C++: Race conditions and Data races

According to this discussion on Stack Overflow, we should make a distinction between race conditions and data races. This blog post defines the terms and gives a neat example where the code is modified to exhibit zero, one or both behaviours.

Here’s an example of a race condition – the ordering of the execution affects the outcome (this can occur if one thread checks a value then performs an action, giving another thread the change to mutate the variable in between):

if (x == 5) // The "Check"
{
   y = x * 2; // The "Act"

   // If another thread changed x in between "if (x == 5)" and "y = x * 2" above,
   // y will not be equal to 10.
}

And here’s an example of a data race – a variable is mutated by multiple threads without synchronisation:

// Execute this function on five threads simultaneously
for ( int i = 0; i < 10000; i++ )
{
   x = x + 1; 
}

In both cases, use of a synchronisation object across all the threads involved (e.g. a mutex) would address the issue.

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Filed under C++, C++ Code, Programming

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