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<!-- Crown Copyright (c) 1998 -->
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<TITLE>C Checker Reference Manual: Common Errors</TITLE>
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<BODY TEXT="#000000" BGCOLOR="#FFFFFF" LINK="#0000FF" VLINK="#400080" ALINK="#FF0000">
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<A NAME=S108>
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<H1>C Checker Reference Manual</H1>
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<H3>January 1998</H3>
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<DL>
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<DT><A HREF="#S109"><B>8.1 </B> - Introduction</A><DD>
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<DT><A HREF="#S110"><B>8.2 </B> - Enumerations controlling switch
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statements</A><DD>
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<DT><A HREF="#S111"><B>8.3 </B> - Incomplete structures and unions</A><DD>
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<DT><A HREF="#S112"><B>8.4 </B> - Variable shadowing</A><DD>
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<DT><A HREF="#S113"><B>8.5 </B> - Floating point equality</A><DD>
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</DL>
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<HR>
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<H1>8  Common Errors</H1>
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<A NAME=S109>
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<HR><H2>8.1  Introduction</H2>
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Tchk is capable of performing a number of checks for common programming
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mistakes. This chapter describes these checks and controlling pragmas.<P>
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<A NAME=S110>
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<HR><H2>8.2  <A NAME=2>Enumerations controlling switch statements</H2>
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Enumerations are commonly used as control expressions in switch statements.
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When case labels for some of the enumeration constant belonging to
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the enumeration type do not exist and there is no default label, the
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switch statement has no effect for certain possible values of the
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control expression. Checks to detect such switch statements are controlled
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by:<P>
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<PRE>
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	#pragma TenDRA enum switch analysis <EM>status</EM>
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</PRE>
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where <EM>status</EM> is <CODE>on</CODE> (raise an error),
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<CODE>warning</CODE> (produce a warning), or <CODE>off</CODE> (the
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default mode when no errors are produced).<P>
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<A NAME=S111>
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<HR><H2>8.3  <A NAME=4>Incomplete structures and unions</H2>
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ISO C allows for structures or unions to be declared but not defined,
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provided they are not used in a context where it is necessary to know
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the complete structure. For example:<P>
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<PRE>
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	struct tag *p;
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</PRE>
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is allowed, despite the fact that struct tag is incomplete. The TenDRA
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C checker has an option to detect such incomplete structures or unions,
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controlled by:<P>
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<PRE>
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	#pragma TenDRA complete struct/union analysis <EM>status</EM>
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</PRE>
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where <CODE>status</CODE> is <CODE>on</CODE> to give an error as an
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incomplete structure or union is detected, <CODE>warning</CODE> to
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give a warning, or <CODE>off</CODE> to disable the check.<P>
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The check can also be controlled by passing the command-line option
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<CODE>-X:complete_struct=</CODE><CODE>state</CODE> to tchk, where
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<CODE>state</CODE> is <CODE>check</CODE>, <CODE>warn</CODE> or <CODE>dont</CODE>.
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<P>
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The only place where the checker can actually detect that a structure
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or union is incomplete is at the end of the source file. This is because
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it is possible to complete a structure after it has been used. For
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example, in:<P>
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<PRE>
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	struct tag *p;
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	struct tag {
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		int a;
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		int b;
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	};
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</PRE>
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struct tag is complete despite the fact that it was incomplete in
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the definition of p.<P>
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<A NAME=S112>
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<HR><H2>8.4  <A NAME=6>Variable shadowing</H2>
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It is quite legal in C to have a variable in an inner scope, with
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the same name as a variable in an outer scope. These variables are
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distinct and whilst in the inner scope, the declaration in the outer
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scope is not visible - it is &quot;shadowed&quot; by the local variable
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of the same name. Confusion can arise if this was not what the programmer
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intended. The checker can therefore be configured to detect shadowing
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in three cases: a local variable shadowing a global variable; a local
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variable shadowing a local variable with a wider scope and a local
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variable shadowing a typedef name, by using:<P>
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<PRE>
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	#pragma TenDRA variable hiding analysis <EM>status</EM>
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</PRE>
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If <EM>status</EM> is <CODE>on</CODE> an error is raised when a local
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variable that shadows another variable is declared, if
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<CODE>warning</CODE> is used the error is replaced by a warning and
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the <CODE>off</CODE> option restores the default behaviour (shadowing
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is permitted and no errors are produced).<P>
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<A NAME=S113>
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<HR><H2>8.5  <A NAME=8>Floating point equality</H2>
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Due to the rounding errors that occur in the handling of floating
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point values, comparison for equality between two floating point values
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is a hazardous and unpredictable operation. Tests for equality of
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two floating point numbers are controlled by:<P>
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<PRE>
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	#pragma TenDRA floating equality <EM>permit</EM>
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</PRE>
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<EM>where</EM><CODE> </CODE><EM>permit</EM><CODE> </CODE><EM>is</EM><CODE>
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allow</CODE>, <CODE>warning</CODE> or <CODE>disallow</CODE>. By default
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the check is switched off.<P>
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<P>
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<!-- FM pgf ignored -->
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<HR>
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<P><I>Part of the <A HREF="../index.html">TenDRA Web</A>.<BR>Crown
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Copyright &copy; 1998.</I></P>
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