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Common Lisp the Language, 2nd Edition
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There are several variables whose values are streams used by many
functions in the Lisp system. These variables and their uses are listed
here. By convention, variables that are expected to hold a stream
capable of input have names ending with -input, and
variables that are expected to hold a stream capable of output have
names ending with -output. Variables expected to hold a
bidirectional stream have names ending with -io.
[Variable]
*standard-input*
In the normal Lisp top-level loop, input is read from
*standard-input* (that is, whatever stream is the value of
the global variable *standard-input*). Many input
functions, including read and read-char, take
a stream argument that defaults to *standard-input*.
[Variable]
*standard-output*
In the normal Lisp top-level loop, output is sent to
*standard-output* (that is, whatever stream is the value of
the global variable *standard-output*). Many output
functions, including print and write-char,
take a stream argument that defaults to
*standard-output*.
[Variable]
*error-output*
The value of *error-output* is a stream to which error
messages should be sent. Normally this is the same as
*standard-output*, but *standard-output* might
be bound to a file and *error-output* left going to the
terminal or to a separate file of error messages.
[Variable]
*query-io*
The value of *query-io* is a stream to be used when
asking questions of the user. The question should be output to this
stream, and the answer read from it. When the normal input to a program
may be coming from a file, questions such as ``Do you really want to
delete all of the files in your directory?’’ should nevertheless be sent
directly to the user; and the answer should come from the user, not from
the data file. For such purposes *query-io* should be used
instead of *standard-input* and
*standard-output*. *query-io* is used by such
functions as yes-or-no-p.
[Variable]
*debug-io*
The value of *debug-io* is a stream to be used for
interactive debugging purposes. This is often the same as the value of
*query-io*, but need not be.
[Variable]
*terminal-io*
The value of *terminal-io* is ordinarily the stream that
connects to the user’s console. Typically, writing to this stream would
cause the output to appear on a display screen, for example, and reading
from the stream would accept input from a keyboard.
It is intended that standard input functions such as
read and read-char, when used with this
stream, would cause ``echoing’’ of the input into the output side of the
stream. (The means by which this is accomplished are of course highly
implementation-dependent.)
[Variable]
*trace-output*
The value of *trace-output* is the stream on which the
trace function prints its output.
The variables *standard-input*,
*standard-output*, *error-output*,
*trace-output*, *query-io*, and
*debug-io* are initially bound to synonym streams that pass
all operations on to the stream that is the value of
*terminal-io*. (See make-synonym-stream.) Thus
any operations performed on those streams will go to the terminal.
X3J13 voted in January 1989 (STANDARD-INPUT-INITIAL-BINDING) to
replace the requirements of the preceding paragraph with the following
new requirements:
The seven standard stream variables, *standard-input*,
*standard-output*, *query-io*,
*debug-io*, *terminal-io*,
*error-output*, and *trace-output*, are
initially bound to open streams. (These will be called the standard
initial streams.)
The streams that are the initial values of
*standard-input*, *query-io*,
*debug-io*, and *terminal-io* must support
input.
The streams that are the initial values of
*standard-output*, *error-output*,
*trace-output*, *query-io*,
*debug-io*, and *terminal-io* must support
output.
None of the standard initial streams (including the one to which
*terminal-io* is initially bound) may be a synonym, either
directly or indirectly, for any of the standard stream variables except
*terminal-io*. For example, the initial value of
*trace-output* may be a synonym stream for
*terminal-io* but not a synonym stream for
*standard-output* or *query-io*. (These are
examples of direct synonyms.) As another example,
*query-io* may be a two-way stream or echo stream whose
input component is a synonym for *terminal-io*, but its
input component may not be a synonym for *standard-input*
or *debug-io*. (These are examples of indirect
synonyms.)
Any or all of the standard initial streams may be direct or indirect synonyms for one or more common implementation-dependent streams. For example, the standard initial streams might all be synonym streams (or two-way or echo streams whose components are synonym streams) to a pair of hidden terminal input and output streams maintained by the implementation.
Part of the intent of these rules is to ensure that it is always safe
to bind any standard stream variable to the value of any other standard
stream variable (that is, unworkable circularities are avoided) without
unduly restricting implementation flexibility.
No user program should ever change the value of
*terminal-io*. A program that wants (for example) to divert
output to a file should do so by binding the value of
*standard-output*; that way error messages sent to
*error-output* can still get to the user by going through
*terminal-io*, which is usually what is desired.
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