Re: [Xen-devel] [PATCH v4 06/14] tools/libxc: add a check in xc_hypercall_buffer_free_pages macro

[Yang Hongyang <yanghy@xxxxxxxxxxxxxx>]

On 05/12/2015 08:10 PM, Andrew Cooper wrote:
> On 12/05/15 12:25, Yang Hongyang wrote:
> > When we use a DECLARE_HYPERCALL_BUFFER_SHADOW define a user
> > pointer '_name' and a shadow xc_hypercall_buffer_t.
> > then call xc_hypercall_buffer_free_pages(_xch, _name, _nr),
> > the complier will report '_name' unused error, it's because
> > xc_hypercall_buffer_free_pages() is a MACRO and '_name'
> > transparently converted to the hypercall buffer. it confused
> > the caller because xc_hypercall_buffer_free_pages() do look
> > like a function and take '_name' as an arg.
> > Add an if check to let the compiler think we are actually
> > using the argument '_name'.
> >
> > Signed-off-by: Yang Hongyang <yanghy@xxxxxxxxxxxxxx>
> > CC: Ian Campbell <Ian.Campbell@xxxxxxxxxx>
> > CC: Ian Jackson <Ian.Jackson@xxxxxxxxxxxxx>
> > CC: Wei Liu <wei.liu2@xxxxxxxxxx>
> > CC: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> > ---
> >   tools/libxc/include/xenctrl.h | 4 +++-
> >   1 file changed, 3 insertions(+), 1 deletion(-)
> >
> > diff --git a/tools/libxc/include/xenctrl.h b/tools/libxc/include/xenctrl.h
> > index 0804257..35fb3ac 100644
> > --- a/tools/libxc/include/xenctrl.h
> > +++ b/tools/libxc/include/xenctrl.h
> > @@ -353,7 +353,9 @@ void xc__hypercall_buffer_free(xc_interface *xch, 
> > xc_hypercall_buffer_t *b);
> >   void *xc__hypercall_buffer_alloc_pages(xc_interface *xch, 
> > xc_hypercall_buffer_t *b, int nr_pages);
> >   #define xc_hypercall_buffer_alloc_pages(_xch, _name, _nr) 
> > xc__hypercall_buffer_alloc_pages(_xch, HYPERCALL_BUFFER(_name), _nr)
> >   void xc__hypercall_buffer_free_pages(xc_interface *xch, xc_hypercall_buffer_t 
> > *b, int nr_pages);
> > -#define xc_hypercall_buffer_free_pages(_xch, _name, _nr) 
> > xc__hypercall_buffer_free_pages(_xch, HYPERCALL_BUFFER(_name), _nr)
> > +#define xc_hypercall_buffer_free_pages(_xch, _name, _nr)        \
> > +    if ( _name )                                                \
> > +        xc__hypercall_buffer_free_pages(_xch, HYPERCALL_BUFFER(_name), _nr)
> This needs to be wrapped in a standard "do { ... } while (0)" for macros
Ok, thank you!

> ~Andrew
>
> >   /*
> >    * Array of hypercall buffers.
> .
--
Thanks,
Yang.

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