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<a name="Qualifiers-implementation"></a>
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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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<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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