Okay, I understood that the offset should be in terms of bytes, and
not of the underlying struct, and that there is no reason I have a
valid size
when retrieving a "naked pointer",

but why is the following code triggering a segfault ?

def do_clustering
  article_vectors = get_article_vectors()

  ncols = article_vectors.size
  nrows = article_vectors.map{ |av| av.map{ |id,tfidf| id }.max }.max
+ 1

  p_size = FFI.type_size :pointer
  d_size = FFI.type_size :double

  data = FFI::MemoryPointer.new :pointer, nrows
  for i in (0..nrows-1)
    row_i = FFI::MemoryPointer.new :double, ncols
    data.put_pointer i*p_size, row_i
  end
  for j in (0..ncols-1)
    col_j = article_vectors[j]
    col_j.each do |i,tfidf|
      c = data.get_pointer i*p_size
      puts "i=#{i} (nrows=#{nrows}), j=#{j} (ncols=#{ncols}), c = #
{c}"
      c.put_double(j*d_size, tfidf)
    end
  end
end