The comma operator can be used to link the related expressions together. A comma-linked list of expressions is evaluated left-to-right and the value of the rightmost expression is the value of the combined expression. It acts as a sequence point.
A sequence point guarantees that all side effects of previous evaluations will have been performed, and no side effects from subsequent evaluations have yet been performed.
So, any expression/assignment will be completed & only then will the next expression to the right be evaluated.
Also, note that the comma operator ranks last in the precedence list of operators in C.