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/slant.c b/slant.c
index 5f9f4f6..5980eb2 100644
--- a/slant.c
+++ b/slant.c
@@ -1683,7 +1683,7 @@ static char *interpret_move(const game_state *state, game_ui *ui,
     } else if (IS_CURSOR_SELECT(button)) {
         if (!ui->cur_visible) {
             ui->cur_visible = 1;
-            return "";
+            return UI_UPDATE;
         }
         x = ui->cur_x;
         y = ui->cur_y;
@@ -1692,7 +1692,7 @@ static char *interpret_move(const game_state *state, game_ui *ui,
     } else if (IS_CURSOR_MOVE(button)) {
         move_cursor(button, &ui->cur_x, &ui->cur_y, w, h, 0);
         ui->cur_visible = 1;
-        return "";
+        return UI_UPDATE;
     } else if (button == '\\' || button == '\b' || button == '/') {
 	int x = ui->cur_x, y = ui->cur_y;
 	if (button == ("\\" "\b" "/")[state->soln[y*w + x] + 1]) return NULL;