mandatory locks arn't needed. (and they are a common extension to the fcntl(2) locking mechanism anyway, at least I do not know of a system which doesn't support them)
open(..., O_RDWR | O_CREAT | O_EXCL, 0600); is what you want, (wrapped in haskell of course) it will create the file if it doesnt exit (O_CREAT) but if it already does exist then it will return an error (EEXIST). this check is done ATOMICALLY, meaning there is no race condition.
Not so; on NFS, the implementation is *not* atomic, and the race remains. See the Linux open(2) man page, for example: O_EXCL When used with O_CREAT, if the file already exists it is an error and the open will fail. In this con text, a symbolic link exists, regardless of where its points to. O_EXCL is broken on NFS file sys tems, programs which rely on it for performing locking tasks will contain a race condition. The solution for performing atomic file locking using a lockfile is to create a unique file on the same fs (e.g., incorporating hostname and pid), use link(2) to make a link to the lockfile. If link() returns 0, the lock is successful. Otherwise, use stat(2) on the unique file to check if its link count has increased to 2, in which case the lock is also suc cessful. The suggested solution still requires a unique filename, so you still need to use the complicated filename-generation technique, and you still can't proof yourself against a hostile user guessing the right name to use at the right moment. Summary: NFS is broken, but it's too late now to fix it. --KW 8-)