This is not strictly necessary since it is essentially a no-op
currently: a cast to void* and value*, even in OCaml 5.0.

However it does make it clearer that what we have here is not a regular
OCaml value, but one allocated with Abstract_tag or Custom_tag,
and follows the example from the manual more closely:
https://v2.ocaml.org/manual/intfc.html#ss:c-outside-head

It also makes it clearer that these modules have been reviewed for
compat with OCaml 5.0.

No functional change.

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

Changes since v2:
- add Acked-by line
---
 tools/ocaml/libs/mmap/xenmmap_stubs.c | 2 +-
 tools/ocaml/libs/xb/xs_ring_stubs.c   | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/tools/ocaml/libs/mmap/xenmmap_stubs.c b/tools/ocaml/libs/mmap/xenmmap_stubs.c
index e2ce088e25..141dedb78c 100644
--- a/tools/ocaml/libs/mmap/xenmmap_stubs.c
+++ b/tools/ocaml/libs/mmap/xenmmap_stubs.c
@@ -28,7 +28,7 @@
 #include <caml/fail.h>
 #include <caml/callback.h>
 
-#define Intf_val(a) ((struct mmap_interface *) a)
+#define Intf_val(a) ((struct mmap_interface *) Data_abstract_val(a))
 
 static int mmap_interface_init(struct mmap_interface *intf,
                                int fd, int pflag, int mflag,
diff --git a/tools/ocaml/libs/xb/xs_ring_stubs.c b/tools/ocaml/libs/xb/xs_ring_stubs.c
index 7a91fdee75..cc9114029f 100644
--- a/tools/ocaml/libs/xb/xs_ring_stubs.c
+++ b/tools/ocaml/libs/xb/xs_ring_stubs.c
@@ -35,7 +35,7 @@
 #include <sys/mman.h>
 #include "mmap_stubs.h"
 
-#define GET_C_STRUCT(a) ((struct mmap_interface *) a)
+#define GET_C_STRUCT(a) ((struct mmap_interface *) Data_abstract_val(a))
 
 /*
  * Bytes_val has been introduced by Ocaml 4.06.1. So define our own version
-- 
2.34.1