.\"     $NetBSD: vfs_hooks.9,v 1.4 2008/04/30 13:10:59 martin Exp $
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.\" by Julio M. Merino Vidal.
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.Dd September 23, 2005
.Dt VFS_HOOKS 9
.Os
.Sh NAME
.Nm vfs_hooks ,
.Nm vfs_hooks_unmount
.Nd VFS hooks interface
.Sh SYNOPSIS
.In sys/param.h
.In sys/mount.h
.Ft void
.Fn vfs_hooks_unmount "struct mount *mp"
.Sh DESCRIPTION
The VFS hooks interface provides a way for different kernel subsystems to
attach custom functions to specific VFS operations.
This enforces code separation by keeping the VFS's core sources uncluttered
and makes all subsystem functionality reside in a single place.
As an example, this interface is used by the NFS server code to automatically
handle the exports list for each mount point.
.Pp
Hooks are described by a
.Ft struct vfs_hooks
object, as seen below:
.Bd -literal
struct vfs_hooks {
	int	(*vh_unmount)(struct mount *);
};
.Ed
.Pp
For simplicity, each field is named after the VFS operation it refers to.
The purpose of each member function, alongside some important notes, is
shown below:
.Bl -tag -width compat
.It Fn vh_unmount "mp"
This hook is executed during the unmount process of a file system.
.El
.Pp
For more information about the purpose of each operation, see
.Xr vfsops 9 .
Note that any of these fields may be a null pointer.
.Pp
After the definition of a
.Ft struct vfs_hooks
object, the kernel has to add it to the
.Va vfs_hooks
link set using the
.Fn VFS_HOOKS_ATTACH "struct vfs_hooks *"
macro.
.Pp
Please note that this interface is incomplete on purpose to keep it in its
smallest possible size (i.e., do not provide a hook that is not used).
If you feel the need to hook a routine to a VFS operation that is not yet
supported by this interface, just add it to the files described in
.Sx CODE REFERENCES .
.Sh FUNCTIONS
The following functions are provided to the VFS code to run the hooked
functions:
.Bl -tag -width compact
.It Fn vfs_hooks_unmount "mp"
Runs all hooks for the VFS unmount operation.
Given that these operations shall not fail, it returns
.Ft void .
.El
.Sh CODE REFERENCES
The VFS hooks interface is implemented within the files
.Pa sys/kern/vfs_hooks.c
and
.Pa sys/sys/mount.h .
.Sh SEE ALSO
.Xr intro 9 ,
.Xr vfs 9 ,
.Xr vfsops 9
.Sh HISTORY
The VFS hooks interface appeared in
.Nx 4.0 .