The vnode is the focus of all file activity in Unix . A vnode is described by
.Vt "struct vnode" . There is a unique vnode allocated for each active file, each current directory, each mounted-on file, text file, and the root.
Each vnode has three reference counts, v_usecount, v_holdcnt and v_writecount. The first is the number of clients within the kernel which are using this vnode. This count is maintained by vref(9), vrele(9) and vput(9). The second is the number of clients within the kernel who veto the recycling of this vnode. This count is maintained by vhold(9) and vdrop(9). When both the v_usecount and the v_holdcnt of a vnode reaches zero then the vnode will be put on the freelist and may be reused for another file, possibly in another file system. The transition to and from the freelist is handled by getnewvnode(9), vfree(9) and vbusy(9). The third is a count of the number of clients which are writing into the file. It is maintained by the open(2) and close(2) system calls.
Any call which returns a vnode (e.g. vget(9), VOP_LOOKUP(9) etc.) will increase the v_usecount of the vnode by one. When the caller is finished with the vnode, it should release this reference by calling vrele(9) (or vput(9) if the vnode is locked).
Other commonly used members of the vnode structure are v_id which is used to maintain consistency in the name cache, v_mount which points at the file system which owns the vnode, v_type which contains the type of object the vnode represents and v_data which is used by file systems to store file system specific data with the vnode. The v_op field is used by the VOP_* macros to call functions in the file system which implement the vnodes functionality.