Sharing unlocked, unchanging data between threads?

[Jonathan M Davis]

On Tuesday, August 10, 2010 10:45:03 Chris Williams wrote:
> I'm writing a fairly large, multithreaded application and some part
> of it is causing periodic access errors.
> 
> Say that I have an associative array like:
> 
> uint[ char[] ] nameToId;
> 
> If I set all values before I start my threads going and never change
> anything after that point -- all access is read only -- is there any
> way that having multiple threads access it at the same time would
> mess anything up? I'm doing this in several places as locking a
> read-only data structure doesn't strike me as something which should
> be necessary. I don't know what D's associative arrays look like
> internally though, so perhaps it is dangerous to do. It would be a
> major hassle to port it all over to locking everything, to test if
> it fixes the problem. So, hopefully someone can say whether this is
> worth checking.

Well, unless it's declared shared, it's going to be thread-local, and then each 
thread is going to have their own copy. Now, if it were declared shared and you 
never changed it after initially setting all of its values, then you shouldn't 
have any problems as far as I'm aware. Just accessing an associative array 
shouldn't change it.

- Jonathan M Davis