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content="faqproc"><title>Question 11.33</title><link href="extensions.html" rev=precedes><link href="compliance.html" rel=precedes><link href="index.html" rev=subdocument></head><body bgcolor="#ffffff"><a href="extensions.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="compliance.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 11.33</h1><p><font face=Helvetica size=8 color=blue><b>Q:</b></font>People seem to make a point of distinguishingbetweenimplementation-defined,unspecified,andundefinedbehavior.What do these mean?</p><p><hr><p><font face=Helvetica size=8 color=blue><b>A:</b></font>First of all,allthree of these represent areasin which the C Standarddoes <em>not</em> specify exactly what a particular construct,or a program which uses it,must do.This looseness in C's definition is traditional and deliberate:it permits compiler writers to(a)makechoiceswhich allow efficient code to be generatedby arranging that various constructsare implemented as ``however the hardware does them''(see also question <a href="../fp/strangefp.html">14.4a</a>),and(b)ignore(that is,avoid worrying aboutgenerating correct code for)certain marginal constructswhich aretoo difficult to define preciselyand which probably aren't useful to well-written programs anyway(see for examplethe code fragments inquestions<a href="../expr/evalorder1.html">3.1</a>,<a href="../expr/evalorder2.html">3.2</a>,and<a href="../expr/ieqiplusplus.html">3.3</a>).</p><p>These three variations on``not precisely defined by the standard''are defined as:</p><p><b>implementation-defined:</b>The implementation must pick some behavior;it may not fail to compile the program.(The program using the construct is not incorrect.)The choice must be documented.The Standard may specifya set of allowable behaviorsfrom which to choose,or it may impose no particular requirements.</p><p><b>unspecified:</b>Like implementation-defined,except that the choice need not be documented.</p><p><b>undefined:</b>Anything at all can happen;the Standard imposes no requirements.The program may fail to compile,or it may execute incorrectly(either crashing or silently generating incorrect results),or it may fortuitously do exactly what the programmer intended.</p><p>Note, too,that since the Standard imposesabsolutely norequirementson the behavior of a compiler faced with an instance of undefined behavior,the compiler(more importantly, any generated code)can do absolutely anything.In particular,there is no guaranteethatat mostthe undefined bit of the program will behave badly,andthat the rest of the program will perform normally.It's perilousto think that you can tolerate undefined behaviorin aprogram,imagining that its undefinedness can't hurt;the undefined behavior can be more undefined than you think it can.(Seequestion <a href="../expr/evalorder2.html">3.2</a>for a relatively simpleexample.)</p><p>Since many people seem to have troublecomprehending the depths to which undefined behavor can descend,it is traditional to come up with eye-catching, outrageous examples.Undefined means that,notwithstanding question <a href="../bool/bool2.html">9.2</a>,<TT>printf("%d", j++ <= j);</TT>can print 42, or ``forty-two.''</p><p>If you'reinterested inwriting portable code,you canignore the distinctions,as you'llusuallywant to avoid code that depends on any of the three behaviors.</p><p>See also questions<a href="../expr/evalorder4.html">3.9</a>,and<a href="appalled.html">11.34</a>.</p><p>(A fourth defined class of not-quite-precisely-defined behavior,without the same stigma attached to it,is <a href="../sx1/index.html#locale-specific"><dfn>locale-specific</dfn></a>.)</p><p>References:ISO Sec. 3.10, Sec. 3.16, Sec. 3.17<br>Rationale Sec. 1.6<br></p><!-- aend --><p><hr><a href="extensions.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="compliance.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 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