diff --git a/src/Pure/General/buffer.ML b/src/Pure/General/buffer.ML
--- a/src/Pure/General/buffer.ML
+++ b/src/Pure/General/buffer.ML
@@ -1,46 +1,48 @@
 (*  Title:      Pure/General/buffer.ML
     Author:     Makarius
 
 Scalable text buffers.
 *)
 
 signature BUFFER =
 sig
   type T
   val empty: T
   val is_empty: T -> bool
   val add: string -> T -> T
+  val length: T -> int
+  val contents: T -> string list
   val content: T -> string
   val build: (T -> T) -> T
   val build_content: (T -> T) -> string
-  val output: T -> (string -> unit) -> unit
   val markup: Markup.T -> (T -> T) -> T -> T
 end;
 
 structure Buffer: BUFFER =
 struct
 
 abstype T = Buffer of string list
 with
 
 val empty = Buffer [];
 
 fun is_empty (Buffer xs) = null xs;
 
 fun add "" buf = buf
   | add x (Buffer xs) = Buffer (x :: xs);
 
-fun content (Buffer xs) = implode (rev xs);
+fun length (Buffer xs) = fold (Integer.add o size) xs 0;
+
+fun contents (Buffer xs) = rev xs;
+val content = implode o contents;
 
 fun build (f: T -> T) = f empty;
 fun build_content f = build f |> content;
 
-fun output (Buffer xs) out = List.app out (rev xs);
-
 end;
 
 fun markup m body =
   let val (bg, en) = Markup.output m
   in add bg #> body #> add en end;
 
 end;
diff --git a/src/Pure/General/file.ML b/src/Pure/General/file.ML
--- a/src/Pure/General/file.ML
+++ b/src/Pure/General/file.ML
@@ -1,182 +1,183 @@
 (*  Title:      Pure/General/file.ML
     Author:     Makarius
 
 File-system operations.
 *)
 
 signature FILE =
 sig
   val standard_path: Path.T -> string
   val platform_path: Path.T -> string
   val bash_path: Path.T -> string
   val bash_paths: Path.T list -> string
   val bash_platform_path: Path.T -> string
   val absolute_path: Path.T -> Path.T
   val full_path: Path.T -> Path.T -> Path.T
   val tmp_path: Path.T -> Path.T
   val exists: Path.T -> bool
   val rm: Path.T -> unit
   val is_dir: Path.T -> bool
   val is_file: Path.T -> bool
   val check_dir: Path.T -> Path.T
   val check_file: Path.T -> Path.T
   val open_dir: (OS.FileSys.dirstream -> 'a) -> Path.T -> 'a
   val open_input: (BinIO.instream -> 'a) -> Path.T -> 'a
   val open_output: (BinIO.outstream -> 'a) -> Path.T -> 'a
   val open_append: (BinIO.outstream -> 'a) -> Path.T -> 'a
   val fold_dir: (string -> 'a -> 'a) -> Path.T -> 'a -> 'a
   val read_dir: Path.T -> string list
   val fold_lines: (string -> 'a -> 'a) -> Path.T -> 'a -> 'a
   val fold_pages: (string -> 'a -> 'a) -> Path.T -> 'a -> 'a
   val read_lines: Path.T -> string list
   val read_pages: Path.T -> string list
   val read: Path.T -> string
   val write: Path.T -> string -> unit
   val append: Path.T -> string -> unit
   val generate: Path.T -> string -> unit
   val output: BinIO.outstream -> string -> unit
   val write_list: Path.T -> string list -> unit
   val append_list: Path.T -> string list -> unit
   val write_buffer: Path.T -> Buffer.T -> unit
   val eq: Path.T * Path.T -> bool
 end;
 
 structure File: FILE =
 struct
 
 (* system path representations *)
 
 val standard_path = Path.implode o Path.expand;
 val platform_path = ML_System.platform_path o standard_path;
 
 val bash_path = Bash.string o standard_path;
 val bash_paths = Bash.strings o map standard_path;
 
 val bash_platform_path = Bash.string o platform_path;
 
 
 (* full_path *)
 
 val absolute_path =
   Path.expand #> (fn path =>
     if Path.is_absolute path then path
     else Path.explode (ML_System.standard_path (OS.FileSys.getDir ())) + path);
 
 fun full_path dir path =
   let
     val path' = Path.expand path;
     val _ = Path.is_current path' andalso error "Bad file specification";
   in absolute_path (dir + path') end;
 
 
 (* tmp_path *)
 
 fun tmp_path path = Path.variable "ISABELLE_TMP" + Path.base path;
 
 
 (* directory entries *)
 
 val exists = can OS.FileSys.modTime o platform_path;
 
 val rm = OS.FileSys.remove o platform_path;
 
 fun test_dir path = the_default false (try OS.FileSys.isDir (platform_path path));
 fun is_dir path = exists path andalso test_dir path;
 fun is_file path = exists path andalso not (test_dir path);
 
 fun check_dir path =
   if is_dir path then path
   else error ("No such directory: " ^ Path.print (Path.expand path));
 
 fun check_file path =
   if is_file path then path
   else error ("No such file: " ^ Path.print (Path.expand path));
 
 
 (* open streams *)
 
 local
 
 fun with_file open_file close_file f =
   Thread_Attributes.uninterruptible (fn restore_attributes => fn path =>
     let
       val file = open_file path;
       val result = Exn.capture (restore_attributes f) file;
     in close_file file; Exn.release result end);
 
 in
 
 fun open_dir f = with_file OS.FileSys.openDir OS.FileSys.closeDir f o platform_path;
 fun open_input f = with_file BinIO.openIn BinIO.closeIn f o platform_path;
 fun open_output f = with_file BinIO.openOut BinIO.closeOut f o platform_path;
 fun open_append f = with_file BinIO.openAppend BinIO.closeOut f o platform_path;
 
 end;
 
 
 (* directory content *)
 
 fun fold_dir f path a =
   check_dir path |> open_dir (fn stream =>
     let
       fun read x =
         (case OS.FileSys.readDir stream of
           NONE => x
         | SOME entry => read (f entry x));
     in read a end);
 
 fun read_dir path = sort_strings (fold_dir cons path []);
 
 
 (* input *)
 
 (*
   scalable iterator:
   . avoid size limit of TextIO.inputAll and overhead of many TextIO.inputLine
   . optional terminator at end-of-input
 *)
 fun fold_chunks terminator f path a = open_input (fn file =>
   let
     fun read buf x =
       (case Byte.bytesToString (BinIO.input file) of
         "" => (case Buffer.content buf of "" => x | line => f line x)
       | input =>
           (case String.fields (fn c => c = terminator) input of
             [rest] => read (Buffer.add rest buf) x
           | line :: more => read_lines more (f (Buffer.content (Buffer.add line buf)) x)))
     and read_lines [rest] x = read (Buffer.add rest Buffer.empty) x
       | read_lines (line :: more) x = read_lines more (f line x);
   in read Buffer.empty a end) path;
 
 fun fold_lines f = fold_chunks #"\n" f;
 fun fold_pages f = fold_chunks #"\f" f;
 
 fun read_lines path = rev (fold_lines cons path []);
 fun read_pages path = rev (fold_pages cons path []);
 
 val read = open_input (Byte.bytesToString o BinIO.inputAll);
 
 
 (* output *)
 
 fun output file txt = BinIO.output (file, Byte.stringToBytes txt);
 
 fun output_list txts file = List.app (output file) txts;
 fun write_list path txts = open_output (output_list txts) path;
 fun append_list path txts = open_append (output_list txts) path;
 
 fun write path txt = write_list path [txt];
 fun append path txt = append_list path [txt];
 fun generate path txt = if try read path = SOME txt then () else write path txt;
 
-fun write_buffer path buf = open_output (Buffer.output buf o output) path;
+fun write_buffer path buf =
+  open_output (fn file => List.app (output file) (Buffer.contents buf)) path;
 
 
 (* eq *)
 
 fun eq paths =
   (case try (apply2 (OS.FileSys.fileId o platform_path)) paths of
     SOME ids => is_equal (OS.FileSys.compare ids)
   | NONE => false);
 
 end;