Make create zero out the new buffer

The new `create_unsafe` function can be used if you want to trade safety
for speed.

Fixes mirage#30

lib/cstruct.ml

@@ -88,7 +88,7 @@ let of_bigarray ?(off=0) ?len buffer =
 let to_bigarray buffer =
   Bigarray.Array1.sub buffer.buffer buffer.off buffer.len
 
-let create len =
+let create_unsafe len =
   let buffer = Bigarray.(Array1.create char c_layout len) in
   { buffer ; len ; off = 0 }
 
@@ -208,6 +208,11 @@ let equal t1 t2 = compare t1 t2 = 0
 (* Note that this is only safe as long as all [t]s are coherent. *)
 let memset t x = unsafe_fill_bigstring t.buffer t.off t.len x
 
+let create len =
+  let t = create_unsafe len in
+  memset t 0;
+  t
+
 let set_uint8 t i c =
   if i >= t.len || i < 0 then err_invalid_bounds "set_uint8" t i 1
   else EndianBigstring.BigEndian.set_int8 t.buffer (t.off+i) c

lib/cstruct.mli

@@ -196,6 +196,10 @@ val to_bigarray: t -> buffer
     sharing of the underlying buffer. *)
 
 val create : int -> t
+(** [create len] is a fresh cstruct of size [len] with an offset of 0,
+    filled with zero bytes. *)
+
+val create_unsafe : int -> t
 (** [create len] is a cstruct of size [len] with an offset of 0.
 
     Note that the returned cstruct will contain arbitrary data,