diff -Naur old/stdlib/scanf.ml new/stdlib/scanf.ml
--- old/stdlib/scanf.ml	2015-05-13 11:26:52.450679862 +0200
+++ new/stdlib/scanf.ml	2015-05-13 11:25:54.198678254 +0200
@@ -411,6 +411,92 @@
 end
 ;;
 
+(* Unsigned int/string conversions *)
+
+let unsigned_int_of_string str =
+  let failure = "unsigned_int_of_string" in
+  let limit = max_int / 5 in
+  let acc = ref 0 in
+  for i = 0 to String.length str - 1 do
+    match str.[i] with
+    | '0' .. '9' as c ->
+      let digit = int_of_char c - int_of_char '0' in
+      let new_acc = !acc * 10 + digit in
+      if !acc >= 0 && (!acc < limit || (!acc = limit && new_acc < 0))
+      then acc := new_acc
+      else failwith failure
+    | '_' -> ()
+    | _ -> failwith failure
+  done;
+  !acc
+
+let unsigned_int32_of_string str =
+  let failure = "unsigned_int32_of_string" in
+  let limit = Int32.div Int32.max_int 5l in
+  let acc = ref 0l in
+  for i = 0 to String.length str - 1 do
+    match str.[i] with
+    | '0' .. '9' as c ->
+      let digit = Int32.of_int (int_of_char c - int_of_char '0') in
+      let new_acc = Int32.add (Int32.mul !acc 10l) digit in
+      if !acc >= 0l && (!acc < limit || (!acc = limit && new_acc < 0l))
+      then acc := new_acc
+      else failwith failure
+    | '_' -> ()
+    | _ -> failwith failure
+  done;
+  !acc
+
+let unsigned_int64_of_string str =
+  let failure = "unsigned_int64_of_string" in
+  let limit = Int64.div Int64.max_int 5L in
+  let acc = ref 0L in
+  for i = 0 to String.length str - 1 do
+    match str.[i] with
+    | '0' .. '9' as c ->
+      let digit = Int64.of_int (int_of_char c - int_of_char '0') in
+      let new_acc = Int64.add (Int64.mul !acc 10L) digit in
+      if !acc >= 0L && (!acc < limit || (!acc = limit && new_acc < 0L))
+      then acc := new_acc
+      else failwith failure
+    | '_' -> ()
+    | _ -> failwith failure
+  done;
+  !acc
+
+let unsigned_nativeint_of_string str =
+  let failure = "unsigned_nativeint_of_string" in
+  let limit = Nativeint.div Nativeint.max_int 5n in
+  let acc = ref 0n in
+  for i = 0 to String.length str - 1 do
+    match str.[i] with
+    | '0' .. '9' as c ->
+      let digit = Nativeint.of_int (int_of_char c - int_of_char '0') in
+      let new_acc = Nativeint.add (Nativeint.mul !acc 10n) digit in
+      if !acc >= 0n && (!acc < limit || (!acc = limit && new_acc < 0n))
+      then acc := new_acc
+      else failwith failure
+    | '_' -> ()
+    | _ -> failwith failure
+  done;
+  !acc
+
+(* To scan native ints, int32 and int64 integers.
+   We cannot access to conversions to/from strings for those types,
+   Nativeint.of_string, Int32.of_string, and Int64.of_string,
+   since those modules are not available to [Scanf].
+   However, we can bind and use the corresponding primitives that are
+   available in the runtime. *)
+external nativeint_of_string : string -> nativeint
+  = "caml_nativeint_of_string"
+;;
+external int32_of_string : string -> int32
+  = "caml_int32_of_string"
+;;
+external int64_of_string : string -> int64
+  = "caml_int64_of_string"
+;;
+
 (* Formatted input functions. *)
 
 type ('a, 'b, 'c, 'd) scanner =
@@ -500,7 +586,7 @@
 let token_int_literal conv ib =
   let tok =
     match conv with
-    | 'd' | 'i' | 'u' -> Scanning.token ib
+    | 'd' | 'i' -> Scanning.token ib
     | 'o' -> "0o" ^ Scanning.token ib
     | 'x' | 'X' -> "0x" ^ Scanning.token ib
     | 'b' -> "0b" ^ Scanning.token ib
@@ -512,29 +598,27 @@
 (* All the functions that convert a string to a number raise the exception
    Failure when the conversion is not possible.
    This exception is then trapped in [kscanf]. *)
-let token_int conv ib = int_of_string (token_int_literal conv ib);;
+let token_int conv ib =
+  if conv = 'u' then unsigned_int_of_string (Scanning.token ib)
+  else int_of_string (token_int_literal conv ib)
+;;
 
 let token_float ib = float_of_string (Scanning.token ib);;
 
-(* To scan native ints, int32 and int64 integers.
-   We cannot access to conversions to/from strings for those types,
-   Nativeint.of_string, Int32.of_string, and Int64.of_string,
-   since those modules are not available to [Scanf].
-   However, we can bind and use the corresponding primitives that are
-   available in the runtime. *)
-external nativeint_of_string : string -> nativeint
-  = "caml_nativeint_of_string"
-;;
-external int32_of_string : string -> int32
-  = "caml_int32_of_string"
+let token_nativeint conv ib =
+  if conv = 'u' then unsigned_nativeint_of_string (Scanning.token ib)
+  else nativeint_of_string (token_int_literal conv ib)
 ;;
-external int64_of_string : string -> int64
-  = "caml_int64_of_string"
+
+let token_int32 conv ib =
+  if conv = 'u' then unsigned_int32_of_string (Scanning.token ib)
+  else int32_of_string (token_int_literal conv ib)
 ;;
 
-let token_nativeint conv ib = nativeint_of_string (token_int_literal conv ib);;
-let token_int32 conv ib = int32_of_string (token_int_literal conv ib);;
-let token_int64 conv ib = int64_of_string (token_int_literal conv ib);;
+let token_int64 conv ib =
+  if conv = 'u' then unsigned_int64_of_string (Scanning.token ib)
+  else int64_of_string (token_int_literal conv ib)
+;;
 
 (* Scanning numbers. *)
 

diff -Naur old/byterun/ints.c new/byterun/ints.c
--- old/byterun/ints.c	2015-05-13 11:27:06.546680251 +0200
+++ new/byterun/ints.c	2015-05-13 11:26:26.938679158 +0200
@@ -23,7 +23,8 @@
 
 static char * parse_sign_and_base(char * p,
                                   /*out*/ int * base,
-                                  /*out*/ int * sign)
+                                  /*out*/ int * sign,
+                                  /*out*/ int * is_signed)
 {
   *sign = 1;
   if (*p == '-') {
@@ -31,8 +32,11 @@
     p++;
   }
   *base = 10;
+  *is_signed = 1;
   if (*p == '0') {
     switch (p[1]) {
+    case 'u': case 'U':
+      *is_signed = 0; p +=2; break;
     case 'x': case 'X':
       *base = 16; p += 2; break;
     case 'o': case 'O':
@@ -60,9 +64,9 @@
 {
   char * p;
   uintnat res, threshold;
-  int sign, base, d;
+  int sign, base, is_signed, d;
 
-  p = parse_sign_and_base(String_val(s), &base, &sign);
+  p = parse_sign_and_base(String_val(s), &base, &sign, &is_signed);
   threshold = ((uintnat) -1) / base;
   d = parse_digit(*p);
   if (d < 0 || d >= base) caml_failwith("int_of_string");
@@ -81,11 +85,17 @@
     caml_failwith("int_of_string");
   }
   if (base == 10) {
-    /* Signed representation expected, allow -2^(nbits-1) to 2^(nbits-1) - 1 */
-    if (sign >= 0) {
-      if (res >= (uintnat)1 << (nbits - 1)) caml_failwith("int_of_string");
-    } else {
-      if (res >  (uintnat)1 << (nbits - 1)) caml_failwith("int_of_string");
+    if (is_signed) {
+      /* Signed representation expected, allow -2^(nbits-1) to 2^(nbits-1)-1 */
+      if (sign >= 0) {
+        if (res >= (uintnat)1 << (nbits - 1)) caml_failwith("int_of_string");
+      } else {
+        if (res >  (uintnat)1 << (nbits - 1)) caml_failwith("int_of_string");
+      }
+    } else if (sign < 0) {
+      caml_failwith("int_of_string");
+    } else if (nbits < 8 * sizeof(uintnat)) {
+      if (res >= (uintnat)1 << nbits) caml_failwith("int_of_string");
     }
   } else {
     /* Unsigned representation expected, allow 0 to 2^nbits - 1
@@ -525,9 +535,9 @@
 {
   char * p;
   uint64 res, threshold;
-  int sign, base, d;
+  int sign, base, is_signed, d;
 
-  p = parse_sign_and_base(String_val(s), &base, &sign);
+  p = parse_sign_and_base(String_val(s), &base, &sign, &is_signed);
   threshold = ((uint64) -1) / base;
   d = parse_digit(*p);
   if (d < 0 || d >= base) caml_failwith("int_of_string");
@@ -548,10 +558,14 @@
   }
   if (base == 10) {
     /* Signed representation expected, allow -2^63 to 2^63 - 1 only */
-    if (sign >= 0) {
-      if (res >= (uint64)1 << 63) caml_failwith("int_of_string");
-    } else {
-      if (res >  (uint64)1 << 63) caml_failwith("int_of_string");
+    if (is_signed) {
+      if (sign >= 0) {
+        if (res >= (uint64)1 << 63) caml_failwith("int_of_string");
+      } else {
+        if (res >  (uint64)1 << 63) caml_failwith("int_of_string");
+      }
+    } else if (sign < 0) {
+      caml_failwith("int_of_string");
     }
   }
   if (sign < 0) res = - res;
diff -Naur old/stdlib/scanf.ml new/stdlib/scanf.ml
--- old/stdlib/scanf.ml	2015-05-13 11:26:54.934679931 +0200
+++ new/stdlib/scanf.ml	2015-05-13 11:26:16.290678864 +0200
@@ -500,7 +500,8 @@
 let token_int_literal conv ib =
   let tok =
     match conv with
-    | 'd' | 'i' | 'u' -> Scanning.token ib
+    | 'd' | 'i' -> Scanning.token ib
+    | 'u' -> "0u" ^ Scanning.token ib
     | 'o' -> "0o" ^ Scanning.token ib
     | 'x' | 'X' -> "0x" ^ Scanning.token ib
     | 'b' -> "0b" ^ Scanning.token ib