On 20/01/2017 17:57, Michael S. Tsirkin wrote: > On Fri, Jan 20, 2017 at 08:42:41AM +0100, Markus Armbruster wrote: >> "Michael S. Tsirkin" <m...@redhat.com> writes: >> >>> There are theoretical concerns that some compilers might not trigger >>> build failures on attempts to define an array of size -1 and make it a >>> variable sized array instead. Let rewrite using a struct with a negative >>> bit field size instead as there are no dynamic bit field sizes. This is >>> similar to what Linux does. >>> >>> Signed-off-by: Michael S. Tsirkin <m...@redhat.com> >>> --- >>> include/qemu/compiler.h | 9 ++++++--- >>> 1 file changed, 6 insertions(+), 3 deletions(-) >>> >>> diff --git a/include/qemu/compiler.h b/include/qemu/compiler.h >>> index 7512082..c6f673e 100644 >>> --- a/include/qemu/compiler.h >>> +++ b/include/qemu/compiler.h >>> @@ -85,9 +85,12 @@ >>> #define typeof_field(type, field) typeof(((type *)0)->field) >>> #define type_check(t1,t2) ((t1*)0 - (t2*)0) >>> >>> -#define QEMU_BUILD_BUG_ON(x) \ >>> - typedef char glue(qemu_build_bug_on__, __LINE__)[(x) ? -1 : 1] \ >>> - __attribute__((unused)) >>> +#define QEMU_BUILD_BUG_ON_STRUCT(x) \ >>> + struct { \ >>> + int qemu_build_bug_on : (x) ? -1 : 1; \ >>> + } >> >> The qemu_build_bug_on name space pollution is harmless, but quite >> unnecessary: the name can be simply omitted (unnamed bit-field). > > I have concerns about it's portability though. I remember > we had to get rid of unnamed fields in some structs at some point > for the sake of some old compiler.
Unnamed bitfields are in C89 and we definitely use unnamed unions. Maybe that was an unnamed struct or scalar. Paolo >>> +#define QEMU_BUILD_BUG_ON(x) typedef QEMU_BUILD_BUG_ON_STRUCT(x) \ >>> + glue(qemu_build_bug_on__, __LINE__) __attribute__((unused)) >>> >>> #if defined __GNUC__ >>> # if !QEMU_GNUC_PREREQ(4, 4)