What about dictionaries? Reading values, adding new ones and the most important: changing an existing value - can it corrupt the state of the dictionary or that it is guaranteed that if I try to read the value of this key, I can only get the old one or the new one, but not something weird instead (because a different thread changed the value while this thread was trying to read it). About your remark that I don't need to check if a.x is True, I do need, and indeed it is acceptable that doSomething() will run, because this logic resides inside a loop, and it is only important for me that it will stop entering the loop sometime (I don't care if there are n iterations or n+1 or even n+m)
Thanks On Wed, Sep 30, 2009 at 10:44 AM, steve <st...@lonetwin.net> wrote: > Hi, > > On 09/30/2009 01:53 PM, Charlie Dickens wrote: > >> Hi, >> if I have a class A that contains a boolean variable named x, is it safe >> to read and change it from different threads without using locks? >> Is it guaranteed that A.x will be always True or False, and not any >> other weird value that that causes it to be inconsistent (assuming I >> only set it to True or False) ? >> >> The guarantee for A.x being only True or False, is independent of whether > you use locks or not. It depends entirely on code that assigns to A.x. > > I have a = A() >> first thread does: >> if a.x is True : >> pass >> >> 2nd thread does: >> a.x = False >> >> is it safe? >> > > what if you have code like this: > > Thread 1 > if a.x is True: > doSomething() > > Thread 2 > a.x == False > > ..and thread 2 executes *after* thread 1's 'if' condition but *before* > doSomething(). If that behavior is acceptable in your application, you > possibly don't even need a.x. > > >> and what if x was a dict ? especially if the only values that are set in >> the dictionary are simple: booleans, integers, floats, strings >> >> Same logic applies. > > hth, > cheers, > - steve > > -- > random non tech spiel: http://lonetwin.blogspot.com/ > tech randomness: http://lonehacks.blogspot.com/ > what i'm stumbling into: http://lonetwin.stumbleupon.com/ >
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