Unsequenced expressionssuggest change
The following expressions are unsequenced:
a + b; a - b; a * b; a / b; a % b; a & b; a | b;
In the above examples, the expression
a may be evaluated before or after the expression
b may be evaluated before
a, or they may even be intermixed if they correspond to several instructions.
A similar rule holds for function calls:
Here not only
b are unsequenced (i.e. the
, operator in a function call does not produce a sequence point) but also
f, the expression that determines the function that is to be called.
Side effects may be applied immediately after evaluation or deferred until a later point.
x++ & x++; f(x++, x++); /* the ',' in a function call is *not* the same as the comma operator */ x++ * x++; a[i] = i++;
x++ & x; f(x++, x); x++ * x; a[i++] = i;
will yield undefined behavior because
- a modification of an object and any other access to it must be sequenced
- the order of evaluation and the order in which side effects1 are applied is not specified.
1 Any changes in the state of the execution environment.