freebsd/share/man/man9/vnode.9

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.Dd May 20, 2003
.Os
.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
The vnode is the focus of all file activity in
.Ux .
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.
.Pp
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
and
.Xr vput 9 .
The second is the number of clients within the kernel who veto
the recycling of this vnode.
This count is
maintained by
.Xr vhold 9
and
.Xr vdrop 9 .
When both the
.Va v_usecount
and the
.Va 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
.Xr getnewvnode 9 ,
.Xr vfree 9
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.
Advisory locking will not work on this.
.It Dv VFIFO
A FIFO (named pipe).
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 .
.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 .