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175 lines
7.5 KiB
HTML
175 lines
7.5 KiB
HTML
4 years ago
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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
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<html>
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<!-- Copyright (C) 1987-2018 Free Software Foundation, Inc.
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Permission is granted to copy, distribute and/or modify this document
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under the terms of the GNU Free Documentation License, Version 1.3 or
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any later version published by the Free Software Foundation. A copy of
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the license is included in the
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section entitled "GNU Free Documentation License".
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This manual contains no Invariant Sections. The Front-Cover Texts are
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(a) (see below), and the Back-Cover Texts are (b) (see below).
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(a) The FSF's Front-Cover Text is:
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A GNU Manual
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(b) The FSF's Back-Cover Text is:
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You have freedom to copy and modify this GNU Manual, like GNU
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software. Copies published by the Free Software Foundation raise
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funds for GNU development. -->
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<!-- Created by GNU Texinfo 6.4, http://www.gnu.org/software/texinfo/ -->
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<head>
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<title>Obsolete Features (The C Preprocessor)</title>
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<meta name="description" content="Obsolete Features (The C Preprocessor)">
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<meta name="keywords" content="Obsolete Features (The C Preprocessor)">
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<meta name="resource-type" content="document">
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<meta name="distribution" content="global">
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<meta name="Generator" content="makeinfo">
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
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<link href="index.html#Top" rel="start" title="Top">
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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="Implementation-Details.html#Implementation-Details" rel="up" title="Implementation Details">
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<link href="Invocation.html#Invocation" rel="next" title="Invocation">
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<link href="Implementation-limits.html#Implementation-limits" rel="prev" title="Implementation limits">
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<style type="text/css">
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<!--
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a.summary-letter {text-decoration: none}
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blockquote.indentedblock {margin-right: 0em}
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blockquote.smallindentedblock {margin-right: 0em; font-size: smaller}
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blockquote.smallquotation {font-size: smaller}
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div.display {margin-left: 3.2em}
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div.example {margin-left: 3.2em}
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div.lisp {margin-left: 3.2em}
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div.smalldisplay {margin-left: 3.2em}
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div.smallexample {margin-left: 3.2em}
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div.smalllisp {margin-left: 3.2em}
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kbd {font-style: oblique}
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pre.display {font-family: inherit}
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pre.format {font-family: inherit}
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pre.menu-comment {font-family: serif}
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pre.menu-preformatted {font-family: serif}
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pre.smalldisplay {font-family: inherit; font-size: smaller}
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pre.smallexample {font-size: smaller}
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pre.smallformat {font-family: inherit; font-size: smaller}
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pre.smalllisp {font-size: smaller}
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span.nolinebreak {white-space: nowrap}
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span.roman {font-family: initial; font-weight: normal}
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span.sansserif {font-family: sans-serif; font-weight: normal}
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ul.no-bullet {list-style: none}
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-->
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</style>
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</head>
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<body lang="en">
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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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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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|
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<div class="header">
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<p>
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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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</body>
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</html>
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