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56 lines
2.7 KiB
TeX
56 lines
2.7 KiB
TeX
\chapter{Environments}
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\label{chap-environments}
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Recall that an \emph{environment} is a mapping from \emph{names} to
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\emph{objects}. This mapping consists of a set of \emph{bindings}.
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When a user is created in the system, a \emph{default global
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environment} is created for that user. The global environment of a
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user consists of a \emph{system-wide} environment and a
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\emph{user-specific} environment.
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The system-wide environment consists of bindings that are themselves
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immutable (i.e., the user is not allowed to alter the binding) such as
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the binding of the symbol \texttt{cl:length} to the function that
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returns the length of a sequence. The objects of these bindings are
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also immutable, such as the length function itself. The system-wide
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environment is the same for every user, allowing the installation of
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software that is immediately visible to all users.
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The user-specific environment consists of bindings that are created by
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the user. These bindings are of three different kinds:
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\begin{itemize}
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\item Bindings created by the user for instance as a result of
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executing a \texttt{defparameter} or \texttt{defun} form.
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\item Default system-wide bindings that can be altered by the user,
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such as the value of \texttt{*print-base*}.
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\item Immutable bindings where the \emph{object} can be modified by
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the user, such as system-defined generic functions to which the user
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is allowed to add specific methods. Each user has a private copy of
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such objects.
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\end{itemize}
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When a function or method object is created as a result of calling
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\texttt{compile} on a lambda expression, or as a result of loading a
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\emph{fasl} file, the object is \emph{linked} to the current global
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environment, in that external references are then resolved. When such
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a function or method object is given to a different user, that
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different user can execute it, but external references in it will
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still refer to the environment into which it was compiled or loaded.
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This mechanism provides an efficient method of protection. A user A
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can grant controlled access to part of his or her global environment
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by allowing a user B to execute a function made available to him or
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her through the \emph{object store}. \seechap{chap-object-store}
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In a traditional modern operating system such as Unix, this kind of
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controlled access required the use of the \emph{setuid} mechanism,
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simply because in such a system there is no way to access an object
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other than through the global file system, and the accessing user must
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have the right permissions to access the object.
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The same mechanism can be used by the system itself to protect objects
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that would be unwise to give users direct access to, such as disks or
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printers.
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