On Wed, May 22, 2013 at 11:04 AM, Michael Powell <mwpowellhtx@gmail.com>wrote:
Hello,
I have some thread synchronization to do, probably will end up using some sort of internally synchronized cancelation or other state to message between parent and child thread.
If the state uses boost::atomic<bool> _canceled, for instance, how do I go about setting/getting the value? I am assuming that we do something like this:
//To get the value. return _canceled.load(boost::memory_order::memory_order_acq_rel);
//To set the value. _canceled.store(canceled, boost::memory_order::memory_order_acq_rel);
Using acquire/release to get in, do what we need with the value, and get out, type thing.
In general, use Acquire if you need to *read* _other_ values (memory locations) as a result of reading this atomic. ie if _canceled == true means that the _canceledReason is set and you need to read that, then you need Acquire. Use Release if you *write* _other_ values that you expect other threads to read. ie if you set _canceledReason, and then set _canceled. If in doubt, use the default - sequential consistency. If in doubt, use locks. Also think about how the code might evolve - will the next person that comes in and adds a read/write understand that the memory ordering needs changing? Also bearing in mind this is targeting ARM architecture (ARM5 I think).
Also, how complex a value can be protected? Are we talking primitive types only? Basic data types like bool, int, float, that sort of thing?
In theory, anything can be made atomic. But only small types (like bool, int) will be lock-free. Others will use a lock to get the same effect. (At least that's how std::atomic works, not sure about boost::atomic.) Tony