doesn't opening a file with O_CREAT|O_EXCL work in posix?

POSIX only guarantees advisory locks; mandatory locks are an optional feature and are not supported in Linux. See for example <https://stackoverflow.com/questions/77931997/linux-mandatory...>.

But that is file locking, not lockfiles...

Creating a file with the exclusive creation mode is a pretty common way to make a lock file and as far as I know that works well in posix.