VNODE(9)						   BSD Kernel Developer's Manual						  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 vnode is described by struct vnode.  There is a unique vnode allocated 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 transition
     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 writ-
     ing 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 fin-
     ished 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   Indicates that the vnode has been reclaimed.

IMPLEMENTATION NOTES

     VFIFO uses the "struct fileops" from /sys/kern/sys_pipe.c.  VSOCK uses the "struct fileops" from /sys/kern/sys_socket.c.  Everything 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 ker-
     nel.

     Calls to malloc(9) or free(9) when holding a vnode interlock, will cause a LOR (Lock Order Reversal) due to the intertwining of VM Objects
     and Vnodes.

SEE ALSO

     malloc(9), VOP_ACCESS(9), VOP_ACLCHECK(9), VOP_ADVLOCK(9), VOP_ATTRIB(9), VOP_BWRITE(9), VOP_CREATE(9), VOP_FSYNC(9), VOP_GETACL(9),
     VOP_GETEXTATTR(9), VOP_GETPAGES(9), VOP_GETVOBJECT(9), VOP_INACTIVE(9), VOP_IOCTL(9), VOP_LINK(9), VOP_LISTEXTATTR(9), VOP_LOCK(9),
     VOP_LOOKUP(9), VOP_OPENCLOSE(9), VOP_PATHCONF(9), VOP_PRINT(9), VOP_RDWR(9), VOP_READDIR(9), VOP_READLINK(9), VOP_REALLOCBLKS(9),
     VOP_REMOVE(9), VOP_RENAME(9), VOP_REVOKE(9), VOP_SETACL(9), VOP_SETEXTATTR(9), VOP_STRATEGY(9), VOP_VPTOCNP(9), VOP_VPTOFH(9), VFS(9)

AUTHORS

     This manual page was written by Doug Rabson.

BSD
								 February 13, 2010							       BSD