On Thu, Mar 27, 2008 at 10:39:22AM -0700, Ian Lance Taylor wrote: > Eus <eus@xxxxxxxxxxxxxx> writes: > > > When browsing the Linux kernel 2.6.21.5's source code, I noticed a variable > > definition like: > > > > typedef __u64 __bitwise __be64; > > > > Being curious about __bitwise, I found it out to be: > > > > #define __bitwise __bitwise__ > > > > that finally turned out to be: > > > > #define __bitwise__ __attribute__((bitwise)) > > > > Since it was __attribute__, I tried to find its documentation in GCC texinfo > > file. But, I couldn't find one. Even I have tried to find it out in the Internet. > > But, I still couldn't find one. > > There is no "bitwise" attribute in standard gcc. > > Looking at the Linux 2.6.24 sources, I see that it is defined as an > attribute only when __CHECKER__ is defined. This has something to do > with sparse--see Documentation/sparse.txt. I don't know the details. Correct. We have in the kernel a number of sparse specific extensions. bitwise, __iomem and __user is the ones most widely used. sparse uses this to do additional checks that gcc not has context to do. The __user annotation is used to annotate pointers to data in userspace which may not be mixed up with pointers in kernel space. Sam
