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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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