What is the meaning of --b is it the value should be deduct or not?

Answer 1

--b uses a predecrement. Since it has a high priority, it decrements b by 1 before most other operations.

Answer 2

If you have an assignment, for example:
b = 1;
v = --b;
it means that v takes the value of b after being decreased. So v will take value 0 and simultaneously b takes value 0. Otherwise, if you had v = b--; v would take value 1and then b would have value 0.

Answer 3

tq so much,guys