Question

LONG __cdecl InterlockedCompareExchange(
  __inout  LONG volatile *Destination,
  __in     LONG Exchange,
  __in     LONG Comparand
);

Return value
The function returns the initial value of the Destination parameter.

Just curious.
Why does InterlockedCompareExchange return initial value? Is there a reason that they designed so?

Answer 1

Here’s a good example from MSDN:

http://msdn.microsoft.com/en-us/library/windows/desktop/ms683560%28v=vs.85%29.aspx

    for(;;)
    {
        // calculate the function
        new_value = Random(old_value);

        // set the new value if the current value is still the expected one
        cur_value = InterlockedCompareExchange(seed, new_value, old_value);

        // we found the expected value: the exchange happened
        if(cur_value == old_value)
            break;

        // recalculate the function on the unexpected value
        old_value = cur_value;
    }

Do you see why it’s important to be able to retain the initial value?