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<HTML><!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"><!-- Created on September, 12 2004 by texi2html 1.64 --><!-- Written by: Lionel Cons <Lionel.Cons@cern.ch> (original author) Karl Berry <karl@freefriends.org> Olaf Bachmann <obachman@mathematik.uni-kl.de> and many others.Maintained by: Olaf Bachmann <obachman@mathematik.uni-kl.de>Send bugs and suggestions to <texi2html@mathematik.uni-kl.de> --><HEAD><TITLE>Autoconf, Automake, and Libtool: Caching results</TITLE><META NAME="description" CONTENT="Autoconf, Automake, and Libtool: Caching results"><META NAME="keywords" CONTENT="Autoconf, Automake, and Libtool: Caching results"><META NAME="resource-type" CONTENT="document"><META NAME="distribution" CONTENT="global"><META NAME="Generator" CONTENT="texi2html 1.64"><script language="Javascript"><!-- // Check the browser version. function checkVersion() { if (navigator.appVersion.charAt(0)>=3) return true; if (navigator.appVersion.charAt(0)>=4) return true; else return false; 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The cache file is itself a Bourne shellscript which is sourced by the <TT>`configure'</TT> script to set any`cache variables' to values that are present in the cache file.</P><P>The next time <TT>`configure'</TT> is run, the cache will be consulted for aprior result. If there is a prior result, the value is re-used and thecode that performs that test is skipped. This speeds up subsequent runsof <TT>`configure'</TT> and configuration of deep trees, which can share acache file in the top-level directory (see section <A HREF="autobook_13.html#SEC13">3. How to run configure and make</A>).</P><P>A custom macro is not required to do caching, though it is consideredbest practice. Sometimes it doesn't make sense for a macro to docaching--tests for system aspects which may frequently change should notbe cached. For example, a test for free disk space should not employcaching as it is a dynamic characteristic.</P><P>The <SAMP>`AC_CACHE_CHECK'</SAMP> macro is a convenient wrapper for caching theresults of tests. You simply provide a description of the test, thename of a cache variable to store the test result to, and the body ofthe test. If the test has not been run before, the cache will be primedwith the result. If the result is already in the cache, then the cachevariable will be set and the test will be skipped. Note that the nameof the cache variable must contain <SAMP>`_cv_'</SAMP> in order to be savedcorrectly.</P><P>Here is the code for an Autoconf macro that ties together many of theconcepts introduced in this chapter:</P><P><TABLE width=100%><tr><td> </td><td class=example bgcolor=#6688aa><br><pre></pre></td></tr></table></P><P><A NAME="Future directions for macro writers"></A></TR></TABLE><BR> <FONT SIZE="-1">This document was generatedby <I>Gary V. Vaughan</I> on <I>September, 12 2004</I>using <A HREF="http://www.mathematik.uni-kl.de/~obachman/Texi2html"><I>texi2html</I></A></BODY></HTML>⌨️ 快捷键说明
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