As mentioned here, the C standard states:
Between the previous and next sequence point an object shall have its
stored value modified at most once by the evaluation of an expression.
Furthermore, the prior value shall be accessed only to determine the
value to be stored.
a = ++b + ++c;
Provided the objects involved (
c) in the expression
a = ++b + ++c; are distinct, that expression is well-defined.
Perhaps, the author meant to use the same variable twice such as
a = ++b + ++b;. I can only speculate. But there's no undefinedness in the given expression.