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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<!-- Created by GNU Texinfo 6.1, http://www.gnu.org/software/texinfo/ -->
<head>
<title>FWEB: Preprocessing</title>

<meta name="description" content="FWEB: Preprocessing">
<meta name="keywords" content="FWEB: Preprocessing">
<meta name="resource-type" content="document">
<meta name="distribution" content="global">
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</head>

<body lang="en">
<a name="Preprocessing"></a>
<div class="header">
<p>
Previous: <a href="Macros-and-formatting.html#Macros-and-formatting" accesskey="p" rel="prev">Macros and formatting</a>, Up: <a href="Macros.html#Macros" accesskey="u" rel="up">Macros</a> &nbsp; [<a href="Parameter-index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-index.html#Concept-index" title="Index" rel="index">Index</a>]</p>
</div>
<hr>

<a name="Preprocessing-1"></a>
<h3 class="section">7.3 Preprocessing</h3>

<a name="index-Macros_002c-preprocessing"></a>
<a name="index-Preprocessing"></a>
<p>Generally, the <small>FWEB</small> preprocessor commands follow a syntax identical
to their C/C++ counterparts.  The one exception is the &lsquo;<samp>@#line</samp>&rsquo;
command.  Whereas the C command takes a line number and file name as
arguments, the <small>FWEB</small> command takes no arguments; its expansion
automatically inserts the current line number and file name.  This
command should be necessary only in rare circumstances.  One of those
involves situations in which an <small>FWEB</small> macro expands to more than one
output line; see <a href="Debugging-with-macros.html#Debugging-with-macros">Debugging with macros</a>.
</p>
<p>The <small>FWEB</small> preprocessor commands may appear in either the definition or the
code parts.  But <em>BEWARE:  No matter where they appear, they are expanded
during INPUT, not output.</em>  (This is probably a design flaw.)  For more
discussion, see <a href="FWEB-macros.html#FWEB-macros">FWEB macros</a>.
</p>
<a name="index-_0040_0023line"></a>
<a name="index-_0040_0023define"></a>
<a name="index-_0040_0023undef"></a>
<a name="index-_0040_0023ifdef"></a>
<a name="index-_0040_0023ifndef"></a>
<a name="index-_0040_0023if"></a>
<a name="index-_0040_0023elif"></a>
<a name="index-_0040_0023endif"></a>

<p>The syntax of each command is as follows:
</p>
<blockquote>
<dl compact="compact">
<dt><code>@#line</code></dt>
<dd><p>&mdash; Insert a <code>#line</code> command.
</p>
</dd>
<dt><code>@#define <var>identifier</var></code></dt>
<dd><p>&mdash; Define an FWEB macro; equivalent to &lsquo;<samp>@m</samp>&rsquo;.
</p></dd>
<dt><code>@#undef <var>identifier</var></code></dt>
<dd><p>&mdash; Undefine an FWEB macro.
</p>
</dd>
<dt><code>@#ifdef <var>identifier</var></code></dt>
<dd><p>&mdash; Is FWEB macro defined?  Equivalent to &lsquo;<samp>@#if&nbsp;defined&nbsp;identifier</samp>&rsquo;<!-- /@w -->.
</p></dd>
<dt><code>@#ifndef <var>identifier</var></code></dt>
<dd><p>&mdash; Is FWEB macro not defined?  Equivalent to &lsquo;<samp>@#if&nbsp;!defined&nbsp;identifier</samp>&rsquo;<!-- /@w -->.
</p>
</dd>
<dt><code>@#if <i>expression</i></code></dt>
<dt><code>@#elif <i>expression</i></code></dt>
<dt><code>@#else</code></dt>
<dt><code>@#endif</code></dt>
</dl>
</blockquote>

<p>In the &lsquo;<samp>@#if</samp>&rsquo; statement, the <i>expression</i> may contain <small>FWEB</small>
macros, but must ultimately evaluate to a number.  If that number is
zero, the expression is false; otherwise, it is true.
</p>
<a name="index-Expression-evaluation"></a>
<p>The <i>expression</i> following constructions such as &lsquo;<samp>@#if</samp>&rsquo; is
evaluated by a built-in expression evaluator that can also be used for
other purposes, such as in macro expansion.  Its behavior is again
motivated by expression 
evaluation in ANSI C; it is not quite as general, but should be more than
adequate.  (One design flaw that will be fixed someday is that the order
of expression evaluation is not necessarily left-to-right, as it is in C.)
It supports both integer and floating-point arithmetic (with type
promotion from integer to floating-point if necessary), and the ANSI
<code>defined</code> operator. Operators with the highest precedence 
(see table below) are evaluated first; as usual, parentheses override the
natural order of evaluation. The unary operator <code>defined</code>
has the highest
precedence; all the other unary operators have the next highest (and equal)
precedence; then come the binary operators. When the operator exists in C,
the action taken by <small>FWEB</small> is precisely that that the C compiler would take.
Arithmetic is done in either <b>long</b> or <b>double</b> variables, as
implemented by the C compiler that compiled <small>FTANGLE</small>.  (This was the
easy choice, not necessarily the most desirable one.)
</p>
<p>The operators, listed from  highest precedence  to lowest, are as
follows
</p>


<hr>
<div class="header">
<p>
Previous: <a href="Macros-and-formatting.html#Macros-and-formatting" accesskey="p" rel="prev">Macros and formatting</a>, Up: <a href="Macros.html#Macros" accesskey="u" rel="up">Macros</a> &nbsp; [<a href="Parameter-index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-index.html#Concept-index" title="Index" rel="index">Index</a>]</p>
</div>



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