📄 autobook_229.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 interface</TITLE><META NAME="description" CONTENT="Autoconf, Automake, and Libtool: Macro interface"><META NAME="keywords" CONTENT="Autoconf, Automake, and Libtool: Macro interface"><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="SEC229"></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_228.html#SEC228" onMouseover="img_act('prev')" onMouseout="img_inact('prev')"><IMG SRC="prev.png" BORDER="0" ALT="Back: Naming macros" ALIGN="MIDDLE" NAME="prev"></A></TD></TR><TR VALIGN="TOP" ALIGN="LEFT"><TD VALIGN="MIDDLE" ALIGN="LEFT"><A HREF="autobook_230.html#SEC230" onMouseover="img_act('next')" onMouseout="img_inact('next')"><IMG SRC="next.png" BORDER="0" ALT="Forward: Implementation specifics" 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_230.html#SEC230" onMouseover="img_act('back')" onMouseout="img_inact('back')"><IMG SRC="back.png" BORDER="0" ALT="FastBack: Implementation specifics" ALIGN="MIDDLE" NAME="back"></A></TD></TR><TR VALIGN="TOP" ALIGN="LEFT"><TD VALIGN="MIDDLE" ALIGN="LEFT"><A HREF="autobook_224.html#SEC224" onMouseover="img_act('up')" onMouseout="img_inact('up')"><IMG SRC="up.png" BORDER="0" ALT="Up: Guidelines for writing macros" ALIGN="MIDDLE" NAME="up"></A></TD></TR><TR VALIGN="TOP" ALIGN="LEFT"><TD VALIGN="MIDDLE" ALIGN="LEFT"><A HREF="autobook_230.html#SEC230" onMouseover="img_act('forward')" onMouseout="img_inact('forward')"><IMG SRC="forward.png" BORDER="0" ALT="FastForward: Implementation specifics" 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> 23.3.5 Macro interface </H3><!--docid::SEC229::--><P>When designing your macro, it is worth spending some time deciding onwhat your macro's interface--the macro's name and argument list--willbe. Often, it will be possible to extract general purposefunctionality into a generic macro and to write a second macro which isa client of the generic one. Like planning the prototype for a Cfunction, this is usually a straightforward process of deciding whatarguments are required by the macro to perform its function. However,there are a couple of further considerations and they are discussedbelow.</P><P>M4 macros refer to their arguments by number with a syntax suchas <CODE>$1</CODE>. It is typically more difficult to read an M4 macrodefinition and understand what each argument's designation is than in aC function body, where the formal argument is referred to by its name.Therefore, it's a good idea to include a standard comment block aboveeach macro that documents the macro and gives an indication of what eachargument is for. Here is an example from the Autoconf source code:</P><P><TABLE width=100%><tr><td> </td><td class=example bgcolor=#6688aa><br><pre># AC_CHECK_FILE(FILE, [ACTION-IF-FOUND], [ACTION-IF-NOT-FOUND])# -------------------------------------------------------------## Check for the existence of FILE.</pre></td></tr></table></P><P>To remain general purpose, the existing Autoconf macros follow theconvention of keeping side-effects outside the definition of the macro.Here, when a user invokes <SAMP>`AC_CHECK_FILE'</SAMP>, they must provide shellcode to implement the side effect that they want to occur if the<SAMP>`FILE'</SAMP> is found or is not found. Some macros implement a basic anddesirable action like defining a symbol like <SAMP>`HAVE_UNISTD_H'</SAMP> if nouser-defined actions are provided. In general, your macros shouldprovide an interface which is consistent with the interfaces provided bythe core Autoconf macros.</P><P>M4 macros may have variable argument lists, so it is possible toimplement macros which have defaults for arguments. By testing eachindividual argument against the empty string with <SAMP>`ifelse'</SAMP>, it ispossible for users to accept the default behavior for individualarguments by passing empty values:</P><P><TABLE width=100%><tr><td> </td><td class=example bgcolor=#6688aa><br><pre>AC_CHECK_FILE([/etc/passwd], [], [AC_MSG_ERROR([something is really wrong])])</pre></td></tr></table></P><P>One final point to consider when designing the interface for a macro ishow to handle macros that are generic in nature and, say, wish to set acache variable whose name is based on one of the arguments. Considerthe <SAMP>`AC_CHECK_HEADER'</SAMP> macro--it defines a symbol and makes an entryin the cache that reflects the result of the test it performs.<SAMP>`AC_CHECK_HEADER'</SAMP> takes an argument -- namely the name of a headerfile to look for. This macro cannot just make a cache entry with a namelike <CODE>ac_cv_check_header</CODE>, since it would only work once; anyfurther uses of this macro in <TT>`configure.in'</TT> would cause anincorrect result to be drawn from the cache. Instead, the name of thesymbol that is defined and the name of the cache variable that is setneed to be computed from one of the arguments: the name of the headerfile being sought. What we really need is to define<CODE>HAVE_UNISTD_H</CODE> and set the cache variable<CODE>ac_cv_header_unistd_h</CODE>. This can be achieved with some <CODE>sed</CODE>and <CODE>tr</CODE> magic in the macro which transforms the filename intouppercase characters for the call to <CODE>AC_DEFINE</CODE> and into lowercasefor the cache variable name. Unknown characters such as <SAMP>`.'</SAMP> needto be transformed into underscores.</P><P>Some existing macros also allow the user to pass in the name of a cachevariable name so that the macro does not need to compute a name. Ingeneral, this should be avoided, as it makes the macro harder to use andexposes details of the caching system to the user.</P><P><A NAME="Implementation specifics"></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 + -