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Variables whose values must be streams are sometimes called stream variables.
These variables are collectively called the standardized I/O customization variables. They can be bound or assigned in order to change the default destinations for input and/or output used by various standardized operators and facilities.
implementation-dependent, but it must be an open stream that is not a generalized synonym stream to an I/O customization variables but that might be a generalized synonym stream to the value of some I/O customization variable.
The value of ‘*standard-input*’, called standard input, is a stream that is used by many operators as a default source of input when no specific input stream is explicitly supplied.
The value of ‘*standard-output*’, called standard output, is a stream that is used by many operators as a default destination for output when no specific output stream is explicitly supplied.
The value of ‘*error-output*’, called error output, is a stream to which warnings and non-interactive error messages should be sent.
The value of ‘*query-io*’, called query I/O, is a bidirectional stream to be used when asking questions of the user. The question should be output to this stream, and the answer read from it.
The value of ‘*debug-io*’, called debug I/O, is a stream to be used for interactive debugging purposes.
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