C# Threading: a race condition example

Question

I am reading http://www.mono-project.com/ThreadsBeginnersGuide.

The first example looks like this:

public class FirstUnsyncThreads {
    private int          


        

Answer 1

I think the writer of the article has confused things.

VoteyDisciple is correct that ++i is not atomic and a race condition can occur if the target is not locked during the operation but this will not cause the issue described above.

If a race condition occurs calling ++i then internal operations of the ++ operator will look something like:-

1. 1st thread reads value 0
2. 2nd thread reads value 0
3. 1st thread increments value to 1
4. 2nd thread increments value to 1
5. 1st thread writes value 1
6. 2nd thread writes value 1

The order of operations 3 to 6 is unimportant, the point is that both the read operations, 1 and 2, can occur when the variable has value x resulting in the same incrementation to y, rather than each thread performing incrementations for distinct values of x and y.

This may result in the following output:-

First runner incrementing i from 0 to 1
Second runner incrementing i from 0 to 1

What would be even worse is the following:-

1. 1st thread reads value 0
2. 2nd thread reads value 0
3. 2nd thread increments value to 1
4. 2nd thread writes value 1
5. 2nd thread reads value 1
6. 2nd thread increments value to 2
7. 2nd thread writes value 2
8. 1st thread increments value to 1
9. 1st thread writes value 1
10. 2nd thread reads value 1
11. 2nd thread increments value to 2
12. 2nd thread writes value 2

This may result in the following output:-

First runner incrementing i from 0 to 1
Second runner incrementing i from 0 to 1
Second runner incrementing i from 1 to 2
Second runner incrementing i from 1 to 2

And so on.

Furthermore, there is a possible race condition between reading i and performing ++i since the Console.WriteLine call concatenates i and ++i. This may result in output like:-

First runner incrementing i from 0 to 1
Second runner incrementing i from 1 to 3
First runner incrementing i from 1 to 2

The jumbled console output which the writer has described can only result from the unpredictability of the console output and has nothing to do with a race condition on the i variable. Taking a lock on i whilst performing ++i or whilst concatenating i and ++i will not change this behaviour.