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| <h4 class="subsection">4.2.1 Ifdef</h4> |
| |
| <p><a name="index-g_t_0023ifdef-85"></a><a name="index-g_t_0023endif-86"></a> |
| The simplest sort of conditional is |
| |
| <pre class="smallexample"> #ifdef <var>MACRO</var> |
| |
| <var>controlled text</var> |
| |
| #endif /* <var>MACRO</var> */ |
| </pre> |
| <p><a name="index-conditional-group-87"></a>This block is called a <dfn>conditional group</dfn>. <var>controlled text</var> |
| will be included in the output of the preprocessor if and only if |
| <var>MACRO</var> is defined. We say that the conditional <dfn>succeeds</dfn> if |
| <var>MACRO</var> is defined, <dfn>fails</dfn> if it is not. |
| |
| <p>The <var>controlled text</var> inside of a conditional can include |
| preprocessing directives. They are executed only if the conditional |
| succeeds. You can nest conditional groups inside other conditional |
| groups, but they must be completely nested. In other words, |
| ‘<samp><span class="samp">#endif</span></samp>’ always matches the nearest ‘<samp><span class="samp">#ifdef</span></samp>’ (or |
| ‘<samp><span class="samp">#ifndef</span></samp>’, or ‘<samp><span class="samp">#if</span></samp>’). Also, you cannot start a conditional |
| group in one file and end it in another. |
| |
| <p>Even if a conditional fails, the <var>controlled text</var> inside it is |
| still run through initial transformations and tokenization. Therefore, |
| it must all be lexically valid C. Normally the only way this matters is |
| that all comments and string literals inside a failing conditional group |
| must still be properly ended. |
| |
| <p>The comment following the ‘<samp><span class="samp">#endif</span></samp>’ is not required, but it is a |
| good practice if there is a lot of <var>controlled text</var>, because it |
| helps people match the ‘<samp><span class="samp">#endif</span></samp>’ to the corresponding ‘<samp><span class="samp">#ifdef</span></samp>’. |
| Older programs sometimes put <var>MACRO</var> directly after the |
| ‘<samp><span class="samp">#endif</span></samp>’ without enclosing it in a comment. This is invalid code |
| according to the C standard. CPP accepts it with a warning. It |
| never affects which ‘<samp><span class="samp">#ifndef</span></samp>’ the ‘<samp><span class="samp">#endif</span></samp>’ matches. |
| |
| <p><a name="index-g_t_0023ifndef-88"></a>Sometimes you wish to use some code if a macro is <em>not</em> defined. |
| You can do this by writing ‘<samp><span class="samp">#ifndef</span></samp>’ instead of ‘<samp><span class="samp">#ifdef</span></samp>’. |
| One common use of ‘<samp><span class="samp">#ifndef</span></samp>’ is to include code only the first |
| time a header file is included. See <a href="Once_002dOnly-Headers.html#Once_002dOnly-Headers">Once-Only Headers</a>. |
| |
| <p>Macro definitions can vary between compilations for several reasons. |
| Here are some samples. |
| |
| <ul> |
| <li>Some macros are predefined on each kind of machine |
| (see <a href="System_002dspecific-Predefined-Macros.html#System_002dspecific-Predefined-Macros">System-specific Predefined Macros</a>). This allows you to provide |
| code specially tuned for a particular machine. |
| |
| <li>System header files define more macros, associated with the features |
| they implement. You can test these macros with conditionals to avoid |
| using a system feature on a machine where it is not implemented. |
| |
| <li>Macros can be defined or undefined with the <samp><span class="option">-D</span></samp> and <samp><span class="option">-U</span></samp> |
| command line options when you compile the program. You can arrange to |
| compile the same source file into two different programs by choosing a |
| macro name to specify which program you want, writing conditionals to |
| test whether or how this macro is defined, and then controlling the |
| state of the macro with command line options, perhaps set in the |
| Makefile. See <a href="Invocation.html#Invocation">Invocation</a>. |
| |
| <li>Your program might have a special header file (often called |
| <samp><span class="file">config.h</span></samp>) that is adjusted when the program is compiled. It can |
| define or not define macros depending on the features of the system and |
| the desired capabilities of the program. The adjustment can be |
| automated by a tool such as <samp><span class="command">autoconf</span></samp>, or done by hand. |
| </ul> |
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