Static Variables and Threads (C)

c multithreading concurrency static mutual-exclusion
dahui picture dahui · Apr 7, 2013 · Viewed 30.5k times · Source

I know that declaring a static variable within a function in C means that this variable retains its state between function invocations. In the context of threads, will this result in the variable retaining its state over multiple threads, or having a separate state between each thread?

Here is a past paper exam question I am struggling to answer:

The following C function is intended to be used to allocate unique identifiers (UIDs) to its callers:

get_uid() 
{
static int i = 0;
return i++;
}

Explain in what way get_uid() might work incorrectly in an environment where it is being called by multiple threads. Using a specific example scenario, give specific detail on why and how such incorrect behaviour might occur.

At the moment I am assuming that each thread has a separate state for the variable, but I am not sure if that is correct or if the answer is more to do with the lack of mutual exclusion. If that is the case then how could semaphores be implemented in this example?

Answer

P.P picture P.P · Apr 7, 2013

Your assumption (threads have their own copy) is not correct. The main problem with code is when multiple threads call that function get_uid(), there's a possible race condition as to which threads increments i and gets the ID which may not be unique.

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