Follow the manual to avoid naked pointers:
https://v2.ocaml.org/manual/intfc.html#ss:c-outside-head

No functional change, except on OCaml 5.0 where it is a bugfix.

Signed-off-by: Edwin Török <edvin.torok@citrix.com>
Acked-by: Christian Lindig <christian.lindig@citrix.com>
---
Reason for inclusion in 4.17:
- bugfix for upcoming OCaml 5.0 compiler (already in beta)

Changes since v2:
- add Acked-by line
---
 tools/ocaml/libs/mmap/mmap_stubs.h  |  5 +++++
 tools/ocaml/libs/xc/xenctrl_stubs.c | 11 ++++++-----
 2 files changed, 11 insertions(+), 5 deletions(-)

diff --git a/tools/ocaml/libs/mmap/mmap_stubs.h b/tools/ocaml/libs/mmap/mmap_stubs.h
index 65e4239890..5c65cc86fb 100644
--- a/tools/ocaml/libs/mmap/mmap_stubs.h
+++ b/tools/ocaml/libs/mmap/mmap_stubs.h
@@ -30,4 +30,9 @@ struct mmap_interface
 	int len;
 };
 
+/* for compatibility with OCaml 4.02.3 */
+#ifndef Data_abstract_val
+#define Data_abstract_val(v) ((void*) Op_val(v))
+#endif
+
 #endif
diff --git a/tools/ocaml/libs/xc/xenctrl_stubs.c b/tools/ocaml/libs/xc/xenctrl_stubs.c
index a8789d19be..8cd11060ec 100644
--- a/tools/ocaml/libs/xc/xenctrl_stubs.c
+++ b/tools/ocaml/libs/xc/xenctrl_stubs.c
@@ -37,7 +37,7 @@
 
 #include "mmap_stubs.h"
 
-#define _H(__h) ((xc_interface *)(__h))
+#define _H(__h) *((xc_interface **) Data_abstract_val(__h))
 #define _D(__d) ((uint32_t)Int_val(__d))
 
 #ifndef Val_none
@@ -70,14 +70,15 @@ static void Noreturn failwith_xc(xc_interface *xch)
 CAMLprim value stub_xc_interface_open(void)
 {
 	CAMLparam0();
-        xc_interface *xch;
+	CAMLlocal1(result);
 
+	result = caml_alloc(1, Abstract_tag);
 	/* Don't assert XC_OPENFLAG_NON_REENTRANT because these bindings
 	 * do not prevent re-entrancy to libxc */
-        xch = xc_interface_open(NULL, NULL, 0);
-        if (xch == NULL)
+	_H(result) = xc_interface_open(NULL, NULL, 0);
+	if (_H(result) == NULL)
 		failwith_xc(NULL);
-        CAMLreturn((value)xch);
+	CAMLreturn(result);
 }
 
 
-- 
2.34.1