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<TITLE>C Checker Reference Manual: Specifying conversions using the
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token syntax</TITLE>
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</HEAD>
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<BODY TEXT="#000000" BGCOLOR="#FFFFFF" LINK="#0000FF" VLINK="#400080" ALINK="#FF0000">
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<A NAME=S173>
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<H1>C Checker Reference Manual</H1>
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<H3>January 1998</H3>
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<IMG SRC="../images/no_next.gif" ALT="next section">
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<A HREF="tdfc1.html"><IMG SRC="../images/top.gif" ALT="current document"></A>
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<A HREF="../index.html"><IMG SRC="../images/home.gif" ALT="TenDRA home page">
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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="#S174"><B>H.1 </B> - Introduction</A><DD>
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<DT><A HREF="#S175"><B>H.2 </B> - User-defined conversions</A>
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<DD>
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<DT><A HREF="#S176"><B>H.3 </B> - Specifying integer promotions</A><DD>
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</DL>
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<HR>
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<H1>H Specifying conversions using the token syntax</H1>
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<A NAME=S174>
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<HR><H2>H.1 Introduction</H2>
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The token syntax described in Annex F can be used to fine-tune the
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conversion analysis and integer range checks described in section
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3.2.2 and chapter 4 respectively.<P>
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<A NAME=S175>
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<HR><H2>H.2 <A NAME=2>User-defined conversions</H2>
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In the example: <P>
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<PRE>
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#pragma token TYPE IP#
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#pragma no_def IP
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#pragma token PROC{ TYPE t ,EXP rvalue : t* : e | EXP e}EXP rvalue:IP:p_to_ip#
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#pragma TenDRA conversion p_to_ip allow
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void f(void){
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IP i, *pip;
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i=pip;
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}
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</PRE>
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the conversion from type pointer to IP to IP would normally result
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in an error. However the pragma: <P>
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<PRE>
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#pragma TenDRA conversion <EM>conv_list</EM> allow
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</PRE>
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allows users to define their own conversions between types that would
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otherwise be incompatible. Each identifier in the <EM>conv_list</EM>
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must be a local identifier for a PROC token (see <A HREF="tdfc20.html#21">F.9
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Procedure tokens</A>), taking exactly one argument which must be an
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expression. The procedure must also deliver an expression and both
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the parameter and the result must be rvalues. When attempting the
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conversion of a value from one type to another, either by assignment
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or casting, if that conversion would not normally be permitted, then,
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for each token introduced as a conversion token by the pragma above:
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<P>
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An attempt is made to resolve the type of the token result to the
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type to which the value is being converted. <P>
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If the result is resolved and the value to be converted is a suitable
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argument for the token procedure, the token procedure is applied to
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implement the conversion. <P>
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If no suitable conversion token can be found, an error will be raised.
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<P>
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<A NAME=S176>
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<HR><H2>H.3 <A NAME=4>Specifying integer promotions</H2>
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Whenever an integral type is used, e.g. in a call to a traditionally
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defined function, its promotion must be computed. If no promotion
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type has been specified, the compiler will raise an error. Although
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programs will generally use the default rules provided in the built-in
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compilation modes, the TenDRA compiler also allows programmers to
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specify their own set of integer promotion rules. Such promotions
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can be split into two categories: literal promotions and computed
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promotions. <P>
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<B>Literal promotions</B>. The promoted type is supplied directly
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using the pragma:<P>
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<PRE>
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#pragma promote <EM>type-name</EM> : <EM>type-name</EM>
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</PRE>
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The second type-name specifies the promoted type of the first type-name.
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Both types must be integral types.<P>
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<B>Computed promotions</B>. The pragma:<P>
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<PRE>
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#pragma compute promote <EM>proc-token</EM>
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</PRE>
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allows the programmer to provide the identification of a procedure
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token (see <A HREF="tdfc20.html#21">F.9 Procedure tokens</A>) for
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computing the promotion type of any integral type. This token is then
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called whenever the promotion of a type without a literal promotion
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is required. The procedure token <EM>proc-token</EM> must be declared
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as:<P>
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<PRE>
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#pragma token PROC ( VARIETY ) VARIETY proc-token #
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</PRE>
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The TenDRA technology provides two pre-defined procedure tokens, identified
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by <CODE>~promote</CODE> and <CODE>~sign-promote</CODE>. These procedures
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perform integer promotions according to the ISO/ANSI promotion rules
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or according to the traditional signed promotion rules respectively.
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The built-in compilation environments (see chapter 2) use these tokens
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to specify the appropriate set of integer promotion rules.<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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