Re^2: Writing to a file atomically without renaming

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?

Comment on Re^2: Writing to a file atomically without renaming

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Re^3: Writing to a file atomically without renaming by salva (Canon) on Jun 30, 2005 at 19:40 UTC
how about using a symbolic link? For instance, suppose you have foo-1 and foo.link pointing to foo-1. To update it, first create foo-2, then change its permissions and finally point foo.link to foo-2. The point is that foo.link can have more relaxed permissions, i.e., 644	[reply]
Re^4: Writing to a file atomically without renaming by nomis80 (Sexton) on Jun 30, 2005 at 19:43 UTC
Ok, but how do I make sure foo-2's owner is the same as foo-1?	[reply]
Re^5: Writing to a file atomically without renaming by salva (Canon) on Jun 30, 2005 at 19:56 UTC
oh, I see your problem now... I think you can't do it without becoming root in some way... like for example, using sudo.	[reply]