vnode(9)

NAME

vnode - internal representation of a file or directory

SYNOPSIS

#include <sys/param.h>
#include <sys/vnode.h>

DESCRIPTION

The vnode is the focus of all file activity in UNIX. A vn

ode is
described by struct vnode. There is a unique vnode allocat

ed 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 transi

tion 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 vnode's functionality.

VNODE TYPES

VNON No type.

VREG A regular file; may be with or without VM object

backing. If you

want to make sure this get a backing object, call
vfs_object_create(9).

VDIR A directory.

VBLK A block device; may be with or without VM object

backing. If you

want to make sure this get a backing object, call
vfs_object_create(9).

VCHR A character device.

VLNK A symbolic link.

VSOCK A socket. Advisory locking will not work on this.

VFIFO A FIFO (named pipe). Advisory locking will not work

on this.

VBAD An old style bad sector map

IMPLEMENTATION NOTES

VFIFO uses the "struct fileops" from /sys/kern/sys_pipe.c.

VSOCK uses
the "struct fileops" from /sys/kern/sys_socket.c. Every

thing else uses
the one from /sys/kern/vfs_vnops.c.

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.

Calls to malloc(9) or free(9) when holding a vnode inter

lock, will cause
a LOR (Lock Order Reversal) due to the intertwining of VM

Objects and
Vnodes.

SEE ALSO

malloc(9), VFS(9)

AUTHORS

This manual page was written by Doug Rabson.

BSD May 20, 2003

BSD