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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 warnings (The C Preprocessor)</title>

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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="Implementation-Details.html#Implementation-Details" rel="next" title="Implementation Details">

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

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

  72. <div class="header">

  73. <p>

  74. Previous: <a href="Traditional-miscellany.html#Traditional-miscellany" accesskey="p" rel="prev">Traditional miscellany</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-warnings-1"></a>

  78. <h3 class="section">10.4 Traditional warnings</h3>

  79. <p>You can request warnings about features that did not exist, or worked

  80. differently, in traditional C with the <samp>-Wtraditional</samp> option.

  81. GCC does not warn about features of ISO C which you must use when you

  82. are using a conforming compiler, such as the &lsquo;<samp>#</samp>&rsquo; and &lsquo;<samp>##</samp>&rsquo;

  83. operators.

  84. </p>

  85. <p>Presently <samp>-Wtraditional</samp> warns about:

  86. </p>

  87. <ul>

  88. <li> Macro parameters that appear within string literals in the macro body.

  89. In traditional C macro replacement takes place within string literals,

  90. but does not in ISO C.

  91. 

  92. </li><li> In traditional C, some preprocessor directives did not exist.

  93. Traditional preprocessors would only consider a line to be a directive

  94. if the &lsquo;<samp>#</samp>&rsquo; appeared in column 1 on the line.  Therefore

  95. <samp>-Wtraditional</samp> warns about directives that traditional C

  96. understands but would ignore because the &lsquo;<samp>#</samp>&rsquo; does not appear as the

  97. first character on the line.  It also suggests you hide directives like

  98. &lsquo;<samp>#pragma</samp>&rsquo; not understood by traditional C by indenting them.  Some

  99. traditional implementations would not recognize &lsquo;<samp>#elif</samp>&rsquo;, so it

  100. suggests avoiding it altogether.

  101. 

  102. </li><li> A function-like macro that appears without an argument list.  In some

  103. traditional preprocessors this was an error.  In ISO C it merely means

  104. that the macro is not expanded.

  105. 

  106. </li><li> The unary plus operator.  This did not exist in traditional C.

  107. 

  108. </li><li> The &lsquo;<samp>U</samp>&rsquo; and &lsquo;<samp>LL</samp>&rsquo; integer constant suffixes, which were not

  109. available in traditional C.  (Traditional C does support the &lsquo;<samp>L</samp>&rsquo;

  110. suffix for simple long integer constants.)  You are not warned about

  111. uses of these suffixes in macros defined in system headers.  For

  112. instance, <code>UINT_MAX</code> may well be defined as <code>4294967295U</code>, but

  113. you will not be warned if you use <code>UINT_MAX</code>.

  114. 

  115. <p>You can usually avoid the warning, and the related warning about

  116. constants which are so large that they are unsigned, by writing the

  117. integer constant in question in hexadecimal, with no U suffix.  Take

  118. care, though, because this gives the wrong result in exotic cases.

  119. </p></li></ul>

  120. 

  121. <hr>

  122. <div class="header">

  123. <p>

  124. Previous: <a href="Traditional-miscellany.html#Traditional-miscellany" accesskey="p" rel="prev">Traditional miscellany</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>

  125. </div>

  126. 

  127. 

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  129. </body>

  130. </html>