1.\" $NetBSD: vfs_hooks.9,v 1.3 2006/01/16 22:05:21 yamt Exp $ 2.\" 3.\" Copyright (c) 2005 The NetBSD Foundation, Inc. 4.\" All rights reserved. 5.\" 6.\" This code is derived from software contributed to The NetBSD Foundation 7.\" by Julio M. Merino Vidal. 8.\" 9.\" Redistribution and use in source and binary forms, with or without 10.\" modification, are permitted provided that the following conditions 11.\" are met: 12.\" 1. Redistributions of source code must retain the above copyright 13.\" notice, this list of conditions and the following disclaimer. 14.\" 2. Redistributions in binary form must reproduce the above copyright 15.\" notice, this list of conditions and the following disclaimer in the 16.\" documentation and/or other materials provided with the distribution. 17.\" 3. All advertising materials mentioning features or use of this software 18.\" must display the following acknowledgement: 19.\" This product includes software developed by the NetBSD 20.\" Foundation, Inc. and its contributors. 21.\" 4. Neither the name of The NetBSD Foundation nor the names of its 22.\" contributors may be used to endorse or promote products derived 23.\" from this software without specific prior written permission. 24.\" 25.\" THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. AND CONTRIBUTORS 26.\" ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED 27.\" TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR 28.\" PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE FOUNDATION OR CONTRIBUTORS 29.\" BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR 30.\" CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF 31.\" SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS 32.\" INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN 33.\" CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) 34.\" ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE 35.\" POSSIBILITY OF SUCH DAMAGE. 36.\" 37.Dd September 23, 2005 38.Dt VFS_HOOKS 9 39.Os 40.Sh NAME 41.Nm vfs_hooks , 42.Nm vfs_hooks_unmount 43.Nd VFS hooks interface 44.Sh SYNOPSIS 45.In sys/param.h 46.In sys/mount.h 47.Ft void 48.Fn vfs_hooks_unmount "struct mount *mp" 49.Sh DESCRIPTION 50The VFS hooks interface provides a way for different kernel subsystems to 51attach custom functions to specific VFS operations. 52This enforces code separation by keeping the VFS's core sources uncluttered 53and makes all subsystem functionality reside in a single place. 54As an example, this interface is used by the NFS server code to automatically 55handle the exports list for each mount point. 56.Pp 57Hooks are described by a 58.Ft struct vfs_hooks 59object, as seen below: 60.Bd -literal 61struct vfs_hooks { 62 int (*vh_unmount)(struct mount *); 63}; 64.Ed 65.Pp 66For simplicity, each field is named after the VFS operation it refers to. 67The purpose of each member function, alongside some important notes, is 68shown below: 69.Bl -tag -width compat 70.It Fn vh_unmount "mp" 71This hook is executed during the unmount process of a file system. 72.El 73.Pp 74For more information about the purpose of each operation, see 75.Xr vfsops 9 . 76Note that any of these fields may be a null pointer. 77.Pp 78After the definition of a 79.Ft struct vfs_hooks 80object, the kernel has to add it to the 81.Va vfs_hooks 82link set using the 83.Fn VFS_HOOKS_ATTACH "struct vfs_hooks *" 84macro. 85.Pp 86Please note that this interface is incomplete on purpose to keep it in its 87smallest possible size (i.e., do not provide a hook that is not used). 88If you feel the need to hook a routine to a VFS operation that is not yet 89supported by this interface, just add it to the files described in 90.Sx CODE REFERENCES . 91.Sh FUNCTIONS 92The following functions are provided to the VFS code to run the hooked 93functions: 94.Bl -tag -width compact 95.It Fn vfs_hooks_unmount "mp" 96Runs all hooks for the VFS unmount operation. 97Given that these operations shall not fail, it returns 98.Ft void . 99.El 100.Sh CODE REFERENCES 101The VFS hooks interface is implemented within the files 102.Pa sys/kern/vfs_hooks.c 103and 104.Pa sys/sys/mount.h . 105.Sh SEE ALSO 106.Xr intro 9 , 107.Xr vfs 9 , 108.Xr vfsops 9 109.Sh HISTORY 110The VFS hooks interface appeared in 111.Nx 4.0 . 112