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| <h3 class="section">2.2 Include Operation</h3> |
| |
| <p>The ‘<samp><span class="samp">#include</span></samp>’ directive works by directing the C preprocessor to |
| scan the specified file as input before continuing with the rest of the |
| current file. The output from the preprocessor contains the output |
| already generated, followed by the output resulting from the included |
| file, followed by the output that comes from the text after the |
| ‘<samp><span class="samp">#include</span></samp>’ directive. For example, if you have a header file |
| <samp><span class="file">header.h</span></samp> as follows, |
| |
| <pre class="smallexample"> char *test (void); |
| </pre> |
| <p class="noindent">and a main program called <samp><span class="file">program.c</span></samp> that uses the header file, |
| like this, |
| |
| <pre class="smallexample"> int x; |
| #include "header.h" |
| |
| int |
| main (void) |
| { |
| puts (test ()); |
| } |
| </pre> |
| <p class="noindent">the compiler will see the same token stream as it would if |
| <samp><span class="file">program.c</span></samp> read |
| |
| <pre class="smallexample"> int x; |
| char *test (void); |
| |
| int |
| main (void) |
| { |
| puts (test ()); |
| } |
| </pre> |
| <p>Included files are not limited to declarations and macro definitions; |
| those are merely the typical uses. Any fragment of a C program can be |
| included from another file. The include file could even contain the |
| beginning of a statement that is concluded in the containing file, or |
| the end of a statement that was started in the including file. However, |
| an included file must consist of complete tokens. Comments and string |
| literals which have not been closed by the end of an included file are |
| invalid. For error recovery, they are considered to end at the end of |
| the file. |
| |
| <p>To avoid confusion, it is best if header files contain only complete |
| syntactic units—function declarations or definitions, type |
| declarations, etc. |
| |
| <p>The line following the ‘<samp><span class="samp">#include</span></samp>’ directive is always treated as a |
| separate line by the C preprocessor, even if the included file lacks a |
| final newline. |
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