- <parameter>path</parameter> parameter is not NULL, it
- is checked whether the message queue is bound to the
- specified name.</para>
+ <parameter>path</parameter> parameter is not
+ <constant>NULL</constant>, it is checked whether the
+ message queue is bound to the specified name.</para>
+
+ <para><function>sd_is_special()</function> may be
+ called to check whether the specified file descriptor
+ refers to a special file. If the
+ <parameter>path</parameter> parameter is not
+ <constant>NULL</constant>, it is checked whether file
+ descriptor is bound to the specified file
+ name. Special files in this context are character
+ device nodes and files in <filename>/proc</filename>
+ or <filename>/sys</filename>.</para>