stdlib: Make gen_fsm callbacks optional

1 files changed, 19 insertions, 0 deletions

diff --git a/lib/stdlib/doc/src/gen_fsm.xml b/lib/stdlib/doc/src/gen_fsm.xml
index 719ab2b558..691a039e34 100644
--- a/lib/stdlib/doc/src/gen_fsm.xml
+++ b/lib/stdlib/doc/src/gen_fsm.xml
@@ -562,6 +562,13 @@ gen_fsm:sync_send_all_state_event -----> Module:handle_sync_event/4
         <v>Extra = term()</v>
       </type>
       <desc>
+        <note>
+          <p>This callback is optional, so callback modules need not export it.
+            If a release upgrade/downgrade with <c>Change={advanced,Extra}</c>
+            specified in the <c>appup</c> file is made when <c>code_change/4</c>
+            isn't implemented the process will crash with an <c>undef</c> exit
+            reason.</p>
+        </note>
         <p>This function is called by a <c>gen_fsm</c> process when it is to
           update its internal state data during a release upgrade/downgrade,
           that is, when instruction <c>{update,Module,Change,...}</c>,
@@ -686,6 +693,13 @@ gen_fsm:sync_send_all_state_event -----> Module:handle_sync_event/4
         <v>&nbsp;Reason = normal | term()</v>
       </type>
       <desc>
+        <note>
+          <p>This callback is optional, so callback modules need not
+            export it. The <c>gen_fsm</c> module provides a default
+            implementation of this function that logs about the unexpected
+            <c>Info</c> message, drops it and returns
+            <c>{next_state, StateName, StateData}</c>.</p>
+        </note>
         <p>This function is called by a <c>gen_fsm</c> process when it receives
           any other message than a synchronous or asynchronous event (or a
           system message).</p>
@@ -899,6 +913,11 @@ gen_fsm:sync_send_all_state_event -----> Module:handle_sync_event/4
         <v>StateData = term()</v>
       </type>
       <desc>
+        <note>
+          <p>This callback is optional, so callback modules need not
+            export it. The <c>gen_fsm</c> module provides a default
+            implementation without cleanup.</p>
+        </note>
         <p>This function is called by a <c>gen_fsm</c> process when it is about
           to terminate. It is to be the opposite of
           <seealso marker="#Module:init/1"><c>Module:init/1</c></seealso>