That case is covered too as long as the program makes use of the dup2 syscall to make the switch instead of close followed by open. But it is probable that some

That case is covered too as long as the program makes use of the dup2 syscall to make the switch instead of close followed by open. But it is probable that some pieces of code that are currently in widespread use do it by close and expecting a call to open to give them fd id for stderr like you say indeed I guess. So the conclusion is that the answer to my original question is probably like you say, we can’t just do that. But it would be interesting to find out how many wide-spread pieces of open source code do it that way, and for all code that does one could submit patches to change them to using the explicit dup2.