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<!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!-- but may not be published or retransmitted without explicit permission.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/nargs.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 15.8</title><link href="oldvarargs.html" rev=precedes><link href="onefixedarg.html" rel=precedes><link href="index.html" rev=subdocument></head><body bgcolor="#ffffff"><a href="oldvarargs.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="onefixedarg.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 15.8</h1><p><font face=Helvetica size=8 color=blue><b>Q:</b></font>How can I discover how many arguments a function was actuallycalled with?</p><p><hr><p><font face=Helvetica size=8 color=blue><b>A:</b></font>This information is not available to a portable program.Some old systems provided a nonstandard <TT>nargs</TT>function,but its use was always questionable,since it typically returned the numberof words passed,not the number of arguments.(Structures,<TT>long int</TT>s,andfloating point valuesare usually passed asseveral words.)</p><p>Any function which takes a variable number of arguments must beable to determine<em>from the arguments themselves</em>how many of themthere are.<TT>printf</TT>-like functions do this by looking for formattingspecifiers (<TT>%d</TT> and the like) in the format string (which is whythese functions fail badly if the format string does not matchthe argument list).Another common technique,applicablewhen the arguments are all of the same type,is to use a sentinelvalue(often 0, -1, or an appropriately-cast null pointer)at the end of the list(see the<TT>execl</TT>and<TT>vstrcat</TT>examplesinquestions<a href="../null/null2.html">5.2</a>and<a href="varargs1.html">15.4</a>).Finally,if thetypes are predictable,you can pass an explicit count of the number of variable arguments(although it's usually a nuisance for the caller to supply).</p><p>References:PCS Sec. 11 pp. 167-8<br></p><!-- aend --><p><hr><a href="oldvarargs.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="onefixedarg.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/nargs.html by HTTrack Website Copier/3.x [XR&CO'2008], Sat, 14 Mar 2009 07:59:00 GMT --></html>⌨️ 快捷键说明
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