Re: [Xen-devel] [PATCH] libs/gnttab: introduce BUILD_BUG_ON

[Wei Liu <wei.liu2@xxxxxxxxxx>]

On Wed, Jul 13, 2016 at 01:16:51PM +0100, Andrew Cooper wrote:
> On 13/07/16 13:07, Wei Liu wrote:
> > On Wed, Jul 13, 2016 at 01:32:10PM +0200, Paulina Szubarczyk wrote:
> >>
> >> On 07/13/2016 11:25 AM, Wei Liu wrote:
> >>> The implementation is taken from libxc.
> >>>
> >>> Signed-off-by: Wei Liu <wei.liu2@xxxxxxxxxx>
> >>> ---
> >>> Cc: Ian Jackson <ian.jackson@xxxxxxxxxxxxx>
> >>> Cc: Paulina Szubarczyk <paulinaszubarczyk@xxxxxxxxx>
> >>>
> >>> I could have put it in a header file accessible to all libraries under
> >>> libs but this construct is only relevant to xengnttab library at the
> >>> moment so it's put under gnttab/private.h. It can be easily moved to
> >>> a common place when other libraries under libs require it.
> >>>
> >>> This patch is necessary to unblock Paulina on her gnttab copy work.
> >>> ---
> >>>  tools/libs/gnttab/private.h | 7 +++++++
> >>>  1 file changed, 7 insertions(+)
> >>>
> >>> diff --git a/tools/libs/gnttab/private.h b/tools/libs/gnttab/private.h
> >>> index d286c86..da487f2 100644
> >>> --- a/tools/libs/gnttab/private.h
> >>> +++ b/tools/libs/gnttab/private.h
> >>> @@ -4,6 +4,13 @@
> >>>  #include <xentoollog.h>
> >>>  #include <xengnttab.h>
> >>>
> >>> +/* Force a compilation error if condition is true */
> >>> +#if __GNUC__ > 4 || (__GNUC__ == 4 && __GNUC_MINOR__ >= 6)
> >>> +#define BUILD_BUG_ON(p) ({ _Static_assert(!(p), "!(" #p ")"); })
> >>> +#else
> >>> +#define BUILD_BUG_ON(p) ((void)sizeof(struct { int:-!!(p); }))
> >>> +#endif
> >>> +
> >>>  struct xengntdev_handle {
> >>>      xentoollog_logger *logger, *logger_tofree;
> >>>      int fd;
> >>>
> >> Hi Wei,
> >>
> >> thank you for the help with the grant copy patch. I have just rebuild Xen
> >> and there is a conflict with definition of BUILD_BUG_ON for mini-os build:
> >>
> >> In file included from minios.c:33:0:
> >> private.h:9:0: error: "BUILD_BUG_ON" redefined [-Werror]
> >>  #define BUILD_BUG_ON(p) ({ _Static_assert(!(p), "!(" #p ")"); })
> >>  ^
> >> In file included from minios.c:25:0:
> >> xen/extras/mini-os-remote/include/lib.h:58:0: note: this is the location of
> >> the previous definition
> >>  #define BUILD_BUG_ON(cond) ({ _Static_assert(!(cond), "!(" #cond ")"); })
> >>  ^
> >> I thought about wrapping the code in #ifndef BUILD_BUG_ON #endif.
> > We could use a prefix to the one used in gnttab.
> >
> > Maybe call it XENGNTTAB_BUILD_BUG_ON for now?
> 
> The contents of gnttabs private.h is entirely up to gnttab.
> 
> It is MiniOS (who is poking about in someone elses private.h) with the
> responsibility not to have a broken build.  It needs to do this by not
> leaking so much stuff into its global namespace.
> 

Yeah, I know. It wouldn't hurt to prefix the name anyway.

The mini-os build system problem can be solve separately.

Wei.

> ~Andrew

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