mongoc_client_read_command_with_opts (mongoc_client_t *client,
                                      const char *db_name,
                                      const bson_t *command,
                                      const mongoc_read_prefs_t *read_prefs,
                                      const bson_t *opts,
                                      bson_t *reply,
                                      bson_error_t *error);

Execute a command on the server, applying logic that is specific to commands that read, and taking the MongoDB server version into account. To send a raw command to the server without any of this logic, use mongoc_client_command_simple().

Use this function for commands that read such as "count" or "distinct". Read concern is applied from opts or else from client. Collation is applied from opts (see example below). Read concern and collation both require MongoDB 3.2 or later, otherwise an error is returned. Read preferences are applied from read_prefs or else from client. No write concern is applied.

To target a specific server, include an integer "serverId" field in opts with an id obtained first by calling mongoc_client_select_server(), then mongoc_server_description_id() on its return value.

reply is always initialized, and must be freed with bson_destroy().


  • client: A mongoc_client_t.
  • db_name: The name of the database to run the command on.
  • command: A bson_t containing the command specification.
  • read_prefs: An optional mongoc_read_prefs_t.
  • opts: A bson_t containing additional options.
  • reply: A location for the resulting document.
  • error: An optional location for a bson_error_t or NULL.


Errors are propagated via the error parameter.


true if successful; otherwise false and error is set.



Demonstrates how to prepare options for mongoc_client_read_command_with_opts and
mongoc_client_write_command_with_opts. First it calls "cloneCollectionAsCapped"
command with "writeConcern" option, then "distinct" command with "collation" and
"readConcern" options,

Start a MongoDB 3.4 replica set with --enableMajorityReadConcern and insert two

$ mongo
MongoDB Enterprise replset:PRIMARY> db.my_collection.insert({x: 1, y: "One"})
WriteResult({ "nInserted" : 1 })
MongoDB Enterprise replset:PRIMARY> db.my_collection.insert({x: 2, y: "Two"})
WriteResult({ "nInserted" : 1 })

Build and run the example:

gcc example-command-with-opts.c -o example-command-with-opts $(pkg-config
--cflags --libs libmongoc-1.0)
./example-command-with-opts [CONNECTION_STRING]
cloneCollectionAsCapped: { "ok" : 1 }
distinct: { "values" : [ 1, 2 ], "ok" : 1 }


#include <mongoc.h>
#include <stdio.h>
#include <stdlib.h>

main (int argc, char *argv[])
   mongoc_client_t *client;
   const char *uristr = "mongodb://";
   bson_t *cmd;
   bson_t *opts;
   mongoc_write_concern_t *write_concern;
   mongoc_read_prefs_t *read_prefs;
   mongoc_read_concern_t *read_concern;
   bson_t reply;
   bson_error_t error;
   char *json;

   mongoc_init ();

   if (argc > 1) {
      uristr = argv[1];

   client = mongoc_client_new (uristr);

   if (!client) {
      fprintf (stderr, "Failed to parse URI.\n");
      return EXIT_FAILURE;

   mongoc_client_set_error_api (client, 2);

   cmd = BCON_NEW ("cloneCollectionAsCapped",
                   BCON_UTF8 ("my_collection"),
                   BCON_UTF8 ("my_capped_collection"),
                   BCON_INT64 (1024 * 1024));

   /* include write concern "majority" in command options */
   write_concern = mongoc_write_concern_new ();
   mongoc_write_concern_set_wmajority (write_concern, 10000 /* wtimeoutMS */);
   opts = bson_new ();
   mongoc_write_concern_append (write_concern, opts);

   if (mongoc_client_write_command_with_opts (
          client, "test", cmd, opts, &reply, &error)) {
      json = bson_as_json (&reply, NULL);
      printf ("cloneCollectionAsCapped: %s\n", json);
      bson_free (json);
   } else {
      fprintf (stderr, "cloneCollectionAsCapped: %s\n", error.message);

   bson_free (cmd);
   bson_free (opts);

   /* distinct values of "x" in "my_collection" where "y" sorts after "one" */
   cmd = BCON_NEW ("distinct",
                   BCON_UTF8 ("my_collection"),
                   BCON_UTF8 ("x"),
                   BCON_UTF8 ("one"),

   read_prefs = mongoc_read_prefs_new (MONGOC_READ_SECONDARY);

   /* "One" normally sorts before "one"; make "One" sort after "one" */
   opts = BCON_NEW ("collation",
                    BCON_UTF8 ("en_US"),
                    BCON_UTF8 ("lower"),

   /* add a read concern to "opts" */
   read_concern = mongoc_read_concern_new ();
   mongoc_read_concern_set_level (read_concern,

   mongoc_read_concern_append (read_concern, opts);

   if (mongoc_client_read_command_with_opts (
          client, "test", cmd, read_prefs, opts, &reply, &error)) {
      json = bson_as_json (&reply, NULL);
      printf ("distinct: %s\n", json);
      bson_free (json);
   } else {
      fprintf (stderr, "distinct: %s\n", error.message);

   bson_destroy (cmd);
   bson_destroy (opts);
   bson_destroy (&reply);
   mongoc_read_prefs_destroy (read_prefs);
   mongoc_read_concern_destroy (read_concern);
   mongoc_write_concern_destroy (write_concern);
   mongoc_client_destroy (client);

   mongoc_cleanup ();

   return EXIT_SUCCESS;