- in which case it will describe the process at the other endpoint
- of a connection.
- </para>
-
- <para><function>sd_bus_creds_get_pid()</function> will retrieve the
- PID (process identifier).</para>
+ in which case they describe the credentials of the creator of a
+ bus, or with
+ <citerefentry><refentrytitle>sd_bus_message_get_creds</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
+ in which case they describe the credentials of the sender of the
+ message.</para>
+
+ <para>Not all credential fields are part of every
+ <literal>sd_bus_creds</literal> object. Use
+ <citerefentry><refentrytitle>sd_bus_creds_get_mask</refentrytitle><manvolnum>3</manvolnum></citerefentry>
+ to determine the mask of fields available.</para>
+
+ <para><function>sd_bus_creds_get_pid()</function> will retrieve
+ the PID (process identifier). Similarly,
+ <function>sd_bus_creds_get_ppid()</function> will retrieve the
+ parent PID. Note that PID 1 has no parent process, in which case
+ -ENXIO is returned.</para>