132 lines
6.2 KiB
HTML
132 lines
6.2 KiB
HTML
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<head>
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<title>The C Preprocessor: Traditional warnings</title>
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<meta name="description" content="The C Preprocessor: Traditional warnings">
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<link href="Index-of-Directives.html#Index-of-Directives" rel="index" title="Index of Directives">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="Traditional-Mode.html#Traditional-Mode" rel="up" title="Traditional Mode">
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<link href="Implementation-Details.html#Implementation-Details" rel="next" title="Implementation Details">
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<link href="Traditional-miscellany.html#Traditional-miscellany" rel="prev" title="Traditional miscellany">
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<body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000">
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<a name="Traditional-warnings"></a>
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<div class="header">
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<p>
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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> [<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>
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</div>
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<hr>
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<a name="Traditional-warnings-1"></a>
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<h3 class="section">10.4 Traditional warnings</h3>
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<p>You can request warnings about features that did not exist, or worked
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differently, in traditional C with the <samp>-Wtraditional</samp> option.
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GCC does not warn about features of ISO C which you must use when you
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are using a conforming compiler, such as the ‘<samp>#</samp>’ and ‘<samp>##</samp>’
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operators.
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</p>
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<p>Presently <samp>-Wtraditional</samp> warns about:
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</p>
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<ul>
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<li> Macro parameters that appear within string literals in the macro body.
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In traditional C macro replacement takes place within string literals,
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but does not in ISO C.
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</li><li> In traditional C, some preprocessor directives did not exist.
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Traditional preprocessors would only consider a line to be a directive
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if the ‘<samp>#</samp>’ appeared in column 1 on the line. Therefore
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<samp>-Wtraditional</samp> warns about directives that traditional C
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understands but would ignore because the ‘<samp>#</samp>’ does not appear as the
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first character on the line. It also suggests you hide directives like
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‘<samp>#pragma</samp>’ not understood by traditional C by indenting them. Some
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traditional implementations would not recognize ‘<samp>#elif</samp>’, so it
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suggests avoiding it altogether.
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</li><li> A function-like macro that appears without an argument list. In some
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traditional preprocessors this was an error. In ISO C it merely means
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that the macro is not expanded.
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</li><li> The unary plus operator. This did not exist in traditional C.
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</li><li> The ‘<samp>U</samp>’ and ‘<samp>LL</samp>’ integer constant suffixes, which were not
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available in traditional C. (Traditional C does support the ‘<samp>L</samp>’
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suffix for simple long integer constants.) You are not warned about
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uses of these suffixes in macros defined in system headers. For
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instance, <code>UINT_MAX</code> may well be defined as <code>4294967295U</code>, but
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you will not be warned if you use <code>UINT_MAX</code>.
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<p>You can usually avoid the warning, and the related warning about
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constants which are so large that they are unsigned, by writing the
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integer constant in question in hexadecimal, with no U suffix. Take
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care, though, because this gives the wrong result in exotic cases.
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</p></li></ul>
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<hr>
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<div class="header">
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<p>
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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> [<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>
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</div>
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</body>
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</html>
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