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211 lines
9.1 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>Pragmas (The C Preprocessor)</title>
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<meta name="description" content="Pragmas (The C Preprocessor)">
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<meta name="keywords" content="Pragmas (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="index.html#Top" rel="up" title="Top">
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<link href="Other-Directives.html#Other-Directives" rel="next" title="Other Directives">
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<link href="Line-Control.html#Line-Control" rel="prev" title="Line Control">
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<style type="text/css">
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a.summary-letter {text-decoration: none}
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div.display {margin-left: 3.2em}
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div.example {margin-left: 3.2em}
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kbd {font-style: oblique}
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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="Pragmas"></a>
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<div class="header">
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<p>
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Next: <a href="Other-Directives.html#Other-Directives" accesskey="n" rel="next">Other Directives</a>, Previous: <a href="Line-Control.html#Line-Control" accesskey="p" rel="prev">Line Control</a>, Up: <a href="index.html#Top" accesskey="u" rel="up">Top</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="Pragmas-1"></a>
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<h2 class="chapter">7 Pragmas</h2>
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<p>The ‘<samp>#pragma</samp>’ directive is the method specified by the C standard
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for providing additional information to the compiler, beyond what is
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conveyed in the language itself. The forms of this directive
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(commonly known as <em>pragmas</em>) specified by C standard are prefixed with
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<code>STDC</code>. A C compiler is free to attach any meaning it likes to other
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pragmas. All GNU-defined, supported pragmas have been given a
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<code>GCC</code> prefix.
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</p>
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<a name="index-_005fPragma"></a>
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<p>C99 introduced the <code><span class="nolinebreak">_Pragma</span><!-- /@w --></code> operator. This feature addresses a
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major problem with ‘<samp>#pragma</samp>’: being a directive, it cannot be
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produced as the result of macro expansion. <code><span class="nolinebreak">_Pragma</span><!-- /@w --></code> is an
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operator, much like <code>sizeof</code> or <code>defined</code>, and can be embedded
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in a macro.
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</p>
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<p>Its syntax is <code><span class="nolinebreak">_Pragma</span> (<var><span class="nolinebreak">string-literal</span></var>)<!-- /@w --></code>, where
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<var>string-literal</var> can be either a normal or wide-character string
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literal. It is destringized, by replacing all ‘<samp>\\</samp>’ with a single
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‘<samp>\</samp>’ and all ‘<samp>\"</samp>’ with a ‘<samp>"</samp>’. The result is then
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processed as if it had appeared as the right hand side of a
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‘<samp>#pragma</samp>’ directive. For example,
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</p>
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<div class="smallexample">
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<pre class="smallexample">_Pragma ("GCC dependency \"parse.y\"")
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</pre></div>
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<p>has the same effect as <code>#pragma GCC dependency "parse.y"</code>. The
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same effect could be achieved using macros, for example
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</p>
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<div class="smallexample">
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<pre class="smallexample">#define DO_PRAGMA(x) _Pragma (#x)
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DO_PRAGMA (GCC dependency "parse.y")
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</pre></div>
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<p>The standard is unclear on where a <code>_Pragma</code> operator can appear.
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The preprocessor does not accept it within a preprocessing conditional
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directive like ‘<samp>#if</samp>’. To be safe, you are probably best keeping it
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out of directives other than ‘<samp>#define</samp>’, and putting it on a line of
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its own.
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</p>
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<p>This manual documents the pragmas which are meaningful to the
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preprocessor itself. Other pragmas are meaningful to the C or C++
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compilers. They are documented in the GCC manual.
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</p>
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<p>GCC plugins may provide their own pragmas.
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</p>
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<dl compact="compact">
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<dt><code>#pragma GCC dependency</code>
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<a name="index-_0023pragma-GCC-dependency"></a>
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</dt>
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<dd><p><code>#pragma GCC dependency</code> allows you to check the relative dates of
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the current file and another file. If the other file is more recent than
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the current file, a warning is issued. This is useful if the current
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file is derived from the other file, and should be regenerated. The
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other file is searched for using the normal include search path.
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Optional trailing text can be used to give more information in the
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warning message.
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</p>
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<div class="smallexample">
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<pre class="smallexample">#pragma GCC dependency "parse.y"
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#pragma GCC dependency "/usr/include/time.h" rerun fixincludes
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</pre></div>
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</dd>
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<dt><code>#pragma GCC poison</code>
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<a name="index-_0023pragma-GCC-poison"></a>
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</dt>
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<dd><p>Sometimes, there is an identifier that you want to remove completely
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from your program, and make sure that it never creeps back in. To
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enforce this, you can <em>poison</em> the identifier with this pragma.
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<code>#pragma GCC poison</code> is followed by a list of identifiers to
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poison. If any of those identifiers appears anywhere in the source
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after the directive, it is a hard error. For example,
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</p>
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<div class="smallexample">
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<pre class="smallexample">#pragma GCC poison printf sprintf fprintf
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sprintf(some_string, "hello");
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</pre></div>
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<p>will produce an error.
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</p>
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<p>If a poisoned identifier appears as part of the expansion of a macro
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which was defined before the identifier was poisoned, it will <em>not</em>
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cause an error. This lets you poison an identifier without worrying
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about system headers defining macros that use it.
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</p>
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<p>For example,
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</p>
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<div class="smallexample">
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<pre class="smallexample">#define strrchr rindex
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#pragma GCC poison rindex
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strrchr(some_string, 'h');
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</pre></div>
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<p>will not produce an error.
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</p>
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</dd>
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<dt><code>#pragma GCC system_header</code>
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<a name="index-_0023pragma-GCC-system_005fheader-1"></a>
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</dt>
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<dd><p>This pragma takes no arguments. It causes the rest of the code in the
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current file to be treated as if it came from a system header.
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See <a href="System-Headers.html#System-Headers">System Headers</a>.
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</p>
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</dd>
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<dt><code>#pragma GCC warning</code>
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<a name="index-_0023pragma-GCC-warning"></a>
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</dt>
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<dt><code>#pragma GCC error</code>
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<a name="index-_0023pragma-GCC-error"></a>
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</dt>
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<dd><p><code>#pragma GCC warning "message"</code> causes the preprocessor to issue
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a warning diagnostic with the text ‘<samp>message</samp>’. The message
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contained in the pragma must be a single string literal. Similarly,
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<code>#pragma GCC error "message"</code> issues an error message. Unlike
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the ‘<samp>#warning</samp>’ and ‘<samp>#error</samp>’ directives, these pragmas can be
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embedded in preprocessor macros using ‘<samp>_Pragma</samp>’.
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</p>
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</dd>
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</dl>
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<hr>
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<div class="header">
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<p>
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Next: <a href="Other-Directives.html#Other-Directives" accesskey="n" rel="next">Other Directives</a>, Previous: <a href="Line-Control.html#Line-Control" accesskey="p" rel="prev">Line Control</a>, Up: <a href="index.html#Top" accesskey="u" rel="up">Top</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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