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 preferences are applied from read_prefs or else from client. No write concern is applied.

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.

opts may be NULL or a BSON document with additional command options:

Consult the MongoDB Manual entry on Database Commands for each command’s arguments.


Errors are propagated via the error parameter.


Returns true if successful. Returns false and sets error if there are invalid arguments or a server or network error.



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_canonical_extended_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_canonical_extended_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;