diff --git a/src/field_5x52_int128_impl.h b/src/field_5x52_int128_impl.h
index f458873..9280bb5 100644
--- a/src/field_5x52_int128_impl.h
+++ b/src/field_5x52_int128_impl.h
@@ -16,7 +16,7 @@
 #endif
 
 SECP256K1_INLINE static void secp256k1_fe_mul_inner(uint64_t *r, const uint64_t *a, const uint64_t * SECP256K1_RESTRICT b) {
-    __int128 c, d;
+    uint128_t c, d;
     uint64_t t3, t4, tx, u0;
     uint64_t a0 = a[0], a1 = a[1], a2 = a[2], a3 = a[3], a4 = a[4];
     const uint64_t M = 0xFFFFFFFFFFFFFULL, R = 0x1000003D10ULL;
@@ -38,13 +38,13 @@ SECP256K1_INLINE static void secp256k1_fe_mul_inner(uint64_t *r, const uint64_t
      *  Note that [x 0 0 0 0 0] = [x*R].
      */
 
-    d  = (__int128)a0 * b[3]
-       + (__int128)a1 * b[2]
-       + (__int128)a2 * b[1]
-       + (__int128)a3 * b[0];
+    d  = (uint128_t)a0 * b[3]
+       + (uint128_t)a1 * b[2]
+       + (uint128_t)a2 * b[1]
+       + (uint128_t)a3 * b[0];
     VERIFY_BITS(d, 114);
     /* [d 0 0 0] = [p3 0 0 0] */
-    c  = (__int128)a4 * b[4];
+    c  = (uint128_t)a4 * b[4];
     VERIFY_BITS(c, 112);
     /* [c 0 0 0 0 d 0 0 0] = [p8 0 0 0 0 p3 0 0 0] */
     d += (c & M) * R; c >>= 52;
@@ -56,11 +56,11 @@ SECP256K1_INLINE static void secp256k1_fe_mul_inner(uint64_t *r, const uint64_t
     VERIFY_BITS(d, 63);
     /* [c 0 0 0 0 d t3 0 0 0] = [p8 0 0 0 0 p3 0 0 0] */
 
-    d += (__int128)a0 * b[4]
-       + (__int128)a1 * b[3]
-       + (__int128)a2 * b[2]
-       + (__int128)a3 * b[1]
-       + (__int128)a4 * b[0];
+    d += (uint128_t)a0 * b[4]
+       + (uint128_t)a1 * b[3]
+       + (uint128_t)a2 * b[2]
+       + (uint128_t)a3 * b[1]
+       + (uint128_t)a4 * b[0];
     VERIFY_BITS(d, 115);
     /* [c 0 0 0 0 d t3 0 0 0] = [p8 0 0 0 p4 p3 0 0 0] */
     d += c * R;
@@ -75,13 +75,13 @@ SECP256K1_INLINE static void secp256k1_fe_mul_inner(uint64_t *r, const uint64_t
     VERIFY_BITS(t4, 48);
     /* [d t4+(tx<<48) t3 0 0 0] = [p8 0 0 0 p4 p3 0 0 0] */
 
-    c  = (__int128)a0 * b[0];
+    c  = (uint128_t)a0 * b[0];
     VERIFY_BITS(c, 112);
     /* [d t4+(tx<<48) t3 0 0 c] = [p8 0 0 0 p4 p3 0 0 p0] */
-    d += (__int128)a1 * b[4]
-       + (__int128)a2 * b[3]
-       + (__int128)a3 * b[2]
-       + (__int128)a4 * b[1];
+    d += (uint128_t)a1 * b[4]
+       + (uint128_t)a2 * b[3]
+       + (uint128_t)a3 * b[2]
+       + (uint128_t)a4 * b[1];
     VERIFY_BITS(d, 115);
     /* [d t4+(tx<<48) t3 0 0 c] = [p8 0 0 p5 p4 p3 0 0 p0] */
     u0 = d & M; d >>= 52;
@@ -92,7 +92,7 @@ SECP256K1_INLINE static void secp256k1_fe_mul_inner(uint64_t *r, const uint64_t
     u0 = (u0 << 4) | tx;
     VERIFY_BITS(u0, 56);
     /* [d 0 t4+(u0<<48) t3 0 0 c] = [p8 0 0 p5 p4 p3 0 0 p0] */
-    c += (__int128)u0 * (R >> 4);
+    c += (uint128_t)u0 * (R >> 4);
     VERIFY_BITS(c, 115);
     /* [d 0 t4 t3 0 0 c] = [p8 0 0 p5 p4 p3 0 0 p0] */
     r[0] = c & M; c >>= 52;
@@ -100,13 +100,13 @@ SECP256K1_INLINE static void secp256k1_fe_mul_inner(uint64_t *r, const uint64_t
     VERIFY_BITS(c, 61);
     /* [d 0 t4 t3 0 c r0] = [p8 0 0 p5 p4 p3 0 0 p0] */
 
-    c += (__int128)a0 * b[1]
-       + (__int128)a1 * b[0];
+    c += (uint128_t)a0 * b[1]
+       + (uint128_t)a1 * b[0];
     VERIFY_BITS(c, 114);
     /* [d 0 t4 t3 0 c r0] = [p8 0 0 p5 p4 p3 0 p1 p0] */
-    d += (__int128)a2 * b[4]
-       + (__int128)a3 * b[3]
-       + (__int128)a4 * b[2];
+    d += (uint128_t)a2 * b[4]
+       + (uint128_t)a3 * b[3]
+       + (uint128_t)a4 * b[2];
     VERIFY_BITS(d, 114);
     /* [d 0 t4 t3 0 c r0] = [p8 0 p6 p5 p4 p3 0 p1 p0] */
     c += (d & M) * R; d >>= 52;
@@ -118,13 +118,13 @@ SECP256K1_INLINE static void secp256k1_fe_mul_inner(uint64_t *r, const uint64_t
     VERIFY_BITS(c, 63);
     /* [d 0 0 t4 t3 c r1 r0] = [p8 0 p6 p5 p4 p3 0 p1 p0] */
 
-    c += (__int128)a0 * b[2]
-       + (__int128)a1 * b[1]
-       + (__int128)a2 * b[0];
+    c += (uint128_t)a0 * b[2]
+       + (uint128_t)a1 * b[1]
+       + (uint128_t)a2 * b[0];
     VERIFY_BITS(c, 114);
     /* [d 0 0 t4 t3 c r1 r0] = [p8 0 p6 p5 p4 p3 p2 p1 p0] */
-    d += (__int128)a3 * b[4]
-       + (__int128)a4 * b[3];
+    d += (uint128_t)a3 * b[4]
+       + (uint128_t)a4 * b[3];
     VERIFY_BITS(d, 114);
     /* [d 0 0 t4 t3 c t1 r0] = [p8 p7 p6 p5 p4 p3 p2 p1 p0] */
     c += (d & M) * R; d >>= 52;
@@ -153,7 +153,7 @@ SECP256K1_INLINE static void secp256k1_fe_mul_inner(uint64_t *r, const uint64_t
 }
 
 SECP256K1_INLINE static void secp256k1_fe_sqr_inner(uint64_t *r, const uint64_t *a) {
-    __int128 c, d;
+    uint128_t c, d;
     uint64_t a0 = a[0], a1 = a[1], a2 = a[2], a3 = a[3], a4 = a[4];
     int64_t t3, t4, tx, u0;
     const uint64_t M = 0xFFFFFFFFFFFFFULL, R = 0x1000003D10ULL;
@@ -169,11 +169,11 @@ SECP256K1_INLINE static void secp256k1_fe_sqr_inner(uint64_t *r, const uint64_t
      *  Note that [x 0 0 0 0 0] = [x*R].
      */
 
-    d  = (__int128)(a0*2) * a3
-       + (__int128)(a1*2) * a2;
+    d  = (uint128_t)(a0*2) * a3
+       + (uint128_t)(a1*2) * a2;
     VERIFY_BITS(d, 114);
     /* [d 0 0 0] = [p3 0 0 0] */
-    c  = (__int128)a4 * a4;
+    c  = (uint128_t)a4 * a4;
     VERIFY_BITS(c, 112);
     /* [c 0 0 0 0 d 0 0 0] = [p8 0 0 0 0 p3 0 0 0] */
     d += (c & M) * R; c >>= 52;
@@ -186,9 +186,9 @@ SECP256K1_INLINE static void secp256k1_fe_sqr_inner(uint64_t *r, const uint64_t
     /* [c 0 0 0 0 d t3 0 0 0] = [p8 0 0 0 0 p3 0 0 0] */
 
     a4 *= 2;
-    d += (__int128)a0 * a4
-       + (__int128)(a1*2) * a3
-       + (__int128)a2 * a2;
+    d += (uint128_t)a0 * a4
+       + (uint128_t)(a1*2) * a3
+       + (uint128_t)a2 * a2;
     VERIFY_BITS(d, 115);
     /* [c 0 0 0 0 d t3 0 0 0] = [p8 0 0 0 p4 p3 0 0 0] */
     d += c * R;
@@ -203,11 +203,11 @@ SECP256K1_INLINE static void secp256k1_fe_sqr_inner(uint64_t *r, const uint64_t
     VERIFY_BITS(t4, 48);
     /* [d t4+(tx<<48) t3 0 0 0] = [p8 0 0 0 p4 p3 0 0 0] */
 
-    c  = (__int128)a0 * a0;
+    c  = (uint128_t)a0 * a0;
     VERIFY_BITS(c, 112);
     /* [d t4+(tx<<48) t3 0 0 c] = [p8 0 0 0 p4 p3 0 0 p0] */
-    d += (__int128)a1 * a4
-       + (__int128)(a2*2) * a3;
+    d += (uint128_t)a1 * a4
+       + (uint128_t)(a2*2) * a3;
     VERIFY_BITS(d, 114);
     /* [d t4+(tx<<48) t3 0 0 c] = [p8 0 0 p5 p4 p3 0 0 p0] */
     u0 = d & M; d >>= 52;
@@ -218,7 +218,7 @@ SECP256K1_INLINE static void secp256k1_fe_sqr_inner(uint64_t *r, const uint64_t
     u0 = (u0 << 4) | tx;
     VERIFY_BITS(u0, 56);
     /* [d 0 t4+(u0<<48) t3 0 0 c] = [p8 0 0 p5 p4 p3 0 0 p0] */
-    c += (__int128)u0 * (R >> 4);
+    c += (uint128_t)u0 * (R >> 4);
     VERIFY_BITS(c, 113);
     /* [d 0 t4 t3 0 0 c] = [p8 0 0 p5 p4 p3 0 0 p0] */
     r[0] = c & M; c >>= 52;
@@ -227,11 +227,11 @@ SECP256K1_INLINE static void secp256k1_fe_sqr_inner(uint64_t *r, const uint64_t
     /* [d 0 t4 t3 0 c r0] = [p8 0 0 p5 p4 p3 0 0 p0] */
 
     a0 *= 2;
-    c += (__int128)a0 * a1;
+    c += (uint128_t)a0 * a1;
     VERIFY_BITS(c, 114);
     /* [d 0 t4 t3 0 c r0] = [p8 0 0 p5 p4 p3 0 p1 p0] */
-    d += (__int128)a2 * a4
-       + (__int128)a3 * a3;
+    d += (uint128_t)a2 * a4
+       + (uint128_t)a3 * a3;
     VERIFY_BITS(d, 114);
     /* [d 0 t4 t3 0 c r0] = [p8 0 p6 p5 p4 p3 0 p1 p0] */
     c += (d & M) * R; d >>= 52;
@@ -243,11 +243,11 @@ SECP256K1_INLINE static void secp256k1_fe_sqr_inner(uint64_t *r, const uint64_t
     VERIFY_BITS(c, 63);
     /* [d 0 0 t4 t3 c r1 r0] = [p8 0 p6 p5 p4 p3 0 p1 p0] */
 
-    c += (__int128)a0 * a2
-       + (__int128)a1 * a1;
+    c += (uint128_t)a0 * a2
+       + (uint128_t)a1 * a1;
     VERIFY_BITS(c, 114);
     /* [d 0 0 t4 t3 c r1 r0] = [p8 0 p6 p5 p4 p3 p2 p1 p0] */
-    d += (__int128)a3 * a4;
+    d += (uint128_t)a3 * a4;
     VERIFY_BITS(d, 114);
     /* [d 0 0 t4 t3 c r1 r0] = [p8 p7 p6 p5 p4 p3 p2 p1 p0] */
     c += (d & M) * R; d >>= 52;
diff --git a/src/scalar_4x64_impl.h b/src/scalar_4x64_impl.h
index 1583b9c..9183006 100644
--- a/src/scalar_4x64_impl.h
+++ b/src/scalar_4x64_impl.h
@@ -7,8 +7,6 @@
 #ifndef _SECP256K1_SCALAR_REPR_IMPL_H_
 #define _SECP256K1_SCALAR_REPR_IMPL_H_
 
-typedef unsigned __int128 uint128_t;
-
 /* Limbs of the secp256k1 order. */
 #define SECP256K1_N_0 ((uint64_t)0xBFD25E8CD0364141ULL)
 #define SECP256K1_N_1 ((uint64_t)0xBAAEDCE6AF48A03BULL)
diff --git a/src/util.h b/src/util.h
index 373a6e5..ea4893c 100644
--- a/src/util.h
+++ b/src/util.h
@@ -84,4 +84,8 @@ static SECP256K1_INLINE void *checked_malloc(size_t size) {
 # endif
 #endif
 
+#if defined(HAVE___INT128)
+typedef unsigned __int128 uint128_t;
+#endif
+
 #endif