📄 autobook_200.html
字号:
<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; } if (checkVersion()) { homeon = new Image(); homeon.src = "homeon.png"; homeoff = new Image(); homeoff.src = "home.png"; tocon = new Image(); tocon.src = "tocon.png"; tocoff = new Image(); tocoff.src = "toc.png"; indexon = new Image(); indexon.src = "indexon.png"; indexoff = new Image(); indexoff.src = "index.png"; helpon = new Image(); helpon.src = "helpon.png"; helpoff = new Image(); helpoff.src = "help.png"; backon = new Image(); backon.src = "backon.png"; backoff = new Image(); backoff.src = "back.png"; forwardon = new Image(); forwardon.src = "forwardon.png"; forwardoff = new Image(); forwardoff.src = "forward.png"; prevon = new Image(); prevon.src = "prevon.png"; prevoff = new Image(); prevoff.src = "prev.png"; nexton = new Image(); nexton.src = "nexton.png"; nextoff = new Image(); nextoff.src = "next.png"; upon = new Image(); upon.src = "upon.png"; upoff = new Image(); upoff.src = "up.png"; } function img_act(imgName) { if (checkVersion()) { imgOn = eval(imgName + "on.src"); document [imgName].src = imgOn; } } function img_inact(imgName) { if (checkVersion()) { imgOff = eval(imgName + "off.src"); document [imgName].src = imgOff; } }// --></SCRIPT></HEAD><BODY LANG="EN" BGCOLOR="#FFFFFF" TEXT="#000000" LINK="#6688AA" VLINK="#336688" ALINK="#808080"><A NAME="SEC200"></A><TABLE BORDER=0 CELLPADDING=0 CELLSPACING=10><TR VALIGN="TOP"><TD ALIGN="MIDDLE" WIDTH=50 BGCOLOR="#e6e6e6"><TABLE CELLPADDING=1 CELLSPACING=1 BORDER=0><TR VALIGN="TOP" ALIGN="LEFT"><TD VALIGN="MIDDLE" ALIGN="LEFT"><A HREF="autobook_199.html#SEC199" onMouseover="img_act('prev')" onMouseout="img_inact('prev')"><IMG SRC="prev.png" BORDER="0" ALT="Back: Discarding input" ALIGN="MIDDLE" NAME="prev"></A></TD></TR><TR VALIGN="TOP" ALIGN="LEFT"><TD VALIGN="MIDDLE" ALIGN="LEFT"><A HREF="autobook_201.html#SEC201" onMouseover="img_act('next')" onMouseout="img_inact('next')"><IMG SRC="next.png" BORDER="0" ALT="Forward: Conditionals" ALIGN="MIDDLE" NAME="next"></A></TD></TR><TR VALIGN="TOP" ALIGN="LEFT"><TD VALIGN="MIDDLE" ALIGN="LEFT"> <TR VALIGN="TOP" ALIGN="LEFT"><TD VALIGN="MIDDLE" ALIGN="LEFT"><A HREF="autobook_201.html#SEC201" onMouseover="img_act('back')" onMouseout="img_inact('back')"><IMG SRC="back.png" BORDER="0" ALT="FastBack: Conditionals" ALIGN="MIDDLE" NAME="back"></A></TD></TR><TR VALIGN="TOP" ALIGN="LEFT"><TD VALIGN="MIDDLE" ALIGN="LEFT"><A HREF="autobook_198.html#SEC198" onMouseover="img_act('up')" onMouseout="img_inact('up')"><IMG SRC="up.png" BORDER="0" ALT="Up: Features of M4" ALIGN="MIDDLE" NAME="up"></A></TD></TR><TR VALIGN="TOP" ALIGN="LEFT"><TD VALIGN="MIDDLE" ALIGN="LEFT"><A HREF="autobook_205.html#SEC205" onMouseover="img_act('forward')" onMouseout="img_inact('forward')"><IMG SRC="forward.png" BORDER="0" ALT="FastForward: Writing macros within the GNU Autotools framework" 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"><H3> 21.4.2 Macro management </H3><!--docid::SEC200::--><P>A number of built-in macros exist in M4 to manage macros. 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>⌨️ 快捷键说明
复制代码
Ctrl + C
搜索代码
Ctrl + F
全屏模式
F11
切换主题
Ctrl + Shift + D
显示快捷键
?
增大字号
Ctrl + =
减小字号
Ctrl + -