| <html lang="en"> |
| <head> |
| <title>Precompiled Headers - Using the GNU Compiler Collection (GCC)</title> |
| <meta http-equiv="Content-Type" content="text/html"> |
| <meta name="description" content="Using the GNU Compiler Collection (GCC)"> |
| <meta name="generator" content="makeinfo 4.13"> |
| <link title="Top" rel="start" href="index.html#Top"> |
| <link rel="up" href="Invoking-GCC.html#Invoking-GCC" title="Invoking GCC"> |
| <link rel="prev" href="Environment-Variables.html#Environment-Variables" title="Environment Variables"> |
| <link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage"> |
| <!-- |
| Copyright (C) 1988, 1989, 1992, 1993, 1994, 1995, 1996, 1997, 1998, |
| 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, |
| 2008 Free Software Foundation, Inc. |
| |
| Permission is granted to copy, distribute and/or modify this document |
| under the terms of the GNU Free Documentation License, Version 1.2 or |
| any later version published by the Free Software Foundation; with the |
| Invariant Sections being ``Funding Free Software'', the Front-Cover |
| Texts being (a) (see below), and with the Back-Cover Texts being (b) |
| (see below). A copy of the license is included in the section entitled |
| ``GNU Free Documentation License''. |
| |
| (a) The FSF's Front-Cover Text is: |
| |
| A GNU Manual |
| |
| (b) The FSF's Back-Cover Text is: |
| |
| You have freedom to copy and modify this GNU Manual, like GNU |
| software. Copies published by the Free Software Foundation raise |
| funds for GNU development.--> |
| <meta http-equiv="Content-Style-Type" content="text/css"> |
| <style type="text/css"><!-- |
| pre.display { font-family:inherit } |
| pre.format { font-family:inherit } |
| pre.smalldisplay { font-family:inherit; font-size:smaller } |
| pre.smallformat { font-family:inherit; font-size:smaller } |
| pre.smallexample { font-size:smaller } |
| pre.smalllisp { font-size:smaller } |
| span.sc { font-variant:small-caps } |
| span.roman { font-family:serif; font-weight:normal; } |
| span.sansserif { font-family:sans-serif; font-weight:normal; } |
| --></style> |
| <link rel="stylesheet" type="text/css" href="../cs.css"> |
| </head> |
| <body> |
| <div class="node"> |
| <a name="Precompiled-Headers"></a> |
| <p> |
| Previous: <a rel="previous" accesskey="p" href="Environment-Variables.html#Environment-Variables">Environment Variables</a>, |
| Up: <a rel="up" accesskey="u" href="Invoking-GCC.html#Invoking-GCC">Invoking GCC</a> |
| <hr> |
| </div> |
| |
| <h3 class="section">3.20 Using Precompiled Headers</h3> |
| |
| <p><a name="index-precompiled-headers-2169"></a><a name="index-speed-of-compilation-2170"></a> |
| 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 users to |
| `precompile' a header file; then, if builds can use the precompiled |
| header file they will be much faster. |
| |
| <p>To create a precompiled header file, simply compile it as you would any |
| other file, if necessary using the <samp><span class="option">-x</span></samp> option to make the driver |
| treat it as a C or C++ header file. You will probably want to use a |
| tool like <samp><span class="command">make</span></samp> to keep the precompiled header up-to-date when |
| the headers it contains change. |
| |
| <p>A precompiled header file will be searched for when <code>#include</code> is |
| seen in the compilation. As it searches for the included file |
| (see <a href="../cpp/Search-Path.html#Search-Path">Search Path</a>) 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><span class="samp">.gch</span></samp>’ appended. If |
| the precompiled header file can't be used, it is ignored. |
| |
| <p>For instance, if you have <code>#include "all.h"</code>, and you have |
| <samp><span class="file">all.h.gch</span></samp> in the same directory as <samp><span class="file">all.h</span></samp>, then the |
| precompiled header file will be used if possible, and the original |
| header will be used otherwise. |
| |
| <p>Alternatively, you might decide to put the precompiled header file in a |
| directory and use <samp><span class="option">-I</span></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>This also works with <samp><span class="option">-include</span></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><span class="option">-include</span></samp> the precompiled header. If the header files |
| have guards against multiple inclusion, they will be skipped because |
| they've already been included (in the precompiled header). |
| |
| <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><span class="file">all.h.gch</span></samp>, and put each precompiled |
| header in the directory, perhaps using <samp><span class="option">-o</span></samp>. It doesn't matter |
| what you call the files in the directory, every precompiled header in |
| the directory will be considered. The first precompiled header |
| encountered in the directory that is valid for this compilation will |
| be used; they're searched in no particular order. |
| |
| <p>There are many other possibilities, limited only by your imagination, |
| good sense, and the constraints of your build system. |
| |
| <p>A precompiled header file can be used only when these conditions apply: |
| |
| <ul> |
| <li>Only one precompiled header can be used in a particular compilation. |
| |
| <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 can |
| even include a precompiled header from inside another header, so long as |
| there are no C tokens before the <code>#include</code>. |
| |
| <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>The precompiled header file must have been produced by the same compiler |
| binary as the current compilation is using. |
| |
| <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><span class="option">-D</span></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><span class="option">-O</span></samp> and <samp><span class="option">-Wdeprecated</span></samp>; the same rule applies to macros |
| defined this way. |
| |
| <li>If debugging information is output when using the precompiled |
| header, using <samp><span class="option">-g</span></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><span class="option">-g</span></samp> can be used in a compilation |
| when no debugging information is being output. |
| |
| <li>The same <samp><span class="option">-m</span></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>Each of the following options must be the same when building and using |
| the precompiled header: |
| |
| <pre class="smallexample"> -fexceptions |
| </pre> |
| <li>Some other command-line options starting with <samp><span class="option">-f</span></samp>, |
| <samp><span class="option">-p</span></samp>, or <samp><span class="option">-O</span></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: |
| |
| <pre class="smallexample"> -fmessage-length= -fpreprocessed -fsched-interblock |
| -fsched-spec -fsched-spec-load -fsched-spec-load-dangerous |
| -fsched-verbose=<number> -fschedule-insns -fvisibility= |
| -pedantic-errors |
| </pre> |
| </ul> |
| |
| <p>For all of these except the last, the compiler will automatically |
| ignore 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>If you do use differing options when generating and using the |
| precompiled header, the actual behavior will be a mixture of the |
| behavior for the options. For instance, if you use <samp><span class="option">-g</span></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. |
| |
| <!-- Copyright (C) 2001, 2002, 2003, 2004, 2006, 2008 --> |
| <!-- Free Software Foundation, Inc. --> |
| <!-- This is part of the GCC manual. --> |
| <!-- For copying conditions, see the file gcc.texi. --> |
| </body></html> |
| |
