tools/ocaml: libxb: Avoid to use String_val() when value is bytes

Commit ec7d54d "ocaml/libs/xb: Use bytes in place of strings for
mutable buffers" switch mutable buffers from string to bytes. However
the C code were still using String_Val() to access them.

While the underlying structure is the same between string and bytes, a
string is meant to be immutable. OCaml 4.06.1 and later will enforce it.
Therefore, it will not be possible to build the OCaml libs when using
-safe-string. This is because String_val() will return a const value.

To avoid plain cast in the code, the code is now switched to use
Bytes_val(). As the macro is not defined in older OCaml version, we need
to provide a stub.

Take the opportunity to switch to const the buffer in
ml_interface_write() as it should not be modified.

Reported-by: Dario Faggioli <[email protected]>
Signed-off-by: Julien Grall <[email protected]>
Acked-by: Christian Lindig <[email protected]>

tools/ocaml/libs/xb/xs_ring_stubs.c

@@ -36,6 +36,14 @@
 
 #define GET_C_STRUCT(a) ((struct mmap_interface *) a)
 
+/*
+ * Bytes_val has been introduced by Ocaml 4.06.1. So define our own version
+ * if needed.
+ */
+#ifndef Bytes_val
+#define Bytes_val(x) ((unsigned char *) Bp_val(x))
+#endif
+
 CAMLprim value ml_interface_read(value ml_interface,
                                  value ml_buffer,
                                  value ml_len)
@@ -44,7 +52,7 @@ CAMLprim value ml_interface_read(value ml_interface,
 	CAMLlocal1(ml_result);
 
 	struct mmap_interface *interface = GET_C_STRUCT(ml_interface);
-	char *buffer = String_val(ml_buffer);
+	unsigned char *buffer = Bytes_val(ml_buffer);
 	int len = Int_val(ml_len);
 	int result;
 
@@ -103,7 +111,7 @@ CAMLprim value ml_interface_write(value ml_interface,
 	CAMLlocal1(ml_result);
 
 	struct mmap_interface *interface = GET_C_STRUCT(ml_interface);
-	char *buffer = String_val(ml_buffer);
+	const unsigned char *buffer = Bytes_val(ml_buffer);
 	int len = Int_val(ml_len);
 	int result;