📄 scanfvsprintf.html
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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!-- but may not be published or retransmitted without explicit permission.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 stdio.sgml dated Wed Dec 21 13:07:57 2005 --><!-- corresponding to FAQ list version 4.0 --><html><!-- Mirrored from c-faq.com/stdio/scanfvsprintf.html by HTTrack Website Copier/3.x [XR&CO'2008], Sat, 14 Mar 2009 07:58:57 GMT --><head><meta name=GENERATOR content="faqproc"><title>Question 12.9</title><link href="varargproto.html" rev=precedes><link href="printftypedef.html" rel=precedes><link href="index.html" rev=subdocument></head><body bgcolor="#ffffff"><a href="varargproto.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="printftypedef.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 12.9</h1><p><font face=Helvetica size=8 color=blue><b>Q:</b></font>Someone told me it was wrong to use <TT>%lf</TT> with <TT>printf</TT>.How can<TT>printf</TT> use <TT>%f</TT> for type <TT>double</TT>,if <TT>scanf</TT> requires <TT>%lf</TT>?</p><p><hr><p><font face=Helvetica size=8 color=blue><b>A:</b></font>It's true that <TT>printf</TT>'s <TT>%f</TT> specifierworks with both <TT>float</TT> and <TT>double</TT>arguments.<a href="fpfmts.html" rel=subdocument>[footnote]</a>Due to the``default argument promotions''(which apply in variable-length argument lists<a href="argpromo.html" rel=subdocument>[footnote]</a>such as<TT>printf</TT>'s,whether or not prototypes are in scope),values of type <TT>float</TT> are promoted to <TT>double</TT>,and <TT>printf</TT> therefore sees only <TT>double</TT>s.(<TT>printf</TT> does accept <TT>%Lf</TT>, for <TT>long double</TT>.)See also question<a href="../varargs/promos.html">15.2</a>.</p><p><TT>scanf</TT>, on the other hand,accepts pointers,and no such promotions apply.Storing into a <TT>float</TT>(via a pointer)is very different fromstoring into a <TT>double</TT>,so <TT>scanf</TT> distinguishes between <TT>%f</TT> and <TT>%lf</TT>.</p><p>Here is a table listing the argument typesexpected by <TT>printf</TT> and <TT>scanf</TT>for the various format specifiers:<blockquote>[TABLE GOES HERE]</blockquote></p><p>(Strictly speaking,<TT>%lf</TT> is undefinedunder <TT>printf</TT>,thoughmanysystems probably accept it.To be portable,alwaysuse <TT>%f</TT>.)</p><p>See also question <a href="scanf2.html">12.13</a>.</p><p>References:K&R1 Sec. 7.3 pp. 145-47, Sec. 7.4 pp. 147-50<br>K&R2 Sec. 7.2 pp. 153-44, Sec. 7.4 pp. 157-59<br>ISO Sec. 7.9.6.1, Sec. 7.9.6.2<br>H&S Sec. 15.8 pp. 357-64, Sec. 15.11 pp. 366-78<br>CT&P Sec. A.1 pp. 121-33<br></p><!-- aend --><p><hr><a href="varargproto.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="printftypedef.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/stdio/scanfvsprintf.html by HTTrack Website Copier/3.x [XR&CO'2008], Sat, 14 Mar 2009 07:58:57 GMT --></html>⌨️ 快捷键说明
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