Question

I need to create a file if it does not exist, in a way that another process trying to create this file would fail. I need the file be considered “created” even before the creating process finished writing the actual data to it.

I read about O_EXCL flag to open(), so it seems that the solution exists, I have a few questions however:

1. do you have experience with this technique? How good is it? (I guess I can’t have a DB-level atomicity, but but good enough is… well, enough)
2. should I immediately close the file after open() so that it is considered created, and then reopen it for writing?
3. are there any subtleties to be aware of?

Answer 1

The open() man page says your method may fail on NFS.

From the section on O_EXCL:

When used with O_CREAT, if the file
already exists it is an error and the
open() will fail. In this context, a
symbolic link exists, regardless of
where it points to. O_EXCL is broken
on NFS file systems; programs which
rely on it for performing locking
tasks will contain a race condition.

And it suggests a more general solution:

The solution for performing atomic
file locking using a lockfile is to
create a unique file on the same file
system (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
successful.

See the “Using Files as Locks” section of this Web page for more details on the various issues and approaches.