Is “a += a” an undifined behaviour such as “i = i++”?

Question

I have understand that i = i++ is an undefined behaviour in C . But i have a doubt about a += a . Is it an undefined behaviour too ?

Answer 1

No, a += a is not undefined. The behavior of i = i++ is not defined by the C standard due to this rule in C 2018 6.5 2:

If a side effect on a scalar object is unsequenced relative to either a different side effect on the same scalar object or a value computation using the value of the same scalar object, the behavior is undefined.

That rule applies because both i++ and i = have side effects of updating i , and they are not sequenced. (Although the value computation of i++ , which produces the value to be used in the rest of the expression is sequenced before the assignment, its side effect of updating i is not sequenced relative to the assignment.)

In a += a , the value computation of the right operand ( a ) occurs before the assignment (according to 6.5.16 3), and then a += has the side effect of updating a . So:

• There is only one side effect, so there are not two unsequenced side effects.
• There is a side effect on a and a value computation of a , but they are sequenced.