Garrett D'Amore wrote: > Peter Cudhea wrote: > > It would be useful to take a definitive stand on whether or not > > operations on two adjacent bool values are atomic with respect to one > > another. E.g. back in the days of the DEC Alpha, there existed no > > instructions that could modify the value of one bool in a word without > > potentially interfering in other uses of a differnt bool in the same > > word. That consideration does not apply to either of the current > > Solaris platforms. But are we introducing a constraint that Solaris > > could never be ported to such a platform? Or are we introducing a > > constraint that any values that should be updatable atomically should > > be stored in a boolean_t and not in a bool? Or does the compiler > > guarantee to allocate bools on word boundaries in platforms such as > > this? Either way, we should spell it out. > > I would suggest that in situations where this matters, there are a > couple of options: > > 1) use locking primitives to guard an associated group of booleans. > 2) find out what C99 says about this (if anything) > 3) if you're really that concerned, such as in kernel space, use boolean_t. > > I raised some of these concerns in private review with Roland before > submitting the case. Personally, I have reservations about using C99 > bool in places where atomicity is a requirement,
What about adding an |atomic_swap_bool()| (e.g. an alias for |atomic_swap_uchar()|) to <atomic.h> ? > or in public headers > where it might force C99, or in structures where strict packing is required. What do you mean with "strict packaging" in this case ? |bool| will behave like the platform's smallest datatype (e.g. |char| (one byte on SPARC+x86)) within a struct and therefore consume a lot less than |boolean_t| (which may result in less data cache usage since |boolean_t| is AFAIK an |enum| which consumes the space of an |int| (four bytes on SPARC+x86)) ... ---- Bye, Roland -- __ . . __ (o.\ \/ /.o) roland.mainz at nrubsig.org \__\/\/__/ MPEG specialist, C&&JAVA&&Sun&&Unix programmer /O /==\ O\ TEL +49 641 3992797 (;O/ \/ \O;)
