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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: Installing Header Files</TITLE><META NAME="description" CONTENT="Autoconf, Automake, and Libtool: Installing Header Files"><META NAME="keywords" CONTENT="Autoconf, Automake, and Libtool: Installing Header Files"><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; 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<TR VALIGN="TOP" ALIGN="LEFT"><TD VALIGN="MIDDLE" ALIGN="LEFT"><A HREF="autobook_97.html#SEC97" onMouseover="img_act('back')" onMouseout="img_inact('back')"><IMG SRC="back.png" BORDER="0" ALT="FastBack: Including Texinfo Documentation" ALIGN="MIDDLE" NAME="back"></A></TD></TR><TR VALIGN="TOP" ALIGN="LEFT"><TD VALIGN="MIDDLE" ALIGN="LEFT"><A HREF="autobook_93.html#SEC93" onMouseover="img_act('up')" onMouseout="img_inact('up')"><IMG SRC="up.png" BORDER="0" ALT="Up: A Large GNU Autotools Project" ALIGN="MIDDLE" NAME="up"></A></TD></TR><TR VALIGN="TOP" ALIGN="LEFT"><TD VALIGN="MIDDLE" ALIGN="LEFT"><A HREF="autobook_99.html#SEC99" onMouseover="img_act('forward')" onMouseout="img_inact('forward')"><IMG SRC="forward.png" BORDER="0" ALT="FastForward: Rolling Distribution Tarballs" ALIGN="MIDDLE" NAME="forward"></A></TD></TR><TR VALIGN="TOP" ALIGN="LEFT"><TD VALIGN="MIDDLE" ALIGN="LEFT"><A HREF="autobook.html#SEC_Top" onMouseover="img_act('home')" onMouseout="img_inact('home')"><IMG SRC="home.png" BORDER="0" ALT="Top: Autoconf, Automake, and Libtool" ALIGN="MIDDLE" NAME="home"></A></TD></TR><TR VALIGN="TOP" ALIGN="LEFT"><TD VALIGN="MIDDLE" ALIGN="LEFT"><A HREF="autobook_toc.html#SEC_Contents" onMouseover="img_act('toc')" onMouseout="img_inact('toc')"><IMG SRC="toc.png" BORDER="0" ALT="Contents: Table of Contents" ALIGN="MIDDLE" NAME="toc"></A></TD></TR><TR VALIGN="TOP" ALIGN="LEFT"><TD VALIGN="MIDDLE" ALIGN="LEFT"><A HREF="autobook_285.html#SEC285" onMouseover="img_act('index')" onMouseout="img_inact('index')"><IMG SRC="index.png" BORDER="0" ALT="Index: Index" ALIGN="MIDDLE" NAME="index"></A></TD></TR><TR VALIGN="TOP" ALIGN="LEFT"><TD VALIGN="MIDDLE" ALIGN="LEFT"><A HREF="autobook_abt.html#SEC_About" onMouseover="img_act('help')" onMouseout="img_inact('help')"><IMG SRC="help.png" BORDER="0" ALT="About: About this document" ALIGN="MIDDLE" NAME="help"></A></TD></TR></TABLE></TD><TD ALIGN="LEFT"><H2> 12.3 Installing Header Files </H2><!--docid::SEC96::--><P><BLOCKQUOTE>One of the more difficult problems with GNU Autotools driven projects isthat each of them depends on <TT>`config.h'</TT> (or its equivalent) and theproject specific symbols that it defines.  The purpose of this file isto be <CODE>#include</CODE>d from all of the project source files.  Thepreprocessor can tailor then the code in these files to the targetenvironment.<P>It is often difficult and sometimes impossible to not introduce adependency on <TT>`config.h'</TT> from one of the project's installableheader files. It would be nice if you could simply install the generated<TT>`config.h'</TT>, but even if you name it carefully or install it to asubdirectory to avoid filename problems, the macros it defines willclash with those from any other GNU Autotools based project which alsoinstalls <EM>its</EM> <TT>`config.h'</TT>.</P><P>For example, if Sic installed its <TT>`config.h'</TT> as<TT>`/usr/include/sic/config.h'</TT>, and had <SAMP>`#include &#60;sic/config.h&#62;'</SAMP>in the installed <TT>`common.h'</TT>, when another GNU Autotools based projectcame to use the Sic library it might begin like this:</P><P><TABLE width=100%><tr><td>&nbsp;</td><td class=example bgcolor=#6688aa><br><pre>#if HAVE_CONFIG_H#  include &#60;config.h&#62;#endif#if HAVE_SIC_H#  include &#60;sic.h&#62;#endifstatic const char version_number[] = VERSION;</pre></td></tr></table></P><P>But, <TT>`sic.h'</TT> says <SAMP>`#include &#60;sic/common.h&#62;'</SAMP>, which in turnsays <SAMP>`#include &#60;sic/config.h&#62;'</SAMP>.  Even though the other project hasthe correct value for <SAMP>`VERSION'</SAMP> in its own <TT>`config.h'</TT>, by thetime the preprocessor reaches the <SAMP>`version_number'</SAMP> definition, ithas been redefined to the value in <TT>`sic/config.h'</TT>.  Imagine themess you could get into if you were using several libraries which eachinstalled their own <TT>`config.h'</TT> definitions.  GCC issues awarning when a macro is redefined to a different value which would helpyou to catch this error.  Some compilers do not issue a warning, andperhaps worse, other compilers will warn even if the repeateddefinitions have the same value, flooding you with hundreds of warningsfor each source file that reads multiple <TT>`config.h'</TT> headers.</P><P>The Autoconf macro <CODE>AC_OUTPUT_COMMANDS</CODE><A NAME="DOCF25" HREF="autobook_fot.html#FOOT25">(25)</A> provides a way to solve this problem.  Theidea is to generate a system specific but installable header from theresults of the various tests performed by <CODE>configure</CODE>.  There isa 1-to-1 mapping between the preprocessor code that relied on theconfigure results written to <TT>`config.h'</TT>, and the new shell codethat relies on the configure results saved in <TT>`config.cache'</TT>.</P><P></BLOCKQUOTE><P>The following code is a snippet from <TT>`configure.in'</TT>, in the body ofthe <CODE>AC_OUTPUT_COMMANDS</CODE> macro:</P><P><TABLE width=100%><tr><td>&nbsp;</td><td class=example bgcolor=#6688aa><br><pre></pre></td></tr></table></P><P>Compare this with the equivalent C pre-processor code from<TT>`sic/common.h'</TT>, which it replaces:</P><P><TABLE width=100%><tr><td>&nbsp;</td><td class=example bgcolor=#6688aa><br><pre>#if STDC_HEADERS || HAVE_STDLIB_H#  include &#60;stdlib.h&#62;#endif#if HAVE_UNISTD_H#  include &#60;unistd.h&#62;#endif#if HAVE_SYS_WAIT_H#  include &#60;sys/wait.h&#62;#endif#if HAVE_ERRNO_H#  include &#60;errno.h&#62;#endif#ifndef errno/* Some systems #define this! */extern int errno;#endif#if HAVE_STRING_H#  include &#60;string.h&#62;#else#  if HAVE_STRING_H#    include &#60;strings.h&#62;#  endif#endif#if HAVE_ASSERT_H#  include &#60;assert.h&#62;#  define SIC_ASSERT assert#else#  define SIC_ASSERT(expr) ((void) 0)#endif</pre></td></tr></table></P><P>Apart from the mechanical process of translating the preprocessor code,there is some plumbing needed to ensure that the <TT>`common.h'</TT> filegenerated by the new code in <TT>`configure.in'</TT> is functionallyequivalent to the old code, and is generated in a correct and timelyfashion.</P><P>Taking my lead from some of the Automake generated <CODE>make</CODE> rulesto regenerate <TT>`Makefile'</TT> from <TT>`Makefile.in'</TT> by calling<TT>`config.status'</TT>, I have added some similar rules to<TT>`sic/Makefile.am'</TT> to regenerate <TT>`common.h'</TT> from<TT>`common-h.in'</TT>.</P><P><TABLE width=100%><tr><td>&nbsp;</td><td class=example bgcolor=#6688aa><br><pre></pre></td></tr></table></P><P>The way that <CODE>AC_OUTPUT_COMMANDS</CODE> works, is to copy the containedcode into <CODE>config.status</CODE> (see section <A HREF="autobook_276.html#SEC276">C. Generated File Dependencies</A>).  It is actually <CODE>config.status</CODE> that creates thegenerated files -- for example, <CODE>automake</CODE> generated<BR><TT>`Makefile'</TT>s are able to regenerate themselves from corresponding<TT>`Makefile.in'</TT>s by calling <CODE>config.status</CODE> if they becomeout of date.  Unfortunately, this means that <CODE>config.status</CODE>doesn't have direct access to the cache values generated while<CODE>configure</CODE> was running (because it has finished its work by thetime <CODE>config.status</CODE> is called).  It is tempting to read in thecache file at the top of the code inside <CODE>AC_OUTPUT_COMMANDS</CODE>, butthat only works if you know where the cache file is saved.  Also thepackage installer can use the <SAMP>`--cache-file'</SAMP> option of<CODE>configure</CODE> to change the location of the file, or turn offcaching entirely with <SAMP>`--cache-file=/dev/null'</SAMP>.</P><P><CODE>AC_OUTPUT_COMMANDS</CODE> accepts a second argument which can be usedto pass the variable settings discovered by <CODE>configure</CODE> into<CODE>config.status</CODE>.  It's not pretty, and is a little error prone.In the first argument to <CODE>AC_OUTPUT_COMMANDS</CODE>, you must be carefulto check that <STRONG>every single</STRONG> configure variable referenced iscorrectly set somewhere in the second argument.</P><P>A slightly stripped down example from the sic project<TT>`configure.in'</TT> looks like this:</P><P><TABLE width=100%><tr><td>&nbsp;</td><td class=example bgcolor=#6688aa><br><pre></pre></td></tr></table></P><P>You will notice that the contents of <TT>`common-h.in'</TT> are copied into<TT>`common.h'</TT> verbatim as it is generated.  It's just an easy way ofcollecting together the code that belongs in <TT>`common.h'</TT>, but whichdoesn't rely on configuration tests, without cluttering<TT>`configure.in'</TT> any more than necessary.</P><P>I should point out that, although this method has served me well for anumber of years now, it is inherently fragile because it relies onundocumented internals of both Autoconf and Automake.  There is a veryreal possibility that if you also track the latest releases ofGNU Autotools, it may stop working.  Future releases of GNU Autotools willaddress the interface problems that force us to use code like this, forthe lack of a better way to do things.</P><P><A NAME="Including Texinfo Documentation"></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>
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