On Sat, Feb 26, 2011 at 12:33 AM, Dave Hylands <dhylands@xxxxxxxxx> wrote: > Hi Zhang, > > On Fri, Feb 25, 2011 at 3:50 AM, Zhang Meng <jammy.linux@xxxxxxxxx> 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 _______________________________________________ Kernelnewbies mailing list Kernelnewbies@xxxxxxxxxxxxxxxxx http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies