Read/Write Slot Protection

You may specify, for each slot in a structure, just which categories of users are allowed to read or write that slot, using a fixed system of four user categories:

root

Users with root privileges on the server. Only root users may restrict root access.

user

The user who created the structure instance holding the slot.

class

The user who created the definition for the structure instance holding the slot -- that is, the user who executed the original ]defstruct.

world

Any user at all.

The point of the class category is to allow the creation of structures which are owned (and counted against the space quota) of a user, but which are maintained by code written by another user.

For example, in a game context it may be desirable for each player to own a per-player state structure containing score and location, but not for each player to be able to randomly update score and location.

As examples, we show first the extreme case of defining a structure type blackhole whose single hadron slot is completely inaccessable (you'll need to be root to have this work properly)

root:
[ 'blackhole
    'hadron
      :rootMayRead    nil
      :rootMayWrite   nil
      :userMayRead    nil
      :userMayWrite   nil
      :classMayRead   nil
      :classMayWrite  nil
      :worldMayRead   nil
      :worldMayWrite  nil
| ]defstruct

and then a structure type blackboard whose single slate slot is both readable and writable by everyone

root:
[ 'blackboard
    'slate
      :worldMayRead   t
      :worldMayWrite  t
| ]defstruct

and finally a structure type spacewar with slots readable by the user but only modifiable by the class:

root:
[ 'spacewar
    'score
      :initval          0
      :userMayRead    t
      :userMayWrite   nil
      :classMayRead   t
      :classMayWrite  t
      :worldMayRead   nil
      :worldMayWrite  nil
    'x-loc
      :initval          0.0
      :userMayRead    t
      :userMayWrite   nil
      :classMayRead   t
      :classMayWrite  t
      :worldMayRead   nil
      :worldMayWrite  nil
    'y-loc
      :initval          0.0
      :userMayRead    t
      :userMayWrite   nil
      :classMayRead   t
      :classMayWrite  t
      :worldMayRead   nil
      :worldMayWrite  nil
| ]defstruct

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