On Tue, Mar 17, 2015 at 10:31:51AM +0100, David Sterba wrote:
> Agreed, but the proposed define is rather cryptic and I was not able to
> understand the meaning on the first glance.
>
> > #define iov_iter_rw(i) ((0 ? (struct iov_iter *)0 : (i))->type & RW_MASK)
>
> This worked for me, does not compile with anything else than
> 'struct iov_iter*' as i:
>
> #define iov_iter_rw(i) ({ \
> struct iov_iter __iter = *(i); \
> (i)->type & RW_MASK; \
> })
>
> The assignment is optimized out.
... and you are getting
a) use of rather lousy gccism when plain C would do
b) double evaluation since you've got it wrong (should've been
__iter.type & RW_MASK, if you do it that way). As it is, if argument has
any side effects, your variant will trigger those twice - even if the
destination of the assignment is never used, the side effects remain.
I agree that it could use /* use ?: for typechecking */, but let's not go into
({...}) land unless we absolutely have to.
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