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[Xen-devel] [PATCH v4 12/27] libxl: ocaml: make Val_defbool GC-proof

In order to avoid newly created OCaml values from being GC'ed, they must be
registered as roots with the GC, before an iteration of the GC may happen. The
Val_* functions potentially allocate new values on the OCaml heap, and may
trigger an iteration of the OCaml GC.

The way to register a value with the GC is to assign it to a variable declared
with a CAMLparam or CAMLlocal macro, which put the value into a struct that
can be reached from a GC root.

This leads to slightly weird looking C code, but avoids hard to find segfaults.

Signed-off-by: Rob Hoes <rob.hoes@xxxxxxxxxx>
Acked-by: David Scott <dave.scott@xxxxxxxxxxxxx>
Acked-by: Ian Campbell <ian.campbell@xxxxxxxxxx>

v4: Updated the commit message a little.
 tools/ocaml/libs/xl/xenlight_stubs.c |   12 +++++++-----
 1 file changed, 7 insertions(+), 5 deletions(-)

diff --git a/tools/ocaml/libs/xl/xenlight_stubs.c 
index 67612f4..94601c4 100644
--- a/tools/ocaml/libs/xl/xenlight_stubs.c
+++ b/tools/ocaml/libs/xl/xenlight_stubs.c
@@ -308,15 +308,17 @@ static int Uuid_val(libxl_uuid *c_val, value v)
 static value Val_defbool(libxl_defbool c_val)
-       CAMLlocal1(v);
+       CAMLlocal2(v1, v2);
+       bool b;
        if (libxl_defbool_is_default(c_val))
-               v = Val_none;
+               v2 = Val_none;
        else {
-               bool b = libxl_defbool_val(c_val);
-               v = Val_some(b ? Val_bool(true) : Val_bool(false));
+               b = libxl_defbool_val(c_val);
+               v1 = b ? Val_bool(true) : Val_bool(false);
+               v2 = Val_some(v1);
-       CAMLreturn(v);
+       CAMLreturn(v2);
 static libxl_defbool Defbool_val(value v)

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