On Sat, Feb 26, 2011 at 12:33 AM, Dave Hylands <dhylands@gmail.com> wrote:
Hi Zhang,
On Fri, Feb 25, 2011 at 3:50 AM, Zhang Meng <jammy.linux@gmail.com> wrote:
Hi ~List,
Could anybody explain the macro below? what does it mean?
Thanks in advance.
#define BUILD_BUG_ON_ZERO(e) (sizeof(struct { int:-!!(e); }))
This is also known as a compile time assert. I think that this particular variant has to be used inside a function.
! is just negation and produces a zero or 1 result. !! just does it twice, so that a non-zero value coming in becomes 1, and a zero value remains as zero.
If e evaluates to false (zero) then -!!(e) evaluates to zero; if e evaluates to true (non-zero) then -!!(e) evaluates to -1.
Declaring a bit field with a size of -1 will cause a compiler error. I'm actually surprised that declaring a bitfield of size 0 works.
The typical declarations of this I've seen usually use arrays and arrange for the size to be -1 or 1 (which is generally more portable). When you use the array style declaration, you can use it outside a function as well.
Dave Hylands
Thanks Dave. My dilemma also comes from declaring a anonymous bitfield of size 0. The following one is easy to understand anyway. #define BUILD_BUG_ON_ZERO(e) (sizeof(char[1 - 2 * !!(e)]) - 1) -- Yours sincerely ZhangMeng