void mongoc_client_set_read_concern (mongoc_client_t *client, const mongoc_read_concern_t *read_concern);
Sets the read concern for the client. This only affects future operations, collections, and databases inheriting from client.
The default read concern is MONGOC_READ_CONCERN_LEVEL_LOCAL. This is the correct read concern for the great majority of applications.
It is a programming error to call this function on a client from a mongoc_client_pool_t. For pooled clients, set the read concern with the instead.