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| <h3 class="section">6.40 When is a Volatile Object Accessed?</h3> |
| |
| <p><a name="index-accessing-volatiles-2722"></a><a name="index-volatile-read-2723"></a><a name="index-volatile-write-2724"></a><a name="index-volatile-access-2725"></a> |
| C has the concept of volatile objects. These are normally accessed by |
| pointers and used for accessing hardware or inter-thread |
| communication. The standard encourages compilers to refrain from |
| optimizations concerning accesses to volatile objects, but leaves it |
| implementation defined as to what constitutes a volatile access. The |
| minimum requirement 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 volatile does |
| not allow you to violate the restriction on updating objects multiple |
| times between two sequence points. |
| |
| <p>Accesses to non-volatile objects are not ordered with respect to |
| volatile accesses. You cannot use a volatile object as a memory |
| barrier to order a sequence of writes to non-volatile memory. For |
| instance: |
| |
| <pre class="smallexample"> int *ptr = <var>something</var>; |
| volatile int vobj; |
| *ptr = <var>something</var>; |
| vobj = 1; |
| </pre> |
| <p>Unless <var>*ptr</var> and <var>vobj</var> can be aliased, it is not guaranteed |
| that the write to <var>*ptr</var> will have occurred by the time the update |
| of <var>vobj</var> has happened. If you need this guarantee, you must use |
| a stronger memory barrier such as: |
| |
| <pre class="smallexample"> int *ptr = <var>something</var>; |
| volatile int vobj; |
| *ptr = <var>something</var>; |
| asm volatile ("" : : : "memory"); |
| vobj = 1; |
| </pre> |
| <p>A scalar volatile object is read when it is accessed in a void context: |
| |
| <pre class="smallexample"> volatile int *src = <var>somevalue</var>; |
| *src; |
| </pre> |
| <p>Such expressions are rvalues, and GCC implements this as a |
| read of the volatile object being pointed to. |
| |
| <p>Assignments are also expressions and have an rvalue. However when |
| assigning to a scalar volatile, the volatile object is not reread, |
| regardless of whether the assignment expression's rvalue is used or |
| not. If the assignment's rvalue is used, the value is that assigned |
| to the volatile object. For instance, there is no read of <var>vobj</var> |
| in all the following cases: |
| |
| <pre class="smallexample"> int obj; |
| volatile int vobj; |
| vobj = <var>something</var>; |
| obj = vobj = <var>something</var>; |
| obj ? vobj = <var>onething</var> : vobj = <var>anotherthing</var>; |
| obj = (<var>something</var>, vobj = <var>anotherthing</var>); |
| </pre> |
| <p>If you need to read the volatile object after an assignment has |
| occurred, you must use a separate expression with an intervening |
| sequence point. |
| |
| <p>As bitfields are not individually addressable, volatile bitfields may |
| be implicitly read when written to, or when adjacent bitfields are |
| accessed. Bitfield operations may be optimized such that adjacent |
| bitfields are only partially accessed, if they straddle a storage unit |
| boundary. For these reasons it is unwise to use volatile bitfields to |
| access hardware. |
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