New name UI_UPDATE for interpret_move's return "".

Now midend.c directly tests the returned pointer for equality to this
value, instead of checking whether it's the empty string.

A minor effect of this is that games may now return a dynamically
allocated empty string from interpret_move() and treat it as just
another legal move description. But I don't expect anyone to be
perverse enough to actually do that! The main purpose is that it
avoids returning a string literal from a function whose return type is
a pointer to _non-const_ char, i.e. we are now one step closer to
being able to make this code base clean under -Wwrite-strings.

1 files changed, 8 insertions, 0 deletions

diff --git a/puzzles.h b/puzzles.h
index 47c164c..f43ee83 100644
--- a/puzzles.h
+++ b/puzzles.h
@@ -684,6 +684,14 @@ extern const int gamecount;
 extern const game thegame;
 #endif
 
+/*
+ * Special string value to return from interpret_move in the case
+ * where the game UI has been updated but no actual move is being
+ * appended to the undo chain. Must be declared as a non-const char,
+ * but should never actually be modified by anyone.
+ */
+extern char UI_UPDATE[];
+
 /* A little bit of help to lazy developers */
 #define DEFAULT_STATUSBAR_TEXT "Use status_bar() to fill this in."