diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index 5ca151003c3..882e420c40c 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,3 +1,7 @@
+2012-02-20  Tristan Gingold  <gingold@adacore.com>
+
+	* darwin-nat.h (enum darwin_msg_state): Add comments.
+
 2012-02-20  Tristan Gingold  <gingold@adacore.com>
 
 	* amd64-darwin-tdep.c (x86_darwin_init_abi_64): Fix jb_pc_offset
diff --git a/gdb/darwin-nat.h b/gdb/darwin-nat.h
index c74af1d3f3d..6c892991070 100644
--- a/gdb/darwin-nat.h
+++ b/gdb/darwin-nat.h
@@ -61,7 +61,17 @@ struct darwin_exception_msg
   integer_t ex_data[2];
 };
 
-enum darwin_msg_state { DARWIN_RUNNING, DARWIN_STOPPED, DARWIN_MESSAGE };
+enum darwin_msg_state
+{
+  /* The thread is running.  */
+  DARWIN_RUNNING,
+
+  /* The thread is stopped.  */
+  DARWIN_STOPPED,
+
+  /* The thread has sent a message and waits for a reply.  */
+  DARWIN_MESSAGE
+};
 
 struct private_thread_info
 {