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<h4 class="subsection">4.2.2 If</h4>
<p>The ‘<samp><span class="samp">#if</span></samp>’ 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>&&</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 ‘<samp><span class="samp">#if</span></samp>’.
<li>Identifiers that are not macros, which are all considered to be the
number zero. This allows you to write <code>#if MACRO<!-- /@w --></code> instead of
<code>#ifdef 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 ‘<samp><span class="samp">#if</span></samp>’.
</ul>
<p>The preprocessor does not know anything about types in the language.
Therefore, <code>sizeof</code> operators are not recognized in ‘<samp><span class="samp">#if</span></samp>’, 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 ‘<samp><span class="samp">#if</span></samp>’ succeeds and the <var>controlled
text</var> is included; otherwise it is skipped.
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