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Liham Pangaplikasyon

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  • September 10, 2012
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Chapter I
1. Race Condition
Race Condition occurs when two threads access a shared variable at the same time. The first thread reads the variable, and the second thread reads the same value from the variable. Then the first thread and second thread perform their operations on the value, and they race to see which thread can write the value last to the shared variable. The value of the thread that writes its value last is preserved, because the thread is writing over the value that the previous thread wrote.

Example:

for ( int i = 0; i < 10000000; i++ )
{
x = x + 1;
}

If you had 5 threads executing this code at once, the value of x WOULD NOT end up being 50,000,000. It would in fact vary with each run. This is because, in order for each thread to increment the value of x, they have to do the following: (simplified, obviously)

Retrieve the value of x
Add 1 to the value of x
Store the value of x

Any thread can be at any step in this process at any time, and they can step on each other when a shared resource is involved. The state of x can be changed by another thread during the time between x is being read and when it is written back. Let's say a thread retrieves the value of x, but hasn't stored it yet. Another thread can also retrieve the samevalue of x (because no thread has changed it yet) and then they would both be storing the samevalue (x+1) back in x!

Example:
Thread 1: reads x, value is 7
Thread 1: add 1 to x, value is now 8
Thread 2: reads x, value is 7
Thread 1: stores 8 in x
Thread 2: adds 1 to x, value is now 8
Thread 2: stores 8 in x

Race conditions can be avoided by employing some sort of locking mechanism before the code that accesses the shared resource:

for ( int i = 0; i < 10000000; i++ )
{
//lock x
x = x + 1;
//unlock x
}

Here, the answer comes out as 50,000,000 every time.

END OF CHAPTER I

Chapter II
2. Virtual...