On Thu, Jan 19, 2017 at 02:33:40PM +0100, Markus Armbruster wrote: > Paolo Bonzini <pbonz...@redhat.com> writes: > > > On 19/01/2017 09:12, Markus Armbruster wrote: > >> "Michael S. Tsirkin" <m...@redhat.com> writes: > >> > >>> QEMU_BUILD_BUG_ON uses a typedef in order to be safe > >>> to use outside functions, but sometimes it's useful > >>> to have a version that can be used within an expression. > >>> Following what Linux does, introduce QEMU_BUILD_BUG_ON_ZERO > >>> that return zero after checking condition at build time. > >> > >> Following Linux's example makes sense, but I can't help but wonder > >> whether we need both QEMU_BUILD_BUG_ON_ZERO() and QEMU_BUILD_BUG_ON(). > > > > I think so, most notably QEMU_BUILD_BUG_ON was added to C11 as > > _Static_assert but QEMU_BUILD_BUG_ON_ZERO wasn't. > > Okay. > > > But we can indeed redefine QEMU_BUILD_BUG_ON to > > (void)QEMU_BUILD_BUG_ON_ZERO(x) like Linux does, until we add optional > > support for _Static_assert. > > Yes, please.
I don't think we can because we use QEMU_BUILD_BUG_ON outside any functions. I don't think you can put 0 there. > >>> Signed-off-by: Michael S. Tsirkin <m...@redhat.com> > >>> --- > >>> include/qemu/compiler.h | 2 ++ > >>> 1 file changed, 2 insertions(+) > >>> > >>> diff --git a/include/qemu/compiler.h b/include/qemu/compiler.h > >>> index 2882470..f4cf13b 100644 > >>> --- a/include/qemu/compiler.h > >>> +++ b/include/qemu/compiler.h > >>> @@ -89,6 +89,8 @@ > >>> typedef char glue(qemu_build_bug_on__,__LINE__)[(x) ? -1 : 1] \ > >>> __attribute__((unused)) > >>> > >>> +#define QEMU_BUILD_BUG_ON_ZERO(x) (sizeof(int[(x) ? -1 : 1]) - > >>> sizeof(int)) > > > > Linux here uses: > > > > #define BUILD_BUG_ON_ZERO(e) (sizeof(struct { int:-!!(e); })) > > > > and the issue is that sizeof(int[(x) ? -1 : 1]) could be > > runtime-evaluated (the type is a variable-length array). > > Let's copy both macros from Linux. I prefer our variant, I don't think it's portable to assume that sizeof(struct {int:0}) is 0. Besides, Linux code is GPLv2 only and this file is 2 or later. -- MST