C language order of precedence

Question

a = 5;
c = (b =a+2) - (a=1);

In book c programming a modern approach by kn king it is written that the effect of executing the second statement will result in 6 or 2 as it is undefined behavior of c but in other books like c by Dennis it is written that it will be executed from left to right. Which one is correct?

Answer 1

In the above case,

 c=(b=a+2) -(a=1);

the value of a is being changed and being read without a sequence point in between, so it is undefined behavior.

Quoting C11, Annex §J.2, Undefined behavior

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.

Also related, from chapter §6.5

The grouping of operators and operands is indicated by the syntax.⁸⁵⁾ Except as specified later, side effects and value computations of subexpressions are unsequenced.⁸⁶⁾

So, there's no guarantee which subexpression will get evaluated first.