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<title>The C Preprocessor: Obsolete Features</title>
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<a name="Obsolete-Features"></a>
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<div class="header">
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<p>
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Next: <a href="Differences-from-previous-versions.html#Differences-from-previous-versions" accesskey="n" rel="next">Differences from previous versions</a>, Previous: <a href="Implementation-limits.html#Implementation-limits" accesskey="p" rel="prev">Implementation limits</a>, Up: <a href="Implementation-Details.html#Implementation-Details" accesskey="u" rel="up">Implementation Details</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="Obsolete-Features-1"></a>
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<h3 class="section">11.3 Obsolete Features</h3>
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<p>CPP has some features which are present mainly for compatibility with
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older programs. We discourage their use in new code. In some cases,
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we plan to remove the feature in a future version of GCC.
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</p>
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<a name="Assertions"></a>
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<h4 class="subsection">11.3.1 Assertions</h4>
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<a name="index-assertions"></a>
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<p><em>Assertions</em> are a deprecated alternative to macros in writing
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conditionals to test what sort of computer or system the compiled
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program will run on. Assertions are usually predefined, but you can
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define them with preprocessing directives or command-line options.
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</p>
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<p>Assertions were intended to provide a more systematic way to describe
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the compiler’s target system and we added them for compatibility with
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existing compilers. In practice they are just as unpredictable as the
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system-specific predefined macros. In addition, they are not part of
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any standard, and only a few compilers support them.
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Therefore, the use of assertions is <strong>less</strong> portable than the use
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of system-specific predefined macros. We recommend you do not use them at
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all.
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</p>
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<a name="index-predicates"></a>
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<p>An assertion looks like this:
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</p>
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<div class="smallexample">
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<pre class="smallexample">#<var>predicate</var> (<var>answer</var>)
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</pre></div>
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<p><var>predicate</var> must be a single identifier. <var>answer</var> can be any
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sequence of tokens; all characters are significant except for leading
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and trailing whitespace, and differences in internal whitespace
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sequences are ignored. (This is similar to the rules governing macro
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redefinition.) Thus, <code>(x + y)</code> is different from <code>(x+y)</code> but
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equivalent to <code>( x + y )<!-- /@w --></code>. Parentheses do not nest inside an
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answer.
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</p>
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<a name="index-testing-predicates"></a>
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<p>To test an assertion, you write it in an ‘<samp>#if</samp>’. For example, this
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conditional succeeds if either <code>vax</code> or <code>ns16000</code> has been
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asserted as an answer for <code>machine</code>.
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</p>
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<div class="smallexample">
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<pre class="smallexample">#if #machine (vax) || #machine (ns16000)
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</pre></div>
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<p>You can test whether <em>any</em> answer is asserted for a predicate by
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omitting the answer in the conditional:
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</p>
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<div class="smallexample">
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<pre class="smallexample">#if #machine
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</pre></div>
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<a name="index-_0023assert"></a>
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<p>Assertions are made with the ‘<samp>#assert</samp>’ directive. Its sole
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argument is the assertion to make, without the leading ‘<samp>#</samp>’ that
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identifies assertions in conditionals.
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</p>
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<div class="smallexample">
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<pre class="smallexample">#assert <var>predicate</var> (<var>answer</var>)
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</pre></div>
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<p>You may make several assertions with the same predicate and different
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answers. Subsequent assertions do not override previous ones for the
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same predicate. All the answers for any given predicate are
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simultaneously true.
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</p>
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<a name="index-assertions_002c-canceling"></a>
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<a name="index-_0023unassert"></a>
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<p>Assertions can be canceled with the ‘<samp>#unassert</samp>’ directive. It
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has the same syntax as ‘<samp>#assert</samp>’. In that form it cancels only the
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answer which was specified on the ‘<samp>#unassert</samp>’ line; other answers
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for that predicate remain true. You can cancel an entire predicate by
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leaving out the answer:
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</p>
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<div class="smallexample">
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<pre class="smallexample">#unassert <var>predicate</var>
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</pre></div>
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<p>In either form, if no such assertion has been made, ‘<samp>#unassert</samp>’ has
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no effect.
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</p>
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<p>You can also make or cancel assertions using command-line options.
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See <a href="Invocation.html#Invocation">Invocation</a>.
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</p>
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<hr>
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<div class="header">
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<p>
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Next: <a href="Differences-from-previous-versions.html#Differences-from-previous-versions" accesskey="n" rel="next">Differences from previous versions</a>, Previous: <a href="Implementation-limits.html#Implementation-limits" accesskey="p" rel="prev">Implementation limits</a>, Up: <a href="Implementation-Details.html#Implementation-Details" accesskey="u" rel="up">Implementation Details</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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