#### 14.9.5 Blocking Mode

An interactive port is always in one of two modes: blocking or non-blocking. This mode is independent of the terminal mode: each can be changed independent of the other. Furthermore, if it is an interactive I/O port, there are separate blocking modes for input and for output.

If an input port is in blocking mode, attempting to read from it when no input is available will cause Scheme to “block”, i.e. suspend itself, until input is available. If an input port is in non-blocking mode, attempting to read from it when no input is available will cause the reading procedure to return immediately, indicating the lack of input in some way (exactly how this situation is indicated is separately specified for each procedure or operation).

An output port in blocking mode will block if the output device is not ready to accept output. In non-blocking mode it will return immediately after performing as much output as the device will allow (again, each procedure or operation reports this situation in its own way).

Interactive ports are initially in blocking mode; this can be changed at any time with the procedures defined in this section.

These procedures represent blocking mode by the symbol `blocking`, and non-blocking mode by the symbol `nonblocking`. An argument called mode must be one of these symbols. A port argument to any of these procedures may be any port, even if that port does not support blocking mode; in that case, the port is not modified in any way.

procedure: port/input-blocking-mode port

  Returns the input blocking mode of port.

procedure: port/set-input-blocking-mode port mode

  Changes the input blocking mode of port to be mode. Returns an unspecified value.

procedure: port/with-input-blocking-mode port mode thunk

  Thunk must be a procedure of no arguments. `port/with-input-blocking-mode` binds the input blocking mode of port to be mode, executes thunk, restores the input blocking mode of port to what it was when `port/with-input-blocking-mode` was called, and returns the value that was yielded by thunk. This binding is performed by `dynamic-wind`, which guarantees that the input blocking mode is restored if thunk escapes from its continuation.

procedure: port/output-blocking-mode port

  Returns the output blocking mode of port.

procedure: port/set-output-blocking-mode port mode

  Changes the output blocking mode of port to be mode. Returns an unspecified value.

procedure: port/with-output-blocking-mode port mode thunk

  Thunk must be a procedure of no arguments. `port/with-output-blocking-mode` binds the output blocking mode of port to be mode, executes thunk, restores the output blocking mode of port to what it was when `port/with-output-blocking-mode` was called, and returns the value that was yielded by thunk. This binding is performed by `dynamic-wind`, which guarantees that the output blocking mode is restored if thunk escapes from its continuation.