Is GSL actually using Atlas?

Question

I want to use Atlas as the BLAS backend for GSL on Raspbian. With these make options:

ldflags = $(shell pkg-config --libs gsl) -L:libatlas.so.3

producing this link step:

gcc -lgsl -lgslcblas -lm -L:libatlas.so.3 -o my-app main.o

The most basic example builds and runs successfully:

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

int
main (void)
{
  double a[] = { 0.11, 0.12, 0.13,
                 0.21, 0.22, 0.23 };

  double b[] = { 1011, 1012,
                 1021, 1022,
                 1031, 1032 };

  double c[] = { 0.00, 0.00,
                 0.00, 0.00 };

  gsl_matrix_view A = gsl_matrix_view_array(a, 2, 3);
  gsl_matrix_view B = gsl_matrix_view_array(b, 3, 2);
  gsl_matrix_view C = gsl_matrix_view_array(c, 2, 2);

  /* Compute C = A B */

  gsl_blas_dgemm (CblasNoTrans, CblasNoTrans,
                  1.0, &A.matrix, &B.matrix,
                  0.0, &C.matrix);

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

  return 0;
}

How can I verify that it's using the Atlas backend? Either making a runtime call, or figuring it out via GDB if necessary.

Answer 1

The answer is basically

1. no, it wasn't using Atlas;
2. The reason is that the library order declaration in ldflags needs to be -L/usr/lib/arm-linux-gnueabihf/atlas -lblas -lgsl -lgslcblas -lm;
3. you can tell by running gdb, b cblas_dgemm, saying yes to future library symbol loading and running. If you land in /usr/lib/arm-linux-gnueabihf/libblas.so.3 - which is a symlink to Atlas via the alternatives system - then yes, it is using Atlas.