[PATCH v3-RESEND 16/28] libxl: ocaml: use the "string option" type for IDL strings

[Rob Hoes <rob.hoes@citrix.com>]

The libxl IDL is based on C type "char *", and therefore "strings" can
by NULL, or be an actual string. In ocaml, it is common to encode such
things as option types.

Signed-off-by: Rob Hoes <rob.hoes@citrix.com>
---
 tools/ocaml/libs/xl/genwrap.py       |    2 +-
 tools/ocaml/libs/xl/xenlight_stubs.c |   21 +++++++++++++++++++++
 2 files changed, 22 insertions(+), 1 deletion(-)

diff --git a/tools/ocaml/libs/xl/genwrap.py b/tools/ocaml/libs/xl/genwrap.py
index acd2728..5cbb748 100644
--- a/tools/ocaml/libs/xl/genwrap.py
+++ b/tools/ocaml/libs/xl/genwrap.py
@@ -8,7 +8,7 @@ import idl
 builtins = {
     "bool":                 ("bool",                   "%(c)s = Bool_val(%(o)s)",           "Val_bool(%(c)s)" ),
     "int":                  ("int",                    "%(c)s = Int_val(%(o)s)",            "Val_int(%(c)s)"  ),
-    "char *":               ("string",                 "%(c)s = dup_String_val(%(o)s)", "caml_copy_string(%(c)s)"),
+    "char *":               ("string option",          "%(c)s = String_option_val(%(o)s)",  "Val_string_option(%(c)s)"),
     "libxl_domid":          ("domid",                  "%(c)s = Int_val(%(o)s)",            "Val_int(%(c)s)"  ),
     "libxl_devid":          ("devid",                  "%(c)s = Int_val(%(o)s)",            "Val_int(%(c)s)"  ),
     "libxl_defbool":        ("bool option",            "%(c)s = Defbool_val(%(o)s)",        "Val_defbool(%(c)s)" ),
diff --git a/tools/ocaml/libs/xl/xenlight_stubs.c b/tools/ocaml/libs/xl/xenlight_stubs.c
index d15a625..152600b 100644
--- a/tools/ocaml/libs/xl/xenlight_stubs.c
+++ b/tools/ocaml/libs/xl/xenlight_stubs.c
@@ -347,6 +347,27 @@ static value Val_hwcap(libxl_hwcap *c_val)
 	CAMLreturn(hwcap);
 }
 
+static value Val_string_option(const char *c_val)
+{
+	CAMLparam0();
+	CAMLlocal2(tmp1, tmp2);
+	if (c_val) {
+		tmp1 = caml_copy_string(c_val);
+		tmp2 = Val_some(tmp1);
+		CAMLreturn(tmp2);
+	}
+	else
+		CAMLreturn(Val_none);
+}
+
+static char *String_option_val(value v)
+{
+	char *s = NULL;
+	if (v != Val_none)
+		s = dup_String_val(Some_val(v));
+	return s;
+}
+
 #include "_libxl_types.inc"
 
 #define _STRINGIFY(x) #x
-- 
1.7.10.4