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| <title>Using the GNU Compiler Collection (GCC): Precompiled Headers</title> |
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| <a name="Precompiled-Headers"></a> |
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| Previous: <a href="Environment-Variables.html#Environment-Variables" accesskey="p" rel="prev">Environment Variables</a>, Up: <a href="Invoking-GCC.html#Invoking-GCC" accesskey="u" rel="up">Invoking GCC</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p> |
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| <a name="Using-Precompiled-Headers"></a> |
| <h3 class="section">3.20 Using Precompiled Headers</h3> |
| <a name="index-precompiled-headers"></a> |
| <a name="index-speed-of-compilation"></a> |
| |
| <p>Often large projects have many header files that are included in every |
| source file. The time the compiler takes to process these header files |
| over and over again can account for nearly all of the time required to |
| build the project. To make builds faster, GCC allows you to |
| <em>precompile</em> a header file. |
| </p> |
| <p>To create a precompiled header file, simply compile it as you would any |
| other file, if necessary using the <samp>-x</samp> option to make the driver |
| treat it as a C or C++ header file. You may want to use a |
| tool like <code>make</code> to keep the precompiled header up-to-date when |
| the headers it contains change. |
| </p> |
| <p>A precompiled header file is searched for when <code>#include</code> is |
| seen in the compilation. As it searches for the included file |
| (see <a href="http://gcc.gnu.org/onlinedocs/cpp/Search-Path.html#Search-Path">Search Path</a> in <cite>The C Preprocessor</cite>) the |
| compiler looks for a precompiled header in each directory just before it |
| looks for the include file in that directory. The name searched for is |
| the name specified in the <code>#include</code> with ‘<samp>.gch</samp>’ appended. If |
| the precompiled header file can’t be used, it is ignored. |
| </p> |
| <p>For instance, if you have <code>#include "all.h"</code>, and you have |
| <samp>all.h.gch</samp> in the same directory as <samp>all.h</samp>, then the |
| precompiled header file is used if possible, and the original |
| header is used otherwise. |
| </p> |
| <p>Alternatively, you might decide to put the precompiled header file in a |
| directory and use <samp>-I</samp> to ensure that directory is searched |
| before (or instead of) the directory containing the original header. |
| Then, if you want to check that the precompiled header file is always |
| used, you can put a file of the same name as the original header in this |
| directory containing an <code>#error</code> command. |
| </p> |
| <p>This also works with <samp>-include</samp>. So yet another way to use |
| precompiled headers, good for projects not designed with precompiled |
| header files in mind, is to simply take most of the header files used by |
| a project, include them from another header file, precompile that header |
| file, and <samp>-include</samp> the precompiled header. If the header files |
| have guards against multiple inclusion, they are skipped because |
| they’ve already been included (in the precompiled header). |
| </p> |
| <p>If you need to precompile the same header file for different |
| languages, targets, or compiler options, you can instead make a |
| <em>directory</em> named like <samp>all.h.gch</samp>, and put each precompiled |
| header in the directory, perhaps using <samp>-o</samp>. It doesn’t matter |
| what you call the files in the directory; every precompiled header in |
| the directory is considered. The first precompiled header |
| encountered in the directory that is valid for this compilation is |
| used; they’re searched in no particular order. |
| </p> |
| <p>There are many other possibilities, limited only by your imagination, |
| good sense, and the constraints of your build system. |
| </p> |
| <p>A precompiled header file can be used only when these conditions apply: |
| </p> |
| <ul> |
| <li> Only one precompiled header can be used in a particular compilation. |
| |
| </li><li> A precompiled header can’t be used once the first C token is seen. You |
| can have preprocessor directives before a precompiled header; you cannot |
| include a precompiled header from inside another header. |
| |
| </li><li> The precompiled header file must be produced for the same language as |
| the current compilation. You can’t use a C precompiled header for a C++ |
| compilation. |
| |
| </li><li> The precompiled header file must have been produced by the same compiler |
| binary as the current compilation is using. |
| |
| </li><li> Any macros defined before the precompiled header is included must |
| either be defined in the same way as when the precompiled header was |
| generated, or must not affect the precompiled header, which usually |
| means that they don’t appear in the precompiled header at all. |
| |
| <p>The <samp>-D</samp> option is one way to define a macro before a |
| precompiled header is included; using a <code>#define</code> can also do it. |
| There are also some options that define macros implicitly, like |
| <samp>-O</samp> and <samp>-Wdeprecated</samp>; the same rule applies to macros |
| defined this way. |
| </p> |
| </li><li> If debugging information is output when using the precompiled |
| header, using <samp>-g</samp> or similar, the same kind of debugging information |
| must have been output when building the precompiled header. However, |
| a precompiled header built using <samp>-g</samp> can be used in a compilation |
| when no debugging information is being output. |
| |
| </li><li> The same <samp>-m</samp> options must generally be used when building |
| and using the precompiled header. See <a href="Submodel-Options.html#Submodel-Options">Submodel Options</a>, |
| for any cases where this rule is relaxed. |
| |
| </li><li> Each of the following options must be the same when building and using |
| the precompiled header: |
| |
| <div class="smallexample"> |
| <pre class="smallexample">-fexceptions |
| </pre></div> |
| |
| </li><li> Some other command-line options starting with <samp>-f</samp>, |
| <samp>-p</samp>, or <samp>-O</samp> must be defined in the same way as when |
| the precompiled header was generated. At present, it’s not clear |
| which options are safe to change and which are not; the safest choice |
| is to use exactly the same options when generating and using the |
| precompiled header. The following are known to be safe: |
| |
| <div class="smallexample"> |
| <pre class="smallexample">-fmessage-length= -fpreprocessed -fsched-interblock |
| -fsched-spec -fsched-spec-load -fsched-spec-load-dangerous |
| -fsched-verbose=<var>number</var> -fschedule-insns -fvisibility= |
| -pedantic-errors |
| </pre></div> |
| |
| </li></ul> |
| |
| <p>For all of these except the last, the compiler automatically |
| ignores the precompiled header if the conditions aren’t met. If you |
| find an option combination that doesn’t work and doesn’t cause the |
| precompiled header to be ignored, please consider filing a bug report, |
| see <a href="Bugs.html#Bugs">Bugs</a>. |
| </p> |
| <p>If you do use differing options when generating and using the |
| precompiled header, the actual behavior is a mixture of the |
| behavior for the options. For instance, if you use <samp>-g</samp> to |
| generate the precompiled header but not when using it, you may or may |
| not get debugging information for routines in the precompiled header. |
| </p> |
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| Previous: <a href="Environment-Variables.html#Environment-Variables" accesskey="p" rel="prev">Environment Variables</a>, Up: <a href="Invoking-GCC.html#Invoking-GCC" accesskey="u" rel="up">Invoking GCC</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p> |
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