@@ -51,9 +51,10 @@
e.g. in a structure initializer (or where-ever else comma expressions
aren't permitted). */
#define BUILD_BUG_ON_ZERO(cond) \
- sizeof(struct { _Static_assert(!(cond), "!(" #cond ")"); })
+ (sizeof(struct { char c; _Static_assert(!(cond), "!(" #cond ")"); }) & 0U)
#else
-#define BUILD_BUG_ON_ZERO(cond) sizeof(struct { int:-!!(cond); })
+#define BUILD_BUG_ON_ZERO(cond) \
+ (sizeof(struct { unsigned u : !(cond); }) & 0U)
#define BUILD_BUG_ON(cond) ((void)BUILD_BUG_ON_ZERO(cond))
#endif
Redefine BUILD_BUG_ON_ZERO to avoid using a compiler extension that gives an acceptable semantics to C99 undefined behavior 58 ("A structure or union is defined as containing no named members (6.7.2.1)"). The first definition includes an additional named field of type char. The chosen ill-formed construct for the second definition is a struct with a named bitfield of width 0 when the condition is true, which prevents the UB without using the compiler extension while keeping the semantics of the construct. The choice of the bitwise AND operation to bring the result to 0 when cond is false boils down to possibly better portability. Signed-off-by: Nicola Vetrini <nicola.vetrini@bugseng.com> --- Changes in V2: - Avoid using a VLA as the compile-time assertion - Do not drop _Static_assert Changes in V3: - Changed the operation to bring the result to 0 when the construct does not lead to a compilation error Changes in V4: - Switched to a shorter construct for the second definition. Changes in V5: - Dropped 'U's from the macro definitions --- xen/include/xen/lib.h | 5 +++-- 1 file changed, 3 insertions(+), 2 deletions(-)