diff --git a/libbf.c b/libbf.c
index abdbece..bc78860 100644
--- a/libbf.c
+++ b/libbf.c
@@ -5365,19 +5365,20 @@ int bf_acos(bf_t *r, const bf_t *a, limb_t prec, bf_flags_t flags)
 #if LIMB_BITS == 64
 
 /* Note: we assume __int128 is available */
+/* uint128_t defined in libbf.h          */
 #define muldq(r1, r0, a, b)                     \
     do {                                        \
-        unsigned __int128 __t;                          \
-        __t = (unsigned __int128)(a) * (unsigned __int128)(b);  \
+        uint128_t __t;                          \
+        __t = (uint128_t)(a) * (uint128_t)(b);  \
         r0 = __t;                               \
         r1 = __t >> 64;                         \
     } while (0)
 
 #define divdq(q, r, a1, a0, b)                  \
     do {                                        \
-        unsigned __int128 __t;                  \
+        uint128_t __t;                  \
         limb_t __b = (b);                       \
-        __t = ((unsigned __int128)(a1) << 64) | (a0);   \
+        __t = ((uint128_t)(a1) << 64) | (a0);   \
         q = __t / __b;                                  \
         r = __t % __b;                                  \
     } while (0)
diff --git a/libbf.h b/libbf.h
index d89d09c..ebf425c 100644
--- a/libbf.h
+++ b/libbf.h
@@ -36,8 +36,8 @@
 #define LIMB_BITS (1 << LIMB_LOG2_BITS)
 
 #if LIMB_BITS == 64
-typedef __int128 int128_t;
-typedef unsigned __int128 uint128_t;
+__extension__ typedef __int128 int128_t;
+__extension__ typedef unsigned __int128 uint128_t;
 typedef int64_t slimb_t;
 typedef uint64_t limb_t;
 typedef uint128_t dlimb_t;