132 lines
6.3 KiB
HTML
132 lines
6.3 KiB
HTML
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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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<!-- Copyright (C) 1988-2016 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; with the
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Invariant Sections being "Funding Free Software", the Front-Cover
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(see below). A copy of the license is included in the section entitled
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"GNU Free Documentation License".
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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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funds for GNU development. -->
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<!-- Created by GNU Texinfo 5.2, http://www.gnu.org/software/texinfo/ -->
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<head>
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<title>Using the GNU Compiler Collection (GCC): Qualifiers implementation</title>
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<meta name="description" content="Using the GNU Compiler Collection (GCC): Qualifiers implementation">
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<meta name="keywords" content="Using the GNU Compiler Collection (GCC): Qualifiers implementation">
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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="Option-Index.html#Option-Index" rel="index" title="Option Index">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="C-Implementation.html#C-Implementation" rel="up" title="C Implementation">
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<link href="Declarators-implementation.html#Declarators-implementation" rel="next" title="Declarators implementation">
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<link href="Structures-unions-enumerations-and-bit_002dfields-implementation.html#Structures-unions-enumerations-and-bit_002dfields-implementation" rel="prev" title="Structures unions enumerations and bit-fields implementation">
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</head>
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<body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000">
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<a name="Qualifiers-implementation"></a>
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<div class="header">
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<p>
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Next: <a href="Declarators-implementation.html#Declarators-implementation" accesskey="n" rel="next">Declarators implementation</a>, Previous: <a href="Structures-unions-enumerations-and-bit_002dfields-implementation.html#Structures-unions-enumerations-and-bit_002dfields-implementation" accesskey="p" rel="prev">Structures unions enumerations and bit-fields implementation</a>, Up: <a href="C-Implementation.html#C-Implementation" accesskey="u" rel="up">C Implementation</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p>
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</div>
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<hr>
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<a name="Qualifiers"></a>
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<h3 class="section">4.10 Qualifiers</h3>
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<ul>
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<li> <cite>What constitutes an access to an object that has volatile-qualified
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type (C90 6.5.3, C99 and C11 6.7.3).</cite>
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<p>Such an object is normally accessed by pointers and used for accessing
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hardware. In most expressions, it is intuitively obvious what is a read
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and what is a write. For example
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</p>
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<div class="smallexample">
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<pre class="smallexample">volatile int *dst = <var>somevalue</var>;
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volatile int *src = <var>someothervalue</var>;
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*dst = *src;
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</pre></div>
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<p>will cause a read of the volatile object pointed to by <var>src</var> and store the
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value into the volatile object pointed to by <var>dst</var>. There is no
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guarantee that these reads and writes are atomic, especially for objects
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larger than <code>int</code>.
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</p>
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<p>However, if the volatile storage is not being modified, and the value of
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the volatile storage is not used, then the situation is less obvious.
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For example
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</p>
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<div class="smallexample">
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<pre class="smallexample">volatile int *src = <var>somevalue</var>;
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*src;
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</pre></div>
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<p>According to the C standard, such an expression is an rvalue whose type
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is the unqualified version of its original type, i.e. <code>int</code>. Whether
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GCC interprets this as a read of the volatile object being pointed to or
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only as a request to evaluate the expression for its side-effects depends
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on this type.
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</p>
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<p>If it is a scalar type, or on most targets an aggregate type whose only
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member object is of a scalar type, or a union type whose member objects
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are of scalar types, the expression is interpreted by GCC as a read of
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the volatile object; in the other cases, the expression is only evaluated
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for its side-effects.
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</p>
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</li></ul>
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<hr>
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<div class="header">
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<p>
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Next: <a href="Declarators-implementation.html#Declarators-implementation" accesskey="n" rel="next">Declarators implementation</a>, Previous: <a href="Structures-unions-enumerations-and-bit_002dfields-implementation.html#Structures-unions-enumerations-and-bit_002dfields-implementation" accesskey="p" rel="prev">Structures unions enumerations and bit-fields implementation</a>, Up: <a href="C-Implementation.html#C-Implementation" accesskey="u" rel="up">C Implementation</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" 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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