I, for one, am quite confident that such a call would be re-entrant.

But at the same time I am very reluctant to write any code that might actually depend on assumptions. If I'm planning to assign a file-handle to a common variable, then you can be sure-as-heck that I'm gonna protect that variable. It doesn't matter to me whether I intend to assign a file-handle to it or the integer "5." If it's a common (shared...) variable, or any other type of shared resource, then I'm gonna protect it. I don't give a hoot about “efficiency,” and I don't care about “what (coincidentally... happens to...) work(s).”