When a non-NULL value is provided, the behavior of the MYSQL_READ_DEFAULT_FILE option roughly corresponds to the behavior of the --defaults-file MariaDB command-line option :
Should MariaDB Connector/C also have an option that roughly corresponds to the behavior of the --defaults-extra-file MariaDB command-line option? This command-line option essentially adds an "extra" option file that is parsed after all others.
It may be useful to have an option like this if we ever want MariaDB Connector/C to parse my.cnf and other option/configuration files for client tools. e.g. see MDEV-19035