Rule 12.2: Accessing volatiles in simple assignment statemnt

[misra-c]

The example:

Code:

x = v + v;

is strictly-speaking compliant with the rule. There are two read accesses of v, each of which will cause a side-effect, but because the + operator is commutative it doesn't matter in which order the two operands are evaluated in this case.

If the example were:

Code:

x = v - v;

then this would be non-compliant because the value of the expression will depend on the order in which the v operands are evaluated, unless they both evaluate to the same value.

It is also worth noting that:

Code:

x = v1 + v2;        // v1 and v2 both volatile

would be non compliant because the value of v1 might be affected by the evaluation of v2 and vice-versa. Any side-effects arising from the evaluation of v1 might be affected by the evaluation of v2 and vice-versa. The order in which any side-effects arising from evaluation of v1 and v2 would depend on the order in which they were evaluated and this might be observable, for example if v1 and v2 are I/O ports.

Having said all that, the rule does contain the recommendation that volatile objects are accessed only in simple assignments such as:

Code:

x = v;

to ensure that a sequence point occurs between each evaluation of a volatile object. Checking tools are not expected to implement recommendations, but they may do so if they wish. If the recommendation is checked then both of:

Code:

x = x + v;
x = v + v;

are non-compliant