+/*
+ * Similar to ALLOC_GROW but handles updating of the nr value and
+ * zeroing the bytes of the newly-grown array elements.
+ *
+ * DO NOT USE any expression with side-effect for any of the
+ * arguments.
+ */
+#define ALLOC_GROW_BY(x, nr, increase, alloc) \
+ do { \
+ if (increase) { \
+ size_t new_nr = nr + (increase); \
+ if (new_nr < nr) \
+ BUG("negative growth in ALLOC_GROW_BY"); \
+ ALLOC_GROW(x, new_nr, alloc); \
+ memset((x) + nr, 0, sizeof(*(x)) * (increase)); \
+ nr = new_nr; \
+ } \
+ } while (0)
+