On Tue, Apr 10, 2018 at 03:00:11PM -0700, Andrew Morton wrote: > On Tue, 10 Apr 2018 14:53:51 -0700 Joe Perches <j...@perches.com> wrote: > > > On Tue, 2018-04-10 at 14:39 -0700, Andrew Morton wrote: > > > On Tue, 10 Apr 2018 11:19:54 -0700 Joe Perches <j...@perches.com> wrote: > > > > > > > A struct with a bool member can have different sizes on various > > > > architectures because neither bool size nor alignment is standardized. > > > > > > What's wrong with bools in structs? > > > > See above. > > Yeah, but so what? `long' has different sizes on different > architectures too.
Right, so we have ILP32/LP64 for all our 32/64 bit archs respectively. So only 2 possible variations to consider, and if you know your bitness you know your layout. (+- some really unfortunate alignment exceptions, the worst of which Arnd recently removed, hooray!) But neither says anything about sizeof(_Bool), and the standard leaves it undefined and only mandates it is large enough to store either 0 or 1 (and I suspect this vagueness is because there are architectures that either have no byte addressibility or it's more expensive than word addressibility). Typically GCC chooses a single byte to represent _Bool, but there are no guarantees. This means that when you care about structure layout (as we all really should) things go wobbly when you use _Bool. If GCC were to guarantee a 1 byte _Bool for all Linux ABIs we could reconsider.
