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, 1 insertions, 1 deletions

diff --git a/lightup.c b/lightup.c
index 4dd46c8..a56f3a9 100644
--- a/lightup.c
+++ b/lightup.c
@@ -1882,7 +1882,7 @@ static char *interpret_move(const game_state *state, game_ui *ui,
     enum { NONE, FLIP_LIGHT, FLIP_IMPOSSIBLE } action = NONE;
     int cx = -1, cy = -1;
     unsigned int flags;
-    char buf[80], *nullret = NULL, *empty = "", c;
+    char buf[80], *nullret = UI_UPDATE, *empty = UI_UPDATE, c;
 
     if (button == LEFT_BUTTON || button == RIGHT_BUTTON) {
         if (ui->cur_visible)