When a shared lock is set on a segment of a file, other processes shall be able to set shared locks on that segment or a portion of it. A shared lock prevents any other process from setting an exclusive lock on any portion of the protected area. A request for a shared lock shall fail if the file descriptor was not opened with read access.

An exclusive lock shall prevent any other process from setting a shared lock or an exclusive lock on any portion of the protected area. A request for an exclusive lock shall fail if the file descriptor was not opened with write access.

When a shared lock is set on a segment of a file, other processes can set shared process-owned locks, and other open file descriptions can be used to set shared OFD-owned locks, on that segment or a portion of it. A shared process-owned lock shall prevent any other process from setting an exclusive process-owned lock, and shall prevent any exclusive OFD-owned lock from being set, on any portion of the protected area. A shared OFD-owned lock shall prevent any other open file description from being used to set an exclusive OFD-owned lock, and shall prevent any exclusive process-owned lock from being set, on any portion of the protected area. A request for a shared lock shall fail if the file descriptor is not open for reading.

An exclusive process-owned lock shall prevent any other process from setting a shared or exclusive process-owned lock, and shall prevent any shared or exclusive OFD-owned lock from being set, on any portion of the protected area. An exclusive OFD-owned lock shall prevent any other open file description from being used to set a shared or exclusive OFD-owned lock, and shall prevent any shared or exclusive process-owned lock from being set, on any portion of the protected area. A request for an exclusive lock shall fail if the file descriptor is not open for writing.