On Sat, Feb 26, 2011 at 12:33 AM, Dave Hylands [off-list ref] wrote:

Hi Zhang,

On Fri, Feb 25, 2011 at 3:50 AM, Zhang Meng [off-list ref] wrote:

quoted

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