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<h4 class="subsection">4.2.2 If</h4>

<p>The &lsquo;<samp><span class="samp">#if</span></samp>&rsquo; directive allows you to test the value of an arithmetic
expression, rather than the mere existence of one macro.  Its syntax is

<pre class="smallexample">     #if <var>expression</var>
     
     <var>controlled text</var>
     
     #endif /* <var>expression</var> */
</pre>
   <p><var>expression</var> is a C expression of integer type, subject to stringent
restrictions.  It may contain

     <ul>
<li>Integer constants.

     <li>Character constants, which are interpreted as they would be in normal
code.

     <li>Arithmetic operators for addition, subtraction, multiplication,
division, bitwise operations, shifts, comparisons, and logical
operations (<code>&amp;&amp;</code> and <code>||</code>).  The latter two obey the usual
short-circuiting rules of standard C.

     <li>Macros.  All macros in the expression are expanded before actual
computation of the expression's value begins.

     <li>Uses of the <code>defined</code> operator, which lets you check whether macros
are defined in the middle of an &lsquo;<samp><span class="samp">#if</span></samp>&rsquo;.

     <li>Identifiers that are not macros, which are all considered to be the
number zero.  This allows you to write <code>#if&nbsp;MACRO<!-- /@w --></code> instead of
<code>#ifdef&nbsp;MACRO<!-- /@w --></code>, if you know that MACRO, when defined, will
always have a nonzero value.  Function-like macros used without their
function call parentheses are also treated as zero.

     <p>In some contexts this shortcut is undesirable.  The <samp><span class="option">-Wundef</span></samp>
option causes GCC to warn whenever it encounters an identifier which is
not a macro in an &lsquo;<samp><span class="samp">#if</span></samp>&rsquo;. 
</ul>

   <p>The preprocessor does not know anything about types in the language. 
Therefore, <code>sizeof</code> operators are not recognized in &lsquo;<samp><span class="samp">#if</span></samp>&rsquo;, and
neither are <code>enum</code> constants.  They will be taken as identifiers
which are not macros, and replaced by zero.  In the case of
<code>sizeof</code>, this is likely to cause the expression to be invalid.

   <p>The preprocessor calculates the value of <var>expression</var>.  It carries
out all calculations in the widest integer type known to the compiler;
on most machines supported by GCC this is 64 bits.  This is not the same
rule as the compiler uses to calculate the value of a constant
expression, and may give different results in some cases.  If the value
comes out to be nonzero, the &lsquo;<samp><span class="samp">#if</span></samp>&rsquo; succeeds and the <var>controlled
text</var> is included; otherwise it is skipped.

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