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<h5 class="subsubsection">2.6.4.2 Character Oriented Interaction</h5>

<p>A character oriented style of interaction involves the function always
returning a data structure of the form <code>(</code><var>state</var><code>,(</code><var>command
lines</var><code>,nil))</code>. The <var>state</var> and <var>command lines</var> fields serve
<a name="index-command-line-236"></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>When this style is used, <code>avram</code> spawns a process and/or sends
<a name="index-spawning-processes-237"></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>(</code><var>state</var><code>,</code><var>character</var><code>)</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>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>Applications using character oriented interaction need to deal with line
<a name="index-line-breaks-238"></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-239"></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.

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