### D.4 Examples

The following program computes the product of two matrices using the Level-3 BLAS function SGEMM,

    [ 0.11 0.12 0.13 ]  [ 1011 1012 ]     [ 367.76 368.12 ]
    [ 0.21 0.22 0.23 ]  [ 1021 1022 ]  =  [ 674.06 674.72 ]
                        [ 1031 1032 ]

The matrices are stored in row major order but could be stored in column major order if the first argument of the call to `cblas_sgemm` was changed to `CblasColMajor`.

    #include <stdio.h>
    #include <gsl/gsl_cblas.h>

    int
    main (void)
    {
      int lda = 3;

      float A[] = { 0.11, 0.12, 0.13,
                    0.21, 0.22, 0.23 };

      int ldb = 2;

      float B[] = { 1011, 1012,
                    1021, 1022,
                    1031, 1032 };

      int ldc = 2;

      float C[] = { 0.00, 0.00,
                    0.00, 0.00 };

      /* Compute C = A B */

      cblas_sgemm (CblasRowMajor,
                   CblasNoTrans, CblasNoTrans, 2, 2, 3,
                   1.0, A, lda, B, ldb, 0.0, C, ldc);

      printf ("[ %g, %g\n", C[0], C[1]);
      printf ("  %g, %g ]\n", C[2], C[3]);

      return 0;
    }

To compile the program use the following command line,

    $ gcc -Wall demo.c -lgslcblas

There is no need to link with the main library `-lgsl` in this case as the CBLAS library is an independent unit. Here is the output from the program,

    $ ./a.out

    [ 367.76, 368.12
      674.06, 674.72 ]