I read about APUE 3rd, 8.16 Process Scheduling, there is an example written to verify that changing nice value of a process will affect its priority, I rewrite the code like below: And the result of the example is shown below: NZERO = 20 parent nice:20 child nice:20, adjusted by 0 child now nice:20 parent count:601089419 child count:603271014 Looks like