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    <p>That is because for most environments, incrementing a memory location is not an atomic operation.</p> <p>The sequence of events would be something like</p> <ul> <li>The compiler places the value 0 at the memory address 0xABCD</li> <li>Thread 1 reads 0xABCD into a register and increments the register value</li> <li>Thread 1 is interrupted by Thread 2</li> <li>Thread 2 reads the value 0 from the memory address 0xABCD into a register</li> <li>Thread 2 increments the value in the register</li> <li>Thread 2 writes the value from the register to 0xABCD</li> <li>Thread 2 reads the value 1 from 0xABCD, increments the register, and writes back again</li> <li>Thread 1 resumes</li> <li>Thread 1 writes the value from its register, 1, to 0xABCD, overwriting the value 2 that Thread 2 had previously written there.</li> </ul> <p>To ensure a consistent result, you must make the increment operation atomic. This is often done by placing a thread lock around the increment operation. For example, in C# that might look like</p> <pre><code>object mylock = new object(); ... lock (mylock) { myInt++; } </code></pre> <p>Alternatively, in the .NET environment, one could use Interlocked.Increment</p> <p><a href="http://msdn.microsoft.com/en-us/library/dd78zt0c.aspx" rel="nofollow">http://msdn.microsoft.com/en-us/library/dd78zt0c.aspx</a></p> <p>Other environments have similar constructs.</p> <p>The best reference I have ever come across for threading in the .NET environment (but is a useful read no matter what your environment) is the following</p> <p><a href="http://www.albahari.com/threading/" rel="nofollow">http://www.albahari.com/threading/</a></p>
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    1. POGlobal variables and threads
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    1. COEven if it was, the variable in question is probably not synchronized between the threads.
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    2. CO@Gray: The question is platform agnostic, but platform specifics come into play (e.g. some compilers require a keyword such as *volatile* to indicate that another thread may change the variable state, while other compilers will re-read the memory location prior to the increment).
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    3. COAll agreed @Eric. I was just pointing out that the synchronization was probably more the problem than the atomic-ness of ++.
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