On an r+b update stream, what must normally separate a successful read from a following w…

On an r+b update stream, what must normally separate a successful read from a following write when that read did not encounter EOF?

1. A successful file-positioning call such as fseek.
2. Only fflush, because it discards unread input portably.
3. A call to feof, even when it returns false.
4. Nothing; + removes every sequencing restriction.