static methods and the call stack in IIS/asp.net

Question

Theoretical question. If you have 100 separate requests coming to an aspx web page that calls the static method below.

    public static GeocodeResult GeocodeQuery(string query)
    {
        int train, tube, dlr = 0;

        // manipulate these ints
        if (train)
        {
            // do something important
        }

    }

Does every request have a separate call stack?

If so - Does this static method call get pushed on these separate call stacks?

If so - Therefore are these ints thread safe? ie. 100 requests, 100 call frames, 300 ints.

Cheers

Answer 1

Yes, each request has it's own call stack. And each stack will contain it's own values for quiery, train, tube, dir, since local variables and method parameters are stored in stack.

So, unless you are accessing some shared state, this method is thread safe.

Answer 2

All static methods are thread safe if the only variables it uses are local variables or parameters.

Answer 3

Note that it's not that a method has a call stack - a thread has a call stack. Methods are invoked by a thread.

Unless the method is inlined, invoking it may push parameters onto the call stack, but will definitely push the return address onto the call stack.

Each thread call stack is independent of the call stack of any other thread. If only the stack is accessed, then the method is thread-safe.

Other storage could be accessed to make the method not thread-safe. That includes static data and instance data as well as objects referenced by them. A static method has no access to instance data, which makes it easier for it to be thread-safe. You only need to watch out for access to static data, or objects referenced by static data:

private static int _balance;
private static void ThreadSafetyIssues()
{
    // Not thread  safe
    int temp = _balance;
    temp ++;
    _balance = temp;

    // Not thread safe
    temp = (int) HttpContext.Current.Session["balance"];
    temp ++;
    HttpContext.Current.Session["balance"] = temp;
}

The HttpContext.Current property is static. This means that any thread can access the context at the same time, including Session. This is just as non-thread-safe as the simple static _balance field above.