Is rename() atomic?

Yes and no.

rename() is atomic assuming the OS does not crash. It cannot be split by any other filesystem op.

If the system crashes you might see a ln() operation instead.

Also note, when operating on a network filesystem, you might get ENOENT when the operation succeeded successfully. Local filesystem can't do that to you.

This is a very late answer, but... yes rename() is atomic but not in the sense of your question. Under Linux, rename(2) says:

However, when overwriting there will probably be a window in which both oldpath and newpath refer to the file being renamed.

But rename() is still atomic in a very important sense: if you use it to overwrite a file, then you will end up with either the old or the new version and nothing else.

[update: but as @jonas-wielicki points out in the comments, you need to make sure the file you are renaming actually has up-to-date contents, using fsync() and friends.]

If newpath already exists it will be atomically replaced (subject to a few conditions; see ERRORS below), so that there is no point at which another process attempting to access newpath will find it missing.

If you see ERRORS, you will find that the rename might fail, but it will never break the atomicity.

This is all from the Linux man page. What I don't know is if you do a rename() on a network file-system where the server runs a different OS. Does the client have a hope in hell of guaranteeing atomicity then? I doubt it.