• collection: A mongoc_collection_t.
• selector: A bson_t containing the query to match the document for updating.
• update: A bson_t containing the update to perform.
• reply: Optional. An uninitialized bson_t populated with the update result, or NULL.
• error: An optional location for a bson_error_t or NULL.

• writeConcern: Construct a mongoc_write_concern_t and use mongoc_write_concern_append to add the write concern to opts. See the example code for mongoc_client_write_command_with_opts.
• sessionId: First, construct a mongoc_client_session_t with mongoc_client_start_session. You can begin a transaction with mongoc_client_session_start_transaction, optionally with a mongoc_transaction_opt_t that overrides the options inherited from collection, and use mongoc_client_session_append to add the session to opts. See the example code for mongoc_client_session_t.
• validate: Construct a bitwise-or of all desired bson_validate_flags_t. Set to false to skip client-side validation of the provided BSON documents.
• bypassDocumentValidation: Set to true to skip server-side schema validation of the provided BSON documents.
• collation: Configure textual comparisons. See Setting Collation Order, and the MongoDB Manual entry on Collation. Collation requires MongoDB 3.2 or later, otherwise an error is returned.
• upsert: When true, creates a new document if no document matches the query.
• arrayFilters: An array of filters specifying to which array elements an update should apply.

#include "mongoc/mongoc.h"
int
main (int argc, char **argv)
{
   bson_t *to_insert = BCON_NEW ("_id", BCON_INT32 (1));
   bson_t *selector = BCON_NEW ("_id", "{", "$gt", BCON_INT32 (0), "}");
   bson_t *update = BCON_NEW ("$set", "{", "x", BCON_INT32 (1), "}");
   const bson_t *next_doc;
   char *to_str;
   bson_error_t error = {0};
   mongoc_cursor_t *cursor;
   mongoc_client_t *client;
   mongoc_collection_t *coll;
   const char *uri_string = "mongodb://localhost:27017/?appname=example-update";
   mongoc_uri_t *uri = mongoc_uri_new_with_error (uri_string, &error);
   if (!uri) {
      fprintf (stderr,
               "failed to parse URI: %s\n"
               "error message:       %s\n",
               uri_string,
               error.message);
      return EXIT_FAILURE;
   }
   client = mongoc_client_new_from_uri (uri);
   if (!client) {
      return EXIT_FAILURE;
   }
   coll = mongoc_client_get_collection (client, "db", "example_coll");
   mongoc_client_set_error_api (client, 2);
   /* insert a document */
   if (!mongoc_collection_insert_one (coll, to_insert, NULL, NULL, &error)) {
      fprintf (stderr, "insert failed: %s\n", error.message);
      return EXIT_FAILURE;
   }
   if (!mongoc_collection_update_one (
          coll, selector, update, NULL, NULL, &error)) {
      fprintf (stderr, "update failed: %s\n", error.message);
      return EXIT_FAILURE;
   }
   to_str = bson_as_relaxed_extended_json (to_insert, NULL);
   printf ("inserted: %s\n", to_str);
   bson_free (to_str);
   cursor = mongoc_collection_find_with_opts (coll, selector, NULL, NULL);
   BSON_ASSERT (mongoc_cursor_next (cursor, &next_doc));
   printf ("after update, collection has the following document:\n");
   to_str = bson_as_relaxed_extended_json (next_doc, NULL);
   printf ("%s\n", to_str);
   bson_free (to_str);
   BSON_ASSERT (mongoc_collection_drop (coll, NULL));
   bson_destroy (to_insert);
   bson_destroy (update);
   bson_destroy (selector);
   mongoc_collection_destroy (coll);
   mongoc_uri_destroy (uri);
   mongoc_client_destroy (client);
   return EXIT_SUCCESS;
}