[Libguestfs] Segmentation fault when trying to add binding

[Richard W.M. Jones]

On Tue, Aug 26, 2014 at 09:10:11AM +0200, Pino Toscano wrote:
> Hi,
> 
> On Tuesday 26 August 2014 09:34:42 Shahar Havivi wrote:
> > I am trying to add xmlXPathRegisterNs() to v2v/xml-c.c but I get a seg
> > fault, the signature should be:
> > http://xmlsoft.org/html/libxml-xpathInternals.html#xmlXPathRegisterNs
> > 
> > I think I am wrong in CAMLparam and CAMLlocal...,
> > Following is the patch:
> > 
> > ======================================================================
> > diff --git a/v2v/xml-c.c b/v2v/xml-c.c
> > index 4c9bc77..a917c24 100644
> > --- a/v2v/xml-c.c
> > +++ b/v2v/xml-c.c
> > @@ -141,6 +141,16 @@ v2v_xml_xpath_new_context (value docv)
> >  }
> > 
> >  value
> > +v2v_xml_xpath_register_ns (value prefix, value uri, value xpathctx)
> > +{
> > +  CAMLparam3 (prefix, uri, xpathctx);
> > +  CAMLlocal1 (retval);
> > +  retval = xmlXPathRegisterNs (BAD_CAST String_val (prefix), BAD_CAST String_val (uri), xpathctx);
> > +
> > +  CAMLreturn (retval);
> > +}
> > +
> 
> First of all, you are using the xpathctx parameter directly as
> parameter for xmlXPathRegisterNs, while it is an OCaml value which
> wraps the actual object; see how it is done in e.g.
> v2v_xml_xpath_eval_expression, using the Xpathctx_val macro.
> 
> Second, the order of the arguments in your xmlXPathRegisterNs is wrong:
> - yours: xmlXPathRegisterNs(prefix, uri, ctxt)
> - actual function: xmlXPathRegisterNs(ctxt, prefix, uri)

This too.

Notice that Val_foo() means convert foo to value, and
Foo_val() means convert value to foo.

In that file, only node_ptr/xmlNodePtr can be cast directly from value
to the C type, and that's for a very special and peculiar reason.  For
everything else you have to convert between value and the C type (and
vice versa) using the appropriate macro.

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
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