The statements are not contradictory, just confusingly worded. A lock is on a file, held by an open file description. Also note that, "open file description"

The statements are not contradictory, just confusingly worded. A lock is on a file, held by an open file description. Also note that, "open file description" != "file descriptor". An open file description exists only in the kernel, and is created on open(2). A file descriptor is the userland-visible handle to this open file description. dup(2) creates a new handle (file descriptor) pointing to this same open file description. Therefore, dup(2), fork(2), etc., do not violate the mutex property of the lock, since they are duplicating the file descriptors (handles), not the open file descriptions (which actually hold the locks). What's not clear from the man pages is what happens to a locked open file description which is passed to another process via a Unix domain socket. I suspect at that point, both "processes" "hold" the lock -- which in reality simply means they both have a file descriptor referring to the same single open file description which holds the lock, thus not violating the mutex property. Though this contradicts the extensive verbiage in flock(2) which refers to "processes" as being subject to the mutex property, I suspect that is a simplification, as later text in flock(2) indicates that open file descriptions are indeed the subject of the mutex property.