freebsd-dev/share/man/man9/vnode.9

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.\" Copyright (c) 1996 Doug Rabson
.\"
.\" All rights reserved.
.\"
.\" This program is free software.
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.\" $FreeBSD$
.\"
.Dd May 20, 2003
.Os
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.Dt VNODE 9
.Sh NAME
.Nm vnode
.Nd internal representation of a file or directory
.Sh SYNOPSIS
.In sys/param.h
.In sys/vnode.h
.Sh DESCRIPTION
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The vnode is the focus of all file activity in
.Ux .
A vnode is described by
.Vt "struct vnode" .
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There is a
unique vnode allocated for each active file, each current directory,
each mounted-on file, text file, and the root.
.Pp
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Each vnode has three reference counts,
.Va v_usecount ,
.Va v_holdcnt
and
.Va v_writecount .
The first is the number of clients within the kernel which are
using this vnode.
This count is maintained by
.Xr vref 9 ,
.Xr vrele 9
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and
.Xr vput 9 .
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The second is the number of clients within the kernel who veto
the recycling of this vnode.
This count is
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maintained by
.Xr vhold 9
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and
.Xr vdrop 9 .
When both the
.Va v_usecount
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and the
.Va v_holdcnt
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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.
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The transition to and from the freelist is handled by
.Xr getnewvnode 9 ,
.Xr vfree 9
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and
.Xr vbusy 9 .
The third is a count of the number of clients which are writing into
the file.
It is maintained by the
.Xr open 2
and
.Xr close 2
system calls.
.Pp
Any call which returns a vnode (e.g.\&
.Xr vget 9 ,
.Xr VOP_LOOKUP 9
etc.)
will increase the
.Va v_usecount
of the vnode by one.
When the caller is finished with the vnode, it
should release this reference by calling
.Xr vrele 9
(or
.Xr vput 9
if the vnode is locked).
.Pp
Other commonly used members of the vnode structure are
.Va v_id
which is used to maintain consistency in the name cache,
.Va v_mount
which points at the file system which owns the vnode,
.Va v_type
which contains the type of object the vnode represents and
.Va v_data
which is used by file systems to store file system specific data with
the vnode.
The
.Va v_op
field is used by the
.Dv VOP_*
macros to call functions in the file system which implement the vnode's
functionality.
.Sh VNODE TYPES
.Bl -tag -width VSOCK
.It Dv VNON
No type.
.It Dv VREG
A regular file; may be with or without VM object backing.
If you want to make sure this get a backing object, call
.Xr vfs_object_create 9 .
.It Dv VDIR
A directory.
.It Dv VBLK
A block device; may be with or without VM object backing.
If you want to make sure this get a backing object, call
.Xr vfs_object_create 9 .
.It Dv VCHR
A character device.
.It Dv VLNK
A symbolic link.
.It Dv VSOCK
A socket.
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Advisory locking will not work on this.
.It Dv VFIFO
A FIFO (named pipe).
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Advisory locking will not work on this.
.It Dv VBAD
An old style bad sector map
.El
.Sh IMPLEMENTATION NOTES
VFIFO uses the "struct fileops" from
.Pa /sys/kern/sys_pipe.c .
VSOCK uses the "struct fileops" from
.Pa /sys/kern/sys_socket.c .
Everything else uses the one from
.Pa /sys/kern/vfs_vnops.c .
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.Pp
The VFIFO/VSOCK code, which is why "struct fileops" is used at all, is
an artifact of an incomplete integration of the VFS code into the
kernel.
.Pp
Calls to
.Xr malloc 9
or
.Xr free 9
when holding a
.Nm
interlock, will cause a LOR (Lock Order Reversal) due to the
intertwining of VM Objects and Vnodes.
.Sh SEE ALSO
.Xr malloc 9 ,
.Xr VFS 9
.Sh AUTHORS
This manual page was written by
.An Doug Rabson .