unsigned int geom_count = viewer::get_geometry(pid, NULL, 0);

if(!geom_count)
    return;

std::vector<viewer::Face> faces;
faces.reserve(geom_count);

for (unsigned int i = 0; i < geom_count; i++)
{
    viewer::Face face;
    viewer::get_geometry(pid, &face, i); //tu się coś rypie
    faces.push_back(face);
}