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<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 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>

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