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| <h3 class="section">2.8 System Headers</h3> |
| |
| <p><a name="index-system-header-files-39"></a> |
| The header files declaring interfaces to the operating system and |
| runtime libraries often cannot be written in strictly conforming C. |
| Therefore, GCC gives code found in <dfn>system headers</dfn> special |
| treatment. All warnings, other than those generated by ‘<samp><span class="samp">#warning</span></samp>’ |
| (see <a href="Diagnostics.html#Diagnostics">Diagnostics</a>), are suppressed while GCC is processing a system |
| header. Macros defined in a system header are immune to a few warnings |
| wherever they are expanded. This immunity is granted on an ad-hoc |
| basis, when we find that a warning generates lots of false positives |
| because of code in macros defined in system headers. |
| |
| <p>Normally, only the headers found in specific directories are considered |
| system headers. These directories are determined when GCC is compiled. |
| There are, however, two ways to make normal headers into system headers. |
| |
| <p>The <samp><span class="option">-isystem</span></samp> command line option adds its argument to the list of |
| directories to search for headers, just like <samp><span class="option">-I</span></samp>. Any headers |
| found in that directory will be considered system headers. |
| |
| <p>All directories named by <samp><span class="option">-isystem</span></samp> are searched <em>after</em> all |
| directories named by <samp><span class="option">-I</span></samp>, no matter what their order was on the |
| command line. If the same directory is named by both <samp><span class="option">-I</span></samp> and |
| <samp><span class="option">-isystem</span></samp>, the <samp><span class="option">-I</span></samp> option is ignored. GCC provides an |
| informative message when this occurs if <samp><span class="option">-v</span></samp> is used. |
| |
| <p><a name="index-g_t_0023pragma-GCC-system_005fheader-40"></a>There is also a directive, <code>#pragma GCC system_header<!-- /@w --></code>, which |
| tells GCC to consider the rest of the current include file a system |
| header, no matter where it was found. Code that comes before the |
| ‘<samp><span class="samp">#pragma</span></samp>’ in the file will not be affected. <code>#pragma GCC system_header<!-- /@w --></code> has no effect in the primary source file. |
| |
| <p>On very old systems, some of the pre-defined system header directories |
| get even more special treatment. GNU C++ considers code in headers |
| found in those directories to be surrounded by an <code>extern "C"<!-- /@w --></code> |
| block. There is no way to request this behavior with a ‘<samp><span class="samp">#pragma</span></samp>’, |
| or from the command line. |
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