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| <h3 class="section">7.1 When is a Volatile Object Accessed?</h3> |
| |
| <p><a name="index-accessing-volatiles-3159"></a><a name="index-volatile-read-3160"></a><a name="index-volatile-write-3161"></a><a name="index-volatile-access-3162"></a> |
| Both the C and C++ standard have the concept of volatile objects. These |
| are normally accessed by pointers and used for accessing hardware. The |
| standards encourage compilers to refrain from optimizations concerning |
| accesses to volatile objects. The C standard leaves it implementation |
| defined as to what constitutes a volatile access. The C++ standard omits |
| to specify this, except to say that C++ should behave in a similar manner |
| to C with respect to volatiles, where possible. The minimum either |
| standard specifies is that at a sequence point all previous accesses to |
| volatile objects have stabilized and no subsequent accesses have |
| occurred. Thus an implementation is free to reorder and combine |
| volatile accesses which occur between sequence points, but cannot do so |
| for accesses across a sequence point. The use of volatiles does not |
| allow you to violate the restriction on updating objects multiple times |
| within a sequence point. |
| |
| <p>See <a href="Qualifiers-implementation.html#Qualifiers-implementation">Volatile qualifier and the C compiler</a>. |
| |
| <p>The behavior differs slightly between C and C++ in the non-obvious cases: |
| |
| <pre class="smallexample"> volatile int *src = <var>somevalue</var>; |
| *src; |
| </pre> |
| <p>With C, such expressions are rvalues, and GCC interprets this either as a |
| read of the volatile object being pointed to or only as request to evaluate |
| the side-effects. The C++ standard specifies that such expressions do not |
| undergo lvalue to rvalue conversion, and that the type of the dereferenced |
| object may be incomplete. The C++ standard does not specify explicitly |
| that it is this lvalue to rvalue conversion which may be responsible for |
| causing an access. However, there is reason to believe that it is, |
| because otherwise certain simple expressions become undefined. However, |
| because it would surprise most programmers, G++ treats dereferencing a |
| pointer to volatile object of complete type when the value is unused as |
| GCC would do for an equivalent type in C. When the object has incomplete |
| type, G++ issues a warning; if you wish to force an error, you must |
| force a conversion to rvalue with, for instance, a static cast. |
| |
| <p>When using a reference to volatile, G++ does not treat equivalent |
| expressions as accesses to volatiles, but instead issues a warning that |
| no volatile is accessed. The rationale for this is that otherwise it |
| becomes difficult to determine where volatile access occur, and not |
| possible to ignore the return value from functions returning volatile |
| references. Again, if you wish to force a read, cast the reference to |
| an rvalue. |
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