Michael Eager <eager@xxxxxxxxxxxx> writes: > > There's nothing to stop the machine from > > reordering memory writes, even if the compiler doesn't. > > Yes, it's safe. It doesn't matter which microsecond that the flag is set. > (Or if it does, then your code should be using a mutex or some other > form of synchronization.) > > Say you have one thread which performs some process when it sees > a shared flag set. It checks this flag every second. If another > process sets this flag, the service thread will notice and take action. > > If the signaling process and the receiving process both try to > access the shared memory at the same time, the processor may > reorder the operations as it chooses. The worst case is that > the server will check and see that the flag is not set, then > the signaling process will set the flag. The result is a short > delay in the receiving process noticing that the flag is set. > > Note that there is a race condition which is problematic. If the > receiving process clears the flag, at the same time that the > signaling process sets it, then these may be reordered. So you > need to insure that only one of the processes ever writes to the > flag. I think that will be OK in practice provided the only memory shared between the two processes is the signalling flag itself. Ian
