The zxid_simple() can be configured in following ways (later ways can override earlier ones).
Built in default configuration
Configuration file, usually /var/zxid/zxid.conf, if any
Configuration string passed as first argument. While configuration string can override all other options (i.e. it is processed last), the PATH specification is parsed before the configuration file is looked up.
Turns out that often the default configuration modified by the configurations string is all you need - you do not need to prepare configuration file.
See section "Configuring and Running" for complete list of configuration options, but generally it is sufficient to specify only a handful:
Where files are kept and configuration file is found.
The URL of the SP
The Common Domain URL (optional, if omitted the Common Domain Cookie processing is disabled)