diff --git a/src/io/CCIO.ml b/src/io/CCIO.ml
index 5f2916c8..5bdc2ec8 100644
--- a/src/io/CCIO.ml
+++ b/src/io/CCIO.ml
@@ -73,7 +73,7 @@ let gen_flat_map f next_elem =
   next
 
 let with_in ?(mode=0o644) ?(flags=[]) filename f =
-  let ic = open_in_gen flags mode filename in
+  let ic = open_in_gen (Open_rdonly::flags) mode filename in
   try
     let x = f ic in
     close_in ic;
@@ -134,8 +134,8 @@ let read_all ?(size=1024) ic =
   with Exit ->
     Bytes.sub_string !buf 0 !len
 
-let with_out ?(mode=0o644) ?(flags=[]) filename f =
-  let oc = open_out_gen flags mode filename in
+let with_out ?(mode=0o644) ?(flags=[Open_creat]) filename f =
+  let oc = open_out_gen (Open_wronly::flags) mode filename in
   try
     let x = f oc in
     close_out oc;
@@ -145,7 +145,7 @@ let with_out ?(mode=0o644) ?(flags=[]) filename f =
     raise e
 
 let with_out_a ?mode ?(flags=[]) filename f =
-  with_out ?mode ~flags:(Open_creat::Open_append::flags) filename f
+  with_out ?mode ~flags:(Open_wronly::Open_creat::Open_append::flags) filename f
 
 let write_line oc s =
   output_string oc s;
diff --git a/src/io/CCIO.mli b/src/io/CCIO.mli
index e338ef16..4ca1b23b 100644
--- a/src/io/CCIO.mli
+++ b/src/io/CCIO.mli
@@ -69,7 +69,8 @@ val with_in : ?mode:int -> ?flags:open_flag list ->
               string -> (in_channel -> 'a) -> 'a
 (** Open an input file with the given optional flag list, calls the function
     on the input channel. When the function raises or returns, the
-    channel is closed. *)
+    channel is closed.
+    @param flags opening flags (default [[Open_rdonly]]) *)
 
 val read_chunks : ?size:int -> in_channel -> string gen
 (** Read the channel's content into chunks of size [size] *)
@@ -92,12 +93,13 @@ val read_all : ?size:int -> in_channel -> string
 
 val with_out : ?mode:int -> ?flags:open_flag list ->
                string -> (out_channel -> 'a) -> 'a
-(** Same as {!with_in} but for an output channel *)
+(** Same as {!with_in} but for an output channel
+    @param flags opening flags (default [[Open_creat; Open_wronly]]) *)
 
 val with_out_a : ?mode:int -> ?flags:open_flag list ->
                   string -> (out_channel -> 'a) -> 'a
-(** Similar to {!with_out} but with the [Open_append] and [Open_creat]
-    flags activated *)
+(** Similar to {!with_out} but with the [[Open_append; Open_creat; Open_wronly]]
+    flags activated, to append to the file *)
 
 val write_line : out_channel -> string -> unit
 (** Write the given string on the channel, followed by "\n" *)