A very handy overview and comparison of the various file-locking mechanisms available on Unix systems (includes Linux 3.15's "open file description locks").

On the futility of Unix file locks (especially on networked file systems).

Note that Linux 3.15 introduced "open file description locks" to fcntl, which combines the semantics of BSD flock (lock associated with open file) with the record-locking and NFS-compatibility of fcntl. That solves everything except for portability.