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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: Automake Conditionals</TITLE><META NAME="description" CONTENT="Autoconf, Automake, and Libtool: Automake Conditionals"><META NAME="keywords" CONTENT="Autoconf, Automake, and Libtool: Automake Conditionals"><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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Aconditional is introduced in <TT>`configure.in'</TT> using the<SAMP>`AM_CONDITIONAL'</SAMP> macro. This macro takes two arguments: the firstis the name of the condition, and the second is a shell expression whichreturns true when the condition is true.</P><P>For instance, here is how to make a condition named <SAMP>`TRUE'</SAMP> which isalways true:</P><P><TABLE width=100%><tr><td> </td><td class=example bgcolor=#6688aa><br><pre>AM_CONDITIONAL(TRUE, true)</pre></td></tr></table></P><P>As another example, here is how to make a condition named<SAMP>`DEBUG'</SAMP> which is true when the user has given the<SAMP>`--enable-debug'</SAMP> option to <CODE>configure</CODE>:</P><P><TABLE width=100%><tr><td> </td><td class=example bgcolor=#6688aa><br><pre>AM_CONDITIONAL(DEBUG, test "$enable_debug" = yes)</pre></td></tr></table></P><P>Once you've defined a condition in <TT>`configure.in'</TT>, you can refer toit in your <TT>`Makefile.am'</TT> using the <SAMP>`if'</SAMP> statement. Here is apart of a sample <TT>`Makefile.am'</TT> that uses the conditions definedabove:</P><P><TABLE width=100%><tr><td> </td><td class=example bgcolor=#6688aa><br><pre>if TRUE## This is always used.bin_PROGRAMS = fooendifif DEBUGAM_CFLAGS = -g -DDEBUGendif</pre></td></tr></table></P><P>It's important to remember that Automake conditionals are<EM>configure-time</EM> conditionals. They don't rely on any specialfeature of <CODE>make</CODE>, and there is no way for the user to affect theconditionals from the <CODE>make</CODE> command line. Automake conditionalswork by rewriting the <TT>`Makefile'</TT> -- <CODE>make</CODE> is unaware thatthese conditionals even exist.</P><P>Traditionally, Automake conditionals have been considered an advancedfeature. However, practice has shown that they are often easier to useand understand than other approaches to solving the same problem. I nowrecommend the use of conditionals to everyone.</P><P>For instance, consider this example:</P><P><TABLE width=100%><tr><td> </td><td class=example bgcolor=#6688aa><br><pre>bin_PROGRAMS = echoif FULL_ECHOecho_SOURCES = echo.c extras.c getopt.celseecho_SOURCES = echo.cendif</pre></td></tr></table></P><P>In this case, the equivalent code without conditionals is more confusingand correspondingly more difficult for the new Automake user to figureout:</P><P><TABLE width=100%><tr><td> </td><td class=example bgcolor=#6688aa><br><pre>bin_PROGRAMS = echoecho_SOURCES = echo.cecho_LDADD = @echo_extras@EXTRA_echo_SOURCES = extras.c getopt.c</pre></td></tr></table></P><P>Automake conditionals have some limitations. One known problem is thatconditionals don't interact properly with <SAMP>`+='</SAMP> assignment. Forinstance, consider this code:</P><P><TABLE width=100%><tr><td> </td><td class=example bgcolor=#6688aa><br><pre>bin_PROGRAMS = zz_SOURCES = z.cif SOME_CONDITIONz_SOURCES += cond.cendif</pre></td></tr></table></P><P>This code appears to have an unambiguous meaning, but Automake 1.4doesn't implement this and will give an error. This bug will be fixedin the next major Automake release.</P><P><A NAME="Language support"></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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