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140 lines
6.4 KiB
HTML
140 lines
6.4 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) 1988-2018 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; with the
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Invariant Sections being "Funding Free Software", the Front-Cover
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Texts being (a) (see below), and with the Back-Cover Texts being (b)
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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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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>C++ Volatiles (Using the GNU Compiler Collection (GCC))</title>
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<meta name="description" content="C++ Volatiles (Using the GNU Compiler Collection (GCC))">
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<meta name="keywords" content="C++ Volatiles (Using the GNU Compiler Collection (GCC))">
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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="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_002b_002b-Extensions.html#C_002b_002b-Extensions" rel="up" title="C++ Extensions">
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<link href="Restricted-Pointers.html#Restricted-Pointers" rel="next" title="Restricted Pointers">
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<link href="C_002b_002b-Extensions.html#C_002b_002b-Extensions" rel="prev" title="C++ Extensions">
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<style type="text/css">
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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="C_002b_002b-Volatiles"></a>
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<div class="header">
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<p>
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Next: <a href="Restricted-Pointers.html#Restricted-Pointers" accesskey="n" rel="next">Restricted Pointers</a>, Up: <a href="C_002b_002b-Extensions.html#C_002b_002b-Extensions" accesskey="u" rel="up">C++ Extensions</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="When-is-a-Volatile-C_002b_002b-Object-Accessed_003f"></a>
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<h3 class="section">7.1 When is a Volatile C++ Object Accessed?</h3>
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<a name="index-accessing-volatiles-1"></a>
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<a name="index-volatile-read-1"></a>
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<a name="index-volatile-write-1"></a>
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<a name="index-volatile-access-1"></a>
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<p>The C++ standard differs from the C standard in its treatment of
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volatile objects. It fails to specify what constitutes a volatile
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access, except to say that C++ should behave in a similar manner to C
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with respect to volatiles, where possible. However, the different
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lvalueness of expressions between C and C++ complicate the behavior.
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G++ behaves the same as GCC for volatile access, See <a href="C-Extensions.html#C-Extensions">Volatiles</a>, for a description of GCC’s behavior.
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</p>
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<p>The C and C++ language specifications differ when an object is
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accessed in a void context:
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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>The C++ standard specifies that such expressions do not undergo lvalue
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to rvalue conversion, and that the type of the dereferenced object may
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be incomplete. The C++ standard does not specify explicitly that it
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is lvalue to rvalue conversion that is responsible for causing an
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access. There is reason to believe that it is, because otherwise
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certain simple expressions become undefined. However, because it
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would surprise most programmers, G++ treats dereferencing a pointer to
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volatile object of complete type as GCC would do for an equivalent
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type in C. When the object has incomplete type, G++ issues a
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warning; if you wish to force an error, you must force a conversion to
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rvalue with, for instance, a static cast.
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</p>
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<p>When using a reference to volatile, G++ does not treat equivalent
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expressions as accesses to volatiles, but instead issues a warning that
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no volatile is accessed. The rationale for this is that otherwise it
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becomes difficult to determine where volatile access occur, and not
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possible to ignore the return value from functions returning volatile
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references. Again, if you wish to force a read, cast the reference to
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an rvalue.
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</p>
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<p>G++ implements the same behavior as GCC does when assigning to a
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volatile object—there is no reread of the assigned-to object, the
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assigned rvalue is reused. Note that in C++ assignment expressions
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are lvalues, and if used as an lvalue, the volatile object is
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referred to. For instance, <var>vref</var> refers to <var>vobj</var>, as
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expected, in the following example:
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</p>
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<div class="smallexample">
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<pre class="smallexample">volatile int vobj;
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volatile int &vref = vobj = <var>something</var>;
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</pre></div>
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<hr>
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<div class="header">
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<p>
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Next: <a href="Restricted-Pointers.html#Restricted-Pointers" accesskey="n" rel="next">Restricted Pointers</a>, Up: <a href="C_002b_002b-Extensions.html#C_002b_002b-Extensions" accesskey="u" rel="up">C++ Extensions</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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