diff --git a/src/bigarray/CCBigstring.ml b/src/bigarray/CCBigstring.ml
index 7993d15c..efa37a74 100644
--- a/src/bigarray/CCBigstring.ml
+++ b/src/bigarray/CCBigstring.ml
@@ -207,13 +207,17 @@ let to_gen_slice a i len =
 let map_file_descr ?pos ?(shared=false) fd len =
   B.map_file fd ?pos Bigarray.char Bigarray.c_layout shared len
 
-let with_map_file ?pos ?(mode=0o644) ?(flags=[Unix.O_RDONLY]) ?shared name len f =
-  let fd = Unix.openfile name flags mode in
-  let a = map_file_descr ?pos ?shared fd len in
+let with_map_file ?pos ?len ?(mode=0o644) ?(flags=[Open_rdonly]) ?shared name f =
+  let ic = open_in_gen flags mode name in
+  let len = match len with
+    | None -> in_channel_length ic
+    | Some n -> n
+  in
+  let a = map_file_descr ?pos ?shared (Unix.descr_of_in_channel ic) len in
   try
     let x = f a in
-    Unix.close fd;
+    close_in ic;
     x
   with e ->
-    Unix.close fd;
+    close_in ic;
     raise e
diff --git a/src/bigarray/CCBigstring.mli b/src/bigarray/CCBigstring.mli
index 32283bd8..952de8a1 100644
--- a/src/bigarray/CCBigstring.mli
+++ b/src/bigarray/CCBigstring.mli
@@ -107,11 +107,12 @@ val to_gen_slice : t -> int -> int -> char gen
 (** {2 Memory-map} *)
 
 val with_map_file :
-  ?pos:int64 -> ?mode:int -> ?flags:Unix.open_flag list -> ?shared:bool ->
-  string -> int -> (t -> 'a) -> 'a
-(** [with_map_file name len f] maps the file into memory, opening it, and
-    call [f] with a slice [pos.... pos+len] of the bytes of the file. When
-    [f] returns, the file is closed.
+  ?pos:int64 -> ?len:int -> ?mode:int -> ?flags:open_flag list -> ?shared:bool ->
+  string -> (t -> 'a) -> 'a
+(** [with_map_file name f] maps the file into memory, opening it, and
+    call [f] with a slice [pos.... pos+len] of the bytes of the file
+    where [len] is the length of the file if not provided.
+    When [f] returns, the file is closed.
     @param pos offset in the file (default 0)
     @param shared if true, modifications are shared between processes that
       have mapped this file (requires the filedescr to be open in write mode).