/*
 * Abstract operations on a vnode.
 *
 * These are used in the form VOP_FOO(vnode, args), which are macros
 * that expands to vnode->vn_ops->vop_foo(vnode, args). The operations
 * "foo" are:
 *
 *    vop_eachopen    - Called on *each* open() of a file. Can be used to
 *                      reject illegal or undesired open modes. Note that
 *                      various operations can be performed without the
 *                      file actually being opened.
 *                      The vnode need not look at O_CREAT, O_EXCL, or
 *                      O_TRUNC, as these are handled in the VFS layer.
 *
 *                      VOP_EACHOPEN should not be called directly from
 *                      above the VFS layer - use vfs_open() to open vnodes.
 *
 *    vop_reclaim     - Called when vnode is no longer in use.
 *
 *****************************************
 *
 *    vop_read        - Read data from file to uio, at offset specified
 *                      in the uio, updating uio_resid to reflect the
 *                      amount read, and updating uio_offset to match.
 *                      Not allowed on directories or symlinks.
 *
 *    vop_readlink    - Read the contents of a symlink into a uio.
 *                      Not allowed on other types of object.
 *
 *    vop_getdirentry - Read a single filename from a directory into a
 *                      uio, choosing what name based on the offset
 *                      field in the uio, and updating that field.
 *                      Unlike with I/O on regular files, the value of
 *                      the offset field is not interpreted outside
 *                      the filesystem and thus need not be a byte
 *                      count. However, the uio_resid field should be
 *                      handled in the normal fashion.
 *                      On non-directory objects, return ENOTDIR.
 *
 *    vop_write       - Write data from uio to file at offset specified
 *                      in the uio, updating uio_resid to reflect the
 *                      amount written, and updating uio_offset to match.
 *                      Not allowed on directories or symlinks.
 *
 *    vop_ioctl       - Perform ioctl operation OP on file using data
 *                      DATA. The interpretation of the data is specific
 *                      to each ioctl.
 *
 *    vop_stat        - Return info about a file. The pointer is a
 *                      pointer to struct stat; see kern/stat.h.
 *
 *    vop_gettype     - Return type of file. The values for file types
 *                      are in kern/stattypes.h.
 *
 *    vop_isseekable  - Check if this file is seekable. All regular files
 *                      and directories are seekable, but some devices are
 *                      not.
 *
 *    vop_fsync       - Force any dirty buffers associated with this file
 *                      to stable storage.
 *
 *    vop_mmap        - Map file into memory. If you implement this
 *                      feature, you're responsible for choosing the
 *                      arguments for this operation.
 *
 *    vop_truncate    - Forcibly set size of file to the length passed
 *                      in, discarding any excess blocks.
 *
 *    vop_namefile    - Compute pathname relative to filesystem root
 *                      of the file and copy to the specified
 *                      uio. Need not work on objects that are not
 *                      directories.
 *
 *****************************************
 *
 *    vop_creat       - Create a regular file named NAME in the passed
 *                      directory DIR. If boolean EXCL is true, fail if
 *                      the file already exists; otherwise, use the
 *                      existing file if there is one. Hand back the
 *                      vnode for the file as per vop_lookup.
 *
 *    vop_symlink     - Create symlink named NAME in the passed directory,
 *                      with contents CONTENTS.
 *
 *    vop_mkdir       - Make directory NAME in the passed directory PARENTDIR.
 *
 *    vop_link        - Create hard link, with name NAME, to file FILE
 *                      in the passed directory DIR.
 *
 *    vop_remove      - Delete non-directory object NAME from passed
 *                      directory. If NAME refers to a directory,
 *                      return EISDIR. If passed vnode is not a
 *                      directory, return ENOTDIR.
 *
 *    vop_rmdir       - Delete directory object NAME from passed
 *                      directory.
 *
 *    vop_rename      - Rename file NAME1 in directory VN1 to be
 *                      file NAME2 in directory VN2.
 *
 *****************************************
 *
 *    vop_lookup      - Parse PATHNAME relative to the passed directory
 *                      DIR, and hand back the vnode for the file it
 *                      refers to. May destroy PATHNAME. Should increment
 *                      refcount on vnode handed back.
 *
 *    vop_lookparent  - Parse PATHNAME relative to the passed directory
 *                      DIR, and hand back (1) the vnode for the
 *                      parent directory of the file it refers to, and
 *                      (2) the last component of the filename, copied
 *                      into kernel buffer BUF with max length LEN. May
 *                      destroy PATHNAME. Should increment refcount on
 *                      vnode handed back.
 */