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

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

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.

~Andrew

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