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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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