Variables whose values must be streams are sometimes called stream variables .
Certain stream variables are defined by this specification to be the proper source of input or output in various situations where no specific stream has been specified instead. A complete list of such standardized stream variables appears in Figure 21--6. The consequences are undefined if at any time the value of any of these variables is not an open stream.
Glossary Term Variable Name debug I/O *debug-io* error output *error-output* query I/O *query-io* standard input *standard-input* standard output *standard-output* terminal I/O *terminal-io* trace output *trace-output*
Figure 21--6: Standardized Stream Variables
Note that, by convention, standardized stream variables have names ending in "-input*" if they must be input streams, ending in "-output*" if they must be output streams, or ending in "-io*" if they must be bidirectional streams.
User programs may assign or bind any standardized stream variable except *terminal-io*.
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