#include <gsl/gsl_vector.h>

void notwhatyouthink(const gsl_vector * v)
{
  v->data[0] = 42.0;
}

int main()
{
  gsl_vector * v = gsl_vector_calloc(32);

  printf("%g\n", v->data[0]);
  notwhatyouthink(v);
  printf("%g\n", v->data[0]);

  gsl_vector_free(v);

  return 0;
}