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<!-- Crown Copyright (c) 1998 -->
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<HEAD>
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<TITLE>C Checker Reference Manual: Conditional Compilation</TITLE>
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<BODY TEXT="#000000" BGCOLOR="#FFFFFF" LINK="#0000FF" VLINK="#400080" ALINK="#FF0000">
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<A NAME=S120>
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<H1>C Checker Reference Manual</H1>
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<H3>January 1998</H3>
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<A HREF="tdfc14.html"><IMG SRC="../images/next.gif" ALT="next section"></A>
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<IMG SRC="../images/no_index.gif" ALT="document index"><P>
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<HR>
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<DL>
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<DT><A HREF="#S121"><B>10.1 </B> - Conditional Compilation</A>
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<DD>
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</DL>
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<HR>
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<H1>10 Conditional Compilation</H1>
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<A NAME=S121>
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<HR><H2>10.1 Conditional Compilation</H2>
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Tchk generally treats conditional compilation in the same way as other
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compilers and checkers. For example, consider:<P>
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<PRE>
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#if expr
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.... /* First branch */
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#else
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.... /* Second branch */
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#endif
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</PRE>
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the expression, expr, is evaluated: if it is non-zero the first branch
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of the conditional is processed; if it is zero the second branch is
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processed instead. <P>
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Sometimes, however, tchk may be unable to evaluate the expression
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statically because of the abstract types and expressions which arise
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from the minimum integer range assumptions or the abstract standard
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headers used by the tool (see target-dependent types in section 4.5).
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For example, consider the following ISO compliant program:<P>
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<PRE>
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#include <stdio.h>
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#include <limits.h>
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int main () {
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#if ( CHAR_MIN == 0 )
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puts ("char is unsigned");
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#else
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puts ("char is signed");
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#endif
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return ( 0 );
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}
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</PRE>
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The TenDRA representation of the ISO API merely states that CHAR_MIN
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- the least value which fits into a char - is a target dependent integral
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constant. Hence, whether or not it equals zero is again target dependent,
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so the checker needs to maintain both branches. By contrast, any conventional
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compiler is compiling to a particular target machine on which CHAR_MIN
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is a specific integral constant. It can therefore always determine
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which branch of the conditional it should compile.<P>
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In order to allow both branches to be maintained in these cases, it
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has been necessary for tchk to impose certain restrictions on the
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form of the conditional branches and the positions in which such target-dependent
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conditionals may occur. These may be summarised as:<P>
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<UL>
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<LI>Target-dependent conditionals may not appear at the outer level.
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If the checker encounters a target-dependent conditional at the outer
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level an error is produced. In order to continue checking in the rest
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of the file an arbitrary assumption must be made about which branch
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of the conditional to process; tchk assumes that the conditional is
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true and the first branch is used;<P>
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<LI>The branches of allowable target-dependent conditionals may not
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contain declarations or definitions.<P>
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</UL>
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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 © 1998.</I></P>
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