Re: [Xen-devel] [PATCH V3] xl: create VFB for PV guest when VNC is specified

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Tue, 2013-12-17 at 11:40 +0000, Wei Liu wrote:
> On Tue, Dec 17, 2013 at 11:36:29AM +0000, Andrew Cooper wrote:
> > On 17/12/13 11:28, Wei Liu wrote:
> > > On Mon, Dec 16, 2013 at 05:46:04PM +0000, Ian Campbell wrote:
> > > [...]
> > >>> Changes in V2:
> > >>> * use macros to reduce code duplication
> > >>> * vfb=[] take precedence over top level VNC options
> > >>> ---
> > >>>  tools/libxl/xl_cmdimpl.c |   89 
> > >>> ++++++++++++++++++++++++++++++++++++----------
> > >>>  1 file changed, 70 insertions(+), 19 deletions(-)
> > >>>
> > >>> diff --git a/tools/libxl/xl_cmdimpl.c b/tools/libxl/xl_cmdimpl.c
> > >>> index bd26bcc..6a22e17 100644
> > >>> --- a/tools/libxl/xl_cmdimpl.c
> > >>> +++ b/tools/libxl/xl_cmdimpl.c
> > >>> @@ -315,6 +315,13 @@ static void *xrealloc(void *ptr, size_t sz) {
> > >>>      return r;
> > >>>  }
> > >>>  
> > >>> +#define ARRAY_EXTEND(array,count)                                      
> > >>>  \
> > >>> +    do {                                                               
> > >>>  \
> > >>> +        (array) = xrealloc((array),                                    
> > >>>  \
> > >>> +                           sizeof(typeof(*(array))) * ((count) + 1));  
> > >>>  \
> > >> sizeof(typeof(*array)) == sizeof(*array), doesn't it?
> > >>
> > >>> +        (count) = (count) + 1;                                         
> > >>>  \
> > >> (count++)
> > >>
> > >> I suppose in a macro like this it is tricky to arrange to only evaluate
> > >> the arguments once, since you need to read and write, but I think you
> > >> can arrange to evaluate array exactly once instead of 3 times and count
> > >> just once.
> > >>
> > > I came up with this. Is it what you asked for?
> > >
> > > #define ARRAY_EXTEND(type_ptr,ptr,type_count,count,ret)                 \
> > >     do {                                                                \
> > >         type_ptr **__ptr = &(ptr);                                      \
> > >         type_count *__count = &(count);                                 \
> > >         *__ptr = xrealloc(*__ptr,                                       \
> > >                           sizeof(**__ptr) * (*__count + 1));            \
> > >         (ret) = *__ptr + *__count;                                      \
> > >         (*__count)++;                                                   \
> > >     } while (0)
> > >
> > > Wei.
> > 
> > Use typeof() rather than passing the types in directly.  It is a GCC
> > extension, which tries its best not to evaluate its argument.
> > 
> 
> Oh I thought typeof((count)) always evaluates its argument, good to
> know. ;-)

Me too, which is why I didn't suggest it earlier.

In hindsight its main use is probably in making macros only evaluate
once, so if it itself did the evaluation it wouldn't be very useful!



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