I need to write something like 64 kB of data atomically in the middle of an existing file. That is all, or nothing should be written. How to achieve that in Linux/C?
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I need to write something like 64 kB of data atomically in the middle of an existing file. That is all, or nothing should be written. How to achieve that in Linux/C?
I don’t think it’s possible, or at least there’s not any interface that guarantees as part of its contract that the write would be atomic. In other words, if there is a way that’s atomic right now, that’s an implementation detail, and it’s not safe to rely on it remaining that way. You probably need to find another solution to your problem.
If however you only have one writing process, and your goal is that other processes either see the full write or no write at all, you can just make the changes in a temporary copy of the file and then use
renameto atomically replace it. Any reader that already had a file descriptor open to the old file will see the old contents; any reader opening it newly by name will see the new contents. Partial updates will never be seen by any reader.