Better than POSIX?

I would also like to see "atomic create" with a new flag to open(). This would make a hidden (ie the inode is not in any directory) file that you can write to. When you close() it it does the equivalent of the atomic rename() to the filename given to open(). If the program crashes then the file disappears as it is not linked to by any directory. Calling fsync() on the file would do the rename that close does at that point, and from then on it would act like a normal file. There may also be a way to abort the file, closing it such that it disappears with no effect.

This will give the atomic operation that everybody is trying to get, without the need to generate a temp filename, without leaving temp files on the disk in case of a crash, and possibly with far better performance as the filesystem could defer the effects of close.

Furthermore I think there would not even need to be a new flag. Instead the flags O_CREAT|O_WRONLY|O_TRUNC (the ones creat() use) would trigger this behavior. This is almost certainly the behavior anybody calling creat() is expecting anyway.