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