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respectively Certain programming patterns such a s producer/ consumer use consistent sharing and transfer of ownership patterns, making the points of publication and privatization more apparent Even then it's easy to trip up and make a mistake, such as treating something private although it is still shared, causing race conditions It's also important to note that the above definitions depend to some degree on modern type safety In the NET Framework this is generally not negotiable, whereas in systems like C++ it is highly encouraged but can be circumvented When any part of the program can manufacture a pointer to any arbitrary address in the process's address space, all data in the entire address space is shared state We will ignore this loophole But when pointer arithmetic is involved in your system, know that many of the same problems we'll look at in this chapter can manifest They can be even more nondeterministic and hard to debug, however To illustrate some of the challenges in identifying shared state, here's a class definition in C++ It has one simple method, f, and two fields, one static ( s_f ) and the other instance ( m_f ) Despite the use of C++ here, the same principles clearly apply to managed code too
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class C { int m_f j public : void f ( int * P y ) { int X j X++ j s_f++ j m_f++ j ( * P Y ) ++ j } }j I I loc a l v a r i a b l e II s t a t i c c l a s s member II c l a s s member I I pointer to somet h i n g
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The method contains four read / increment/ write operations (via C++'s
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++ unary operator) In a concurrent system, it is possible that multiple
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threads could be invoking f on the same instance of c concurrently with one another Some of these increments will be safe to perform while others are not Others still might only be safe if f is called in certain ways We'll see many detailed examples of what can go wrong with this example Simply
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put, any increments of shared data are problematic This is not strictly true because higher level programming conventions and constructs may actu ally prevent problematic shared interactions, but given the information above, we have no choice but to assume the worst By simply looking at the class definition above, how do we determine what state is shared Unfortunately we can't We need more information The answer to this question depends on how instances of C are used in addition to where the py pointer came from We can quickly label the operations that do not act on shared state because there are so few (just one) The only memory location not shared with other threads is the x variable, so the x++ statement doesn't modify shared state (Similar to the statement above about type safety, we are relying on the fact that we haven't previously shared the address of x on the thread's stack with another thread Of course, another thread might have found an address to the stack through some other means and could perform address arithmetic to access x indirectly, but this is a remote possibility Again, we will assume some reasonable degree of type safety) Though it doesn't appear in this example, if there was a statement to increment the value of py, ie, py++, it would not affect shared state because py is passed by value The s_f++ statement affects shared state because, by the definition of static variables, the class's static memory is visible to multiple threads run ning at once Had we used a static local variable in f in the above example, it would fall into this category too Here's where it becomes complicated The m_f++ line might, at first glance, appear to act on private memory, but we don't have enough infor mation to know Whether it modifies shared state or not depends on if the caller has shared the instance of C across multiple threads (or itself received the pointer from a caller that has shared the instance) Remember, m_f++ is a pointer dereference internally, ( t h i s - >m_f ) ++ The t h i s pointer might refer to an object allocated on the current thread's stack or allocated dynam ically on the heap and may or may not be shared among threads in either case
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class D { s t a t i c C S_C j C m_c j II i n i t i a l i zed el sewhere II a l s o i n i t i a l i zed e l s ewhere
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