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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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