3 files changed, 27 insertions, 0 deletions
diff --git a/system/doc/design_principles/events.xml b/system/doc/design_principles/events.xml
index 5579f1e459..fab9e8305e 100644
--- a/system/doc/design_principles/events.xml
+++ b/system/doc/design_principles/events.xml
@@ -217,5 +217,22 @@ terminate(_Args, Fd) ->
 ok</pre>
     </section>
   </section>
+  <section>
+    <title>Handling Other Messages</title>
+    <p>If the gen_event should be able to receive other messages than
+      events, the callback function <c>handle_info(Info, StateName, StateData)</c>
+      must be implemented to handle them. Examples of
+      other messages are exit messages, if the gen_event is linked to
+      other processes (than the supervisor) and trapping exit signals.</p>
+    <code type="none">
+handle_info({'EXIT', Pid, Reason}, State) ->
+    ..code to handle exits here..
+    {ok, NewState}.</code>
+    <p>The code_change method also has to be implemented.</p>
+    <code type="none">
+code_change(OldVsn, State, Extra) ->
+    ..code to convert state (and more) during code change
+    {ok, NewState}</code>
+  </section>
 </chapter>
 
diff --git a/system/doc/design_principles/fsm.xml b/system/doc/design_principles/fsm.xml
index 7cdd62057b..c3e9027274 100644
--- a/system/doc/design_principles/fsm.xml
+++ b/system/doc/design_principles/fsm.xml
@@ -308,6 +308,11 @@ terminate(normal, _StateName, _StateData) ->
 handle_info({'EXIT', Pid, Reason}, StateName, StateData) ->
     ..code to handle exits here..
     {next_state, StateName1, StateData1}.</code>
+    <p>The code_change method also has to be implemented.</p>
+    <code type="none">
+code_change(OldVsn, StateName, StateData, Extra) ->
+    ..code to convert state (and more) during code change
+    {ok, NextStateName, NewStateData}</code>
   </section>
 </chapter>
 
diff --git a/system/doc/design_principles/gen_server_concepts.xml b/system/doc/design_principles/gen_server_concepts.xml
index 8131c47a69..231333da0e 100644
--- a/system/doc/design_principles/gen_server_concepts.xml
+++ b/system/doc/design_principles/gen_server_concepts.xml
@@ -264,6 +264,11 @@ terminate(normal, State) ->
 handle_info({'EXIT', Pid, Reason}, State) ->
     ..code to handle exits here..
     {noreply, State1}.</code>
+    <p>The code_change method also has to be implemented.</p>
+    <code type="none">
+code_change(OldVsn, State, Extra) ->
+    ..code to convert state (and more) during code change
+    {ok, NewState}.</code>
   </section>
 </chapter>