A file is opened with Share Reservations in order to prevent other programs from later opening the file in ways that conflict with these reservations. Share Reservations are expressed in terms of denying read or write access by using two bits in the open_flags that are passed on vn_open: O_DENYRD=0x00020000 and O_DENYWRT=0x00010000.

If a file is opened with an access intent of read or write that conflicts with an already established Share Reservation, the open is rejected with EBUSY. Conversely, if a file is already opened for an access intent that another open is trying to deny, the later open fails with EBUSY.

See v_open (BPX1VOP, BPX4VOP) — Open or create a file for information about Share Reservations from the NFS Server's point of view.

A PFS indicates that it supports Share Reservations by setting the pfsi_sharessupported flag as part of its output from PFS initialization. The access and deny modes are always passed to the PFS on vn_open, but when the PFS supports Share Reservations the LFS does not monitor or enforce the reservations.