void
mongoc_read_prefs_set_tags (mongoc_read_prefs_t *read_prefs,
                            const bson_t *tags);

#include <mongoc/mongoc.h>
static void
run_query_with_read_prefs_tags (mongoc_collection_t *collection)
{
   char *str;
   const bson_t *doc;
   bson_t filter = BSON_INITIALIZER;
   bson_error_t error;
   mongoc_cursor_t *cursor;
   mongoc_read_prefs_t *read_prefs;
   /*  Create a tagset representing
    *  [
    *    {"dc": "ny", "rack": "1" }, // Any node in rack1 in the ny datacenter
    *    {"dc": "ny", "rack": "2" }, // Any node in rack2 in the ny datacenter
    *    {"dc": "ny" },              // Any node in the ny datacenter
    *    {}                          // If all else fails, just any available node
    * ]
    */
   bson_t *tags = BCON_NEW (
      "0", "{", "dc", BCON_UTF8("ny"), "rack", BCON_UTF8("1"), "}",
      "1", "{", "dc", BCON_UTF8("ny"), "rack", BCON_UTF8("2"), "}",
      "2", "{", "dc", BCON_UTF8("ny"), "}",
      "3", "{", "}"
   );
   read_prefs = mongoc_read_prefs_new (MONGOC_READ_SECONDARY);
   mongoc_read_prefs_set_tags (read_prefs, tags);
   bson_destroy (tags);
   cursor =
      mongoc_collection_find_with_opts (collection, &filter, NULL, read_prefs);
   while (mongoc_cursor_next (cursor, &doc)) {
      str = bson_as_canonical_extended_json (doc, NULL);
      printf ("%s\n", str);
      bson_free (str);
   }
   if (mongoc_cursor_error (cursor, &error)) {
      fprintf (stderr, "Cursor error: %s\n", error.message);
   }
   mongoc_cursor_destroy (cursor);
   mongoc_read_prefs_destroy (read_prefs);
   bson_destroy (doc);
}
int main (void)
{
   mongoc_client_t *client;
   mongoc_collection_t *collection;
   mongoc_init ();
   client =
      mongoc_client_new ("mongodb://localhost/?appname=rp_tags&replicaSet=foo");
   mongoc_client_set_error_api (client, 2);
   collection = mongoc_client_get_collection (client, "dbname", "collname");
   run_query_with_read_prefs_tags (collection);
   mongoc_collection_destroy (collection);
   mongoc_client_destroy (client);
   mongoc_cleanup();
}