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    <p><code>mmap</code> can't be used for this purpose. There's no way to prevent data from being written to disk. In practice, using <code>mlock()</code> to make the memory unswappable <em>might</em> have a side effect of preventing it from getting written to disk except when you ask for it to be written, but there's no guarantee. Certainly if another process opens the file, it's going to see the copy cached in memory (with your latest changes), not the copy on physical disk. In many ways, what you should do depends on whether you're trying to do synchronization with other processes or just for safety in case of crash or power failure.</p> <p>If your data size is small, you might try a number of other methods for atomic syncing to disk. One way is to store the entire dataset in a filename and create an empty file by that name, then delete the old file. If 2 files exist at startup (due to extremely unlikely crash time), delete the older one and resume from the newer one. <code>write()</code> <em>may</em> also be atomic if your data size is smaller than a filesystem block, page size, or disk block, but I don't know of any guarantee to that effect right off. You'd have to do some research.</p> <p>Another very standard approach that works as long as your data isn't so big that 2 copies won't fit on disk: just create a second copy with a temporary name, then <code>rename()</code> it over top of the old one. <code>rename()</code> is always atomic. This is probably the best approach unless you have a reason not to do it that way.</p>
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    1. COThere's no need to synchronize processes. It's more of a backup. I would also like to avoid doing any kind of copying if possible. The data is also at least 50MB.
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