Re: [Libguestfs] [nbdkit PATCH v3 1/4] plugins: Don't use bogus errno from non-C plugins
On Thu, Jan 26, 2017 at 08:58:34PM -0600, Eric Blake wrote:
diff --git a/plugins/ocaml/ocaml.c b/plugins/ocaml/ocaml.c
index e2b433e..5d7aeeb 100644
--- a/plugins/ocaml/ocaml.c
+++ b/plugins/ocaml/ocaml.c
@@ -499,6 +499,17 @@ SET(pwrite)
SET(flush)
SET(trim)
+/* We can't guarantee that errno is stable across language binding
+ * glue code, so this callback is implemented in C only, and not
+ * exposed in OCaml.
+ */
+static int
+plugin_ocaml_errno_is_reliable (void *handle)
+{
+ return 0;
+}
Actually OCaml is a real compiled language, and the call from C to
OCaml code (via caml_callback_exn) is a short piece of asm which
preserves errno.
However you'll need to save errno around caml_enter_blocking_section
since that unblocks and processes signals.
IOW:
static int
pread_wrapper (void *h, void *buf, uint32_t count, uint64_t offset)
{
CAMLparam0 ();
CAMLlocal3 (rv, strv, offsetv);
+ int saved_errno;
...
rv = caml_callback3_exn (pread_fn, *(value *) h, strv, offsetv);
+ saved_errno = errno;
if (Is_exception_result (rv)) {
nbdkit_error ("%s", caml_format_exception (Extract_exception (rv)));
caml_enter_blocking_section ();
+ errno = saved_errno;
CAMLreturnT (int, -1);
}
Rich.
--
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
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