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<HTML><HEAD><TITLE>Preprocessing</TITLE></HEAD><BODY><H1><A NAME="Preprocessing">Preprocessing</A></H1><HR><P>The translator processes each source file in a series of phases.<B>Preprocessing</B> constitutes the earliest phases, which produce a<A HREF="#translation unit">translation unit</A>.Preprocessing treats a source file as a sequence of<A HREF="lib_file.html#text lines">text lines</A>. You can specify<B><A NAME="directives">directives</A></B> and<B><A NAME="macros">macros</A></B>that insert, delete, and alter source text.</P><P>This document describes briefly just those aspectsof preprocessing most relevant to the use of the Standard C library:</P><P>The macro <CODE><A NAME="__FILE__">__FILE__</A></CODE>expands to a<A HREF="charset.html#string literal">string literal</A>that gives the remembered<A HREF="lib_over.html#filename">filename</A>of the current source file. You can alter the value of this macroby writing a<A HREF="#line directive"><I>line</I> directive</A>.</P><P>The macro <CODE><A NAME="__LINE__">__LINE__</A></CODE>expands to a decimal integer constantthat gives the remembered line number within the current source file.You can alter the value of this macro by writing a<A HREF="#line directive"><I>line</I> directive</A>.</P><P>A <B><A NAME="define directive"><I>define</I> directive</A></B>defines a name as a macro.Following the directive name <CODE>define</CODE>,you write one of two forms:</P><UL><LI>a name <I>not</I> immediately followed by a left parenthesis,followed by any sequence of preprocessing tokens -- to define amacro without parameters<LI>a name immediately followed by a left parenthesis with <I>no</I>intervening white space, followed by zero or more distinct <I>parameternames</I> separated by commas, followed by a right parenthesis, followedby any sequence of preprocessing tokens -- to define a macro withas many parameters as names that you write inside the parentheses</UL><P>You can selectively skip groups of lineswithin source files by writing an<B><A NAME="if directive"><I>if</I> directive</A></B>,or one of the other <B>conditional directives</B>,<I>ifdef</I> or <I>ifndef.</I>You follow the conditional directive by the first group of linesthat you want to selectively skip.Zero or more <I>elif</I> directives follow this first group of lines,each followed by a group of lines that you want to selectively skip.An optional <I>else</I> directive follows all groups of lines controlledby <I>elif</I> directives, followed by the last group of lines you wantto selectively skip.The last group of lines ends with an <I>endif</I> directive.</P><P>At most one group of lines is retained in the translation unit --the one immediately preceded by a directive whose<A HREF="#if expression"><I>#if</I> expression</A> has a nonzero value.For the directive:</P><PRE>#ifdef X</PRE><P>this expression is <CODE>defined (X)</CODE>, and for the directive:</P><PRE>#ifndef X</PRE><P>this expression is <CODE>!defined (X)</CODE>.</P><P>A <B><A NAME="if expression"><I>#if</I> expression</A></B> is aconditional expression that the preprocessor evaluates.You can write only<A HREF="express.html#integer constant expression">integer constant expressions</A>, with the followingadditional considerations:</P><UL><LI>The expression <CODE>defined X</CODE>, or <CODE>defined (X)</CODE>,is replaced by 1 if <CODE>X</CODE> is defined as a macro, otherwise 0.<LI>You cannot write the<A HREF="express.html#sizeof operator"><I>sizeof</I></A>or <I>type cast</I> operators. (The translator expands all macronames, then replaces each remaining name with 0,before it recognizes keywords.)<LI>The translator may be able to represent a broader range of integersthan the target environment.<LI>The translator represents type <I>int</I> the same as <I>long,</I>and <I>unsigned int</I> the same as <I>unsigned long.</I><LI>The translator can translate character constants to a set ofcode values different from the set for the target environment.</UL><P>An <B><A NAME="include directive"><I>include</I> directive</A></B>includes the contents of a<A HREF="lib_over.html#standard headers">standard header</A>or another source file in a translation unit. The contents ofthe specified standard header or source file replace the <I>include</I>directive. Following the directive name <CODE>include</CODE>,write one of the following:</P><UL><LI>a standard header name between angle brackets<LI>a filename between double quotes<LI>any other form that expands to one of the two previous formsafter macro replacement</UL><P>A <B><A NAME="line directive"><I>line</I> directive</A></B> altersthe source line number and filename used by the predefined macros<A HREF="#__FILE__"><CODE>__FILE__</CODE></A> and<A HREF="#__FILE__"><CODE>__FILE__</CODE></A>.Following the directive name <CODE>line</CODE>,write one of the following:</P><UL><LI>a decimal integer (giving the new line number of the line following)<LI>a decimal integer as before, followed by a string literal (givingthe new line number and the new source filename)<LI>any other form that expands to one of the two previous formsafter macro replacement</UL><P>Preprocessing translates each source file in a series of distinct<B><A NAME="Phases of Translation">phases</A></B>.The first few phases of translation:terminate each line with a newline character (<CODE><I>NL</I></CODE>),convert trigraphs to their single-character equivalents,and concatenate each line ending in a backslash (<CODE>\</CODE>)with the line following. Later phases process<A HREF="#include directive">include directives</A>,expand macros, and so on to produce a<B><A NAME="translation unit">translation unit</A></B>.The translator combines separate translation units,with contributions as needed from the<A HREF="index.html#Standard C Library">Standard C library</A>, at<B><A NAME="link time">link time</A></B>, to form the executable<B><A NAME="program">program</A></B>.</P><P> An <B><A NAME="undef directive"><I>undef</I> directive</A></B>removes a macro definition. You might want toremove a macro definition so that you can define it differently witha <I>define</I> directive or to unmask any other meaning given tothe name. The name whose definition you want to removefollows the directive name <CODE>undef</CODE>.If the name is not currently defined as a macro,the <I>undef</I> directive has no effect.</P><HR><P>See also the<B><A HREF="index.html#Table of Contents">Table of Contents</A></B> and the<B><A HREF="_index.html">Index</A></B>.</P><P><I><A HREF="crit_pb.html">Copyright</A> © 1989-2002by P.J. Plauger and Jim Brodie. All rights reserved.</I></P><!--V4.01:1125--></BODY></HTML>⌨️ 快捷键说明
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