mongoc_session_opts_set_default_transaction_opts (
   mongoc_session_opt_t *opts, const mongoc_transaction_opt_t *txn_opts);

Set the default options for transactions started with this session. The txn_opts argument is copied and can be freed after calling this function.

When a session is first created with mongoc_client_start_session(), it inherits from the client the read concern, write concern, and read preference with which to start transactions. Each of these fields can be overridden independently. Create a mongoc_transaction_opt_t with mongoc_transaction_opts_new(), and pass a non-NULL option to any of the mongoc_transaction_opt_t setter functions:

Pass the resulting transaction options to mongoc_session_opts_set_default_transaction_opts(). Each field set in the transaction options overrides the inherited client configuration. There is an opportunity to override each one of these fields again by passing a mongoc_transaction_opt_t to mongoc_client_session_start_transaction().