dpans/concept-files.tex

% -*- Mode: TeX -*-

%% 23.0.0 6 was left out.

This section describes the \clisp\ interface to file systems.
%% 23.0.0 5
The model used by this interface assumes 
     that \newtermidx{files}{file} are named by \newtermidx{filenames}{filename},
     that a \term{filename} can be represented by a \term{pathname} \term{object}, 
 and that given a \term{pathname} a \newterm{stream} can be constructed 
      that connects to a \term{file} whose \term{filename} it represents.

For information about opening and closing \term{files},
and manipulating their contents, \seechapter\Streams.

\Thenextfigure\ lists some \term{operators} 
that are applicable to \term{files} and directories.

\displaythree{File and Directory Operations}{
compile-file&file-length&open\cr
delete-file&file-position&probe-file\cr
directory&file-write-date&rename-file\cr
file-author&load&with-open-file\cr
}

\beginsubsection{Coercion of Streams to Pathnames}
\DefineSection{StreamsToPathnames}

A \newterm{stream associated with a file} is either a \term{file stream}
or a \term{synonym stream} whose target is a \newterm{stream associated with a file}.
Such streams can be used as \term{pathname designators}.

Normally, when a \term{stream associated with a file} is used as a
\term{pathname designator}, it denotes the \term{pathname} used to 
open the \term{file}; this may be, but is not required to be, the
actual name of the \term{file}.

Some functions, such as \funref{truename} and \funref{delete-file},
coerce \term{streams} to \term{pathnames} in a different way that 
involves referring to the actual \term{file} that is open, which might
or might not be the file whose name was opened originally.  Such special
situations are always notated specifically and are not the default.

\endsubsection%{Coercion of Streams to Pathnames}

\beginsubsection{File Operations on Open and Closed Streams}
\DefineSection{OpenAndClosedStreams}

\issue{CLOSED-STREAM-FUNCTIONS:ALLOW-INQUIRY}

Many \term{functions} that perform \term{file} operations accept either
\term{open} or \term{closed} \term{streams} as \term{arguments};
\seesection\StreamArgsToStandardizedFns.

Of these, the \term{functions} in \thenextfigure\ treat \term{open} and 
\term{closed} \term{streams} differently.

\displaythree{File Functions that Treat Open and Closed Streams Differently}{
delete-file&file-author&probe-file\cr
directory&file-write-date&truename\cr
}

Since treatment of \term{open} \term{streams} by the \term{file system} 
may vary considerably between \term{implementations}, however, 
a \term{closed} \term{stream} might be the most reliable kind of
\term{argument} for some of these functions---in particular, those in
\thenextfigure.  For example, in some \term{file systems}, 
\term{open} \term{files} are written under temporary names 
and not renamed until \term{closed}
and/or are held invisible until \term{closed}.
In general, any code that is intended to be portable should
use such \term{functions} carefully.

\displaythree{File Functions where Closed Streams Might Work Best}{
directory&probe-file&truename\cr
}
\endissue{CLOSED-STREAM-FUNCTIONS:ALLOW-INQUIRY}

\endsubsection%{File Operations on Open and Closed Streams}

\beginsubsection{Truenames}
\DefineSection{Truenames}

Many \term{file systems} permit more than one \term{filename} to designate 
a particular \term{file}.

Even where multiple names are possible, most \term{file systems} have a convention
for generating a canonical \term{filename} in such situations.  Such a canonical
\term{filename} (or the \term{pathname} representing such a \term{filename}) is
called a \newterm{truename}.  

%This came from the CLtL description of PROBE-FILE
The \term{truename} of a \term{file} may differ from other \term{filenames}
for the file because of
     symbolic links,
     version numbers,
% Added to distinguish from Sandra's addition that follows. -kmp 12-Dec-91
     logical device translations in the \term{file system},
% Added per Sandra. -kmp 12-Dec-91
     \term{logical pathname} translations within \clisp,
  or other artifacts of the \term{file system}.

The \term{truename} for a \term{file} is often, but not necessarily, unique for
each \term{file}.  For instance, a Unix \term{file} with multiple hard links 
could have several \term{truenames}.

\beginsubsubsection{Examples of Truenames}

For example, a DEC TOPS-20 system with \term{files} \f{PS:<JOE>FOO.TXT.1} 
and \f{PS:<JOE>FOO.TXT.2} might permit the second \term{file} to be referred
to as \f{PS:<JOE>FOO.TXT.0}, since the ``\f{.0}'' notation denotes ``newest''
version of several \term{files}.
In the same \term{file system}, a ``logical device'' ``\f{JOE:}'' might be 
taken to refer to \f{PS:<JOE>}'' and so the names \f{JOE:FOO.TXT.2} or
\f{JOE:FOO.TXT.0} might refer to \f{PS:<JOE>FOO.TXT.2}.
In all of these cases, the \term{truename} of the file would probably be
\f{PS:<JOE>FOO.TXT.2}.

If a \term{file} is a symbolic link to another \term{file} (in a \term{file system}
permitting such a thing), it is conventional for the \term{truename} to be
the canonical name of the \term{file} after any symbolic links have been followed;
that is, it is the canonical name of the \term{file} whose contents would
become available if an \term{input} \term{stream} to that \term{file} were 
opened.

In the case of a \term{file} still being created (that is, of an \term{output}
\term{stream} open to such a \term{file}), the exact \term{truename} of the file
might not be known until the \term{stream} is closed.  In this case, 
\thefunction{truename} might return different values for such a \term{stream}
before and after it was closed.  In fact, before it is closed, the name returned
might not even be a valid name in the \term{file system}---for example, while a
file is being written, it might have version \kwdref{newest} and might only take on 
a specific numeric value later when the file is closed even in a \term{file system}
where all files have numeric versions.

\endsubsubsection%{Examples of Truenames}

\endsubsection%{Truenames}