<html lang="en">
<head>
<title>Qualifiers implementation - Using the GNU Compiler Collection (GCC)</title>
<meta http-equiv="Content-Type" content="text/html">
<meta name="description" content="Using the GNU Compiler Collection (GCC)">
<meta name="generator" content="makeinfo 4.13">
<link title="Top" rel="start" href="index.html#Top">
<link rel="up" href="C-Implementation.html#C-Implementation" title="C Implementation">
<link rel="prev" href="Structures-unions-enumerations-and-bit_002dfields-implementation.html#Structures-unions-enumerations-and-bit_002dfields-implementation" title="Structures unions enumerations and bit-fields implementation">
<link rel="next" href="Declarators-implementation.html#Declarators-implementation" title="Declarators implementation">
<link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage">
<!--
Copyright (C) 1988-2015 Free Software Foundation, Inc.
Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.3 or
any later version published by the Free Software Foundation; with the
Invariant Sections being ``Funding Free Software'', the Front-Cover
Texts being (a) (see below), and with the Back-Cover Texts being (b)
(see below). A copy of the license is included in the section entitled
``GNU Free Documentation License''.
(a) The FSF's Front-Cover Text is:
A GNU Manual
(b) The FSF's Back-Cover Text is:
You have freedom to copy and modify this GNU Manual, like GNU
software. Copies published by the Free Software Foundation raise
funds for GNU development.-->
<meta http-equiv="Content-Style-Type" content="text/css">
<style type="text/css"><!--
pre.display { font-family:inherit }
pre.format { font-family:inherit }
pre.smalldisplay { font-family:inherit; font-size:smaller }
pre.smallformat { font-family:inherit; font-size:smaller }
pre.smallexample { font-size:smaller }
pre.smalllisp { font-size:smaller }
span.sc { font-variant:small-caps }
span.roman { font-family:serif; font-weight:normal; }
span.sansserif { font-family:sans-serif; font-weight:normal; }
--></style>
</head>
<body>
<div class="node">
<a name="Qualifiers-implementation"></a>
<p>
Next: <a rel="next" accesskey="n" href="Declarators-implementation.html#Declarators-implementation">Declarators implementation</a>,
Previous: <a rel="previous" accesskey="p" href="Structures-unions-enumerations-and-bit_002dfields-implementation.html#Structures-unions-enumerations-and-bit_002dfields-implementation">Structures unions enumerations and bit-fields implementation</a>,
Up: <a rel="up" accesskey="u" href="C-Implementation.html#C-Implementation">C Implementation</a>
<hr>
</div>
<h3 class="section">4.10 Qualifiers</h3>
<ul>
<li><cite>What constitutes an access to an object that has volatile-qualified
type (C90 6.5.3, C99 and C11 6.7.3).</cite>
<p>Such an object is normally accessed by pointers and used for accessing
hardware. In most expressions, it is intuitively obvious what is a read
and what is a write. For example
<pre class="smallexample"> volatile int *dst = <var>somevalue</var>;
volatile int *src = <var>someothervalue</var>;
*dst = *src;
</pre>
<p class="noindent">will cause a read of the volatile object pointed to by <var>src</var> and store the
value into the volatile object pointed to by <var>dst</var>. There is no
guarantee that these reads and writes are atomic, especially for objects
larger than <code>int</code>.
<p>However, if the volatile storage is not being modified, and the value of
the volatile storage is not used, then the situation is less obvious.
For example
<pre class="smallexample"> volatile int *src = <var>somevalue</var>;
*src;
</pre>
<p>According to the C standard, such an expression is an rvalue whose type
is the unqualified version of its original type, i.e. <code>int</code>. Whether
GCC interprets this as a read of the volatile object being pointed to or
only as a request to evaluate the expression for its side-effects depends
on this type.
<p>If it is a scalar type, or on most targets an aggregate type whose only
member object is of a scalar type, or a union type whose member objects
are of scalar types, the expression is interpreted by GCC as a read of
the volatile object; in the other cases, the expression is only evaluated
for its side-effects.
</ul>
</body></html>