On Tue, Apr 23, 2013 at 12:04 PM, Paul Smith <address@hidden
> I'm not sure if the lock locks the FD (so that if you dup'd the FD but
> it still pointed to the same output, you could take exclusive locks on
> both), or if it locks the underlying resource.
I'm pretty sure it's the underlying resource that gets locked. The man page is not quite explicit but implicitly fairly clear. The opening paragraph says:
"Apply, test or remove a POSIX lock on a section of an open file. The file is specified by fd, a file descriptor open for writing...".