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, 2 insertions, 2 deletions

diff --git a/flip.c b/flip.c
index c7126fb..68979fe 100644
--- a/flip.c
+++ b/flip.c
@@ -951,7 +951,7 @@ static char *interpret_move(const game_state *state, game_ui *ui,
             tx = ui->cx; ty = ui->cy;
             ui->cdraw = 1;
         }
-        nullret = "";
+        nullret = UI_UPDATE;
 
         if (tx >= 0 && tx < w && ty >= 0 && ty < h) {
             /*
@@ -985,7 +985,7 @@ static char *interpret_move(const game_state *state, game_ui *ui,
         ui->cx = min(max(ui->cx, 0), state->w - 1);
         ui->cy = min(max(ui->cy, 0), state->h - 1);
         ui->cdraw = 1;
-        nullret = "";
+        nullret = UI_UPDATE;
     }
 
     return nullret;