8.4 struct vnodeops

An instance of the vnode operations structure (struct vnodeops, listed in Figure fig-vnodeops-h) exists for each different type of file system. For each vnode, the vnode field v_op is set to the pointer of the operations vector of the underlying file system.

  

Figure: SunOS 5.x Vnode Operations Interface

Figure: SunOS 5.x Vnode Operations Interface

typedef struct vnodeops {
  int     (*vop_open)();
  int     (*vop_close)();
  int     (*vop_read)();
  int     (*vop_write)();
  int     (*vop_ioctl)();
  int     (*vop_setfl)();
  int     (*vop_getattr)();
  int     (*vop_setattr)();
  int     (*vop_access)();
  int     (*vop_lookup)();
  int     (*vop_create)();
  int     (*vop_remove)();
  int     (*vop_link)();
  int     (*vop_rename)();
  int     (*vop_mkdir)();
  int     (*vop_rmdir)();
  int     (*vop_readdir)();
  int     (*vop_symlink)();
  int     (*vop_readlink)();
  int     (*vop_fsync)();
  void    (*vop_inactive)();


  int     (*vop_fid)();
  void    (*vop_rwlock)();
  void    (*vop_rwunlock)();
  int     (*vop_seek)();
  int     (*vop_cmp)();
  int     (*vop_frlock)();
  int     (*vop_space)();
  int     (*vop_realvp)();
  int     (*vop_getpage)();
  int     (*vop_putpage)();
  int     (*vop_map)();
  int     (*vop_addmap)();
  int     (*vop_delmap)();
  int     (*vop_poll)();
  int     (*vop_dump)();
  int     (*vop_pathconf)();
  int     (*vop_pageio)();
  int     (*vop_dumpctl)();
  void    (*vop_dispose)();
  int     (*vop_setsecattr)();
  int     (*vop_getsecattr)();
} vnodeops_t;

4.5in

typedef struct vnodeops {
  int     (*vop_open)();
  int     (*vop_close)();
  int     (*vop_read)();
  int     (*vop_write)();
  int     (*vop_ioctl)();
  int     (*vop_setfl)();
  int     (*vop_getattr)();
  int     (*vop_setattr)();
  int     (*vop_access)();
  int     (*vop_lookup)();
  int     (*vop_create)();
  int     (*vop_remove)();
  int     (*vop_link)();
  int     (*vop_rename)();
  int     (*vop_mkdir)();
  int     (*vop_rmdir)();
  int     (*vop_readdir)();
  int     (*vop_symlink)();
  int     (*vop_readlink)();
  int     (*vop_fsync)();
  void    (*vop_inactive)();


  int     (*vop_fid)();
  void    (*vop_rwlock)();
  void    (*vop_rwunlock)();
  int     (*vop_seek)();
  int     (*vop_cmp)();
  int     (*vop_frlock)();
  int     (*vop_space)();
  int     (*vop_realvp)();
  int     (*vop_getpage)();
  int     (*vop_putpage)();
  int     (*vop_map)();
  int     (*vop_addmap)();
  int     (*vop_delmap)();
  int     (*vop_poll)();
  int     (*vop_dump)();
  int     (*vop_pathconf)();
  int     (*vop_pageio)();
  int     (*vop_dumpctl)();
  void    (*vop_dispose)();
  int     (*vop_setsecattr)();
  int     (*vop_getsecattr)();
} vnodeops_t;

Each field of the structure is assigned a pointer to a function that implements a particular operation on the file system in question:

• vop_open opens the requested file and returns a new vnode for it.

• vop_close closes a file.

• vop_read reads data from the opened file.

• vop_write writes data to the file.

• vop_ioctl performs miscellaneous I/O control operations on the file, such as setting non-blocking I/O access.

• vop_setfl is used to set arbitrary file flags.

• vop_getattr gets the attributes of a file, such as the mode bits, user and group ownership, etc.

• vop_setattr sets the attributes of a file.

• vop_access checks to see if a particular user, given the user's credentials, is allowed to access a file.

• vop_lookup looks up a directory for a file name. If found, a new vnode is returned.

• vop_create creates a new file.

• vop_remove removes a file from the file system.

• vop_link makes a hard-link to an existing file.

• vop_rename renames a file.

• vop_mkdir makes a new directory.

• vop_rmdir removes an existing directory.

• vop_readdir reads a directory for entries within.

• vop_symlink creates a symbolic-link to a file.

• vop_readlink reads the value of a symbolic link, that is, what the link points to.

• vop_fsync writes out all cached information for a file.

• vop_inactive signifies to the vnode layer that this file is no longer in use, that all its references had been released, and that it can now we deallocated.

• vop_fid returns a unique file identifier fid for a vnode. This call works in conjunction with the vfs operation vfs_vget described in Appendix section sec-appendix-vnode-vfsops.

• vop_rwlock locks a file before attempting to read from or write to it.

• vop_rwunlock unlocks a file after having read from or wrote to it.

• vop_seek sets the read/write head to a particular point within a file, so the next read/write call can work from that location in the file.

• vop_cmp compares two vnodes and returns true/false.

• vop_frlock perform file and record locking on a file.

• vop_space frees any storage space associated with this file.

• vop_realvp for certain file systems, returns the ``real'' vnode. This is useful in stackable vnodes, where a higher layer may request the real/hidden vnode underneath, so it can operate on it.

• vop_getpage reads a page of a memory-mapped file.

• vop_putpage writes to a page of a memory-mapped file.

• vop_map maps a file into memory. See [Gingell87a,Gingell87b] for more details.

• vop_addmap adds more pages to a memory-mapped file.

• vop_delmap removes some pages from a memory-mapped file.

• vop_poll polls for events on the file. This is mostly useful when the vnode is of type ``socket'' or ``fifo,'' and replaces the older vop_select vnode operation. This operation is often used to implement the select(2) system call.

• vop_dump dumps the state of the kernel (memory buffers, tables, variables, registers, etc.) to a given vnode, usually a swap-device. This is used as the last action performed when a kernel panics and needs to save state for post-mortem recovery by tools such as crash [SMCC95].

• vop_pathconf supports the POSIX path configuration standard. This call returns various configurable file or directory variables.

• vop_pageio performs I/O directly on mapped pages of a file.

• vop_dumpctl works in conjunction with vop_dump. It is used to prepare a file system before a dump operation by storing data structures that might otherwise get corrupted shortly after a panic had occurred, and deallocates these private dump data structures after a successful dump.

• vop_dispose removes a mapped page from memory.

• vop_setsecattr is used to set Access Control Lists (ACLs) on a file.

• vop_getsecattr is used to retrieve the ACLs of a file.

Vnode operations get invoked transparently via macros that dereference the operations vector's field for that operation, and pass along the vnode and the arguments it needs. Each vnode operation has a macro associated with it, located in <sys/vnode.h>. Figure fig-vnode-mac shows as an example, the definitions for some of these calls.

  

Figure: Some Vnode Macros

Figure: Some Vnode Macros

#define VOP_OPEN(vpp, mode, cr)        (*(*(vpp))->v_op->vop_open)(vpp, mode, cr)
#define VOP_CLOSE(vp, f, c, o, cr)     (*(vp)->v_op->vop_close)(vp, f, c, o, cr)
#define VOP_READ(vp, uiop, iof, cr)    (*(vp)->v_op->vop_read)(vp, uiop, iof, cr)
#define VOP_MKDIR(dp, p, vap, vpp, cr) (*(dp)->v_op->vop_mkdir)(dp, p, vap, vpp, cr)
#define VOP_GETATTR(vp, vap, f, cr)    (*(vp)->v_op->vop_getattr)(vp, vap, f, cr)
#define VOP_LOOKUP(vp, cp, vpp, pnp, f, rdir, cr) \
                            (*(vp)->v_op->vop_lookup)(vp, cp, vpp, pnp, f, rdir, cr)
#define VOP_CREATE(dvp, p, vap, ex, mode, vpp, cr) \
                          (*(dvp)->v_op->vop_create)(dvp, p, vap, ex, mode, vpp, cr)

6.3in

#define VOP_OPEN(vpp, mode, cr)        (*(*(vpp))->v_op->vop_open)(vpp, mode, cr)
#define VOP_CLOSE(vp, f, c, o, cr)     (*(vp)->v_op->vop_close)(vp, f, c, o, cr)
#define VOP_READ(vp, uiop, iof, cr)    (*(vp)->v_op->vop_read)(vp, uiop, iof, cr)
#define VOP_MKDIR(dp, p, vap, vpp, cr) (*(dp)->v_op->vop_mkdir)(dp, p, vap, vpp, cr)
#define VOP_GETATTR(vp, vap, f, cr)    (*(vp)->v_op->vop_getattr)(vp, vap, f, cr)
#define VOP_LOOKUP(vp, cp, vpp, pnp, f, rdir, cr) \
                            (*(vp)->v_op->vop_lookup)(vp, cp, vpp, pnp, f, rdir, cr)
#define VOP_CREATE(dvp, p, vap, ex, mode, vpp, cr) \
                          (*(dvp)->v_op->vop_create)(dvp, p, vap, ex, mode, vpp, cr)

When any piece of file system code, that has a handle on a vnode, wants to call a vnode operation on it, it simply dereferences the macro, as depicted in Figure fig-vnode-mac-ex.

  

Figure: Vnode Macros Usage Example

Figure: Vnode Macros Usage Example

int foo(vnode_t *dp, char *name,
        vattr_t *vap, vnode_t **vpp, cred_t *cr)
{
  int error;

  error = VOP_MKDIR(dp, name, vap, vpp, cr);
  if (error)
    return (error);
}

3.5in

int foo(vnode_t *dp, char *name,
        vattr_t *vap, vnode_t **vpp, cred_t *cr)
{
  int error;

  error = VOP_MKDIR(dp, name, vap, vpp, cr);
  if (error)
    return (error);
}