Graphviz Library node position

I am making an example program to understand the feasibility of an idea.
What I’m trying to do is to create a graph and use graphviz to calculate the node position.
I wrote this piece of code but it doesn’t seem to work.
What am I doing wrong?
Can someone help me out?

#include <graphviz/gvc.h>

int main(int argc, const char * argv []) {
  GVC_t *gvc = gvContext();

  // Create a simple digraph
  Agraph_t *g = agopen("g", Agdirected, 0);
  Agnode_t *n = agnode(g, "n", 1);
  Agnode_t *m = agnode(g, "m", 1);
  Agedge_t *e = agedge(g, n, m, 0, 1);

  // Set an attribute - in this case one that affects the visible rendering
  agsafeset(n, "color", "red", "");

  // Use the directed graph layout engine
  gvLayout(gvc, g, "dot");
  printf("%s\n", agget(n, "pos"));
  // Free layout data
  gvFreeLayout(gvc, g);
  // Free graph structures

  // close output file, free context, and return number of errors


In particular, function agget() returns me an empty string.