| Definite Access of Volatile Objects

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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 that it might perform on non-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.
In most expressions, it is intuitively obvious what is a read and what is a write. For instance

    volatile int *dst = somevalue;
    volatile int *src = someothervalue;
    *dst = *src;

will cause a read of the volatile object pointed to by src and stores the value into the volatile object pointed to by dst. There is no guarantee that these reads and writes are atomic, especially for objects larger than `int`.
Less obvious expressions are where something which looks like an access is used in a void context. An example would be,

    volatile int *src = somevalue;
    *src;

With C, such expressions are rvalues, and as rvalues cause a read of the object, GCC interprets this as a read of the volatile being pointed to.