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# Input and Output Ports
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A _port_ encapsulates an I/O stream, normally for just one direction. An
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_input port_ reads from a stream, and an _output port_ writes to a
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string.
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For many procedures that accept a port argument, the argument is
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optional, and it defaults to either the _current input port_ or _current
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output port_. For `mzscheme`, the current ports are initialized to the
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process’s stdin and stdout. The `current-input-port` and
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`current-output-port` procedures, when called with no arguments, return
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the current output and input port, respectively.
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Examples:
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```racket
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> (display "hello world\n")
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hello world
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> (display "hello world\n" (current-output-port))
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hello world
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```
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Ports are created by various procedures that specific to the different
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kinds of streams. For example, `open-input-file` creates an input port
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for reading from a file. Procedures like `with-input-from-file` both
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create a port and install it as the current port while calling a given
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body procedure.
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See \[missing\] for information about using ports.
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