#define atomic_sub(b,a) do {(a)->counter -= b;} while (0)
#define atomic_sub_return(n,a) ((a)->counter -= n)
#define atomic_dec_return(a) atomic_sub_return(1,a)
+#define atomic_add_unless(v, a, u) ((v)->counter != u ? (v)->counter += a : 0)
+#define atomic_inc_not_zero(v) atomic_add_unless((v), 1, 0)
#ifdef HAVE_LIBPTHREAD