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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: Macro management</TITLE><META NAME="description" CONTENT="Autoconf, Automake, and Libtool: Macro management"><META NAME="keywords" CONTENT="Autoconf, Automake, and Libtool: Macro management"><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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Weshall examine the most common ones that you're likely to encounter.There are others and you should consult the GNU M4 manual forfurther information.</P><P>The most obvious one is <CODE>define</CODE>, which defines a macro. Itexpands to the empty string:</P><P><TABLE><tr><td> </td><td class=smallexample><FONT SIZE=-1><pre>define([foo], [bar])dnldefine([combine], [$1 and $2])dnl</FONT></pre></td></tr></table></P><P>It is worth highlighting again the liberal use of quoting. We wish todefine a pair of macros whose names are <EM>literally</EM> <CODE>foo</CODE> and<CODE>combine</CODE>. If another macro had been previously defined witheither of these names, <CODE>m4</CODE> would have expanded the macroimmediately and passed the expansion of <CODE>foo</CODE> to <CODE>define</CODE>,giving unexpected results.</P><P>The <CODE>undefine</CODE> macro will remove a macro's definition fromM4's macro table. It also expands to the empty string:</P><P><TABLE><tr><td> </td><td class=smallexample><FONT SIZE=-1><pre>undefine([foo])dnlundefine([combine])dnl</FONT></pre></td></tr></table></P><P>Recall that once removed from the macro table, unmatched text will oncemore be passed through to the output.</P><P>The <CODE>defn</CODE> macro expands to the definition of a macro, named by thesingle argument to <CODE>defn</CODE>. It is quoted, so that it can be used asthe body of a new, renamed macro:</P><P><TABLE><tr><td> </td><td class=smallexample><FONT SIZE=-1><pre>define([newbie], defn([foo]))dnlundefine([foo])dnl</FONT></pre></td></tr></table></P><P>The <CODE>ifdef</CODE> macro can be used to determine if a macro name has anexisting definition. If it does exist, <CODE>ifdef</CODE> expands to thesecond argument, otherwise it expands to the third:</P><P><TABLE><tr><td> </td><td class=smallexample><FONT SIZE=-1><pre>ifdef([foo], [yes], [no])dnl</FONT></pre></td></tr></table></P><P>Again, <CODE>yes</CODE> and <CODE>no</CODE> have been quoted to prevent expansiondue to any pre-existing macros with those names. <EM>Always</EM> considerthis a real possibility!</P><P>Finally, a word about built-in macros: these macros are all defined foryou when <CODE>m4</CODE> is started. One common problem with these macrosis that they are not in any kind of name space, so it's easier toaccidentally invoke them or want to define a macro with an existingname. One solution is to use the <CODE>define</CODE> and <CODE>defn</CODE>combination shown above to rename all of the macros, one by one. Thisis how Autoconf makes the distinction clear.</P><P><A NAME="Conditionals"></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>?? 快捷鍵說明
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