Page 324 - DCAP404 _Object Oriented Programming

Unit 14: Advanced Concept in C++

As is evident, the shared variable is checked and updated as an atomic operation so that the race Notes
condition does not occur.

Deadlock

The concurrency may lead to a situation where one or more threads wait for resources that can
never become available. Such a condition is called a deadlock. Consider the following code that
illustrates a deadlock.
mutex X;

mutex Y;
void ThreadThree()
{
int ct = 0;
while(true)
{
mutex::lock(X);

mutex::lock(Y);
cout << “ThreadThree in action” <<++ct<< “\n”;
}
}
void ThreadFour()
{
int ct = 0;
while(true)
{

mutex::lock(Y);
mutex::lock(X);
cout << “ThreadFour in action” << ++ct<< “\n”;
}
}
When this code is run a deadlock is possible. The two threads may enter a situation where
ThreadThree is waiting for ThreadFour to release the locks while ThreadFour waits for
ThreadThree to release the locks.
To resolve this simple deadlock all we need to do is to ensure that we lock resources in a
consistent order. Thus, changing ThreadFour to lock X before Y ensures there will be no deadlock.

Self Assessment

Fill in the blanks:
12. A ……………………….. is a sequence of instructions to be executed within a program.

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