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  1. <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">

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  10. 

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  26. <title>Traditional macros (The C Preprocessor)</title>

  27. 

  28. <meta name="description" content="Traditional macros (The C Preprocessor)">

  29. <meta name="keywords" content="Traditional macros (The C Preprocessor)">

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  35. <link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">

  36. <link href="Traditional-Mode.html#Traditional-Mode" rel="up" title="Traditional Mode">

  37. <link href="Traditional-miscellany.html#Traditional-miscellany" rel="next" title="Traditional miscellany">

  38. <link href="Traditional-lexical-analysis.html#Traditional-lexical-analysis" rel="prev" title="Traditional lexical analysis">

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  70. <body lang="en">

  71. <a name="Traditional-macros"></a>

  72. <div class="header">

  73. <p>

  74. Next: <a href="Traditional-miscellany.html#Traditional-miscellany" accesskey="n" rel="next">Traditional miscellany</a>, Previous: <a href="Traditional-lexical-analysis.html#Traditional-lexical-analysis" accesskey="p" rel="prev">Traditional lexical analysis</a>, Up: <a href="Traditional-Mode.html#Traditional-Mode" accesskey="u" rel="up">Traditional Mode</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Index-of-Directives.html#Index-of-Directives" title="Index" rel="index">Index</a>]</p>

  75. </div>

  76. <hr>

  77. <a name="Traditional-macros-1"></a>

  78. <h3 class="section">10.2 Traditional macros</h3>

  79. 

  80. <p>The major difference between traditional and ISO macros is that the

  81. former expand to text rather than to a token sequence.  CPP removes

  82. all leading and trailing horizontal whitespace from a macro&rsquo;s

  83. replacement text before storing it, but preserves the form of internal

  84. whitespace.

  85. </p>

  86. <p>One consequence is that it is legitimate for the replacement text to

  87. contain an unmatched quote (see <a href="Traditional-lexical-analysis.html#Traditional-lexical-analysis">Traditional lexical analysis</a>).  An

  88. unclosed string or character constant continues into the text

  89. following the macro call.  Similarly, the text at the end of a macro&rsquo;s

  90. expansion can run together with the text after the macro invocation to

  91. produce a single token.

  92. </p>

  93. <p>Normally comments are removed from the replacement text after the

  94. macro is expanded, but if the <samp>-CC</samp> option is passed on the

  95. command-line comments are preserved.  (In fact, the current

  96. implementation removes comments even before saving the macro

  97. replacement text, but it careful to do it in such a way that the

  98. observed effect is identical even in the function-like macro case.)

  99. </p>

  100. <p>The ISO stringizing operator &lsquo;<samp>#</samp>&rsquo; and token paste operator

  101. &lsquo;<samp>##</samp>&rsquo; have no special meaning.  As explained later, an effect

  102. similar to these operators can be obtained in a different way.  Macro

  103. names that are embedded in quotes, either from the main file or after

  104. macro replacement, do not expand.

  105. </p>

  106. <p>CPP replaces an unquoted object-like macro name with its replacement

  107. text, and then rescans it for further macros to replace.  Unlike

  108. standard macro expansion, traditional macro expansion has no provision

  109. to prevent recursion.  If an object-like macro appears unquoted in its

  110. replacement text, it will be replaced again during the rescan pass,

  111. and so on <em>ad infinitum</em>.  GCC detects when it is expanding

  112. recursive macros, emits an error message, and continues after the

  113. offending macro invocation.

  114. </p>

  115. <div class="smallexample">

  116. <pre class="smallexample">#define PLUS +

  117. #define INC(x) PLUS+x

  118. INC(foo);

  119.      &rarr; ++foo;

  120. </pre></div>

  121. 

  122. <p>Function-like macros are similar in form but quite different in

  123. behavior to their ISO counterparts.  Their arguments are contained

  124. within parentheses, are comma-separated, and can cross physical lines.

  125. Commas within nested parentheses are not treated as argument

  126. separators.  Similarly, a quote in an argument cannot be left

  127. unclosed; a following comma or parenthesis that comes before the

  128. closing quote is treated like any other character.  There is no

  129. facility for handling variadic macros.

  130. </p>

  131. <p>This implementation removes all comments from macro arguments, unless

  132. the <samp>-C</samp> option is given.  The form of all other horizontal

  133. whitespace in arguments is preserved, including leading and trailing

  134. whitespace.  In particular

  135. </p>

  136. <div class="smallexample">

  137. <pre class="smallexample">f( )

  138. </pre></div>

  139. 

  140. <p>is treated as an invocation of the macro &lsquo;<samp>f</samp>&rsquo; with a single

  141. argument consisting of a single space.  If you want to invoke a

  142. function-like macro that takes no arguments, you must not leave any

  143. whitespace between the parentheses.

  144. </p>

  145. <p>If a macro argument crosses a new line, the new line is replaced with

  146. a space when forming the argument.  If the previous line contained an

  147. unterminated quote, the following line inherits the quoted state.

  148. </p>

  149. <p>Traditional preprocessors replace parameters in the replacement text

  150. with their arguments regardless of whether the parameters are within

  151. quotes or not.  This provides a way to stringize arguments.  For

  152. example

  153. </p>

  154. <div class="smallexample">

  155. <pre class="smallexample">#define str(x) &quot;x&quot;

  156. str(/* <span class="roman">A comment</span> */some text )

  157.      &rarr; &quot;some text &quot;

  158. </pre></div>

  159. 

  160. <p>Note that the comment is removed, but that the trailing space is

  161. preserved.  Here is an example of using a comment to effect token

  162. pasting.

  163. </p>

  164. <div class="smallexample">

  165. <pre class="smallexample">#define suffix(x) foo_/**/x

  166. suffix(bar)

  167.      &rarr; foo_bar

  168. </pre></div>

  169. 

  170. <hr>

  171. <div class="header">

  172. <p>

  173. Next: <a href="Traditional-miscellany.html#Traditional-miscellany" accesskey="n" rel="next">Traditional miscellany</a>, Previous: <a href="Traditional-lexical-analysis.html#Traditional-lexical-analysis" accesskey="p" rel="prev">Traditional lexical analysis</a>, Up: <a href="Traditional-Mode.html#Traditional-Mode" accesskey="u" rel="up">Traditional Mode</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Index-of-Directives.html#Index-of-Directives" title="Index" rel="index">Index</a>]</p>

  174. </div>

  175. 

  176. 

  177. 

  178. </body>

  179. </html>