Figure 7.10 The queue lies on the thread boundary, which makes it susceptible to concurrency issues. in C#.NET

Print Code-128 in C#.NET Figure 7.10 The queue lies on the thread boundary, which makes it susceptible to concurrency issues.

Figure 7.10 The queue lies on the thread boundary, which makes it susceptible to concurrency issues.
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WHAT DOES THREAD-SAFE MEAN
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ATOMIC OPERATIONS
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One of my earliest programming assignments was to write the pseudocode to make a peanut butter and jelly sandwich. When everyone had submitted solutions, the instructor took the class to the cafeteria and followed our pseudocode. It was quite humorous to see the results. Many of the steps were omitted or too general to be followed. The point of the exercise was to teach us to think in small units of work. Most actions are made up of many smaller actions. For example, opening the jar of peanut butter involves grasping the lid, holding it under a constant amount of pressure, and rotating the jar. In conversation, we simply tell someone to open the jar. We understand that the operation actually is made up of many smaller operations. This is just as true in software development as it is in making a sandwich. When one of those smaller operations is guaranteed to be completed without being interrupted, we call that operation atomic. Recall from chapter 1 that when multiple threads are executing on a processor there are many interrupts to an individual thread s execution. A thread that is executing is periodically interrupted and moved from the processor. A different thread is then given a chance to work. This is called a context switch. If a context switch can occur during an operation, that operation is not atomic. It is best to assume that operations are not atomic and that a context switch can occur during processing. In the following sections we ll cover the necessary mechanisms to protect data.
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Atomic Operations
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Atomic operations are statements that will always complete without interruption. This ensures that they will complete as expected without need for synchronization.
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To understand what is happening we can look at the MSIL that corresponds to instructions. A detailed discussion of IL is beyond the scope of this book, but it does offer some valuable insight. Consider the following instruction:
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X += 1
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This produces the following MSIL:
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ldloc.0 ldc.i4.1 add.ovf stloc.0 Load local variable in location zero onto the stack Load the value 1 into the stack Add the top two elements of the stack and put the result back onto the stack Save the top value in the stack to local variable in location zero
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As you can see, what we think of as being a single instruction is in reality four. It is quite likely that after the MSIL is compiled to machine instructions that this will change, although it may not. The point is that an instruction that on the surface seems to be quite simple may actually be doing many things. Figure 7.11 shows how two increment operations can interact to yield incorrect results. 120
CONCURRENCY CONTROL
Figure 7.11 If operations are not atomic, a value can be changed on one thread without another thread s knowledge.
If steps 1a and 1b happen at the same time, or roughly the same time, a race condition will occur because the increment operation is not an atomic operation. In order for the class variable to be incremented, it must first be on the thread s stack. During the time it is on the stack of a thread, a different thread may get a copy of the value. If the operation were atomic, Thread B would not be able to access the class variable until Thread A had completed its interaction with it.
ILDASM
ILDASM is a tool for disassembling a .NET program to IL. This allows analysis
of the code generated by the compiler, and can yield insight, such as what operations are likely to be thread-safe.
To view the IL of a program, use the ildasm.exe program that s located in the Framework s bin directory. It allows a compiled .NET program to be reduced to IL for analysis. 7.2.1 The Interlocked class We saw in the last section how the += and ++ operators are not atomic and thus not thread-safe. The Interlocked class provides several static methods that perform atomic operations. When more than one thing is interlocked, it means that an action on one is constrained or restricted by actions on the other. For example, if two people are locked together with handcuffs and one tries to go north while the other tries to go south, there will be a constraining result.
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