Mercurial > ~dholland > hg > ag > index.cgi

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2//EN">
<HTML>
<HEAD>
<TITLE>Expression Evaluator - Macro preprocessor and C Parser </TITLE>
</HEAD>


<BODY BGCOLOR="#ffffff" BACKGROUND="tilbl6h.gif"
 TEXT="#000000" LINK="#0033CC"
 VLINK="#CC0033" ALINK="#CC0099">

<P>
<IMG ALIGN="right" SRC="../../images/agrsl6c.gif" ALT="AnaGram"
         WIDTH=124 HEIGHT=30 >
<BR CLEAR="all">
Back to :
<A HREF="../../index.html">Index</A> |
<A HREF="index.html">Macro preprocessor overview</A>
<P>


<IMG ALIGN="bottom" SRC="../../images/rbline6j.gif" ALT="----------------------"
        WIDTH=1010 HEIGHT=2  >
<P>
<H1> Expression Evaluator - Macro preprocessor and C Parser   </H1>

<IMG ALIGN="bottom" SRC="../../images/rbline6j.gif" ALT="----------------------"
        WIDTH=1010 HEIGHT=2  >
<P>
<BR>

<H2>Introduction</H2>
<P>          The Expression Evaluator module, <tt>ex.syn</tt>, exists only to
          evaluate the conditional expressions in <tt>#if</tt> and
	  <tt>#elif</tt>
          statements. The syntax for these expressions has been
          extracted from a full C syntax and reduced to that which is
          necessary for this purpose.
<P>
          The rules for the evaluation of constant expressions are
          given in Kernighan and Ritchie, sections A7.19 and A12.5
<P>
<BR>

<H2>Theory of Operation</H2>
          The only unusual feature of this grammar is the
          interposition of a token called "nonzero" to avoid division
          by zero. Otherwise the grammar is a conventional expression
          grammar.
<P>
          The parser is implemented as an event driven parser. The
          initializer is <tt>init_ex()</tt>. The parser itself is
          <tt>ex()</tt>. The
          parser has been interfaced to the rest of the program by a
          class definition, <tt>expression_evaluator</tt>.
<P>
          The class definition for <tt>expression_evaluator</tt> is given in
          <tt>mpp.h</tt>. Each instance of the class has its own parser control
          block. Therefore the parser makes all of its references
          indirectly through a pointer, <tt>ex_pcb</tt>. Whenever it is
          necessary to invoke <tt>init_ex()</tt> or <tt>ex()</tt>,
          therefore, it is
          necessary first to make sure <tt>ex_pcb</tt> is correctly set.
<P>
          Input to the parser consists of tokens as defined in
          <tt>token.h</tt>. Since the input does not consist of simple
          characters, the <b>input values</b> switch is set. The input
          procedures store the full token into the input_value field
          of the pcb and the id field into input_code.

<P>

<BR>


<IMG ALIGN="bottom" SRC="../../images/rbline6j.gif" ALT="----------------------"
      WIDTH=1010 HEIGHT=2 >
<P>
<IMG ALIGN="right" SRC="../../images/pslrb6d.gif" ALT="Parsifal Software"
                WIDTH=181 HEIGHT=25>
<BR CLEAR="right">

<P>
Back to :
<A HREF="../../index.html">Index</A> |
<A HREF="index.html">Macro preprocessor overview</A>

<P>
<ADDRESS><FONT SIZE="-1">
                  AnaGram parser generator - examples<BR>
                  Expression Evaluator - Macro preprocessor and C Parser <BR>
                  Copyright &copy; 1993-1999, Parsifal Software. <BR>
                  All Rights Reserved.<BR>
</FONT></ADDRESS>

</BODY>
</HTML>
author	David A. Holland
date	Mon, 13 Jun 2022 00:04:38 -0400
parents	13d2b8934445
children