📄 seqpoints.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!-- but may not be published or retransmitted without explicit permission.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 21:47:45 2005 by faqproc version 2.7 --><!-- from source file expr.sgml dated Sat Jul 3 17:10:33 2004 --><!-- corresponding to FAQ list version 4.0 --><html><!-- Mirrored from c-faq.com/expr/seqpoints.html by HTTrack Website Copier/3.x [XR&CO'2008], Sat, 14 Mar 2009 07:58:47 GMT --><head><meta name=GENERATOR content="faqproc"><title>Question 3.8</title><link href="comma.html" rev=precedes><link href="evalorder4.html" rel=precedes><link href="index.html" rev=subdocument></head><body bgcolor="#ffffff"><a href="comma.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="evalorder4.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 3.8</h1><p><font face=Helvetica size=8 color=blue><b>Q:</b></font>How can I understandcomplex expressions like the ones in this section,and avoid writing undefined ones?What's a ``sequence point''?</p><p><hr><p><font face=Helvetica size=8 color=blue><b>A:</b></font>A sequence pointisapointin timeat whichthe dusthas settledandall side effectswhich have been seen so farare guaranteed to be complete.The sequence points listed in the C standard are:<UL><li>at the end ofthe evaluation ofa <a href="../sx1/index.html#full expression"><dfn>full expression</dfn></a>(a full expression is an expression statement,or any other expression which is nota subexpression withinany larger expression);<li>at the <TT>||</TT>, <TT>&&</TT>, <TT>?:</TT>, and comma operators;and<li>at a function call(after the evaluation of all the arguments,andjust before the actual call).</UL></p><p>TheStandard states that<blockquote>Between the previous and next sequence pointan object shall have its stored value modifiedat most once by the evaluation of an expression.Furthermore,the prior value shall be accessed onlyto determine the value to be stored.</blockquote></p><p>These two rather opaque sentencessay several things.First,they talk about operations bounded bythe``previous and next sequence points'';such operationsusually correspond tofull expressions.(In an expression statement,the ``next sequence point''isusuallyat the terminating semicolon,and the ``previous sequence point''isat the end of the previousstatement.An expression may also contain intermediatesequence points,as listed above.)</p><p>The first sentence rules out both the examples<pre> i++ * i++</pre>and<pre> i = i++</pre>from questions <a href="evalorder2.html">3.2</a>and <a href="ieqiplusplus.html">3.3</a>--in both cases,<TT>i</TT> has its value modified twice within the expression,i.e. between sequence points.(If we were to write a similar expressionwhichdidhave an internal sequence point,such as<pre> i++ && i++</pre>it <em>would</em> be well-defined,if questionably useful.)</p><p>The second sentencecan bequitedifficult to understand.It turns out that it disallows code like<pre> a[i] = i++</pre>from question <a href="evalorder1.html">3.1</a>.(Actually, the other expressions we've been discussingare in violation of the second sentence, as well.)To see why,let's first look more carefullyat what the Standard is trying to allow and disallow.</p><p>Clearly,expressions like<pre> a = b</pre>and<pre> c = d + e</pre>which readsome values and use them to write others,are well-defined andlegal.Clearly,<a href="fn16.html" rel=subdocument>[footnote]</a>expressions like<pre> i = i++</pre>which modify the same value twiceare abominationswhich needn't be allowed(or in any case,needn't be well-defined,i.e. we don't have to figure out a way to say what they do,and compilers don't have to support them).Expressionslike theseare disallowed by the first sentence.</p><p>It's also clear<a href="fn16.html" rel=subdocument>[footnote]</a>that we'd like to disallow expressions like<pre> a[i] = i++</pre>which modify <TT>i</TT> <em>and</em> use it along the way,but not disallow expressions like<pre> i = i + 1</pre>whichuse and modify <TT>i</TT>but only modify itlaterwhen it's reasonably easy to ensurethat the final store of the final value(into <TT>i</TT>, in this case)doesn't interfere withtheearlieraccesses.</p><p>And that'swhat the second sentence says:if an object is written to within a full expression,anyand allaccesses to it within the same expressionmust bedirectly involved in the computation ofthe value to be written.This rule effectively constrains legal expressionsto those in which the accesses demonstrably precedethemodification.For example,the old standby<TT>i = i + 1</TT> isallowed,because the access of <TT>i</TT> is used to determine <TT>i</TT>'s final value.The example<pre> a[i] = i++</pre>is disallowed because one of the accessesof <TT>i</TT>(the one in <TT>a[i]</TT>)has nothing to dowith the value which ends up being stored in <TT>i</TT>(which happens over in <TT>i++</TT>),and so there's no good way todefine--either for our understandingor the compiler's--whether the access should take place before or after the incremented value is stored.Since there's no good way to define it,the Standard declares that it is undefined,and that portable programssimply must not use such constructs.</p><p>See alsoquestions <a href="evalorder4.html">3.9</a> and <a href="confused.html">3.11</a>.</p><p>References:ISO Sec. 5.1.2.3, Sec. 6.3, Sec. 6.6, Annex C<br>Rationale Sec. 2.1.2.3<br>H&S Sec. 7.12.1 pp. 228-9<br></p><!-- aend --><p><hr><a href="comma.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="evalorder4.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 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