diff -ur cil-1.7.3.old/ocamlutil/errormsg.ml cil-1.7.3/ocamlutil/errormsg.ml
--- cil-1.7.3.old/ocamlutil/errormsg.ml	2013-07-24 16:07:11.000000000 +0100
+++ cil-1.7.3/ocamlutil/errormsg.ml	2017-11-22 14:55:09.549734942 +0000
@@ -210,23 +210,24 @@
   let str1 = 
     if str <> "" && String.get str 0 = '"' (* '"' ( *) 
     then rem_quotes str else str in
-  let l = String.length str1 in
+  let str1 = Bytes.of_string str1 in
+  let l = Bytes.length str1 in
   let rec loop (copyto: int) (i: int) = 
     if i >= l then 
-      String.sub str1 0 copyto
+      Bytes.sub str1 0 copyto
      else 
-       let c = String.get str1 i in
+       let c = Bytes.get str1 i in
        if c <> '\\' then begin
-          String.set str1 copyto c; loop (copyto + 1) (i + 1)
+          Bytes.set str1 copyto c; loop (copyto + 1) (i + 1)
        end else begin
-          String.set str1 copyto '/';
-          if i < l - 2 && String.get str1 (i + 1) = '\\' then
+          Bytes.set str1 copyto '/';
+          if i < l - 2 && Bytes.get str1 (i + 1) = '\\' then
               loop (copyto + 1) (i + 2)
           else 
               loop (copyto + 1) (i + 1)
        end
   in
-  loop 0 0
+  Bytes.to_string (loop 0 0)
 
 let readingFromStdin = ref false
 
diff -ur cil-1.7.3.old/ocamlutil/pretty.ml cil-1.7.3/ocamlutil/pretty.ml
--- cil-1.7.3.old/ocamlutil/pretty.ml	2017-11-22 13:17:54.112327112 +0000
+++ cil-1.7.3/ocamlutil/pretty.ml	2017-11-22 14:56:24.658729757 +0000
@@ -725,8 +725,9 @@
               invalid_arg ("dprintf: unimplemented format " 
 			   ^ (String.sub format i (j-i+1)));
 	    let j' = succ j in (* eat the d,i,x etc. *)
-	    let format_spec = "% " in
-	    String.set format_spec 1 (fget j'); (* format_spec = "%x", etc. *)
+	    let format_spec = Bytes.of_string "% " in
+	    Bytes.set format_spec 1 (fget j'); (* format_spec = "%x", etc. *)
+            let format_spec = Bytes.to_string format_spec in
             Obj.magic(fun n ->
               collect (dctext1 acc
                          (Int64.format format_spec n))
@@ -735,8 +736,9 @@
 	    if j != i + 1 then invalid_arg ("dprintf: unimplemented format " 
 					    ^ (String.sub format i (j-i+1)));
 	    let j' = succ j in (* eat the d,i,x etc. *)
-	    let format_spec = "% " in
-	    String.set format_spec 1 (fget j'); (* format_spec = "%x", etc. *)
+	    let format_spec = Bytes.of_string "% " in
+	    Bytes.set format_spec 1 (fget j'); (* format_spec = "%x", etc. *)
+            let format_spec = Bytes.to_string format_spec in
             Obj.magic(fun n ->
               collect (dctext1 acc
                          (Int32.format format_spec n))
@@ -745,8 +747,9 @@
 	    if j != i + 1 then invalid_arg ("dprintf: unimplemented format " 
 					    ^ (String.sub format i (j-i+1)));
 	    let j' = succ j in (* eat the d,i,x etc. *)
-	    let format_spec = "% " in
-	    String.set format_spec 1 (fget j'); (* format_spec = "%x", etc. *)
+	    let format_spec = Bytes.of_string "% " in
+	    Bytes.set format_spec 1 (fget j'); (* format_spec = "%x", etc. *)
+            let format_spec = Bytes.to_string format_spec in
             Obj.magic(fun n ->
               collect (dctext1 acc
                          (Nativeint.format format_spec n))
diff -ur cil-1.7.3.old/src/cil.ml cil-1.7.3/src/cil.ml
--- cil-1.7.3.old/src/cil.ml	2013-07-24 16:07:11.000000000 +0100
+++ cil-1.7.3/src/cil.ml	2017-11-22 14:53:42.922740924 +0000
@@ -5033,19 +5033,19 @@
 (* Take the name of a file and make a valid symbol name out of it. There are 
  * a few characters that are not valid in symbols *)
 let makeValidSymbolName (s: string) = 
-  let s = String.copy s in (* So that we can update in place *)
-  let l = String.length s in
+  let s = Bytes.of_string s in (* So that we can update in place *)
+  let l = Bytes.length s in
   for i = 0 to l - 1 do
-    let c = String.get s i in
+    let c = Bytes.get s i in
     let isinvalid = 
       match c with
         '-' | '.' -> true
       | _ -> false
     in
     if isinvalid then 
-      String.set s i '_';
+      Bytes.set s i '_';
   done;
-  s
+  Bytes.to_string s
 
 let rec addOffset (toadd: offset) (off: offset) : offset =
   match off with
diff -ur cil-1.7.3.old/src/formatlex.mll cil-1.7.3/src/formatlex.mll
--- cil-1.7.3.old/src/formatlex.mll	2013-07-24 16:07:11.000000000 +0100
+++ cil-1.7.3/src/formatlex.mll	2017-11-22 14:52:59.217743941 +0000
@@ -145,11 +145,11 @@
  * We convert L"Hi" to "H\000i\000" *)
 let wbtowc wstr =
   let len = String.length wstr in 
-  let dest = String.make (len * 2) '\000' in 
+  let dest = Bytes.make (len * 2) '\000' in 
   for i = 0 to len-1 do 
-    dest.[i*2] <- wstr.[i] ;
+    Bytes.set dest (i*2) wstr.[i] ;
   done ;
-  dest
+  Bytes.to_string dest
 
 (* This function converst the "Hi" in L"Hi" to { L'H', L'i', L'\0' } *)
 let wstr_to_warray wstr =