Thank you for your reply, but I don't want atomic writing to solve a mutual access problem. I already lock my files using flock. What I need is to maintain the integrity of the file at all times (that is, exception safety). I don't want to start overwriting the file and then in the middle run out of space. I want some kind of two-phase commit system. And I want it to preserve the ownership of the file.
Maybe I could do it by mmapping the file, writing my new stuff at the end so that I can erase it if things go wrong. Then when I'm done I just move my stuff up to the beginning of the file, erasing what was already there. That can't fail, I guess, so I think it could work. What do you think of that?
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