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| <h2 class="chapter">2 Header Files</h2> |
| |
| <p><a name="index-header-file-25"></a>A header file is a file containing C declarations and macro definitions |
| (see <a href="Macros.html#Macros">Macros</a>) to be shared between several source files. You request |
| the use of a header file in your program by <dfn>including</dfn> it, with the |
| C preprocessing directive ‘<samp><span class="samp">#include</span></samp>’. |
| |
| <p>Header files serve two purposes. |
| |
| <ul> |
| <li><a name="index-system-header-files-26"></a>System header files declare the interfaces to parts of the operating |
| system. You include them in your program to supply the definitions and |
| declarations you need to invoke system calls and libraries. |
| |
| <li>Your own header files contain declarations for interfaces between the |
| source files of your program. Each time you have a group of related |
| declarations and macro definitions all or most of which are needed in |
| several different source files, it is a good idea to create a header |
| file for them. |
| </ul> |
| |
| <p>Including a header file produces the same results as copying the header |
| file into each source file that needs it. Such copying would be |
| time-consuming and error-prone. With a header file, the related |
| declarations appear in only one place. If they need to be changed, they |
| can be changed in one place, and programs that include the header file |
| will automatically use the new version when next recompiled. The header |
| file eliminates the labor of finding and changing all the copies as well |
| as the risk that a failure to find one copy will result in |
| inconsistencies within a program. |
| |
| <p>In C, the usual convention is to give header files names that end with |
| <samp><span class="file">.h</span></samp>. It is most portable to use only letters, digits, dashes, and |
| underscores in header file names, and at most one dot. |
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| <li><a accesskey="1" href="Include-Syntax.html#Include-Syntax">Include Syntax</a> |
| <li><a accesskey="2" href="Include-Operation.html#Include-Operation">Include Operation</a> |
| <li><a accesskey="3" href="Search-Path.html#Search-Path">Search Path</a> |
| <li><a accesskey="4" href="Once_002dOnly-Headers.html#Once_002dOnly-Headers">Once-Only Headers</a> |
| <li><a accesskey="5" href="Alternatives-to-Wrapper-_0023ifndef.html#Alternatives-to-Wrapper-_0023ifndef">Alternatives to Wrapper #ifndef</a> |
| <li><a accesskey="6" href="Computed-Includes.html#Computed-Includes">Computed Includes</a> |
| <li><a accesskey="7" href="Wrapper-Headers.html#Wrapper-Headers">Wrapper Headers</a> |
| <li><a accesskey="8" href="System-Headers.html#System-Headers">System Headers</a> |
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