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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!-- but may not be published or retransmitted without explicit permission.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 ansi.sgml dated Wed Aug 11 00:43:45 2004 --><!-- corresponding to FAQ list version 4.0 --><html><!-- Mirrored from c-faq.com/ansi/argpromos.html by HTTrack Website Copier/3.x [XR&CO'2008], Sat, 14 Mar 2009 07:58:54 GMT --><head><meta name=GENERATOR content="faqproc"><title>Question 11.3</title><link href="updates.html" rev=precedes><link href="mixoldandnew.html" rel=precedes><link href="index.html" rev=subdocument></head><body bgcolor="#ffffff"><a href="updates.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="mixoldandnew.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 11.3</h1><p><font face=Helvetica size=8 color=blue><b>Q:</b></font>My ANSI compilercomplains about a mismatch when it sees<pre> extern int func(float); int func(x) float x; { ...</pre></p><p><hr><p><font face=Helvetica size=8 color=blue><b>A:</b></font>You have mixed the new-styleprototypedeclaration ``<TT>extern int func(float);</TT>''with the old-style definition ``<TT>int func(x) float x;</TT>''.It is usuallypossibleto mix thetwostyles(see question<a href="mixoldandnew.html">11.4</a>),but not in this case.</p><p>Old C(and ANSI C, in the absence of prototypes,and invariable-length argument lists;see question <a href="../varargs/promos.html">15.2</a>)``widens'' certain arguments when they are passed to functions.<TT>float</TT>sarepromoted to <TT>double</TT>,andcharacters andshort integersare promoted to <TT>int</TT>.(For old-style function definitions,the values are automatically converted backto the corresponding narrower typeswithin the body of the called function,if they are declared that way there.)Therefore,the old-style definition aboveactually says that <TT>func</TT> takes a <TT>double</TT>(which will be converted to <TT>float</TT> inside the function).</p><p>This problem can be fixed either by using new-style syntaxconsistently in the definition:<pre> int func(float x) { ... }</pre></p><p>or by changing the new-style prototype declaration to match the old-styledefinition:<pre> extern int func(double);</pre>(In this case, it would be clearest to change the old-styledefinition to use <TT>double</TT> as well.<a href="fn49.html" rel=subdocument>[footnote]</a>)</p><p>Itis arguably muchsaferto avoid ``narrow''(<TT>char</TT>, <TT>short int</TT>, and <TT>float</TT>)function arguments and returntypesaltogether.</p><p>See also question<a href="../decl/implfdecl.html">1.25</a>.</p><p>References:K&R1 Sec. A7.1 p. 186<br>K&R2 Sec. A7.3.2 p. 202<br>ISO Sec. 6.3.2.2, Sec. 6.5.4.3<br>Rationale Sec. 3.3.2.2, Sec. 3.5.4.3<br>H&S Sec. 9.2 pp. 265-7, Sec. 9.4 pp. 272-3<br></p><!-- aend --><p><hr><a href="updates.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="mixoldandnew.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/ansi/argpromos.html by HTTrack Website Copier/3.x [XR&CO'2008], Sat, 14 Mar 2009 07:58:54 GMT --></html>⌨️ 快捷键说明
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