Keep string alive over C function call (simonjbeaumont#21)

* Keep string alive over C function call

To avoid the GC removing a string where C code writes a result, use it
after the call. See also

xapi-project/xen-api@bc053bb

In the commit above, Sys.opaque_identity was used to keep the value
alive. However, it is not available in the OCaml 4.02.3 compiler.

Signed-off-by: Christian Lindig <christian.lindig@citrix.com>

* Use "List.hd [s]" for using s

Signed-off-by: Christian Lindig <christian.lindig@citrix.com>

lib/pci.ml

@@ -83,7 +83,11 @@ let with_string ?(size=1024) f =
    * to the lifetime of the input parameter, which can be moved
    * by the GC and cause problems. *)
   let buf = CArray.make char ~initial:'\x00' size in
-  f (CArray.start buf) size
+  let s = CArray.start buf in
+  let r = f s size in
+  (* Keep `s` alive through the C binding invocation in `f` *)
+  ignore (List.hd [s] |> Obj.repr |> Obj.tag);
+  r
 
 let lookup_class_name pci_access class_id =
   with_string (fun buf size ->