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- <a name="Character-Oriented-Interaction"></a>
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- <h4 class="subsubsection">2.6.4.2 Character Oriented Interaction</h4>
- <p>A character oriented style of interaction involves the function always
- returning a data structure of the form <code>(<var>state</var>,(<var>command
- lines</var>,nil))</code>. The <var>state</var> and <var>command lines</var> fields serve
- <a name="index-command-line-5"></a>
- exactly the same purposes respectively as they do in the case of line
- oriented interaction. The field that would be occupied by the
- <var>prompt strings</var> list in the case of line oriented interaction is
- identically <code>nil</code> in this style.
- </p>
- <p>When this style is used, <code>avram</code> spawns a process and/or sends
- <a name="index-spawning-processes-2"></a>
- command lines to it as in the case of line oriented interaction, but
- attempts to read only a single character from it per iteration. When the
- character is received, <code>avram</code> applies the function to the pair
- <code>(<var>state</var>,<var>character</var>)</code> in order to obtain the next state
- and the next list of command lines. If the process has terminated, a
- <code>nil</code> value is used in place of the character. If the process is
- quiescent, deadlock ensues.
- </p>
- <p>The character oriented style is a lower level protocol that shifts more
- of the burden of analyzing the process’s output to the virtual code
- application. It can do anything line oriented interaction can do except
- proceeding immediately without waiting to receive any output from the
- process. It may also allow more general criteria (in effect) than the
- matching of a fixed prompt string to delimit the received data, for
- those pathological processes that may require such things.
- </p>
- <p>Applications using character oriented interaction need to deal with line
- <a name="index-line-breaks"></a>
- breaks explicitly among the received characters, unlike the case with
- line oriented interaction, where the line breaks are implicit in the
- <a name="index-Unix-6"></a>
- list of received strings. Contrary to the convention for Unix text
- files, line breaks in the output of a process are indicated by character
- code 13 followed by character code 10.
- </p>
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