Add example on SYSCTL_NODE expansion for _bsd_sysctl__net_children

libbsd.txt

@@ -272,3 +272,46 @@ time and the various initialization routines will thus be executed in'
 the correct order.
 
 XXX This needs more details.
+
+=== SYSCTL_NODE Example
+
+During development, we had an undefined reference to
+_bsd_sysctl__net_children that we had trouble tracking down. Thanks to
+Chris Johns, we located it. He explained how to read SYSCTL_NODE
+definitions. This line from freebsd/netinet/in_proto.c is attempting
+to add the "inet" node to the parent node "_net".
+
+[listing]
+----
+SYSCTL_NODE(_net,      PF_INET,         inet,   CTLFLAG_RW, 0,
+        "Internet Family");
+----
+
+Our problem was that we could not find where _bsd_sysctl__net_children
+was defined. Chris suggested that when in doubt compile with -save-temps
+and look at the preprocessed .i files. But he did not need that. He
+explained that this the symbol name _bsd_sysctl__net_children was
+automatically generated by a SYSCTL_NODE as follows:
+
+* _bsd_ - added by RTEMS modifications to SYSCTL_NODE macro
+* sysctl_ - boilerplace added by SYSCTL_NODE macro
+* "" - empty string for parent node
+* net - name of SYSCTL_NODE
+* children - added by SYSCTL macros
+  
+This was all generated by a support macro declaring the node as this:
+
+[listing]
+----
+struct sysctl_oid_list SYSCTL_NODE_CHILDREN(parent, name);
+----
+
+Given this information, we located this SYSCTL_NODE declaration in 
+kern/kern_mib.c
+
+[listing]
+----
+SYSCTL_NODE(, CTL_KERN,   kern,   CTLFLAG_RW, 0,
+        "High kernel, proc, limits &c");
+----
+