...'ve looked up the corresponding handle. Since it's only a table lookup it's fast and no thread spends much time in that critical section. So, unit numbers don't really contributte much to the overhead of handling threads. Yes, though presumably you could, as usual, design some pathological examples where it would be significant. E.g. as explained by Robert Corbett. -- Janne ...

...unlocked as soon as you've looked up the corresponding handle. Since it's only a table lookup it's fast and no thread spends much time in that critical section. So, unit numbers don't really contributte much to the overhead of handling threads. (Yes, the table has to be locked longer during OPEN and CLOSE since you're modifying it. And yes, things that don't modify the table could be let...