📄 funcptr.html
字号:
<!DOCTYPE HTML PUBLIC "-//W3O//DTD W3 HTML 2.0//EN"><!-- This collection of hypertext pages is Copyright 1995-2005 by Steve Summit. --><!-- Content from the book "C Programming FAQs: Frequently Asked Questions" --><!-- (Addison-Wesley, 1995, ISBN 0-201-84519-9) is made available here by --><!-- permission of the author and the publisher as a service to the community. --><!-- It is intended to complement the use of the published text --><!-- and is protected by international copyright laws. --><!-- The on-line content may be accessed freely for personal use --><!-- but may not be published or retransmitted without explicit permission. --><!-- --><!-- this page built Sat Dec 24 21:47:46 2005 by faqproc version 2.7 --><!-- from source file varargs.sgml dated Sun Nov 25 08:09:42 2001 --><!-- corresponding to FAQ list version 4.0 --><html><!-- Mirrored from c-faq.com/varargs/funcptr.html by HTTrack Website Copier/3.x [XR&CO'2008], Sat, 14 Mar 2009 07:59:00 GMT --><head><meta name=GENERATOR content="faqproc"><title>Question 15.11</title><link href="float.html" rev=precedes><link href="handoff.html" rel=precedes><link href="index.html" rev=subdocument></head><body bgcolor="#ffffff"><a href="float.html" rev=precedes><img src="../images/buttonleft.gif" alt="prev"></a><a href="index.html" rev=subdocument><img src="../images/buttonup.gif" alt="up"></a><a href="handoff.html" rel=precedes><img src="../images/buttonright.gif" alt="next"></a> <a href="../index-2.html"><img src="../images/buttontop.gif" alt="top/contents"></a><a href="../search.html"><img src="../images/buttonsrch.gif" alt="search"></a><hr><p><!-- qbegin --><h1>comp.lang.c FAQ list<font color=blue>·</font><!-- qtag -->Question 15.11</h1><p><font face=Helvetica size=8 color=blue><b>Q:</b></font>I can't get <TT>va_arg</TT> to pull in an argumentof type pointer-to-function.</p><p><hr><p><font face=Helvetica size=8 color=blue><b>A:</b></font>Try using a <TT>typedef</TT> for the function pointer type.</p><p>The type-rewriting games whichthe <TT>va_arg</TT> macrotypicallyplays are stymiedby overly-complicated types such aspointer-to-function.To illustrate,a simplified implementation of <TT>va_arg</TT> is<pre> #define va_arg(argp, type) \ (*(type *)(((argp) += sizeof(type)) - sizeof(type)))</pre>where <TT>argp</TT>'stype(<TT>va_list</TT>)is <TT>char *</TT>.When you attempt to invoke<pre> va_arg(argp, int (*)())</pre>the expansion is<pre> (*(int (*)() *)(((argp) += sizeof(int (*)())) - sizeof(int (*)())))</pre>which is a syntax error(the first cast <TT>(int (*)() *)</TT> is meaningless).<a href="opaque.html" rel=subdocument>[footnote]</a></p><p>If youusea <TT>typedef</TT>for the function pointer type, however, all will be well.Given<pre> typedef int (*funcptr)();</pre>the expansion of<pre> va_arg(argp, funcptr)</pre>is<pre> (*(funcptr *)(((argp) += sizeof(funcptr)) - sizeof(funcptr)))</pre>which works correctly.</p><p>See alsoquestions <a href="../decl/typedefvsdefine.html">1.13</a>, <a href="../decl/pfitypedef.html">1.17</a>, and <a href="../decl/cdecl1.html">1.21</a>.</p><p>References:ISO Sec. 7.8.1.2<br>Rationale Sec. 4.8.1.2<br></p><!-- aend --><p><hr><a href="float.html" rev=precedes><img src="../images/buttonleft.gif" alt="prev"></a><a href="index.html" rev=subdocument><img src="../images/buttonup.gif" alt="up"></a><a href="handoff.html" rel=precedes><img src="../images/buttonright.gif" alt="next"></a> <a href="../questions.html"><img src="../images/buttontop.gif" alt="contents"></a><a href="../search.html"><img src="../images/buttonsrch.gif" alt="search"></a><br><!-- lastfooter --><a href="../about.html">about this FAQ list</a> <a href="../eskimo.html">about eskimo</a> <a href="../search.html">search</a> <a href="../feedback.html">feedback</a> <a href="copyright.html">copyright</a><p>Hosted by<a href="http://www.eskimo.com/"><img src="../../www.eskimo.com/img/link/eskitiny.gif" alt="Eskimo North"></a></body><!-- Mirrored from c-faq.com/varargs/funcptr.html by HTTrack Website Copier/3.x [XR&CO'2008], Sat, 14 Mar 2009 07:59:00 GMT --></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 + -