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101 lines
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<title>Traditional warnings - The C Preprocessor</title>
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<a name="Traditional-warnings"></a>
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<p>
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Previous: <a rel="previous" accesskey="p" href="Traditional-miscellany.html#Traditional-miscellany">Traditional miscellany</a>,
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Up: <a rel="up" accesskey="u" href="Traditional-Mode.html#Traditional-Mode">Traditional Mode</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><span class="option">-Wtraditional</span></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><span class="samp">#</span></samp>’ and ‘<samp><span class="samp">##</span></samp>’
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operators.
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<p>Presently <samp><span class="option">-Wtraditional</span></samp> warns about:
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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>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><span class="samp">#</span></samp>’ appeared in column 1 on the line. Therefore
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<samp><span class="option">-Wtraditional</span></samp> warns about directives that traditional C
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understands but would ignore because the ‘<samp><span class="samp">#</span></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><span class="samp">#pragma</span></samp>’ not understood by traditional C by indenting them. Some
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traditional implementations would not recognize ‘<samp><span class="samp">#elif</span></samp>’, so it
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suggests avoiding it altogether.
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<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>The unary plus operator. This did not exist in traditional C.
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<li>The ‘<samp><span class="samp">U</span></samp>’ and ‘<samp><span class="samp">LL</span></samp>’ integer constant suffixes, which were not
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available in traditional C. (Traditional C does support the ‘<samp><span class="samp">L</span></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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</ul>
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