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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 ]