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| <title>The C Preprocessor: Computed Includes</title> |
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| <a name="Computed-Includes-1"></a> |
| <h3 class="section">2.6 Computed Includes</h3> |
| <a name="index-computed-includes"></a> |
| <a name="index-macros-in-include"></a> |
| |
| <p>Sometimes it is necessary to select one of several different header |
| files to be included into your program. They might specify |
| configuration parameters to be used on different sorts of operating |
| systems, for instance. You could do this with a series of conditionals, |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">#if SYSTEM_1 |
| # include "system_1.h" |
| #elif SYSTEM_2 |
| # include "system_2.h" |
| #elif SYSTEM_3 |
| … |
| #endif |
| </pre></div> |
| |
| <p>That rapidly becomes tedious. Instead, the preprocessor offers the |
| ability to use a macro for the header name. This is called a |
| <em>computed include</em>. Instead of writing a header name as the direct |
| argument of ‘<samp>#include</samp>’, you simply put a macro name there instead: |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">#define SYSTEM_H "system_1.h" |
| … |
| #include SYSTEM_H |
| </pre></div> |
| |
| <p><code>SYSTEM_H</code> will be expanded, and the preprocessor will look for |
| <samp>system_1.h</samp> as if the ‘<samp>#include</samp>’ had been written that way |
| originally. <code>SYSTEM_H</code> could be defined by your Makefile with a |
| <samp>-D</samp> option. |
| </p> |
| <p>You must be careful when you define the macro. ‘<samp>#define</samp>’ saves |
| tokens, not text. The preprocessor has no way of knowing that the macro |
| will be used as the argument of ‘<samp>#include</samp>’, so it generates |
| ordinary tokens, not a header name. This is unlikely to cause problems |
| if you use double-quote includes, which are close enough to string |
| constants. If you use angle brackets, however, you may have trouble. |
| </p> |
| <p>The syntax of a computed include is actually a bit more general than the |
| above. If the first non-whitespace character after ‘<samp>#include</samp>’ is |
| not ‘<samp>"</samp>’ or ‘<samp><</samp>’, then the entire line is macro-expanded |
| like running text would be. |
| </p> |
| <p>If the line expands to a single string constant, the contents of that |
| string constant are the file to be included. CPP does not re-examine the |
| string for embedded quotes, but neither does it process backslash |
| escapes in the string. Therefore |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">#define HEADER "a\"b" |
| #include HEADER |
| </pre></div> |
| |
| <p>looks for a file named <samp>a\"b</samp>. CPP searches for the file according |
| to the rules for double-quoted includes. |
| </p> |
| <p>If the line expands to a token stream beginning with a ‘<samp><</samp>’ token |
| and including a ‘<samp>></samp>’ token, then the tokens between the ‘<samp><</samp>’ and |
| the first ‘<samp>></samp>’ are combined to form the filename to be included. |
| Any whitespace between tokens is reduced to a single space; then any |
| space after the initial ‘<samp><</samp>’ is retained, but a trailing space |
| before the closing ‘<samp>></samp>’ is ignored. CPP searches for the file |
| according to the rules for angle-bracket includes. |
| </p> |
| <p>In either case, if there are any tokens on the line after the file name, |
| an error occurs and the directive is not processed. It is also an error |
| if the result of expansion does not match either of the two expected |
| forms. |
| </p> |
| <p>These rules are implementation-defined behavior according to the C |
| standard. To minimize the risk of different compilers interpreting your |
| computed includes differently, we recommend you use only a single |
| object-like macro which expands to a string constant. This will also |
| minimize confusion for people reading your program. |
| </p> |
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