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159 lines
8.3 KiB
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<!DOCTYPE HTML PUBLIC "-//W3O//DTD W3 HTML 2.0//EN">
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<!Converted with LaTeX2HTML 0.6.5 (Tue Nov 15 1994) by Nikos Drakos (nikos@cbl.leeds.ac.uk), CBLU, University of Leeds >
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<HEAD>
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<TITLE>21.1. Standard Streams</TITLE>
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<BODY>
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<meta name="description" value=" Standard Streams">
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<meta name="keywords" value="clm">
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<meta name="resource-type" value="document">
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<meta name="distribution" value="global">
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<P>
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<b>Common Lisp the Language, 2nd Edition</b>
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<BR> <HR><A NAME=tex2html3812 HREF="node184.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html3810 HREF="node182.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html3804 HREF="node182.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html3814 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html3815 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html3813 HREF="node184.html"> Creating New Streams</A>
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<B>Up:</B> <A NAME=tex2html3811 HREF="node182.html"> Streams</A>
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<B> Previous:</B> <A NAME=tex2html3805 HREF="node182.html"> Streams</A>
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<HR> <P>
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<H1><A NAME=SECTION002510000000000000000>21.1. Standard Streams</A></H1>
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<P>
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There are several variables whose values are streams used by many
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functions in the Lisp system. These variables and their uses are
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listed here. By convention, variables that are expected to hold a
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stream capable of input have names ending with <tt>-input</tt>, and
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variables that are expected to hold a
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stream capable of output have names ending with <tt>-output</tt>.
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Variables expected to hold a
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bidirectional stream have names ending with <tt>-io</tt>.
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<P>
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<BR><b>[Variable]</b><BR>
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<tt>*standard-input*</tt><P>In the normal Lisp top-level loop, input is read from
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<tt>*standard-input*</tt> (that is, whatever stream is the value of the global
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variable <tt>*standard-input*</tt>). Many input functions, including
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<tt>read</tt> and <tt>read-char</tt>, take a stream argument that defaults to
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<tt>*standard-input*</tt>.
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<P>
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<BR><b>[Variable]</b><BR>
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<tt>*standard-output*</tt><P>In the normal Lisp top-level loop, output is sent to
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<tt>*standard-output*</tt> (that is, whatever stream is the value of the global
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variable <tt>*standard-output*</tt>). Many output functions, including
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<tt>print</tt> and <tt>write-char</tt>, take a stream argument that defaults
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to <tt>*standard-output*</tt>.
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<P>
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<BR><b>[Variable]</b><BR>
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<tt>*error-output*</tt><P>The value of <tt>*error-output*</tt> is a stream to which error messages
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should be sent. Normally this is the same as <tt>*standard-output*</tt>,
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but <tt>*standard-output*</tt> might be bound to a file and <tt>*error-output*</tt>
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left going to the terminal or to a separate file of error messages.
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<P>
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<BR><b>[Variable]</b><BR>
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<tt>*query-io*</tt><P>The value of <tt>*query-io*</tt> is a stream to be used when
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asking questions of the user. The question should be output to this
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stream, and the answer read from it. When
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the normal input to a program may be coming from a file, questions such
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as ``Do you really want to delete all of the files in your directory?'' should
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nevertheless
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be sent directly to the user; and the answer should come from the user,
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not from the data file. For such purposes <tt>*query-io*</tt> should be
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used instead of <tt>*standard-input*</tt> and <tt>*standard-output*</tt>.
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<tt>*query-io*</tt> is used by such functions
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as <tt>yes-or-no-p</tt>.
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<P>
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<BR><b>[Variable]</b><BR>
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<tt>*debug-io*</tt><P>The value of <tt>*debug-io*</tt> is a stream to be used for interactive
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debugging purposes. This is often the same as the value of <tt>*query-io*</tt>,
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but need not be.
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<P>
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<BR><b>[Variable]</b><BR>
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<tt>*terminal-io*</tt><P>The value of <tt>*terminal-io*</tt> is ordinarily
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the stream that connects to the user's console.
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Typically, writing to this stream would cause the output to appear
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on a display screen, for example, and reading from the stream would
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accept input from a keyboard.
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<P>
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It is intended
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that standard input functions such as <tt>read</tt> and <tt>read-char</tt>,
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when used with this stream, would cause ``echoing'' of the input
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into the output side of the stream. (The means by which this is
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accomplished are of course highly implementation-dependent.)
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<P>
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<BR><b>[Variable]</b><BR>
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<tt>*trace-output*</tt><P>The value of <tt>*trace-output*</tt> is the stream on which the <tt>trace</tt>
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function prints its output.
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<P>
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The variables
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<tt>*standard-input*</tt>, <tt>*standard-output*</tt>,
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<tt>*error-output*</tt>,
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<tt>*trace-output*</tt>,
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<tt>*query-io*</tt>, and <tt>*debug-io*</tt>
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are initially bound to synonym streams that pass all
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operations on to the stream that is the value of <tt>*terminal-io*</tt>.
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(See <tt>make-synonym-stream</tt>.)
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Thus any operations performed on those streams will go to the terminal.
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<P>
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<img align=bottom alt="change_begin" src="gif/change_begin.gif"><br>
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X3J13 voted in January 1989
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(STANDARD-INPUT-INITIAL-BINDING) <A NAME=19383> </A>
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to replace the requirements of the preceding
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paragraph with the following new requirements:
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<P>
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The seven standard stream variables,
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<tt>*standard-input*</tt>, <tt>*standard-output*</tt>, <tt>*query-io*</tt>,
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<tt>*debug-io*</tt>, <tt>*terminal-io*</tt>,
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<tt>*error-output*</tt>, and
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<tt>*trace-output*</tt>,
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are initially bound to open streams. (These will be called
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<i>the standard initial streams</i>.)
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<P>
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The streams that are the initial values of
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<tt>*standard-input*</tt>, <tt>*query-io*</tt>, <tt>*debug-io*</tt>, and <tt>*terminal-io*</tt>
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must support input.
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<P>
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The streams that are the initial values of
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<tt>*standard-output*</tt>,
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<tt>*error-output*</tt>,
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<tt>*trace-output*</tt>, <tt>*query-io*</tt>, <tt>*debug-io*</tt>, and <tt>*terminal-io*</tt>
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must support output.
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<P>
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None of the standard initial streams (including the one to which
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<tt>*terminal-io*</tt> is initially bound) may be a synonym, either directly
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or indirectly, for any of the standard stream variables
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except <tt>*terminal-io*</tt>. For example, the initial value of
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<tt>*trace-output*</tt> may be a synonym stream for <tt>*terminal-io*</tt>
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but not a synonym stream for <tt>*standard-output*</tt> or <tt>*query-io*</tt>.
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(These are examples of direct synonyms.) As another example,
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<tt>*query-io*</tt> may be a two-way stream or echo stream whose
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input component is a synonym for <tt>*terminal-io*</tt>,
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but its input component may not be a synonym for <tt>*standard-input*</tt>
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or <tt>*debug-io*</tt>. (These are examples of indirect synonyms.)
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<P>
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Any or all of the standard initial streams may be direct or indirect
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synonyms for one or more common implementation-dependent streams.
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For example, the standard initial streams might all be synonym streams
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(or two-way or echo streams whose components are synonym streams)
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to a pair of hidden terminal input and output streams maintained by
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the implementation.
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<P>
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Part of the intent of these rules is to ensure that it is always safe
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to bind any standard stream variable to the value of any other
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standard stream variable (that is, unworkable circularities are
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avoided) without unduly restricting implementation flexibility.
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<br><img align=bottom alt="change_end" src="gif/change_end.gif">
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<P>
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No user program should ever change the value of <tt>*terminal-io*</tt>. A program
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that wants (for example) to divert output to a file should do so by binding
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the value of <tt>*standard-output*</tt>; that way error messages sent to
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<tt>*error-output*</tt> can still get to the user by going through <tt>*terminal-io*</tt>,
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which is usually what is desired.
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<P>
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<BR> <HR><A NAME=tex2html3812 HREF="node184.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html3810 HREF="node182.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html3804 HREF="node182.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html3814 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html3815 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html3813 HREF="node184.html"> Creating New Streams</A>
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<B>Up:</B> <A NAME=tex2html3811 HREF="node182.html"> Streams</A>
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<B> Previous:</B> <A NAME=tex2html3805 HREF="node182.html"> Streams</A>
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<HR> <P>
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<HR>
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<P><ADDRESS>
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AI.Repository@cs.cmu.edu
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</ADDRESS>
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