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, 3 insertions, 3 deletions

diff --git a/magnets.c b/magnets.c
index 553ca0d..354c00a 100644
--- a/magnets.c
+++ b/magnets.c
@@ -1804,11 +1804,11 @@ static char *interpret_move(const game_state *state, game_ui *ui,
     if (IS_CURSOR_MOVE(button)) {
         move_cursor(button, &ui->cur_x, &ui->cur_y, state->w, state->h, 0);
         ui->cur_visible = 1;
-        return "";
+        return UI_UPDATE;
     } else if (IS_CURSOR_SELECT(button)) {
         if (!ui->cur_visible) {
             ui->cur_visible = 1;
-            return "";
+            return UI_UPDATE;
         }
         action = (button == CURSOR_SELECT) ? CYCLE_MAGNET : CYCLE_NEUTRAL;
         gx = ui->cur_x;
@@ -1817,7 +1817,7 @@ static char *interpret_move(const game_state *state, game_ui *ui,
                (button == LEFT_BUTTON || button == RIGHT_BUTTON)) {
         if (ui->cur_visible) {
             ui->cur_visible = 0;
-            nullret = "";
+            nullret = UI_UPDATE;
         }
         action = (button == LEFT_BUTTON) ? CYCLE_MAGNET : CYCLE_NEUTRAL;
     } else if (button == LEFT_BUTTON && is_clue(state, gx, gy)) {