
      outMatrix = mxGetPr(plhs[0]);

      TestBlas(inMatrix,outMatrix,(mwSize)ncols);
  }

The compiler warnings I receive are:

>> mex -largeArrayDims TestBlas.c -lmwblas
Building with 'Xcode with Clang'.
/Users/jhess/Dropbox/Uni/E-Technik Master/Forschungspraxis Machine Learning/Matlab/TestBlas.c:9:23: warning: passing 'mwSize *' (aka 'unsigned long *') to parameter of type 'const ptrdiff_t *' (aka 'const long *') converts between pointers to integer types with different sign [-Wpointer-sign]
    double sum = ddot(&n,L,&one,L,&one);
                      ^~
/Applications/MATLAB_R2015a.app/extern/include/blas.h:559:22: note: passing argument to parameter 'n' here
    const ptrdiff_t *n,
                     ^
/Users/jhess/Dropbox/Uni/E-Technik Master/Forschungspraxis Machine Learning/Matlab/TestBlas.c:9:28: warning: incompatible pointer types passing 'int *' to parameter of type 'const ptrdiff_t *' (aka 'const long *') [-Wincompatible-pointer-types]
    double sum = ddot(&n,L,&one,L,&one);
                           ^~~~
/Applications/MATLAB_R2015a.app/extern/include/blas.h:561:22: note: passing argument to parameter 'incx' here
    const ptrdiff_t *incx,
                     ^
/Users/jhess/Dropbox/Uni/E-Technik Master/Forschungspraxis Machine Learning/Matlab/TestBlas.c:9:35: warning: incompatible pointer types passing 'int *' to parameter of type 'const ptrdiff_t *' (aka 'const long *') [-Wincompatible-pointer-types]
    double sum = ddot(&n,L,&one,L,&one);
                                  ^~~~
/Applications/MATLAB_R2015a.app/extern/include/blas.h:563:22: note: passing argument to parameter 'incy' here
    const ptrdiff_t *incy
                     ^
3 warnings generated.

MEX completed successfully.

It seems to be something like I am passing wrong variable-types (or pointer-types?) to the BLAS-function but I just couldn't figure out how to fix it. Can somebody have a look please? Many thanks!

Answer 1

[James Tursa]

You need to use the correct integer types for BLAS/LAPACK calls. E.g.,

      mwSignedIndex one = 1;  // <-- Or ptrdiff_t one = 1;

and

void TestBlas(double *L, double *R, mwSignedIndex n)  // <-- Or ptrdiff_t n

Also, your output is a scalar, so

      plhs[0] = mxCreateDoubleMatrix(1,1,mxREAL);

Then, of course, to get the output returned to the caller,

      *R = ddot(&n,L,&one,L,&one);

Finally, to make this code robust, you should consider putting in some checks to make sure the inputs are as expected (double class, real, etc.)

[Luke Skywalker]

Thanks a bunch, works perfectly :)

[James Tursa]

You're welcome. May the Force be with you ...